Atmospheric Degradation of Ozone Depleting Substances, Their

Review pubs.acs.org/CR

Atmospheric Degradation of Ozone Depleting Substances, Their Substitutes, and Related Species James B. Burkholder,*,† R. A. Cox,‡ and A. R. Ravishankara§ †

Chemical Sciences Division, Earth System Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305, United States ‡ Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP, United Kingdom § Departments of Chemistry and Atmospheric Science, Colorado State University, 1872 Campus Delivery, Fort Collins, Colorado 80523, United States 5.7. Perfluoroketones 5.8. Perfluorocarbons (PFCs) and Persistent Greenhouse Gases 5.9. Hydrochlorocarbons and CCl4 5.10. Hydrobromocarbons and Hydroiodocarbons 6. Degradation Products − Photolysis 6.1. Photolysis Processes Involving Inorganic Degradation Products of CFCs and Halons 6.1.1. Photolysis of ClO Dimer (ClOOCl) 6.1.2. Photolysis of OIO 6.1.3. Photolysis of Dihalogen Compounds 6.1.4. Photolysis of Halogen Oxyacids, Nitrates, and Nitryl Compounds 6.2. Photolysis of Halogenated Carbonyl Compounds 7. Degradation Products − Heterogeneous Processes 7.1. Removal of Degradation Products in Aqueous Droplets 7.1.1. Tropospheric Lifetimes Based on Laboratory Measurements of Uptake Coefficients 7.1.2. Partitioning and Uptake of Halogenated Species on Sulfuric Acid Surfaces 7.2. Removal of Halogen-Containing Degradation Products on Ice Surfaces 7.2.1. Physical Removal of HCl 7.2.2. Physical Scavenging of Organic Species 7.2.3. Heterogeneous Scavenging of Partially Fluorinated Alcohols (PFA) 7.2.4. Reactive Scavenging of Halogenated Carbonyls on Ice 7.3. Heterogeneous Reactions Leading to Halogen Activation 7.3.1. Heterogeneous Halogen Activation on Liquid Surfaces (Water, Salts, H2SO4) 7.3.2. Cl Activation by N2O5 7.3.3. Activation and Scavenging of Bromine

CONTENTS 1. Introduction 2. The Chemistry−Climate Connection for Ozone Depleting Substances and Substitutes 3. Degradation Mechanisms of ODSs and Substitutes 4. Atmospheric Loss Processes 4.1. Gas-Phase Reactions 4.1.1. Atmospheric Oxidants, Their Distribution, and Abundance 4.2. UV and VUV Photolysis 4.3. Heterogeneous Processes 4.3.1. Reactions of Importance in ODS Chemistry 4.3.2. Removal of Degradation Products in Aqueous Droplets 4.3.3. Uptake of Degradation Products on Solid Surfaces 4.3.4. Scavenging by Adsorption on Solid Particles 4.3.5. Characteristic Heterogeneous Reactions 4.3.6. Aerosols of Importance in the Stratosphere 4.3.7. Aerosols of Importance in the Troposphere 4.3.8. Kinetics of Heterogeneous Reaction in Aerosol Particles 5. Atmospheric Source Gases 5.1. Chlorofluorocarbons (CFCs) 5.2. Halons 5.3. Hydrochlorofluorocarbons (HCFCs) 5.4. Hydrofluorocarbons (HFCs) 5.5. Halo-olefins 5.5.1. Hydrofluoro-olefins (HFOs) and Perfluoro-olefins (PFOs) 5.5.2. Chloro-olefins 5.5.3. Bromo-olefins 5.6. Hydrofluoroethers (HFEs) and Hydrochlorofluoroethers

© XXXX American Chemical Society

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Special Issue: 2015 Chemistry in Climate

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Received: November 28, 2014

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DOI: 10.1021/cr5006759 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 7.3.4. Heterogeneous Halogen Activation on Solid Surfaces (Ice, NAT, and SAT) 8. Conclusions and Future Directions 9. Halocarbon Nomenclature 9.1. Uppercase Lettering 9.2. Numbering 9.3. Lowercase Lettering (Isomers) 9.4. Bromofluorocarbons and Bromochlorofluorocarbons Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

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1. INTRODUCTION Ozone depleting substances (ODSs), listed in the Annexes to the Montreal Protocol (1987), are manmade chemicals emitted into the atmosphere that deplete the stratospheric ozone layer. There are many substances that can deplete the ozone layer, but here we focus on those listed in the Protocol and those suggested or currently used as substitutes for these ODSs. ODSs listed in the Protocol contain chlorine and bromine. The majority of ODSs are also fluorinated substances; the key exceptions are 1,1,1-trichlorethane (methyl chloroform, CH3CCl3), methyl bromide (CH3Br), and methyl chloride (CH 3 Cl). CH 3 Br and CH 3 Cl have both natural and anthropogenic sources. Chlorofluorocarbons (CFCs) are probably the best-known ODSs and were the first to be linked to ozone layer depletion by the seminal paper of Molina and Rowland.1 Other major classes of ODSs include Halons, which are fully halogenated substances containing bromine, and hydrochlorofluorocarbons (HCFCs), which were for the most part introduced as substitutes for CFCs. CHClF2 (HCFC-22, see section 9 on nomenclature) is a key exception in that it was available and used commercially at the same time as the CFCs. ODSs are sufficiently stable that they can be used in various appliances, can be easily transported, and are safe to store. Their inherent stability, however, also means that these compounds are quite inert in the atmosphere and accumulate in the atmosphere if emitted. For example, CFCs and Halons have negligible, or no, reaction with the OH radical, Cl atom, NO3 radical, and O3, the most prevalent atmospheric oxidants in the lower atmosphere (troposphere). CFCs are also not photolyzed in the lower atmosphere. Their primary loss is via transport to the stratosphere followed by destruction there by UV photolysis via the absorption of short-wavelength radiation that is not present in the lower atmosphere. Similarly, Halons are also stable in the lower atmosphere (although there is some loss via UV photolysis) and are photolyzed in the stratosphere. Stratospheric photolysis of CFCs and Halons liberates chlorine and bromine, which take part in catalytic ozone destruction as illustrated for chlorine in Figure 1. More reactive ODSs, such as CH 3Br, react in the troposphere, and only a fraction of the amount emitted reaches the stratosphere. The fraction transported to the stratosphere depends on their rate of removal in the troposphere and the transport time to the stratosphere. More recently, attention has shifted to molecules that are very short-lived, and thus only a very small fraction of their emission reaches the stratosphere. Understanding the ODS loss processes and lifetimes in the

Figure 1. Schematic of CFC and HCFC degradation in the atmosphere and chlorine ozone depletion cycles in the stratosphere. The diagram shows the source gas loss processes (initiation step) leading to the production of reactive chlorine, stratospheric chlorine chemistry, and the atmospheric processing of halogenated coproducts leading to the ultimate removal of halogen from the atmosphere.

atmosphere is therefore an important factor in evaluating the impact of ODSs on stratospheric ozone. When the ozone layer depletion by CFCs and Halons was identified, the main question regarding of the degradation of ODSs was: how rapidly does atmospheric degradation lead to the release of chlorine and bromine? In general, stratospheric degradation of these compounds leads to rapid release of chlorine and bromine. Now, the interest has expanded to understanding the complete degradation of these compounds to stable end-products and the potential impact of the degradation products on other environmental issues such as climate change and toxic chemical formation. Further, our understanding of transport to the stratosphere has increased, and it is known that even degradation products formed in the troposphere can reach the stratosphere depending on when and where they are emitted. These interests have been driven by the proposed use of very short-lived substitutes (VSLS) that are expected to be primarily degraded in the troposphere. The primary goal of the Montreal Protocol (1987) was to protect the ozone layer against destruction by anthropogenic ODSs by limiting their production and emissions. The initial protocol was amended and adjusted multiple times as evidence for ozone layer depletion became stronger. A multistep strategy for moving to a nonozone-layer depleting world was adopted and implemented; this strategy is shown in Figure 2. Today, CFCs and methyl chloroform (CH3CCl3) are phased out of production, Halons are not being manufactured to any significant extent, and other ODSs such as CH3Br are almost completely phased out. On the basis of the Montreal Protocol’s strategy, ODSs collectively are decreasing in the atmosphere, and the ozone layer is expected to return to its benchmark 1980 levels around 2030 in the midlatitudes and the Antarctic ozone hole is not expected to occur after roughly 2060.3 HCFCs were introduced to replace the phased out ODSs in various uses. The strategy in using HCFCs was to exploit their shorter atmospheric lifetime due to the presence of H atoms in the molecules, thus ensuring that a smaller fraction of the emitted amount is transported to the stratosphere. The B


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are commonly referred to as very short-lived substances (VSLSs) in the literature. These substitutes, for example, hydrofluoro-olefins (HFOs), are substances with lifetimes of days to weeks, which is much shorter than the time scale for mixing between the hemispheres and transport to the stratosphere. They are not well mixed in the troposphere; their short-lifetime effectively reduces the fraction of their emission reaching the stratosphere and their accumulation in the atmosphere as compared to the longer-lived HCFCs and HFCs. The short lifetime of a molecule arises from an enhanced reactivity with tropospheric free radicals and/or due to its rapid tropospheric solar photolysis. The variability of free radical abundance and solar flux with location, season, and time of day means that the rate of loss of the substitute will change significantly with tropospheric conditions.8 Thus, the transport of the substitute from one region of the troposphere to another becomes an important consideration. Because of these factors, it is not feasible to define a single atmospheric lifetime for a short-lived substance, and atmospheric models are typically used to evaluate a substitute’s impact on stratospheric ozone layer depletion and climate change. As mentioned earlier, the main agents for tropospheric degradation are the ubiquitous OH radicals as well as Cl atoms, NO3 radicals, and solar photolysis. If the molecule reaches the stratosphere, photolysis in the short-wavelength UV and reaction with O(1D) atoms also contribute to ODS loss. Reactions with O 3 , which is present throughout the atmosphere, may be important in special cases. The overarching goal of the use of substitutes has been to minimize ozone layer depletion, have as little impact as possible on climate, and be biochemically and environmentally safe in other ways. Further, in this effort, very reactive substances have been introduced. Therefore, we are not only interested in the release of halogen atoms from the parent molecule, but also in the products generated by the degradation. Potential issues related to the degradation of the substitutes include: (1) the possible formation of longer-lived atmospheric species following the atmospheric degradation of a short-lived species and their potential role as greenhouse gases or in transporting halogen to the stratosphere; (2) formation of potentially environmentally harmful species, for example, trifluoroacetic acid (CF3C(O)OH, TFA) or other fluoroacids; and (3) production of tropospheric ozone from reactions of the substitutes, both in the vicinity and away from the emission regions. This Review discusses the connections of ODSs and substitutes to the depletion of the ozone layer, influence on climate, and impact on the environment through formation of toxic species (e.g., tropospheric ozone and toxic chemicals in water). It starts with an examination of the processes that initiate removal of ODSs and their substitutes. We then discuss the identification of the major first generation halogen containing products, followed by the identification of the major stable halogenated end-products, and the fate of the stable products. Rate coefficients for the initiation are not reviewed, but references to such relevant material are provided.

Figure 2. Diagram of the strategy used by the Montreal Protocol to transition from CFCs and other potent ODSs to substitute substances that are essentially benign to the ozone layer. The main path was to use shorter-lived lower ODP chemicals such as hydrochlorofluorocarbons (HCFCs). The HCFCs were then replaced by hydrofluorocarbons (HFCs), as shown by the gray arrows. However, substitution of ODSs to HFCs and alternate technologies were also implemented as shown by the blue arrows. The concern over the climate impact of ODSs and their substitutes was also a factor in the development of the substitution strategy and thus led to attempts to avoid high global warming potential (GWP) HFCs (a current concern) via use of low-GWP HFCs or alternate technologies, as shown by the thin gray lines. Figure adapted from UNEP HFC report.2

presence of H atoms allows these molecules to react with OH in the troposphere, and a fraction of their emissions are degraded in the troposphere minimizing the fraction that reaches the stratosphere. Therefore, the ozone depletion potential of HCFCs is much smaller than those of the CFCs and Halons they replaced even though they contain chlorine. The lower ozone depletion potential, ODP, of HCFCs, coupled with their ability to be used in existing equipment and other applications, allowed for a smooth transition out of using CFCs. (Ozone depletion potential, ODP, is a metric for measuring the effectiveness of an ozone depleting substance X relative to CFC-11 in depleting the stratospheric ozone layer; this metric has been described and discussed previously.4) In some applications, CFCs were replaced by hydrofluorocarbons (HFCs) that have been shown to be nonozone-layerdepleting,5 and thus completely bypassed the issue of ozone depletion. The removal of the HCFCs in the troposphere has an additional benefit. For equal emissions of a CFC and an HCFC, there is a smaller buildup of the emitted molecules in the atmosphere.6 Therefore, HCFCs would also contribute less to climate change. Starting around 2007, even HCFCs were designated for a phasedown. In response to these phase-outs and phasedowns, the use of HFCs in their place has increased significantly. However, HCFCs and many of the HFCs are also potent greenhouse gases. It is projected that the climate effects of the continued and increasing use of the current mix of HFCs would be harmful to climate;2,7 indeed, as measured in terms of the radiative forcing metric, the emissions going forward from now on could be 25% of the emissions of CO2 by 2050.2,3,7 Therefore, there are efforts underway to utilize shorter-lived and, hence, lower global warming potential (GWP) substitutes in place of the high-GWP HFCs. (The concept of GWP is similar to that of ODP, and it is a metric for evaluating the global warming by the emission of a greenhouse gas. Global warming potentials are discussed in WMO/UNEP 20064b and Ravishankara et al. in this issue.) Clearly, since the recognition of the ozone layer depletion by ODSs, there has been a continued and persistent shift toward the use of shorter and shorter-lived substances. Molecules with lifetimes of the order of days are now being considered; they

2. THE CHEMISTRY−CLIMATE CONNECTION FOR OZONE DEPLETING SUBSTANCES AND SUBSTITUTES There are four primary reasons for thinking about the degradation of the ODSs and their substitutes in connection to climate and climate change: C


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(1) How rapidly do the ODSs and their substitutes that are also greenhouse gases degrade in the atmosphere and thus determine their atmospheric abundances? (2) Does the degradation of ODSs and their substitutes lead to formation of chemicals that are longer lived than the parent compounds and, thus, potentially lead to more climate forcing than a consideration of only the parent compounds would suggest? (3) Where and when do these substances degrade and thus determine the extent of ozone depletion (or production in the troposphere) and thus influence climate, noting that ozone itself is also a greenhouse gas and it affects abundances of other climate forcing agents? (4) How does climate change alter the rates of degradation of these chemicals and identities of products formed? This is because changes in atmospheric circulation and changes in water vapor are some of the major drivers of this change. One of the driving factors in finding suitable substitutes for CFCs and Halons was to ensure that they are also not greenhouse gases with larger GWPs. Simply put, this was to ensure that the Montreal Protocol did not have a negative impact on another environmental issue while trying to rectify the ozone layer depletion. Two of the potential classes of substitutes that were considered were HFCs and perfluorocarbons, PFCs. It was clear that the use of PFCs was going to be detrimental to climate because their lifetimes are very long and they have very large GWPs (approaching tens of thousands). Therefore, PFCs fell out of consideration. On the other hand, HFCs were deemed acceptable substitutes for many uses. Their use has increased significantly. However, now it is becoming clear that their continued and increasing use will harm climate, and thus there is a drive to find substitutes that are more benign to climate.

Figure 4. A conceptual construct for the process by which a molecule emitted into the atmosphere (X) is converted to stable products that are subsequently removed from the atmosphere.

3. DEGRADATION MECHANISMS OF ODSs AND SUBSTITUTES The Earth’s atmosphere (see Figure 3) is to a first approximation an oxidizing environment; any molecule that is emitted into the atmosphere is oxidized. Atmospheric oxidation of a chemical emitted at the surface or the lower atmosphere consists of three primary steps (see Figure 4): (1) initiation, the step that destroys (removes) the emitted species; (2) propagation, a sequence of processes that continues the degradation and describes the fate of the radicals or reactive species formed in previous steps; and (3) termination, processes that lead to the formation of stable species. It is important to note that the formation of a “stable” species does not mean that that molecule will not be degraded further in the atmosphere. In this Review, we consider termination to be when the molecule formed requires further reactions with energetic free radicals and/or has a lifetime longer than the time scale for substantial atmospheric transport, that is, lifetimes of days or greater. The majority of the ODSs and their substitutes are carbonbased chemicals. Therefore, it is customary to follow the fate of carbon centric entities in the degradation. Of course, because of the importance of the liberated halogens, the course of oxidation to the formation of the halogenated species is also noted. Most often, the rate-determining step in the degradation of ODSs and their substitutes is the initiation step. As discussed below, initiation occurs for the most part via gas-phase reaction with reactive free radicals or by UV photolysis. In general, heterogeneous processes do not initiate the degradation, unless oxidizing species are present in the condensed phase. Propagation consists of a large set of simultaneous possible reactions that usually produce oxidized reactive free radicals. The individual propagation steps are usually very rapid as compared to the initiation step. Finally, reactions between free radicals and the formation of a stable product lead to termination. For the ODSs and their substitutes, the stable end-products are likely to be species such as CO, CO2, and halogenated aldehydes or ketones.

Figure 3. Temperature (lower scale) structure and the partial pressure of O2 (upper scale) in the Earth’s atmosphere. The three main regions of the atmosphere are shown. They are marked by the changes in temperature gradients. The locations of those changes are referred to as tropopause (around 20 km) and stratopause (around 50 km). The troposphere holds about 82% of the atmosphere, while the stratosphere and troposphere together account for roughly 99.9% of the atmosphere. The rough range of temperature encountered in the troposphere and stratosphere is shown as the teal shaded area. D


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droplets or polar stratospheric clouds (PSCs) in the very cold polar regions. Even within the troposphere, there are some distinctions between the boundary layer (close to emissions, close to deposition surfaces, and where water vapor is high), the free troposphere, and the upper part of the free troposphere. Under high NOx conditions, RO2 radicals predominantly react with NO:

The development of a comprehensive degradation mechanism requires consideration of all of the individual steps as outlined in the reaction tree in Figure 4, as well as their dependence on atmospheric conditions. There is often a competition between different processes in some steps, and the final outcome depends on the characteristics of the source molecule (X) and the rates of the degradation processes under relevant atmospheric conditions. Therefore, different oxidants may lead to different reaction products. The formation of a specific reaction product depends on the relative contribution of that process to the overall rate. Also, minor processes that may not be significant for the removal of a species may be important for the formation of unique products. There have been extensive measurements carried out to characterize the degradation of ODSs and their substitutes. In general, although a great deal of effort has been expended to characterize and quantify the initiation step, the subsequent degradation steps are not always well characterized experimentally. They are often inferred from the observation of stable end-products that are sometimes obtained only under a limited range of experimental conditions, that is, temperature and pressure as well as O2 and NOx abundances. The majority of stable end-product yield studies have been performed at room temperature with major product yield uncertainties on the order of ±10%. Mass balance between reactant loss and product formation of >80% is typical. End-products with yields of CC< double bond, ozonolysis, to form a primary ozonide, for example:

local and global photolysis lifetimes. Absorption spectra for ODSs and replacement compounds are typically measured experimentally using single wavelength (monochromator) and broadband (e.g., diode array and CCD) methods. Compre-

which thermally decomposes to form an aldehyde, for example, CF3CFO or CH2O in this example, and a Criegee biradical intermediate (CI), CH2OO or CF3CFOO. The CI could be halogenated depending on the composition of the source gas. In general, halo-olefin ozonolysis reactions are relatively slow (significantly less than the analogous alkene ozonolysis), but the higher O3 concentration over other atmospheric oxidants requires its consideration. The reactions of the OH radical and, to a lesser extent, Cl atom reactions represent the predominant loss process for halo-olefin source gases, although the O3 reaction can play a role under some circumstances. For example, the [O3]/[OH] ratio in the troposphere has a value of ≤1 × 106. Therefore, the kOH/kO3 ratio would need to be C−Br > C−I. For example, the bond energies and wavelength thresholds for the photodissociation of CH3F, CH3Cl, CH3Br, and CH3I are 111.1 kcal mol−1 (∼257 nm), 83.6 kcal mol−1 (∼342 nm), 70.5 kcal mol−1 (∼405 nm), and 57.4 kcal mol−1 (∼498 nm), respectively, for the formation of a halogen atom + CH3 radical. Although energetically accessible the majority of ODSs do not actually photodissociate in the atmosphere at, or near, their threshold wavelength due to negligible absorption in those wavelength regions. The rate of atmospheric photolysis depends on the overlap of the absorption spectrum of the molecule, the quantum yield for the dissociation process to occur, and the solar flux. The first-order photolysis rate coefficient, J (s−1), for the removal of a molecule via photolysis is given by J(s−1) =

∫ J(λ) dλ = ∫ σ(λ , T )ϕ(λ , T )ψ (λ , Z , χ ) dλ (E4.2-1)

where σ(λ,T) is the absorption cross section at wavelength λ of the molecule at temperature T, ϕ(λ,P,T) is the molecule’s photolysis quantum yield at the given wavelength, pressure (P), and temperature, and ψ(λ,Z,χ) is the solar flux (photons cm−2 s−1 nm−1), which is a function of wavelength, altitude (Z), albedo, overhead column abundances of absorbing species, and solar zenith angle (SZA, χ). Laboratory measurements provide the fundamental molecular data needed to calculate atmospheric photolysis rates and J


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The solar flux term in eq E4.2-1 is obtained using radiative transfer models. Figure 7 shows representative solar flux altitude and wavelength dependence obtained using the TUV calculator33 for midlatitude summer conditions. The solar flux shows a strong wavelength and altitude dependence in the 190−400 nm region due to the UV absorption of O3 (see O3 profile in Figure 5). The key wavelength regions for atmospheric photolysis are 190−230 nm (in the stratosphere) and >290 nm (in both the troposphere and the stratosphere). Molecules that exhibit only short wavelength UV absorption, for example, CFCs and HCFCs, will be photolyzed exclusively in the stratosphere. Not shown in Figure 7 is the solar flux contribution in the vacuum ultraviolet (VUV) wavelength region, λ < 170 nm. VUV radiation only penetrates the upper atmosphere, Z > 60 km, due to absorption by O2 and N2, and photodissociation at these altitudes occurs primarily at the H atom Lyman-α line (121.567 nm), which is in a “window” in the O2 absorption spectrum. Experimental data for Lyman-α absorption cross sections, σ(L-α), are rather sparse. SPARC15 provided an evaluation of σ(L-α) for a number of key ODSs and greenhouse gases and also showed that VUV photolysis loss was only significant for persistent gases, that is, compounds with atmospheric lifetimes greater than ∼300 years, for example, NF3 and PFCs.15 Photolysis rate coefficients for representative CFCs, HCFCs, Halons, and a few other compounds are given in Figure 8. For CFCs and Halons, UV photolysis is the predominant atmospheric loss process and the mechanism for the release of reactive halogen. CFCs are removed by short wavelength UV photolysis with the majority of the loss by photolysis in the 195−220 nm region, while Halons are removed by both shortand long-wavelength photolysis (the relative importance of the short- and long-wavelength regions is altitude dependent and

In most cases, photodissociation of ODSs and their substitutes leads to the direct formation of reactive halogen, Cl or Br-atoms, as well as molecular halogenated radical products. The halogenated molecular radicals degrade in the atmosphere following pathways analogous to the degradation initiated via free radical reactions. Photolysis of a source gas may, however, lead to the formation of different intermediate halogenated species than would otherwise be obtained via chemical reaction. Thus, photolysis and chemical reaction removal of a source gas can have different atmospheric impacts

Figure 7. Representative altitude dependence of the solar flux. Fluxes from the NCAR TUV calculator33 for midlatitude summer conditions.

depending on the region of source gas degradation and the fate of the subsequent degradation products.

Figure 8. Global annually averaged atmospheric photolysis rate coefficients for representative chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), Halons, and related compounds. 2-D model calculation data from the SPARC lifetime report.15 K


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best quantified using atmospheric models). For the majority of the HCFCs, photodissociation occurs in the short-wavelength UV region (stratosphere), but is typically a secondary loss process compared to loss via free radical reactions. HFCs have very weak absorption at wavelengths >169 nm such that atmospheric photolysis in this wavelength region makes a negligible contribution to their atmospheric loss. HFCs are only removed photolytically in the upper stratosphere and mesosphere by Lyman-α photolysis. Hydrochlorocarbons and hydrobromocarbons have very minor tropospheric photolysis losses. CCl4 also has negligible tropospheric photolysis loss even though it is highly chlorinated.

partition equilibrium coefficients are fundamental (often independently measurable) parameters that are used in Table 2. Degradation Products Involved in Heterogeneous Processes species X = F, Cl, Br, I

heterogeneous reaction/hydrolysis

HX

√, reaction

√, water, ice aerosol

XNO2

√, hydrolysis

XONO2 X2 XY

√, water (PGI)b √, water, ice

√, reaction √, hydrolysis √, hydrolysis Carbonyl Products √, hydrolysis √, water (PGI) √, water (PGI)

CX2O CXYO HCXO CX3CHO

XC(O)OH CX3C(O)OH FC(O)OF CF3COF CF3COCl CX3C(O)OONO2 CX3OONO2 CF3CH2OH CF3CH2OC(O)H CH3C(O)OCH2CF3 CF3C(O)OCH3 C2H5OC(O)CF3 a b

Inorganic Products √,a reaction √, water, ice

HOX I2O5

4.3. Heterogeneous Processes

Heterogeneous processes are often the termination of the degradation of ODSs and their substitutes. More importantly, they lead to removal of the oxidized species from the atmosphere and thus represent the end of the oxidation. In the case of the ODSs, the eventual end-products are CO2, CF2O, and HCl/HBr/HI. All of these are removed to varying extent via heterogeneous processes. In addition, heterogeneous reactions hydrolyze species such as CF2O to lead to very stable HF and also promote halogen activation reactions that lead to increased concentrations of catalytic species. Over the past two decades, the introduction of shorter-lived substitutes has led to additional interest in heterogeneous processes for the following reasons: (1) the extent of transport of short-lived ODSs and their degradation products to the stratosphere critically depends on their interactions on liquid or solid particles encountered during transport of species to the stratosphere; and (2) the formation of potentially toxic byproducts of the oxidation of the substitutes is of major concern. The toxic chemicals, for example TFA, are deposited by heterogeneous and multiphase processes. So, there is enhanced attention on heterogeneous processes associated with the degradation of CFC substitutes. Heterogeneous reactions on atmospheric particles can also play an indirect role by increasing local concentrations of oxidizing free radical species, or by modification of photolysis rates of stable species through their adsorption on particle surfaces. It is known that the absorption spectra of molecules adsorbed on atmospheric mineral dust surface are shifted to longer wavelengths, which could significantly decrease the photolysis lifetimes of these long-lived ODS. This possibility was investigated following the ozone depletion hypothesis in 197434 and was found to be quite small. Similar conclusions apply for HCFC-22. The ultimate step in the removal of halogen produced from ODSs and their substitutes is removal from the stratosphere in the form of the most stable gas-phase molecules, that is, HX (X = F, Cl, or Br), or in the case of X = I, as I2O5 particles. In certain cases, exceptionally stable halogenated organics, for example, halogenated carbonyls and carboxylic acids, are formed and can be removed from the stratosphere, along with HX, by downward transport. Once they enter the troposphere, these molecules partition readily onto aerosol particles or into hydrometeors (rain or snow), and ultimately are deposited in precipitation and removed from the air. The heterogeneous processing of trace gases in the atmosphere involves their partitioning into the condensed phase, the extent of which depends on the nature and surface area/volume of particulate matter, and on the solubility or surface adsorption equilibria of the specific trace gas molecules involved. The

physical removal

comment X = F, Cl, Br, I X = Cl, Br, I particle nucleation X = Cl X = Cl, Br, I X = Cl, Br, I

NASA/JPL11 NASA/JPL11

√, hydrolysis

√, water, ice (PGI) Organic Acids

√, water, ice Acid Halides √, hydrolysis √, hydrolysis √, water √, hydrolysis √, water Peroxynitrates (PGI) (PGI) Fluoro-alcohols √ Fluoro-esters √ √ √ √

NASA/JPL11

NASA/JPL11 NASA/JPL11

√ = products for which heterogeneous reactions have been studied. PGI = potential for product gas injection.

quantitative treatment of these heterogeneous processes. The degradation product types that are involved in heterogeneous reactions are given in Table 2. 4.3.1. Reactions of Importance in ODS Chemistry. The majority of the halogen loading currently in the stratosphere is accounted for by longer-lived ODS. However, there is now evidence suggesting that halogenated very short-lived substances (VSLS, lifetimes less than ∼0.5 yr) and their degradation products contribute to halogen in the stratosphere, through a process known as product gas injection (PGI), where halogen containing products from the degradation of the ODSs are transported. To assess the importance of this process, estimates of the relative rates of physical scavenging and oxidation of the VSLS and active halogen release from their degradation products in the upper troposphere lower stratosphere (UTLS) region are required. Both of these processes are influenced by heterogeneous reactions. L


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4.3.2. Removal of Degradation Products in Aqueous Droplets. In the following section, we briefly give the equations for processes involved in the heterogeneous reactions. Readers are referred to material in publications for the details of these processes and the derivation of these equations.35 The driving force for gas uptake into liquid droplets is its solubility. This is expressed in terms of the Henry’s law constant, H (M atm−1), which is the equilibrium constant for the gas partitioned between the gas and liquid phases. The rate of gas−liquid transfer under quasi-steady state conditions is determined by the uptake coefficient, γ, which is the fraction of gas molecules hitting the surface that enter the condensed phase. γ for uptake into a liquid depends on exposure time, t, and can be described (assuming no restrictions due to gas-phase diffusion) by the resistance model.36 1 1 1 = + γ αb Γsol

Γsol =

Dl t

4HRT c̅ π

Kw is the autoprotolysis constant of H2O, Kdiss is the acid dissociation constant of the trace gas in water, and [H+]e is the equilibrium hydrogen ion concentration at the droplet surface. Removal rates of halogenated degradation products in the aqueous phase depend primarily on the species solubility and rate of hydrolysis to form their corresponding acids. For such a case, three parameters are needed to evaluate heterogeneous uptake rates: the Henry’s law coefficient, H (M atm−l), which describes solubility; the hydrolysis rate coefficient, khyd (s−l); and the liquid-phase diffusion coefficient Dl (cm2 s−l). An example is the hydrolysis of carbonyl halides as represented by X 2CO (g) → X 2CO (aq)

X 2CO + H 2O → CO2 + 2X− + 2H+

(E4.3-1)

1 1 1 = + γeff γ Γdiff

The magnitude of Γdiff depends on the surface geometry and the gas-phase diffusivity of the arriving molecules. Diffusion coefficients are given in a recent evaluation by Tang et al.37 When trace gas uptake to the bulk of liquid (usually aqueous) particles is followed by chemical reaction, the process is not usually reversible. The reactive uptake coefficient, γr, is given by (E4.3-2)

τD1/2 =

kIrxn

where is the pseudo-first-order rate coefficient for reaction of species taken up into the bulk. For small droplets the concept of the diffuso-reactive length is important.35c This is the average distance beyond the surface of the particle in which reactions take place and is given by l=

(E4.3-3)

For spherical particles, eq E4.3-2 can be modified to account for this with 1 1 c̅ = + I ⎡ γr αb 4HRT D l k rxn ⎣coth

( rl ) − ( rl )⎤⎦

τcloud =

d 6D l1/2

(E4.3-6)

1 −1/2 (k hyd + τD1/2) 1/2 Lcfc RTH(k hyd)

(E4.3-7)

where L c is the average cloud liquid water content (approximately 0.3 g/m3 = 3 × 10−7cm3/cm3) and fc is the average fraction of the global tropospheric volume containing clouds fc = 0.151. The overall lifetime against heterogeneous removal must also take into account uptake into the oceans, which can be described by a lifetime expression analogous to eq E4.3-7. Heterogeneous reaction can also determine the rate of uptake

(E4.3-4)

Note that for a molecule that can dissociate in the aqueous phase, an effective solubility, H*, is used whereby ⎛ 1 + Kdiss[H+]e ⎞ H* = H × ⎜ ⎟ Kw ⎝ ⎠

(4.3-2)

For a typical cloud droplet of 10 μm in diameter, 1/τD = 500 s−1. Because most halogenated species under consideration have khyd < 500 s−1, their tropospheric cloud processing will be limited by the product Hkhyd. The lifetime of a species from global cloud processing in the troposphere is given by

Dl I k rxn

k hyd

(4.3-1)

For the acid halides (RCOX), the hydrolysis products are X−, RCO2−, and 2H+. The hydrolysis step is assumed to be irreversible, which is to be expected for the acid products (e.g., HX or trifluoroacetic acid, TFA). While liquid-phase diffusion coefficients can be calculated with adequate reliability by techniques such as the Wilke−Chang method,38 values for H and khyd generally have to be obtained from laboratory measurements. It is also important to note that reaction of a species with more than one reactant in the same droplet is not additive but rather is the square root of the sum. This is a consequence of eq E4.3-2 and is a manifestation of the competition between diffusion and reaction. For determinating rates of heterogeneous atmospheric removal, the liquid-phase diffusion time (τD) of dissolved species within a cloud droplet has to be taken into account. If the hydrolysis rate is fast as compared to diffusion (khyd ≫ 1/ τD), then the uptake coefficient and the overall cloud processing rate are governed by the product Hkhyd1/2. If khyd < 1/τD, tropospheric clouds can be treated as continuous “processors” of the species. A constant fraction of the gas will be dissolved in aqueous solution in clouds, determined by H and the cloud liquid water content. The dissolved gas will be hydrolyzed at a rate determined by khyd, making the overall cloud processing rate proportional to Hkhyd. The liquid-phase diffusion time (τD) is expressed in terms of the liquid diffusion coefficient Dl and the cloud droplet diameter d:35a

Here, αb is the accommodation coefficient for gas molecules entering the surface and 1/Γsol is the uptake resistance term due to solubility, c is the mean speed of the gas molecules, R is the gas constant (l atm mol−1 K−1), and Dl is the liquid-phase diffusion coefficient (cm2 s−1).35a,b Once equilibrium is reached the net uptake is zero. This process is reversible. It should also be noted that if initial uptake at the surface is rapid then gasphase diffusion can be rate limiting, especially at pressures near 1 atm. In this case, eq E4.3-1 contains an additional term for the gas-phase diffusion resistance, that is:

1 1 1 4HRT I = + with Γb = D l k rxn γr αb Γb c̅

(X = H or halogen)

(E4.3-5) M


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molecule−1 s−1). 1/Γs can be considered the resistance for the surface reaction. Parameterization for the reactive uptake coefficient, γgs, for gas-phase species X reacting directly with surface species, Y, upon collision (ER type mechanism) is given by

on soils, vegetation, and snowpack, but the overall deposition velocity is usually controlled by gas-phase transfer to the bulk surfaces. Often, these are simply represented as empirically determined deposition velocities. Also, not all clouds lead to rainout. If the loss rate is limited by the degree of partitioning to the liquid phase, for each incremental altitude drop the raindrop equilibrates with the surrounding air so species (both water and trace gas) may evaporate and be redistributed rather than rained out. This is the normal outcome for cirrus clouds, ice, or water. Furthermore, if reaction forming less soluble, photolabile products occurs in the droplets, they can transfer to the gas phase and undergo photolysis. This is important for activation of halogen radicals in the troposphere. 4.3.3. Uptake of Degradation Products on Solid Surfaces. On solid surfaces, gas molecules adsorb on the surface but cannot pass into the bulk of the particle because the bulk phase diffusion coefficients in the solid phase are orders of magnitude lower than those in the liquid phase (e.g., see Ravishankara35d); the adsorbed molecules may desorb again. The equilibrium fractional surface coverage, Θ, can be described in terms of the Langmuir equilibrium coefficient, KLangC, by θ=

KLangC[X]g N = Nmax 1 + KLangC[X]g

KLangC[X]g =

αsc 4kdesNmax

γ = γgs =

KLinC(T )[X]gas Ns = Nmax Nmax

Nmax(Y)

= γgs(X)θ(Y)

(E4.3-12)

Here, γgs is the elementary reaction probability that a gas-phase molecule X colliding with surface component Y reacts with it. Nmax(Y) denotes the maximum coverage of Y for a volatile species Y in equilibrium with the gas phase. γ can be calculated for a given gas-phase concentration of Y if values of KLangC(Y) and γgs are available. 4.3.4. Scavenging by Adsorption on Solid Particles. Uptake onto solid surfaces can occur at all altitudes, longitudes, and latitudes on aerosol particles. Removal rates due to uptake on a solid particle, followed by loss via gravitational settling, can be estimated using a method similar to that used for rainout. Because atmospheric concentrations of degradation products are expected to be very low, the Langmuir isotherm may be reduced to a simplified form, eq E4.3-10. The fraction of trace species retained in the solid, R, may be obtained from the equation:

(E4.3-8)

RX = (E4.3-9)

NS KlinC(T )NmaxD = NS + NG KlinC(T )NmaxD + 1

(4.3-1)

where NS and NG are the total number of molecules in the solid and gas phases in a given volume, and D is the surface area densities of ice, for example, for a thick cirrus cloud D ≈ 10−4 cm−1, and the temperature dependence of KlinC. If chemical conversion takes place on the solid surface, removal of the trace gas is determined by the reactive uptake coefficient, given by an expression similar to eq 4.3-1. In the case of water, the stable condensed phase is ice in the UT, but at lower altitudes liquid water or solutions become thermodynamically stable. Therefore, a trace gas may be initially scavenged by ice but will have to equilibrate with the stable condensed phase at each altitude drop. The factors that determine the magnitude of KLinC are not well understood, but are a function of adsorbate, surface, and temperature. 4.3.5. Characteristic Heterogeneous Reactions. Three types of heterogeneous reaction are important for halogen containing species produced by degradation of ODS: (1) Solvation to form halide ions, which are the most stable form of halogen in the environment, for example, HCl (aq) + H2O → H3O+ + Cl−; solvation of HCl leads to enhanced [Cl−], leading to increase in condensed phase reaction rates involving chloride ions. (2) Hydrolysis reactions, which produce new stable products, such as hydrates, that can increase wet deposition rates; a very important process is hydrolysis of N2O5, which converts reactive nitrogen oxides to stable nitric acid. Most halogenated carbonyls are hydrolyzed but the rates vary with type and halogen. (3) Redox reactions, which lead to change in oxidation state of the halogen, for example:

Here, N is the number of adsorbed molecules per unit surface area and Nmax is the number when the surface is saturated (usually 1 monolayer, approximately (2−4) × 1014 molecules cm−2). At low trace gas pressure, it is convenient to use the partition coefficient KLinC (cm), which is equal to KLangCNmax. Thus θ=

γgs(X)[Y]s

(E4.3-10)

Partition onto ice, which is the predominant solid particulate surface in the UTLS region, is complicated by a further factor, that is, surface modification following uptake of strongly adsorbing gases leading to a quasi liquid layer on the ice surface39 and “burial” of adsorbed gases by the large flux of adsorbing/desorbing H2O molecules.40 Both of these factors influence uptake kinetics and tend to increase effective partitioning above that derived from the Langmuir model, which accounts for adsorption on the surface. The following partition heterogeneous reactions can come into play, which may lead to formation of new degradation products that may be photolabile and more (or less) stable with respect to removal. The parametrization for the reactive uptake coefficient, γs, for gas-phase species X reacting with surface species, Y, after adsorption on the surface (LH type mechanism) is given by 4ks[Y ]s KLangC(X)Nmax 1 1 1 = + where Γs = γs αs Γs c ̅(1 + KLangC(X)[X]g ) (E4.3-11) −2

Here, [Y]s is the surface concentration (molecules cm ) of species Y and ks is the surface reaction rate coefficient (cm2 N

XONO2 + HX (aq) → XCl + HNO3 (aq)

(4.3-3)

HOX + X− (aq) → X 2 + OH− (aq)

(4.3-4)

N2O5 + X− (aq) → ClNO2 + NO3− (aq)

(4.3-5)


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The viscosity calculated from these parametrizations can be used to obtain an estimate for the diffusion coefficient using the Stokes−Einstein equation:

These three important examples involve oxidation of halide ions by oxidizing agents present in/on aerosol particles. The products, dihalogens or halogen nitroxy compounds, are readily photolyzed to produce reactive halogen atoms. This process leads to several important and unexpected phenomena: ozone depletion in the stratosphere (e.g., the spring−late winter polar ozone depletions from Cl2 formation and photolysis), the “bromine explosion” in polar boundary layer air from Br2 formation and subsequent ozone loss, reactive bromine production in volcanic plumes, and halogen release from sea salt aerosol in the marine boundary layer with consequent effects on atmospheric reactions. Treatment of rate processes for heterogeneous reactions on atmospheric particulate matter requires knowledge of the kinetics and mechanism of the processes occurring in the condensed phase or at the interface. The impact of these reactions on air composition depends on the products formed. Both of these depend on the chemical composition and physical properties (phase and size) of the particles. Water is present on all atmospheric particles either in liquid form or adsorbed on the surface; the amount of water in the condensed phase depends on relative humidity of the air and the hygroscopicity of the particulate material. Water is either a reaction partner or an agent in solvation. 4.3.6. Aerosols of Importance in the Stratosphere. The most widely distributed aerosol in the lower stratosphere is aqueous sulfuric acid, formed by oxidation of gaseous sulfur dioxide. Stratospheric aerosols consist mainly of submicron super cooled size liquid particles containing aqueous concentrated H2SO4 (50−80 wt %) at low temperature (185−260 K). The water content depends in a predictable way on the relative humidity, which is defined by the temperature and the absolute water concentration. Substrates with well-characterized surface area and acid content are relatively straightforward to investigate experimentally in the laboratory, either as bulk substrates or as an aerosol of submicrometer particles of defined size distribution. The latter form minimizes the limiting effects of gas-phase diffusion on the uptake rates, which can be a problem when bulk surfaces are used and γ values are large. Despite the chemical simplicity of this substrate, parametrization of uptake and reaction of trace gases on and within sulfuric acid is fairly complex due to the strong dependence of physical and acidic properties of sulfuric acid on composition, which itself is a strong function of humidity and temperature. The thermodynamic basis for describing sulfuric acid composition for atmospheric conditions has been given in detail by Carslaw et al.41 H2SO4 content is mostly given in weight percent (wt) or mole fraction X, which can be interconverted using X = wt/(wt + (100 − wt)98/18)

D l = cT/η

(4.3-16)

The constant c has been either determined from fits to experimentally measured diffusion coefficients or estimated. For details of these parametrizations, see the introductory material given in IUPAC.42 The sulfuric acid aerosol in the stratosphere consists of very fine particles ( J(CFC-12) > J(CFC-13); see Figure 8. In addition, CFCs with 2 Cl atoms on the same carbon have greater cross sections than molecules with the chlorine atoms distributed between carbon atoms, J(CFC-11) > J(CFC-113) and J(CFC-12) > J(CFC-114). The UV absorption spectra for the c-C4Cl2F6 (R-316c) stereioisomers (Cl on adjacent carbon atoms) fall within the range of CFC-12 and CFC-113 spectra, while the differences in the stereoisomer spectra result in significantly different atmospheric photolysis rate coefficients; see Table 3. This trend further illustrates that the molecular structure of the CFCs has a significant impact on atmospheric photolysis lifetimes. Atmospheric photolysis of CFCs occurs predominantly in the stratosphere between 20 and 35 km; see Figure 7. Weaker UV absorbing compounds photolyze at higher altitudes. Photolysis at Lyman-α is only important above 60 km and thus not a significant loss process for compounds with lifetimes 295 nm wavelength ranges. The O(1D) reaction represents a minor stratospheric loss process. Photolysis and the O(1D) reaction lead to the direct formation of inorganic bromine (Br or BrO radicals) and inorganic chlorine (ClO radicals) in the case of CBrClF2. The elimination of bromine or chlorine from the Halon via photolysis or the O(1D) reaction essentially leads to the same radical products and subsequent degradation. Therefore, the total loss, that is, the lifetime of the source gas, is of primary concern. UV photolysis can also yield molecular halogen, for example, Br2, in some cases, although the yields are small.57 The major first generation halogenated end-products formed following the removal of the Halon source gas are given in Table 4. The major products include inorganic bromine and chlorine, and CF2O, while the degradation of CBrF3 (Halon1301) leads to the formation of CF3OH as a major endproduct. The fate of the halogenated end-products depends on the region of the atmosphere where they are formed. CBrF3 (Halon-1301) is primarily removed in the stratosphere, 98%.15 CBrClF2 (Halon-1211) is removed in both the troposphere, 60%, and the stratosphere, 40%.15 CBr2F2 (Halon-1202) is primarily removed in the troposphere, 93%, with a minor stratospheric loss, 7%.15 CBrF2CBrF2 (Halon-2402) is removed in both the troposphere, 32%, and the stratosphere, 68%.15 The fate of the halogenated end-products is discussed in sections 6 and 7.

OH + CHClF2 (HCFC‐22) → H 2O + CClF2

(5-12)

This reaction produces a halogenated radical product, that is, CClF2 in the example of HCFC-22. HCFC removal via reaction with a Cl atom would lead to the same halogenated radical product and HCl as the reaction coproduct. Abstraction of a Cl or F atom is endothermic and does not occur under atmospheric conditions. For HCFCs with multiple reaction sites, the site-specific reactivity for OH and Cl reactions most likely differs and may result in a different proportion of stable end-products. In general, Cl atom reactions are less site-specific. Although the HCFCs in Table 5 are long-lived, they are removed from the atmosphere over a large range of time scales and regions of the atmosphere depending on their level of reactivity. Figure 9 displays the rate coefficients for the OH + HCFC reactions compared to the approximate time scales associated with atmospheric mixing. First, Figure 9 illustrates how the OH reactivity systematically decreases with increasing fluorine substitution. Reactivity of HCFCs toward OH and their partial lifetime with respect to OH reactive loss varies greatly with the extent and position of halogen substitution. The HCFCs are well mixed throughout the troposphere, and their lifetimes with respect to OH reactive loss are sufficiently long that they are transported into the stratosphere and contribute reactive chlorine to the destruction of O3. Short wavelength UV photolysis and the O(1D) reaction also contribute to the removal of the HCFC source gas in the stratosphere. UV photolysis leads to the direct formation of reactive Cl, for example:

5.3. Hydrochlorofluorocarbons (HCFCs)

Hydrochlorofluorocarbons (HCFCs) are, for the most part, the transitional substitutes for CFCs and Halons. They are currently increasing in the atmosphere, and their atmospheric abundance is expected to reduce over the next decade. HCFCs are ozone depleting substances (ODSs), but their ODPs are much less than those of CFCs (by factors of 5−30). Unlike CFCs, many of the HCFCs are removed in the troposphere almost exclusively via OH reaction. They are degraded in the stratosphere via OH and O(1D) reactions and by short-

CHClF2 (HCFC‐22) + hν → Cl + CHF2

(5-13)

The UV absorption spectra of the HCFCs follow the same general trends as the CFCs discussed earlier. Room-temperature absorption spectra for a number of HCFCs are displayed in Figure 10. The dependence of the HCFC absorption cross S


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Table 5. Hydrochlorofluorocarbon (HCFC) Lifetimes, Major Loss Processes, Key Degradation Pathways, and Major First Generation Halogenated End-Products

T


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Table 5. continued

a Global steady-state lifetimes taken from the WMO-20143 ozone assessment report unless noted otherwise. bHalogen containing radical reaction products are highlighted in italics. cCH2ClF (HCFC-31), Bhatnagar and Carr59 established the unimolecular rate coefficient for Cl-atom elimination from CHClFO to be >3 × 103 s−1 over the pressure range 6−25 Torr (O2/N2 mixture) at 299 K. The HC(O)F yield in the Cl initiated degradation was measured to be 100% under atmospheric conditions at 298 K.60 dCHClF2 (HCFC-22), Wu and Carr14 measured the rate coefficient for unimolecular Cl-atom elimination from the CF2ClO radical to be 6.4 × 104 and 5.0 × 104 s−1 at 298 and 238 K, respectively. The CF2O yield from the CF2ClO radical was determined in several studies to be ∼100% in 1 atm of air at 298 K (see Calvert et al.13b). UV photolysis and O(1D) reaction account for 0.4% and 1.4% of the total HCFC-22 loss, respectively.15 eCHCl2F (HCFC-21), Wu and Carr14 measured the rate coefficient for unimolecular Cl-atom elimination from the CCl2FO radical to be 1.2 × 105 and 1.0 × 105 s−1 at 298 and 238 K, respectively. The ClC(O)F yield in the Cl atom initiated degradation of HCFC-21 in 1 atm of air at 298 K was found to be ∼100% in several studies (see Calvert et al.13b). fCH3CCl2F (HCFC-141b), reaction of the CCl2FCH2O radical with O2 dominates its atmospheric loss (see Calvert et al.13b for a summary of available experimental studies). UV photolysis and O(1D) reaction account for 9.4% and 0.5% of the total HCFC-141b loss, respectively.15 gCH3CClF2 (HCFC-142b), the O2 reaction (formation of CF2ClCHO) is the dominant pathway for the loss of the CCl2FCH2O radical at 298 K in 1 atm of air (see Calvert et al.13b). Mors et al.61 report a rate coefficient for the O2 reaction of 2.5 × 10−15 cm3 molecule−1 s−1 at 293 K and a rate coefficient for the thermal decomposition of the CCl2FCH2O radical of 95% (see Calvert et al.13b). The stratospheric loss of HFC-245fa is ∼7% of its total global loss.3 UV photolysis and O(1D) reaction account for 0.1% and 0.8% of the total HFC-245fa loss, respectively.15 vCH2FCF2CF3 (HFC-236cb), the degradation of the CF3CF2CHFO radical is not established. The stratospheric loss of HFC-236cb is estimated to be ∼8% of its total global loss.3 wCHF2CHFCF3 (HFC-236ea), both H atom abstraction reaction sites will lead to the formation of perfluoroaldehydes. The stratospheric loss of HFC-236ea is ∼8% of its total global loss.3 xCF3CH2CF3 (HFC-236fa), the dominant loss process for the CF3CH(O)CF3 radical is reaction with O2 to form perfluoroacetone (see Calvert et al.13b). The stratospheric loss of HFC-236fa is ∼4% of its total global loss.3 yCF3CHFCF3 (HFC-227ea), although the C−C bond scission is a relatively slow unimolecular process, 2 × 103 s−1, it is the primary loss process for the CF3CF(O)CF3 radical (see Calvert et al.13b). The stratospheric loss of HFC-227ea is ∼12% of its total global loss.3 UV photolysis and O(1D) reaction account for 0.1% and 0.5% of the total HFC-227ea loss, respectively.15 zCF3CF2CF2H (HFC-227ca), the propagation degradation steps of a perfluorodalkoxy radical are expected to lead to the stepwise formation of CF2O and eventually a CF3O radical. aaCH3CF2CH2CF3 (HFC-365mfc), H abstraction from the terminal −CH3 site has been reported to be the major reactive channel (see Calvert et al.13b). The stratospheric loss of HFC-365mfc is ∼7% of its total global loss.3 ab CH2FCH2CF2CF3 (HFC-356mcf), the site reactivity and stable halogenated end-products are not well established. The stratospheric loss of HFC356mcf is ∼3% of its total global loss.3 acCF3CH2CH2CF3 (HFC-356mff), the most likely end-product is a partially fluorinated ketone. The stratospheric loss of HFC-356mff is ∼7% of its total global loss.3 adCHF2CF2CF2CHF2 (HFC-338pcc), the stratospheric loss of HFC-338pcc is ∼8% of its total global loss.3 aeCHF2CF2CF2CF3 (HFC-329p), the stratospheric loss of HFC-329p is ∼11% of its total global loss.3 af CF3CHFCHFCF2CF3 (HFC-43-10mee), perfluoroaldehyde end-products are expected following H atom abstraction from both reaction sites. The stratospheric loss of HFC-43-10mee is ∼10% of its total global loss.3 agCF3CH2CF2CH2CF3 (HFC-458mfcf), a partially fluorinated ketone is the most likely end-product. The stratospheric loss of HFC-458mfcf is ∼10% of its total global loss.3 ahCF3CF2CH2CH2CF2CF3 (HFC-55-10mcff), a partially fluorinated ketone is the most likely end-product. The stratospheric loss of HFC-55-10mcff is ∼6% of its total global loss.3 ai CHF2(CF2)4CF3 (HFC-52-13p), the propagation degradation steps of a perfluorodalkoxy radical are expected to lead to the stepwise formation of CF2O and eventually a CF3O radical. The stratospheric loss of HFC-52-13p is ∼12% of its total global loss.3

Figure 11. Trends in hydrofluorocarbon (HFC) OH reactivity with respect to molecular size and halogen composition. The symbols represent the rate coefficient value at 272 K. The whiskers represent the range of values over the temperature range 200−298 K where the lowest value corresponds to the lowest temperature. Rate coefficient data from the NASA/JPL11 recommendation. The text scale to the right approximates the corresponding time scales and regions of the atmosphere where HFC source gas removal occurs.

The HFOs considered in this Review are summarized in Table 7. A representative HFO atmospheric degradation mechanism is given in Scheme 3, which highlights the OH

initiation reaction and the major propagation steps for CF3CFCH2 (HFO-1234yf). The OH addition reaction is, in most cases, the predominant initial reactive step in the Z


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Table 7. Hydrofluoro-olefin (HFO) and Perfluoro-olefin (PFO) Room-Temperature OH and Cl Atom Reaction Rate Coefficients, Key Atmospheric Loss Pathways, and Major First Generation Haloginated Products Following the Addition of OH Radicals or Cl Atoms to the HFO >CC< Double Bond

AA


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Table 7. continued

kOH(298 K) taken from Sander et al.11 unless noted otherwise, units of 10−12 cm3 molecule−1 s−1. bkCl(298 K) taken from the literature as given in the notes, units of 10−12 cm3 molecule−1 s−1. cIf no experimental end-product studies are available, the expected most likely degradation products are listed in the table, including the possible formation of chlorinated products. dCH2CHF, kCl(298 K) taken from Iyer et al.64 Iyer et al. also reported a preferred Cl addition to CH2 versus CHF of 1.9 ± 0.1. eCH2CF2, kCl(298 K) taken from Dong et al.65 Addition to the CH2 group expected to be preferred, anti-Markovnikov addition. fCF2CF2, Acerboni et al.29 report nearly total conversion to CF2O in their OH initiated degradation experiments. The same products and yields are expected for Cl atom initiated degradation. g CH2CHCH2F, kCl(298 K) taken from Albaladejo et al.66 hCH2CHCF3, kCl(298 K) taken from Sulbaek Andersen et al.67 iCH2CFCF3, kCl(298 K) taken from work of Papadimitriou et al.31 and Kaiser and Wallington.68 Hurley et al.69 and Papadimitriou et al.31 report a 100% CF3CFO yield in the Cl atom initiated degradation at 298 K with ∼60% addition to the terminal carbon atom.31 Hurley et al.69 reported a ∼90% CF3CFO yield in the OH initiated degradation at 298 K. jCF2CFCH2F, no kinetic or end-product experimental studies available. kCF2CHCHF2, no kinetic or end-product experimental studies available. lCHFCFCHF2, no kinetic or end-product experimental studies available. mCF2CHCF3, no kinetic or end-product experimental studies available. n(E)-CHFCHCF3, kCl(298 K) taken from Sondergaard et al.70 o(E)-CHFCFCF3, kCl(298 K) taken from Hurley et al.71 Hurley et al.71 report 100% yields of CF3CHO and HC(O)F in both the OH and the Cl initiated degradation at 296 K. p(Z)-CHFCFCF3, kCl(298 K) taken from Hurley et al.71 Hurley et al.71 report 100% yields of CF3CHO and HC(O)F in the OH and Cl initiated degradation at 296 K. Papadimitriou et al.31 obtained similar results in their Cl initiated degradation study. qCF2CFCF3, kCl(298 K) taken from Mashino et al.72 Mashino et al.72 report 100% yields of CF3CFO and CF2O in the OH and Cl initiated degradation at 296 K. Acerboni et al.32 obtained similar results in their OH initiated degradation study. rCH2CHCF2CF3, kCl(298 K) taken from Sulbaek Andersen et al.67 sCH2CHCF2CF2CF3, kOH(298 K) and kCl(298 K) taken from Sulbaek Andersen et al.67 t(E)-CF3CHCHCF3, no kinetic or end-product experimental studies available. u(Z)-CF3CHCHCF3, kOH(298 K) taken from Baasandorj et al.73 vCF2CFCFCF2, kOH(298 K) taken from Acerboni et al.32 Acerboni et al.32 reported a ∼150% yield of CF2O in the OH initiated degradation in 1 atm of air at 298 K; also Wallington and Hurley.56a w(E)-CF3CHCHCF2CF3, no kinetic or end-product experimental studies available. a

AB


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Scheme 3. Degradation Scheme for HFO-1234yf, CF3CFCH2, Initiated by the OH Radical Reactiona

a

Stable end-products are enclosed in boxes. In this case, both OH radical addition sites lead to the formation of CF3CFO as a major product.

example, CF2O and CF3CHO. The atmospheric fate of the stable halogenated degradation end-products is described in sections 6 and 7. The reaction intermediates and end-products formed in the HFO degradation initiated by addition of Cl, O3, or NO3 may differ from the OH initiated mechanism described above. Of particular interest is the Cl atom initiated degradation in that it may lead to the formation of chlorinated end-products, that is, products that may have an impact on the depletion of stratospheric ozone. In general, the reaction rate coefficients for Cl atom addition are much greater than the analogous OH rate coefficients (see Table 7), which may partially offset the lower atmospheric abundance of Cl to make it a significant HFO loss process in some environments. Further research is needed to evaluate the role of Cl + HFO chemistry in the atmosphere. The O3 reactions are discussed in section 5.5.1.1. 5.5.1.1. Ozonolysis Reactions. Ozonolysis occurs via electrophilic addition of O3 to a >CC< double bond. The presence of fluorine or fluorinated groups, CF3 or CxF2x+1, around the double bond decreases the ozonolysis reaction rate coefficient significantly; that is, halo-olefin reactivity is much less than the analogous alkene reactivity. Ozonolysis reactions generally represent a minor, or negligible, atmospheric loss process for halo-olefin source gases. The exception is for haloolefins with a CH2CH− group, which are sufficiently reactive

atmospheric degradation of HFOs. The regioselectivity of the initiation addition reaction is an important first and critical step because the different radical addition sites may lead to the formation of different stable end-products depending on the composition of the molecule. Similar to the atmospheric degradation of other classes of molecules, O2 rapidly adds to the radical center carbon atom leading to the formation of a semistable hydroxy-haloperoxy radical. There are several atmospheric degradation channels available for the peroxy radical, although not all are shown in the scheme. In the troposphere, a halo-peroxynitrate represents the formation of a temporary halogen and NOx reservoir species. The halo-peroxynitrates are expected to thermally decompose back to reactants, as discussed earlier, on a time scale shorter than other possible loss processes. Stable hydroperoxyhalocarbons and halo-acids can be formed in HO2 and RO2 chemistry and may undergo multiphase loss processes; see section 7. The halo-peroxy radical reacts with NO to form a halo-alkoxy radical under tropospheric conditions. Finally, stable halo-aldehyde end-products are formed when the haloalkoxy radical either thermally decomposes, for example, via C−C bond scission or halogen elimination, or reacts with O2. The primary alkoxy radical degradation pathways and the major first generation stable end-products are included in Table 7. In general, the major end-products are halo-aldehydes, for AC


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Table 8. Ozonolysis: Halo-olefin Room-Temperature Rate Coefficients, Atmospheric Lifetimes, and Most Likely First Generation End-Products compound

kO3(298 K)a (10−20 cm3 molecule−1 s−1)

ozonolysis lifetime (years)b

ref

(E)-CHFCHCF3

0.281

Hydrofluoro-olefins (HFOs) Ljubic and Sabljic74 Sulbaek Andersen et al.67 Nielsen et al.;75 Papadimitriou et al.31 Sondergaard et al.70

(Z)-CHFCHCF3

0.165

Nilsson et al.76

(E)-CHFCFCF3

1.98

Hurley et al.71

(Z)-CHFCFCF3

0.145

Hurley et al.71

CF2CFCF3

0.062

Acerboni et al.32e

CH2CHF CH2CHCF3 CH2CFCF3

2.6d 35 0.277

HC(O)F CF3CHO CF3CFO

11 19

HC(O)F CF3CHO HC(O)F CF3CHO HC(O)F CF3CFO HC(O)F CF3CFO CF2O CF3CFO CF3CF2CHO CF3CHO CF2O CF2CFCFO

22 51

CH2CHCF2CF3 (Z)-CF3CHCHCF3 CF2CFCFCF2

20 1 × 105 s−1.95 fCH3CH2Cl, H atom abstraction from the −CH2Cl group is expected to be the dominant loss process. HCl is a major product and CH3CClO is a minor end-product. gCH2ClCH2Cl, both HCl and CH2ClCHO have been observed as significant products in the degradation of the CH2ClCHCl(O) radical. hCH3CCl3, CCl3CHO is the major end-product, >90%, with CCl2O formed as a minor end-product. iCH3CH2CH2Cl, H atom abstraction from the −CH2Cl group is expected to be the major degradation pathway. jCH3CHClCH3, H atom abstraction from the −CHCl− group is expected to be the major degradation pathway.

stratosphere (referred to in the literature as source gas injection, SGI), particularly if emissions are in the subtropics or tropics, for example, Asian tropical regions; see Brioude et al.8b and references within. Source gas degradation is also of interest because inorganic bromine or iodine as well as halogenated end-products may also be transported into the stratosphere and deplete ozone (referred to as product gas injection, PGI). A general tropospheric halogen degradation mechanism is given in Scheme 5. The key atmospheric ODS bromine compounds, degradation pathways, and first generation end-products are given in Table

13. Among these compounds, CH3Br (methyl bromide) contributes the most reactive bromine to the stratosphere, while CH2Br2 (dibromomethane) and CHBr3 (bromoform) contribute the most from the short-lived brominated compounds. UV photolysis and reaction with the OH radical contribute to the degradation of the source gases. For monobromo compounds, atmospheric loss is predominantly determined by reactive loss, while for multihalogenated compounds UV photolysis can become an important loss process. The available experimental end-product studies for CH3Br, CH2Br2, CHBr3, CH2BrCl, CHBrCl2, CHBr2Cl, AJ


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Scheme 5. Inorganic Halogen Tropospheric Chemistry97

Other species for which photolysis plays an important role are hypohalous acids (HOCl, HOBr, HOI), which undergo photolysis throughout the troposphere and stratosphere, and their photolysis is important for multiphase halogen reactivation chemistry and make them temporary reservoirs in the stratosphere. Halo-nitro species (FNO, FNO2, FONO2; ClNO, ClNO2 (ClONO), ClONO2; BrNO2 (BrONO), BrONO2; INO, INO2, IONO2) are reservoir species and active in catalytic ozone destruction cycles for ClONO2 and BrONO2. The principal organic degradation products that undergo atmospheric photolysis are the carbonyl compounds such as carbonyl halides, halogenated aldehydes and ketones, and halogenated organic acids and nitrates. Photolysis often is the only stratospheric sink for fully halogenated oxygenated VOC degradation products formed in the stratosphere, because their reaction with OH is immeasurably slow, and downward transport controls their residence time in the stratosphere. In the troposphere, photolysis of halogenated oxygenated VOC degradation products competes with removal by multiphase processes, and the relative efficiency of these processes determines their atmospheric loss rate.

CH3CH2Br, and n-C3H7Br are summarized in Table 13, and the available literature is discussed in Calvert et al.13b The majority of iodocarbons are emitted naturally from the oceans. Table 14 lists the key atmospheric iodine source gases, including CF3I and CF3CF2CF2I, which are man-made and released into the atmosphere during commercial use. CH3I is the most abundant atmospheric iodocarbon, while a number of other iodocarbons have also been observed in the MBL in significant amounts. Iodocarbon source gases are removed from the atmosphere by UV photolysis and to a much lesser extent by reaction with OH and Cl atoms. Iodocarbon lifetimes range from days for monoiodine containing molecules, for example, CH3I, to minutes for multihalogenated molecules, for example, CH2I2. UV photolysis leads to the direct release of inorganic iodine, although other minor photolysis channels are also possible in some cases. Multiphase chemistry of inorganic degradation products, see section 7, represents an important removal process for reactive iodine and the iodo-degradation products in the troposphere. These processes limit the accumulation of inorganic iodine in the troposphere, which impacts the efficiency with which reactive iodine is transported to the stratosphere.

6.1. Photolysis Processes Involving Inorganic Degradation Products of CFCs and Halons

All of these molecules photolyze in the near UV/vis region. Therefore, their is little altitude dependence to their loss, and photolysis is important for halogen chemistry in both the stratosphere and the troposphere. The processes are rapid, and these molecules are generally in quasi-steady state during daytime. The photolysis of the principal inorganic degradation products involved in stratospheric and tropospheric chemistry is shown in Table 15. The photochemistry of fluorine analogues of these compounds has little significance because the analogous inorganic FOx species are not released from degradation products. The approximate local photolysis lifetimes are given in Table 15. The data illustrate the generally rapid photolysis rates of these species, allowing them to participate in cyclic reactions leading to ozone loss and halogen recycling. The notable exceptions are chlorine nitrate and HOCl, which photolyze slowly and are the only significant reservoir products in this category. Some recent new results obtained for these species, which have been highlighted in the NASA/JPL and IUPAC data evaluations, are indicated in the table; their significance is discussed briefly below. 6.1.1. Photolysis of ClO Dimer (ClOOCl). The photolysis of the ClOOCl molecule is central to the chemistry of ozone depletion by the ClO dimer catalytic cycles in the springtime polar lower stratosphere. Photodissociation can occur by two pathways following absorption in the UV/vis region:

6. DEGRADATION PRODUCTS − PHOTOLYSIS The principal inorganic degradation products that remove halogen from the atmosphere are the hydrogen halides, HX (X = F, Cl, Br, I). These molecules absorb only weakly in the UV, and photolysis is not important in determining their atmospheric lifetime. All other inorganic halogen-containing degradation products are either free radical species that participate in catalytic ozone loss cycles, or are photochemically active closed shell molecules, which play an important part in creating these radicals. The catalytic radical species include: halogenated radicals, F, FO, FO2, CF3, CF3O, CF3O2; Cl, ClO, ClO2, ClCO; Br, BrO; I, IO; and halogen oxides, OClO, OBrO; OIO. The photolytically active closed shell molecules include halogens and halogen oxides (F2, Cl2, Br2, BrCl, I2 and Cl2O, Cl2O2, Cl2O3). Of these, dihalogens are formed mostly from multiphase mechanisms, and the halogen oxides from gas-phase reactions; they all undergo rapid photolysis throughout the troposphere and stratosphere. The ClO dimer is especially important in polar ozone loss.

ClOOCl + hν → ClO + ClO → ClOO + Cl

(6‐1a) (6‐1b)

Channel 6-1a leads to a null cycle with no ozone loss; channel 6-1b leads to ozone loss via reaction of Cl + O3 because ClOO decomposes rapidly to liberate a second Cl atom. The atmospheric photolysis rate coefficient has been subject to uncertainty because of the uncertainties in the absorption cross sections of ClOOCl. The contribution of Cl2, which is always present as an impurity, has to be accounted for. This is particularly difficult because ClOOCl appears to have a spectral feature that is very similar to the Cl2 spectrum. This difficulty is AK


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Table 13. Hydrobromocarbons and Related Compounds: Atmospheric Loss Processes, Major Degradation Pathways, and Halogenated First Generation End-Products

a

Fractional loss values taken from WMO-200397 ozone assessment. bThe predominant loss process for the CH2BrO, CHBr2O, and CBr3O radicals in 1 atm of air at 296 K has been shown to be Br atom elimination. Br atom elimination is also the dominant loss process for brominated alkoxy radical containing chlorine. cCH3Br, both OH reaction and UV photolysis lead to the formation of inorganic bromine. dCH2Br2, Orlando et al.98 reported unit formation of HC(O)Br in 700 Torr air and the unimolecular rate coefficient for Br atom elimination from the CHBr2O radical to be >4 × 106 s−1 at 298 K. Br2 formation via UV photolysis is a minor process.57 e CHBr3, Kamboures et al.99 identified CBr2O as a major product following the Cl atom reactive loss of CHBr3. Estimated fractional loss was taken from Papanastasiou et al.100 fCH2BrCl, UV photolysis and H atom abstraction lead to the same halogenated end-products. Bilde et al.101 reported a ∼100% yield of HC(O)Cl at 298 K following the Cl atom reactive loss of CHBr3. gCHBrCl2, Bilde et al.102 observed the effect of chemical activation in the unimolecular decomposition of the CBrCl2O radical via an increased yield of the BrC(O)Cl end-product in the presence of NO. hCHBr2Cl, UV photolysis and chemical reaction lead to different end-products. Bilde et al.102 did not observe evidence for chemical activation in the unimolecular decomposition of the CBr2ClO radical. iCH3CH2Br, H atom abstraction from the −CH2Br group is expected to be the major reactive channel. jCH2BrCH2Br, CH2BrCHO is expected to be the major halogenated end-product. kn-C3H7Br, all three reactive channels are active with 32%, 56%, and 12% reactivity at the α, β, and γ sites, respectively.103 Bromoacetone was identified as a major product under atmospheric conditions, ∼50% yield. liso-C3H7Br, H atom abstraction from the −CHBr− group is expected to be the major reactive channel with inorganic bromine being the expected major halogen end-product. mCHBrClCF3, its atmosopheric lifetime is 1 year. Unimolecular Br atom elimination from the CF3CBrClO radical is its predominant loss process (see Calvert et al.13b). nCHBrF2, its atmospheric lifetime is 5.1 years. The same halogenated end-products are formed in both the OH and the photolysis loss processes. AL


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Table 14. Hydroiodocarbons and Related Compounds: Atmospheric Loss Processes, Major Degradation Pathways, and Halogenated First Generation End-Products

a

Estimated fractional losses taken from the WMO-200297 ozone assessment unless noted otherwise. bHalogen containing radical reaction products are highlighted in italics. cCH3I, UV photolysis and OH reaction both lead to the formation of inorganic iodine. dCF3I, fractional loss estimated from the OH rate coefficient from Sander et al.11 and the photolysis lifetime in Solomon et al.104 eCH2ClI, the photolysis quantum yield is expected to be unity, although product channel branching ratios have not been reported.11 The Cl atom photolysis product channel would lead to CH2OO (Criegee) biradical formation. fCH2BrI, I atom elimination is expected to be the dominant photolysis channel, although branching ratios have not been reported.11 gCH2I2, I atom elimination from the CH2IOO radical would lead to the formation of the CH2OO (Criegee) biradical. hCH3CH2I, there are several pathways possible for reactive loss. For the OH reaction, H atom abstraction from the CH2I group is expected to dominate.13b,105 Abstraction from the −CH3 group has been shown to form inorganic iodine, CH2CH2 + I,106 while CH2ICHO is a possible minor product.97 i CH3CH2CH2I, H atom abstraction from all reactive sites is expected with the majority of the reaction occurring at the −CH2I group.13b,105 j CH3CHICH3, H atom abstraction is expected to occur predominantly at the −CHI− group.13b,105 Abstraction at the −CH3 group was found to lead to the formation of inorganic iodine, CH3CHCH2 + I, with a significant yield.106,107 kCF3CF2CF2I, UV photolysis is expected to be the major loss process. The sequential degradation of the CF3CF2CF2 radical will lead to the production of two CF2O and a CF3O radical.

dissociation cross section for Cl atom formation at 248, 308, and 352 nm with correction for Cl2 impurity by Wilmouth et al.,113 or (d) using high-resolution DOAS subtraction of Cl2 impurity in Cl2 + ClOOCl spectra obtained by cavity enhanced long-path absorption measurements in the structured region of the Cl2 spectrum at 509−570 nm by Young et al.114 The cross sections reported by these methods are in good agreement and show little or no temperature dependence over the atmospheric range. The preferred values from NASA/JPL and IUPAC evaluations are those reported by Papanastasiou et al.109 The

thought to have led to the very low cross sections reported by Pope et al.108 at wavelengths longer that ∼290 nm. The spectral interference issues has been addressed using several different experimental approaches: (a) taking into account all absorbers using a multiple isosbestic point analysis, Papanastasiou et al.,109 (b) by determining the photodissociation cross section via relative loss of ClOOCl and a reference molecule gas with known cross section and quantum yield by Chen et al.110 at 308, 351 nm, Lien et al.111 at 248 nm, and Jin et al.112 at 330 nm, (c) by measuring a photoAM


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Table 15. Photolysis Products and Lifetimes of Inorganic Degradation Products photolysis lifetime (s)a molecule ClO OClO

BrO OBrO IO OIO Cl2 Br2 I2 BrCl ICl ClOOCl HOCl HOBr HOI ClONO2 BrONO2 IONO2 ClNO2

photolysis products 3

Cl + O( P) Cl + O(1D) ClO + O(3P) Cl + O2(3Σ) ClOO Br + O(3P) BrO + O(3P) I + O(3P) I + O2(3Σ) O + IO Cl(2P3/2) + Cl(2P3/2) Br(2P3/2) + Br(2P3/2) I(2P3/2) + I(2P3/2) Br + Cl I(2P3/2) + Cl(2P3/2) Cl + ClOO ClO + ClO OH + Cl OH + Br OH + I Cl + NO3 ClO + NO2 Br + NO3 BrO + NO2 I + NO3 IO + NO2 Cl + NO2

photolysis threshold (nm) 445 262 476 5500 all 504 573 498 all 493 493 620 792 548 569 1323 1614 513 571 561 692 1063 831 1044 808 1018 839

Z=0

Z = 20 km

no spectrum data recommended for λ < 246 nm

12 100

3450

20

10

40 2.5 6.5 6

20 2 4.5 5

610 37 7 120 55 1100

320 26 5.5 80 40 520

5400 610 155 30 250

2650 350 95 14 700

1000

550

30

15

3200

1500

comment

no known atmospheric source I observed as photolysis product; no recommended quantum yield T dependence in vis spectrum reported

absorption near 500 nm recently observed

disagreement between studies of the long wavelength quantum yield

large disagreement in UV/vis spectrum between studies recent T-dependent spectrum and quantum yield data

Approximate local photolysis lifetime at altitude Z calculated at midlatitude (∼40° N) for the summer. Solar fluxes from the NCAR TUV calculator33 and the room-temperature absorption cross section data from the NASA/JPL11 data evaluation.

a

study of Young et al.114b confirms the weak ClOOCl absorption near 525 nm as shown in Figure 12. Pulsed photolysis at isolated laser wavelengths has shown that channel 6-1b dominates with Cl atoms formed with near unity quantum yield, which agrees with the interpretation of the steady-state photolysis experiments of Cox and Hayman.115

6.1.2. Photolysis of OIO. Degradation of iodocarbons releases I atoms in the atmosphere, which can lead to ozone loss via the reaction I + O3 → IO + O2

(6-2)

The IO can be converted to OIO by reaction with XO when X = Br and I, but not with X = HO Cl or NO: (6-3)

IO + XO → X + OIO

OIO photolysis leads to two possible product channels: OIO + hν → O + IO → O2 + I

(6‐4a) (6‐4b)

OIO has a large absorption cross section in the 500−650 nm region, and a potentially important catalytic cycle for ozone loss results in the case of channel 6-4b. Gomez Martin et al.116 measured the 6-4b channel branching ratio to be Φ(I) = 1.07 ± 0.15 using pulsed laser photolysis of OIO and resonance fluorescence detection of I atoms. The concentration of OIO was measured using CRDS with recommended absorption cross sections. Their results show the catalytic cycle (reactions 6-2 + 6-3 + 6-4) to be an efficient ozone loss process in the marine boundary layer (MBL) and also in the lower stratosphere, if significant IOx is released there. Photochemical box modeling of halogen chemistry and observed amounts of IO and OIO in the equatorial, midocean MBL show that the photochemistry of OIO should make a significant contribution to O3 depletion over much of the

Figure 12. ClOOCl UV/vis absorption spectrum. Data from the NASA/JPL data evaluation11 and the results from the most recent laboratory study of Young et al.114b See Sander et al.11 for a discussion of earlier studies and basis for the NASA/JPL recommendations. AN


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Table 16. Photolysis Products, Thresholds, and Representative Local Photolysis Lifetimes for Several Halogenated Carbonyl Compounds photolysis lifetimeb molecule HC(O)F

HC(O)Cl HC(O)Br HC(O)I ClC(O)F

BrC(O)F FC(O)I BrC(O)Cl CF2O CCl2O

CBr2O CF3CFO CHF2CHO

CH2FCFO C2F5CHO

n-C3F7CHO

n-C4F9CHO

CF3CH2CHO n-C6F13CH2CHO

CF3CClO

CF3OH CH3OCl

CF2ClCHO

CFCl2CHO

CCl3CHO

CH3CClO

photolysis products HF + CO H + FCO F + HCO Cl + HCO Br + HCO I + HCO Cl + FCO F + ClCO F + Cl + CO Br + FCO F + Br + CO I + FCO Br + ClCO Cl + BrCO F + FCO CO + Cl2 Cl + ClCO 2Cl + CO Br + BrCO 2Br + CO CF3 + FCO CHF2 + HCO CHF2CO + H CH2F2 + CO CH2F + FCO C2F5 + HCO C2F5CO + H CH2F2 + CO n-C3F7 + HCO n-C3F7CO + H n-C3F7H + CO n-C4F9 + HCO n-C4F9CO + H n-C4F9H + CO CF3CH2 + HCO CF3CH3 + CO n-C6F13CH2 + HCO n-C6F13CH2CO + H n-C6F13CH3 + CO CF3 + ClCO CF3 + Cl + CO CF3CO + Cl CF3Cl + CO CF3 + OH CF3O + H CH3O + Cl CH3 + ClO CH2OCl + H CF2Cl + HCO CF2ClCO + H CHF2Cl + CO CFCl2 + HCO CFCl2CO + H CHFCl2 + CO CCl3 + HCO H + CCl3CO CCl3H + CO CCl3 + HCO

photolysis thresholds (nm)a all 282 237.2

Z=0

Z = 20 km

−d

26 years

3.7 years 2 days

0.54 years 0.6 days

−d

21 years

commentc

no spectrum data recommended for λ < 240 nm no spectrum data recommended for λ < 240 nm no UV spectrum data reported

no UV spectrum data reported no UV spectrum data reported no UV spectrum data reported 226 1084 377 339

392 320 all

249 243.5 589 383 287

341 306 5300

−d 136 years

440 years 8 years

10 days

3.3 days

no spectrum data recommended for λ < 240 nm

−d 0.45 days

17 years 0.2 day

uncertainty in Φ(λ,M); Φ = 0.3 recommended in IUPAC

0.6 days

0.3 days

no UV spectrum data reported uncertainty in Φ(λ,M); Φ = 0.15 used in calculation

3 days

1.5 days

uncertainty in Φ(λ,M); Φ = 0.02 used in calculation

1.9 days

0.8 days

uncertainty in Φ(λ,M); Φ = 0.03 used in calculation

10 days

4 days


0.6 days

0.25 days


60 days

16 days

−d

>2000 years

cross section upper-limit of 1 × 10−21 cm2 molecule−1

3.9 h

2h

produced in gas-phase ClO + CH3CO2 reaction

>0.06 days

>0.024 days

no Φ(λ,M) data reported; Φ = 1 upper-limit used in calculation

>0.09 days

>0.035 days

no Φ(λ,M) data reported; Φ = 1 upper-limit used in calculation

0.3 days

0.1 days

34 years

3.2 years

AO


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Table 16. continued photolysis lifetimeb molecule CCl3CClO

photolysis products H + CCl3CO CCl3 + ClCO Cl + CCl3CO

photolysis thresholds (nm)a

Z=0 4 days

Z = 20 km

commentc

1.4 days

a

Photolysis thresholds calculated using the thermochemical data available in the NASA/JPL and IUPAC data evaluations. bApproximate local photolysis lifetime at altitude Z calculated at midlatitude for the summer using the NCAR TUV calculator33 fluxes and the room-temperature absorption cross section data recommended in the NASA/JPL11 and IUPAC12 data evaluations. cPhotolysis quantum yield, Φ(λ,M), of unity (independent of wavelength and pressure) was used in the lifetime calculation unless noted otherwise. dNo significant photolysis loss.

case largely dominated by photodissociation to form the same products.

world’s oceans. However, in the lower stratosphere, the upper limits to IO and OIO concentrations based on measurements suggest that catalytic cycles involving iodine make only a minor contribution to total ozone loss.117 6.1.3. Photolysis of Dihalogen Compounds. The photochemistry of dihalogen compounds is significant due to their production in heterogeneous reactions both in the stratosphere (e.g., ClONO2 + HCl producing Cl2) and in the troposphere (e.g., HOBr + Cl−/Br− producing BrCl or Br2; HOI + Br− producing IBr). The spectra of the dihalogens are well-known; a broad continuous absorption occurs throughout the visible and near UV region, with banded structure superimposed in the visible regions. Photolysis leads to halogen atoms, which react rapidly with ozone to give XO radicals, leading to ozone loss. Cl and Br also react with H-containing organic compounds, initiating their oxidation. Production of halogen atoms thus augments the oxidizing capacity of the troposphere. Br chemistry in the troposphere has been a particular focus as photochemical release of Br from BrCl or Br2 leads to recycling of reactive BrOx produced in heterogeneous reactions of sea salt and volcanic aerosols. There is abundant evidence for this process from observations of BrO in the marine boundary layer and in volcanic plumes.118 6.1.4. Photolysis of Halogen Oxyacids, Nitrates, and Nitryl Compounds. Halogen oxyacids, HOX, are produced in the reaction of HO2 with halogen oxide radicals. They undergo photolysis to produce halogen atoms and OH radicals. Halogen oxide radicals also react with NO2 to form halogen nitrates, which act as temporary reservoirs for radical species. They undergo photolysis to produce halogen atoms and NO3 radicals, or XO + NO2. The photolysis rate of halogen nitrates is quite well-defined, except for that of IONO2, where there is conflict between two reported values of the absorption cross sections. The lower values of Joseph et al.119 are preferred over the higher values obtained by Moessinger et al.120 as their technique was less likely to have suffered from attenuation by aerosol scattering due to iodine oxide secondary products. Photolysis of nitryl chloride, ClNO2, formed in the heterogeneous reaction of N2O5 with aerosols containing chloride also plays a role in release of Cl atoms in the troposphere. IUPAC recommends the absorption cross-section values reported by Ghosh et al.,121 which cover the relevant atmospheric wavelength range and also provide information on its temperature dependence. At wavelengths of >300 nm, photodissociation to form Cl and NO2 occurs with a quantum yield of close to unity. The impact of a small branching ratio to O(3P) + ClNO or the prompt dissociation of NO2 to O(3P) + NO will be slight as the fate of both ClNO and NO2 is in any

6.2. Photolysis of Halogenated Carbonyl Compounds

The photolysis lifetimes of halogenated VOC degradation products fall into two categories: (a) the halogenated aldehydes, which have photolysis lifetimes of ∼1 day and are photolyzed in the troposphere, and (b) the carbonyl halides and fully halogenated compounds, that have photolysis lifetimes of >1 year, which are photolyzed mainly in the stratosphere, if they are either made there or transported from the troposphere. The second category also includes the halogenated esters, which are produced in the degradation of halo-ethers. By analogy to nonhalogenated esters, these compounds would only photolyze at wavelengths 240b 0.96, 3.0 2.0 8900 59 0.96, 3.0 2.0/0.27 0.55 ± 0.04 0.58 ± 0.04 0.12 ± 0.04 0.09 ± 0.01

0.06

HOI

see IUPAC datasheet for γss

CF2O Reaction HOCl + HCl → H2O + Cl2 HOBr + HCl → BrCl + H2O ClONO2 + HCl → HONO2 + Cl2 ClONO2 + H2O → HONO2 + HOCl HONO + HCl → ClNO + H2O N2O5 + HCl → HONO2 + ClNO2 BrONO2+ H2O → HONO2 + HOBr HONO + HBr → BrNO + H2O HOBr + HBr → Br2 + H2O a

sourcea IUPAC

240−295 HHOCl = 1.91 × 10−6 exp(5862.4/T) exp(−SHOClMH2SO4) M atm−1, where SHOCl = 0.0776 + 59.18/T M−1, MH2SO4 = H2SO4 molar conc HHOBr = 5.22 × 10−5 exp(5427/T) ln H* = a1 + (b1 + b2 wt)/T, a1 = −11.695 ± 0.537, b1 = 11 101 ± 163, b2 = −90.7 ± 1.2

NASAJPL IUPAC NASAJPL IUPAC

10−2 M

200−220

IUPAC

0.8

Γb = 0.11 + exp(29.2−0.40 W)

210−300

IUPAC

1.0

0.1

200−300

0.3

kI = 5 × 104 (s−1)

228

From IUPAC42 or NASA/JPL;11 η = viscosity in cP; W = weight % H2SO4; S = Setchenow coefficient.

encountered in the “ice stability” region, uptake rate is time dependent, and saturates at a surface coverage, θ, of about 3.0 × 1014 molecules cm−2 and can be described quite well using a Langmuir model.130 However, competitive adsorption with the more abundant HNO3 has to be taken into account in partitioning of HCl and OVOC compounds.131 7.2.2. Physical Scavenging of Organic Species. In addition to HNO3 and HCl, there are many trace species that are removed by partitioning to ice surfaces including inorganic and OVOC degradation products of halogenated ODS. Many of the inorganic species undergo heterogeneous reactions on ice, which play a role in halogen activation and ozone depletion. Partitioning of OVOC affects mainly their redistribution and physical scavenging. To assess the significance of partitioning for removal of OVOC, the partition coefficients, KLinC, of the gaseous oxygenated organics to ice particles at upper tropospheric temperatures have been measured in a number of laboratories in recent years. These results indicate a wide range of affinity for surface uptake ranging from strongly adsorbing organic acids similar to inorganic mineral acids (HNO3, HCl) to moderately strongly adsorbing organic acids,132 to weakly absorbing nonpolar molecules such as

alcohols,133 halogenated carbonyls,134 and long-chain halogenated alcohols.135 Several groups have attempted analysis of experimental data to obtain a correlation between Langmuir partition constants, KLinC, and thermodynamic constants for the surface or phase change. Pouvesle et al.136 showed that, provided the correlation was restricted to regions where reversible Langmuir adsorption prevailed (e.g., for trace gases that hydrogen-bond to ice surfaces), a good correlation exists between the Langmuir partition constant, K LinC (T), and the free energy of condensation, ΔGg−l(T), which was first suggested by Sokolov and Abbatt.133 The result is also intuitive because vapors with high negative values of ΔGg−1 (i.e., low vapor pressure) are known to be sticky and have an affinity for polar surfaces. This correlation is shown in Figure 14 where data at 228 K for KLinC(T) for a range of OVOC are plotted versus ΔGg−l(T). The values for the aliphatic OVOC and H2O2 were calculated using recommended temperature-dependent expressions for KLinC from a recent evaluation by the IUPAC panel.50b The experimental data show considerable scatter due to uncertainty in both parameters, but are approximated by a linear function. From data sets at different temperatures, a parametrization can AS


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Table 19. Langmuir Parameters for Uptake of Halogenated Degradation Products on Ice Surfaces species

KLinC (cm)

KLinC (228 K) (cm) 6086 414 000 229 000

HOCl HOBr CCl2O CFClO CF2O CHClO CHFO CF3CHO CCl3CHO CF3COOH CF3CH2OH CF3CF2CH2 OH CF3CF2CF2CH2OH CF3COF CF3COCl

0.0219 exp(2858/T) 4.14 × 105 2.29 × 105 0.54 × 105 3.6 × 10−8 exp(4760/T) no data no data no data no data no data no data 1.06 × 10−2 exp(904/T) 7.52 × 10−4 exp(2069/T) 1150 4.2 × 10−10 exp(5390/T) 4.2 × 10−10 exp(5390/T) 4.2 × 10−10 exp(5390/T) no data no data

HF HCl HBr HI

42

0.56 6.6 1150 7.8

temp (K)

0.085 2.3 × 10−3

205−230 188 188 195 180−220 190−209

>3 × 10−6

200

208−228 208−228 200−228 208−228 208−228 208−228

that the correlation can be used for halogenated species. For a third species, CF3C(O)OH (TFA), the only experimental measurement of KLinC for TFA on ice from Symington et al.134 shows gross divergence from the value calculated from the correlation with the ΔGg−l(T) correlation line. As noted in section 7.2.3, the adsorption characteristics of TFA departed from the reversible uptake involving H-bonding to the surface and exhibited irreversible strong uptake similar to the inorganic acids, which is attributed to hydrate formation. On the basis of laboratory measurements of the partition coefficient of acetic acid, up to 5% of CH3C(O)OH can be expected to be adsorbed on ice surfaces in cold dense cirrus clouds (D ≈ 10−4 cm−1) at 230 K.132b Acetic acid adsorbs moderately strongly to ice as compared to other OVOC, but less so than the inorganic acids, nitric acid, and HX. It can be concluded that physical removal of halogenated carbonyls and organic acids in the UTLS by partitioning to ice is rather inefficient, with the exception of TFA and possibly other fluorinated carboxylic acids. 7.2.3. Heterogeneous Scavenging of Partially Fluorinated Alcohols (PFA). Partially fluorinated alcohols and carbonyl compounds are present in the atmosphere but at lower mixing ratios than their aliphatic analogues. They originate from degradation of CFCs, CFC replacement compounds, and a multitude of fluorinated solvents and surface coating applicants. The presence of CH bonds in these molecules imparts reactivity toward OH and hence shorter lifetimes than fully halogenated substances, but they are longer lived than aliphatic OVOC. The concentrations of these substances are known to be increasing, and the possibility of their removal by heterogeneous processes has been investigated. Partitioning parameters (KLinC) onto ice have been measured recently for a limited number of fluorinated OVOC on ice at UTLS temperatures. The available data are given in Table 20. The partitioning parameters may be used to calculate the relative amount of CF 3 CH 2 OH, CF 3 CF 2 CH 2 OH, or CF3CF2CF2CH2OH scavenged from the gas phase due to adsorption processes at the ice surface of cirrus clouds.135,137

Figure 14. KLinC(T) values for a range of OVOCs are plotted versus ΔGg−l(T). Data from Pouvesle et al.136 (●), Symington et al.132a (CF3C(O)OH), and Symington et al.134 (CF3CH2OH, CCl3CHO, and CF3CHO).

be derived for calculating KLinC at any temperature, for which a value of ΔGg−l(T) is available (e.g., from vapor pressure data or tabulated values of thermodynamic properties). The expression given by Pouvesle et al.136 is ln KLinC = −AΔGg − l(T ) − B

γ

(E7.2-1)

where A and B are temperature-dependent parameters given by A = −1.08 × 10−6T + 6.232 × 10−4 and B = −4.643 × 10−4T2 + 0.2696T − 33.925 (note: two typographical errors in Pouvesle et al.136 are corrected here). Also shown in Figure 14 are experimental values for KLinC and values calculated using expression E7.2-1 for halogenated compounds for which data for ΔGg−l(T) exist. For two species CF3CH2OH and CCl3CHO where both parameters are available, there is reasonable agreement, giving some confidence AT


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Table 20. Summary of Uptake Coefficients for Reactions on Ice Surfaces at 180−230 K from the IUPAC Data Evaluation50b reaction HOCl + HCl HOCl + HBr ClONO2 + H2O ClONO2 + HCl ClONO2 + HBr N2O5 + HCl N2O5 + HBr HOBr + HCl HOBr + HBr BrONO2 + H2O BrONO2 + HCl BrONO2 + HBr a

αgs/γER 0.22 0.22 × θHCl 0.3 1/0.3 + 2.7/θHBr 0.5 0.24 0.24 × θHCl 0.56 0.56 × θHBr

KlinC(X) (cm−1)

kr (cm2 s−1)

KlinC = 0.0219 exp(2858/T) (Y = HCl)

E−R mechanism 3.3 × 10−15 1.2 × 104 (X = ClONO2) 1.04 exp(2032/T)

θ(Y)a (molecules cm−2)

5 × 10−17 5 × 10−17

θHBr: KlinC = 4.14 × 10−10 [HBr]0.88 θH2O = 1015 (1−0.81qHNO3), θHNO3: KlinC = 7.5 × 10−5 exp(4585/T)

E−R mechanism

KlinC = 0.0219 exp(2858/T) (Y = HCl)

E−R mechanism

θHBr = 4.14 × 10−10 [HBr]0.88

no rec. no rec. 0.24 4.8 × 10−4 exp(1240/T) 5.3 × 10−4 exp(1100/T) 0.3 6.6 × 10−3 exp(700/T)

θ(Y) = fractional surface coverage of reagent species.

For the densest cirrus clouds, with D values around 1 × 10−4 cm2 cm−3,138 between 223−203 K roughly 0.1−1.5% for CF 3 CH 2 OH and CF 3 CF 2 CH 2 OH, and 0.2−2.9% for CF3CF2CF2CH2OH are taken up. Thus, ice surfaces do not constitute a significant sink for these fluorinated alcohols. In contrast, the uptake of PFAs to liquid droplets has been suggested as a permanent sink for halogenated alcohols due to their fast reactions with OH in the liquid phase.139 The end-product of oxidation of PFAs is trifluoroacetic acid (TFA, CF3C(O)OH), which has a much higher partitioning parameter. Trifluoroacetic acid, which is a stronger acid (pKa = 0.3 at 298 K), interacts more strongly with the surface than acetic or formic acid, and uptake is only partially reversible ((Nads/Ndes = 0.6 ± 0.2) on the time scale of the experiments in the laboratory flow system (Symington et al.132a), which is attributed to hydrate formation. In this respect, it resembles the stronger inorganic acids (HNO3 and HCl). The fraction of CF3C(O)OH partitioned locally to dense cirrus clouds is ∼23%, determined using the experimental KLinC values at 208 K. Lawrence and Crutzen140 showed that in addition to precipitation scavenging ice crystals could reduce soluble trace gas lifetimes by dissipation, that is, sedimentation/ advective transport followed by revolatilization of adsorbed trace gases at lower altitudes. Most (∼90%) of the cirrus ice crystals are too small to reach the surface without dissipating, and adsorption of inorganic strong acids such as HNO3 and HCl on these smaller crystals can efficiently deplete the gas phase at temperatures prevailing in the upper troposphere. At lower altitudes, the revolatilized gases can be lost by wet deposition. Lawrence and Crutzen suggested that organic acids redistributed to lower altitudes by adsorption on cirrus clouds140 can be lost by wet deposition at lower altitudes. This mechanism would be effective for any soluble organic species (e.g., CF3C(O)OH) that exhibits comparable partition constants to the aliphatic acids, which are known to be common constituents of rain. 7.2.4. Reactive Scavenging of Halogenated Carbonyls on Ice. Halogenated aldehydes formed as degradation products of ODS containing two or more C atoms are removed mainly by photolysis or OH reaction. In the case of fluoral and chloral,

heterogeneous reaction with water (liquid or ice) can lead to the formation of stable gem-diols.141 CX3CHO + H 2O → CX3CH(OH)2

(7-1)

This leads to modification of the photochemistry by removal of the CO chromophore resulting in closure of the normal photolysis channel. These products can be isolated as stable solid (fluoral hydrate) or liquid (chloral hydrate) at room temperature. Symington et al.134 determined the rates of surface reaction and suggested this likely to be the major loss process for fluoral and a major loss process for chloral (at nighttime). If formed at UT temperatures, they would be expected to remain in the condensed phase, introducing an efficient physical removal process for gas-phase halocarbon degradation products. On the other hand, the increased photochemical stability of the gas phase for chloral offers the possibility of PGI of Cl from the troposphere. The impact of this process has not been assessed in models. 7.3. Heterogeneous Reactions Leading to Halogen Activation

Heterogeneous reactions leading to activation of inorganic chlorine from its reservoirs formed from degradation of ODS are an important feature of stratospheric chemistry. The most significant reaction is that of chlorine nitrate with HCl leading to the formation of Cl2: ClONO2 + HCl → HNO3 + Cl 2

(7-2)

Photolysis of Cl2 releases atomic Cl, leading to catalytic destruction of ozone. This reaction has been shown to be the primary cause for ozone destruction in the Antarctic and Arctic polar vortices in SH and NH springtime. It is now considered to be important at other latitudes, the rate of the processes depending on the nature and extent of the local stratospheric aerosol amounts and composition. For example, activation of ClOx on the surface of cirrus clouds in the UTLS region has been a focus of research in the past decade. The heterogeneous ClONO2 + HCl reaction is now a wellcharacterized reaction; the rate coefficients for its occurrence on aqueous H2SO4 aerosols, on ice, and on solid hydrates of NAT and SAT have been investigated and evaluated.11,42,50b AU


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The reaction also occurs on solid surfaces such as silica and Al2O3,142 which is of interest in the context of introduction of aerosol into the stratosphere for geo-engineering by solar radiation management (SRM). Other halogen activation reactions of HCl and HBr are those involving HOCl and HOBr. For example, the conversion of HOBr to BrCl is relatively efficient in the UTLS region, where NOx is less abundant and where cirrus clouds offer surfaces area comparable to those of PSCs. HOBr + HCl → H 2O + BrCl

(7-4)

(7-5)

(7-10)

BrO + HO2 → HOBr + O2

(7-11)

HOBr + hν → OH + Br

(7-12)

OH + O3 → HO2 + O2

(7-13)

Br + O3 → BrO + O2

(7-14)

net: 2O3 → 3O2

Inorganic Br is released from the degradation of Halons in the stratosphere and may also enter the stratosphere by transport from the troposphere, where it is produced from diverse manmade and natural sources. Heterogeneous uptake of HBr and HOBr can influence this cycle by removal of active bromine Br and BrO. However, the following acid-catalyzed redox reaction occurring in aqueous phase or on ice surfaces:

(7-6)

In the stratosphere the predominant surface is sulfuric acid aerosol. In the absence of HCl, uptake of N2O5 on aqueous H2SO4 aerosol leads solely to hydrolysis, producing HNO3. There is compelling evidence that under these conditions this occurs by an acid-catalyzed mechanism:145 N2O5 + H3O+ → HNO3 + NO2+

(7-9)

As p(HCl) increases, the observed yield of ClNO2 product increases due to competition between reaction of H2NO3+ with H2O and Cl−. No formation of Cl2, which is produced from reaction of N2O5 in aqueous salt solutions containing chloride at higher temperatures and low pH, was observed on cold H2SO4/H2O surfaces. The possibility of Cl activation by N2O5 on ice surfaces has been considered in the context of polar boundary layer chemistry, where active halogen in the Arctic has become well characterized by observations. The reaction of N2O5 on frozen mixed halide salt solutions (Cl and Br) as a function of temperature and composition was investigated recently in the laboratory147 using a coated wall flow tube technique coupled to a chemical ionization mass spectrometer (CIMS). The molar yield of photolabile halogen compounds was near unity for almost all conditions studied, with the observed reaction products being nitryl chloride (ClNO2) and/or molecular bromine (Br2). The relative yield of ClNO2 and Br2 depended on the ratio of bromide to chloride ions in the solutions used to form the ice. At a bromide to chloride ion molar ratio greater than 1/30 (i.e., 20× higher than in seawater) in the starting solution, Br2 was the dominant product. Otherwise ClNO2 was primarily produced on these near pH-neutral brines. These results provide new experimental confirmation that the chemical environment of the brine layer changes with temperature and that these changes can directly affect multiphase chemistry. 7.3.3. Activation and Scavenging of Bromine. Ozone destruction by the following cycle involving the BrO + HO2 cycle is of particular significance in the lower stratosphere:

(7-3)

which may catalyze O3 destruction in the stratosphere, or react with organic species, leading to their oxidation, adding to the oxidation capacity in the troposphere. This chlorine activation process has been established to occur in the marine boundary layer, where Cl− is derived from sea salt aerosol. However, it has been considered to be a reaction of minor significance for activation of Cl from HCl on stratospheric sulfuric acid aerosols. Recent measurements of ClNO2 in the troposphere show that this molecule is rather widespread at nighttime, and heterogeneous chlorine activation could be a significant component of oxidizing capacity in the global troposphere.143 The formation of ClNO2 can occur on any atmospheric surface/particulate with Cl− present, including aqueous aerosols, ice and snow particles, and mineral particles exposed to Cl-containing gases. However, where water is present on the surface, N2O5 reaction with Cl− competes with hydrolysis of N2O5 to form HNO3. Reaction of N2O5 in aqueous salt solutions containing chloride at ambient temperatures and low pH was observed to produce both ClNO2 and Cl2 as chlorine-containing products, the relative amounts depending on Cl− concentration.144 Formation of Cl2 is attributed to further reaction of ClNO2 with Cl−: ClNO2 + Cl− → Cl 2 + NO2−

H 2NO3+ + H 2O → HNO3 + H3O+

H 2NO3+ + HCl → ClNO2 + H3O+

Nitryl chloride undergoes UV photolysis releasing Cl atoms: ClNO2 + hν → Cl + NO2

(7-8)

In the presence of HCl on H2SO4/H2O surfaces, the uptake leads to formation of HNO3 and ClNO2 products.146 It is suggested that the following reaction occurs:

These activation reactions have all been investigated experimentally in recent years to try and establish quantitative kinetic data and resolve some remaining issues of reaction mechanisms. 7.3.1. Heterogeneous Halogen Activation on Liquid Surfaces (Water, Salts, H2SO4). In recent years, activation of halogen radicals from ODS degradation products has focused on processes occurring in the UTLS region and in the troposphere. Both Cl and Br activations occur in these domains. The following sections describe some recent developments of interest for ozone chemistry and oxidizing capacity. 7.3.2. Cl Activation by N2O5. The heterogeneous reaction of N2O5 with Cl− present in atmospheric particles leads to the formation of nitryl chloride (ClNO2) in the reaction: N2O5 + Cl− → NO3− + ClNO2

NO2+ + H 2O → H 2NO3+

HOBr + Br −+H+ → H 2O + Br2

(7-15)

Br2 + hν → Br + Br

(7-16)

serves to regenerate the active Br species from HBr or Br− present in aerosols. Chloride can also be activated via the coupled reaction:

(7-7) AV


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HOBr + Cl−+H+ → H 2O + BrCl

(7-17)

BrCl + hν → Br + Cl

(7-18)

recycling in the UTLS region, these kinetic issues need to be resolved experimentally. 7.3.4. Heterogeneous Halogen Activation on Solid Surfaces (Ice, NAT, and SAT). Parameterization for the reactive uptake coefficient, γgs, for gas-phase species reacting at a solid surface with surface species, Y, after adsorption on the surface (LH type mechanism) was given earlier, eq E4.3-12 in section 4.3.4. Values of ksKLangC(X) have been determined experimentally from measurements of γ as a function of surface coverage [Y]s. Equation E4.3-12 demonstrates that γ depends on the gas-phase concentration of X, if KLangC(X)[X]g is similar to or larger than 1 (i.e., at high coverage). This is especially important when interpreting data from laboratory experiments performed using gas-phase reactant concentrations that lead to significant surface coverage. The IUPAC summary sheets (http://iupac.pole-ether.fr/index.html) provide preferred values for KLangC(X), αs, and ks. The data, although exhibiting some uncertainty, allow quantitative treatment of the heterogeneous chemical reactions, which leads to the following conclusions: (1) Reactions on ice that are important in both the polar stratosphere and the UTLS regions are kinetically well-defined and can account for the activation of chlorine radicals leading to ozone loss. (2) Heterogeneous hydrolysis of chlorine and bromine nitrates releases hypohalous acids, which also participate in halogen activation in the stratosphere and the troposphere. (3) HOBr is a key intermediate leading to recycling of active BrO radicals in the UTLS and in the polar regions of the troposphere, and can explain the appearance of BrO in the polar boundary layer and excess bromine in the lower stratosphere, caused by PGI from the tropospheric degradation of bromocarbons.

In the stratosphere, this reaction occurs efficiently on sulfuric acid aerosols at temperature of 200−230 K and composition ranging from 50 to 70 wt % H2SO4 and also on ice surfaces. In the troposphere, Br can be activated from liquid particles from sources such as volcanic HBr or sea-salt derived Br−, as well as on cirrus ice. The impact of this process on atmospheric chemistry has been assessed using kinetic data obtained in laboratory experiments, which show a large accommodation coefficient for HOBr (α = 0.6, Wachsmuth et al.148) and an acid-catalyzed reaction mechanism proposed by Eigen and Kustin.149 For the low temperature and high wt % H2SO4 stratospheric conditions, the mechanism likely involves protonation of HOBr followed by reaction of H2OBr+ with Br− or Cl−,150 but for tropospheric conditions the general acidassisted mechanism is favored. Roberts et al.151 have re-evaluated the aqueous reaction kinetics of HOBr for tropospheric conditions according to the general acid-assisted mechanism. They deduce that the rate of reaction of HOBr with halide ions becomes independent of pH at high acidity yielding an acid-independent second-order rate constant, kII. The limit of acid-saturation is poorly constrained by available experimental data, but it is suggested that for the reaction HOBr + Br−(aq) + H+(aq) it is at pH ≲ 1, with kIIsat = 108 −109 M−1 s−1, and the rate of reaction HOBr(aq) + Cl−(aq) + H+(aq) saturates to become acid-independent at pH ≤ 6, with kIIsat ≈ 104 M−1 s−1. The rate constant for Cl− is many orders of magnitude lower (a factor of 103 at pH = 3 and a factor of 106 at pH = 0) than has been currently assumed in numerical models of tropospheric BrO chemistry. The new revised kinetics is shown to reconcile the discrepancies in different reactive uptake coefficients reported from laboratory experiments. The re-evaluation confirms HOBr reactive uptake is rapid on moderately acidified sea-salt aerosol but predicts very low reactive uptake coefficients on highly acidified submicron particles. This explains the reported Br-enhancement (relative to Na) in sulfate-rich submicrometer particles in the marine environment.118b Predictions of high (accommodation limited) HOBr uptake coefficients in concentrated (>1 μmol/mol SO2) plume environments supports the explanation for rapid BrO formation observed in halogen-rich volcanic plumes throughout the troposphere.118a For the low temperature and high wt % H2SO4 stratospheric conditions, the recommended HOBr uptake coefficients in the presence of HCl and HBr can be calculated using values of kII given by the resistance model parametrization, which has been recommended by IUPAC, based on the work of Abbatt and coworkers152 and Hanson.153 In this case, the values of kII were those derived by Hanson, who gives an expression for kII for stratospheric conditions (W = wt % H2SO4):

8. CONCLUSIONS AND FUTURE DIRECTIONS In the beginning of the issue of ozone layer depletion due to the ozone depleting substances, mostly CFCs, Halons, methylchloroform, CH3Br, and CH3Cl, the key question relating to the degradation of these chemicals was the rate of release of reactive halogen (Cl or Br) once their degradation started. Now, it is clear that once the degradation started, due to the photolysis of the ODSs or their reaction with OH and O(1D), they release most of the reactive halogen and also form halogenated end-products very rapidly as compared to the rate at which they are transported from the region where the initiation started. Indeed, this rapid release is true for all of the ODSs and halogenated substitutes considered to date. The major steps in the atmospheric degradation of ODSs and some replacements are reasonably well understood. Therefore, atmospheric models used to forecast stratospheric ozone recovery can confidently deal with the rate of change of the catalysts in the stratosphere. The models can also confidently calculate the influence of ODSs on climate, both directly in their role as greenhouse gases and via the ozone layer depletion they induce. It is known that the halogens released from ODSs in the stratosphere eventually return to the troposphere, where they are removed by deposition (wet and dry). In addition, chlorine and bromine are also released from ODSs (with the exception of CFCs and most Halons) and their substitutes in the troposphere due to their oxidation in this region. However, the contribution to tropospheric halogen from such release is

k II(M−1 s−1) = exp(154 − 1.63 × W ) exp( −(38 500 − 478 × W )/T )

(E7-2)

This equation reproduces the experimentally derived kII values at 210−228 K, but the predicted values at 238 and 250 K are overestimated by a factor of 3 and 6, respectively. This suggests that saturation of the rate coefficient also occurs in the low temperature, low diffusivity regime in the concentrated acid aerosol. In view of the importance of BrAW


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from its principal degradation end-product, which has not previously been considered in the overall lifecycle of atmospheric chlorine. This is a new, additional, source of Cl that has potential significance to tropospheric oxidation rates. Therefore, attention has to be paid to the potential role of Cl initiated oxidation, especially because Cl atom reaction rate coefficients with halo-olefins (next generation substitutes) are significantly greater than in the analogous OH reaction (primarily due to suppression of the OH reactivity). There are indications that Cl atom concentrations can be as much as 104 atom cm−3 in some regions. Because Cl atoms react more rapidly with various halo-olefins, their reactions can rival those of OH at least over limited times and regions. If Cl atom initiated degradation were to occur, it may play a role in determining the rate and location of their degradation in the troposphere. That is, Cl initiated chemistry may enhance regional degradation and environmental impacts as HFOs gain more prevalent usage. In addition, this pathway could conceivably lead to chlorinated end-products that are longer lived than the original nonchlorine containing compound. If this were to occur, a nonozone depleting compound would produce an ozone depleting chemical! Care has to be exercised to examine this potential pathway. One of the key issues related to end-products of concern is the production and regional accumulation of toxic substances (e.g., halo-acids) in the environment. Our knowledge of extent and prevalence of this production and accumulation depends critically on the understanding of the degradation and removal mechanisms in all of their facets. For example, in the case of haloacids such as trifluoroacetic acid (TFA), one needs to consider all degradation pathways that can lead to the precursor of the haloacid, uptake of haloacids in clouds and other condensed matter, conversion of the haloacid precursors in the condensed matter, and the eventual removal via rain, snow, etc., need to be considered. Sources of TFA, other than the recognized source of HFC-134a and HFO-1234yf, have been noted in this Review. The review of material relating to heterogeneous processes confirms the picture that the stable halogen-containing degradation products resulting from oxidation of ODS are removed from the atmosphere by multiphase processes. Earlier experimental data for partition to the liquid phase have been extended to include partition to the ice phase, allowing evaluation of removal rates from the cirrus clouds in the upper troposphere-lower stratosphere (UTLS) region and by polar stratospheric cloud particles. The latter is also important for assessing the extent of transport of degradation products of the short-lived substitutes into the stratosphere. In this regard, it is worth noting that multiphase processes also lead to chemical reactions, which modify the composition of aged air in the tropopause region and the lower stratosphere; the important reactions are hydrolysis of degradation products leading to the formation of more stable oxygenated organic halogen species, and redox reactions in which halogen reservoirs are transformed into photochemically active molecules, which lead to recycling of reactive halogen radicals. The framework for thinking about stratospheric degradation of ODSs needs to consider the formation of halogenated products (e.g., CCl2O) that can transport unreleased halogen from one part of the stratosphere to another. However, because the time constant for the removal of species from the stratosphere is many years, transport of halogenated products is not a concern for all of the current ODSs and their

expected to be much smaller than that from natural sources (e.g., sea salt for Cl and brominated organics for Br). Over the past decades, it has become clear that the Montreal Protocol also helped mitigate climate change by reducing the levels of ODSs, which are also greenhouse gases. Ever since this realization, attempts have been made to ensure that the substitutes for ODSs are not only safe for the ozone layer but also have as little influence on climate as possible. Therefore, ozone depletion and climate change properties of substitutes have often been investigated together. What was not emphasized during the initial decades of ODS and substitute use was the production of species (often not containing chlorine or bromine) that can have other influences on the atmosphere, for example, radiative forcing of climate by greenhouse gases or persistent toxic chemicals. Over the past decade, because of the considerations noted above, the interest has enlarged from the release of halogens to include consideration of degradation mechanisms and the production of stable end-products. This is particularly true in the case of the substitutes, including ones that do not contain chlorine or bromine. This wider inquiry has further expanded to include possible production of unwanted toxic substances and the possible production of urban ozone by the use of reactive substitutes. New substitute compounds (or for that matter any new halogenated compound) are still being introduced commercially. Although the degradation of the various classes of ODSs and replacement compounds is reasonably well understood, an evaluation of the impact of all new compounds on the environment (ozone layer depletion, climate change, and issues of deposition of deleterious chemicals) is needed on a case-by-case basis prior to approvals for their use and commercialization (e.g., a recent study of the R-316c substitute stereoisomers (see Table 1) demonstrated their potential deleterious impact on the ozone layer). A majority of the information on degradation has come from laboratory-based evaluations of kinetics, photochemistry, heterogeneous processes, and mechanisms. They are still needed to aid making informed decisions with regards to the use of new compounds. The advances in atmospheric observations of various end-products (e.g., CF2O, total chlorine, BrO, etc.) have enabled validation of the degradation mechanisms. However, given the uncertainties in the observations and lack of measurements of all of the needed products, there is still a great deal of reliance on laboratorybased information. New experimental data on reaction kinetics and mechanisms have extended our understanding of these complex multiphase physicochemical processes, leading to greater confidence in the interpretation of field observations of atmospheric composition, and a more quantitative description of the impact and removal processes of ODS and their degradation products. Until the introduction of halo-olefins as substitutes, for all practical purposes, initiation of the degradation of the substitutes in the troposphere was known to be controlled by their reactions with OH and UV photolysis, with minor contribution from other reactions. A novel photochemically active species nitryl chloride (ClNO2) was shown to be widely produced in the troposphere. The molecule is formed principally during nighttime by a heterogeneous reaction of N2O5 with HCl (or Cl−) in aqueous aerosol particles, and its photolysis leads to release of atomic Cl. (Formation of other photolabile species such as ClNO and Cl2 has also been investigated.) This process constitutes an activation of chlorine AX


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ozone layer, climate, production of toxic substances, and production of other unintended chemicals. In this Review, we have noted the potential role of Cl atom initiated oxidation of reactive chemicals, especially olefinic substitutes. There is need for the characterization of products of Cl atom initiated oxidation under appropriate conditions and examining minor products that could impact either the ozone layer or the climate. The conversion of halogenated species to haloacids has been noted here. This conversion is likely to occur in clouds and condensed matter, and it may not lead to wet or dry deposition of the species, at least not immediately. It is very likely that a part of the haloacids is released into the gas phase. Therefore, studies of the gas-phase chemistry of haloacids are important for a full characterization of haloacids and their impacts.

substitutes. However, the formation of longer lived (more inert) degradation products such as CCl2O leads to a gradation in chlorine levels with the stratosphere. This is especially important when considering new substitutes. Even though there have been significant advances in our understanding of the degradation of the ODSs and their substitutes, there remain some uncertainties that are noted here. Uncertainties in loss process parameters, for example, reaction rate coefficients, absorption cross sections, etc., exist but overall do not significantly limit our understanding of the atmospheric impact of ODSs and substitutes. The exception is for newly proposed compounds and the identification of some possible minor degradation end-products. Minor degradation pathways including chemical reaction of degradation intermediates (propagation steps) with HO2, RO2, and NO2 were not examined in detail in this Review, although it was noted that these reactions represent additional pathways for the removal of halogen from the atmosphere via heterogeneous processing. Distinction is apparent between the inorganic chlorinecontaining products that have high affinity for transfer and reaction on atmospheric particles, and organic halogen species that are less reactive and less soluble. Overall there is a large range of atmospheric lifetimes encountered in the degradation products resulting from competing chemical and physical processes. However, there is no evidence for significant modification of the ODP of ODSs and their substitutes used to date. Degradation of Br-containing ODS leads to products with higher heterogeneous reactivity leading to activation of Br. Inorganic bromine in the lower stratosphere is ∼3 ppt more than expected from the degradation of Halons and CH3Br in the stratosphere, which suggests a contribution of Br x originating in the troposphere. This could be explained by multiphase recycling of reactive bromine in competition with removal of stable reservoirs such as HBr from the UTLS in precipitation. A similar process may account for the persistence of BrO in volcanic plumes in the troposphere. However, the issue of bromine from short-lived substances (and more recently chlorine from short-lived chlorine substances) still needs some clarification to quantify the contributions of Cl and Br from non-ODS sources, especially as the level of man-made ODSs decreases and the halogen loading also decreases. Iodine compounds have been considered as potential substitutes for certain ODSs. The ozone layer depletion from such use of iodine compounds has also been evaluated. The major uncertainty with regards to the role of iodine-catalyzed ozone destruction is the heterogeneous chemistry of iodine in the stratosphere and its possible loss into condensed matter. Similarly, iodine-catalyzed ozone depletion in the troposphere was noted as a possibility more than three decades ago.154 New laboratory measurements of the photolysis products of iodine monoxide, OIO, indicate that IOx catalytic cycles could be even more efficient for ozone destruction (as compared to Cl and Br) than previously believed. The release of iodocarbons in the stratosphere could, therefore, lead to enhanced ozone depletion and should be avoided. From the above discussions, it is clear that much is known about the degradation of ODSs and their substitutes that are currently in use. However, it is very important to be vigilant toward the use of substitutes whose degradation pathways and impacts of the degradation products are not evaluated. New substitutes have to be evaluated for the potential impacts on the

9. HALOCARBON NOMENCLATURE Shorthand notation for hydrogen, fluorine, and chlorine containing halocarbons consists of a series of uppercase letters that designate the elements present in the molecule combined with a series of numbers that represent the quantity of the carbon, hydrogen, and fluorine elements (the chlorine content is inferred), and lowercase letters that designate the isomeric form of the molecule, if necessary, or functional groups and structure for more complex halocarbons. 9.1. Uppercase Lettering

An H for hydrogen is always the first uppercase letter, if hydrogen is present in the molecule. An uppercase C is always added at the end to designate carbon. The other elements are included alphabetically: C and F are used to designate chlorine and fluorine. Perfluorinated molecules use a P prefix. 9.2. Numbering

The first number is the no. of double bonds (omitted if zero). The second number is the no. of carbon atoms − 1 (omitted if zero). The third number is the no. of hydrogen atoms + 1. The fourth number is the no. of fluorine atoms. The number of Cl atoms is the balance of the atoms in the molecule. Examples include CCl2F2 (CFC-12), CHClF2 (HCFC-22), CHF2CF3 (HFC-125), and CF4 (PFC-14). 9.3. Lowercase Lettering (Isomers)

For C2 compounds, the isomer with the smallest difference in the sum of the masses of the substituents on each carbon has no letter, the isomer with next smallest difference uses “a”, the isomer with the next smallest difference uses “b”, and so on. Examples include CH2ClCHF2 (HCFC-142), CHClFCH2F (HCFC-142a), and CH3CClF2 (HCFC-142b). Halocarbons with carbon numbers ≥3 follow similar rules to designate their isomeric form, although in these cases multiple letters are necessary to uniquely describe the molecule as outlined in Table 21. For >C3 compounds, lettering begins at the end of the molecule and includes each carbon group (minimizing the number of letters and combinations of letters using letters that appear earliest in the alphabet). Examples include CHCl 2 CF 2 CF 3 (HCFC-225ca), CF 3CHFCH2 F (HFC-245eb), CF3CF2 CH 2CH2F (HFC356mcf), and CF3CHFCHFCF2CF3 (HFC-43-10mee). An unsaturated halocarbon, containing fluorine, is labeled a hydrofluoro-olefin (HFO). Examples include (E)-CF3CHCHCl (HFO-1233zd(E)) and CF3CFCH2 (HFO-1234yf). AY


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Biographies

Table 21. Suffix Lettering for Chlorine and Fluorine Substituted Isomers appended lettera group substitution

letterb

−CCl2− −CClF− −CF2− −CHCl− −CHF− −CH2−

a b c d e f

propanes: second appended letterc

>C3: first appended letter

mass difference

letter

group substitution

letter

smallest • • greatest

a • • z

−CCl3 −CCl2F −CClF2 −CF3 −CHCl2 −CH2Cl −CHF2 −CH2F −CHClF −CH3

j k l m n o p q r s

James B. Burkholder was born and raised in western New York, and received a B.S. (1976) from the State University College at Fredonia, NY, and Ph.D. (1982) in physical chemistry from Indiana University under the direction of Prof. E. J. Bair. He joined the Cooperative Institute for Environmental Science (CIRES) and the National Oceanic and Atmospheric Administration (NOAA) laboratory as a postdoctoral researcher (1983−1986) with Dr. C. J. Howard in the Chemical Kinetics Group. He continued research at CIRES/NOAA and was promoted to CIRES Senior Research Scientist in 1998. He joined the research staff at the NOAA Aeronomy laboratory (now Chemical Sciences Division) in 1999 where he is currently the Program Leader for the Chemical Processes and Instrument Development group. He has participated extensively in laboratory studies designed to elucidate aspects of the chemical processes (e.g., kinetics, photochemistry, and heterogeneous processes) undertaken by trace species in the Earth’s atmosphere. He is a coeditor for Atmospheric Chemistry and Physics (2008−present). He is a panel member of the NASA Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies Evaluation (2008−present).

a First letter for propanes. bLetters are in order of decreasing mass of the group substitution. cDetermined by the mass difference between the terminal carbon groups.

Table 22. Suffix Lettering for Halopropene Isomers

a

first appended letter

second appended letter

refers to the central carbon substitution

refers to the terminal methylene carbon substitution

group substitution

lettera

group substitution

lettera

−Cl −F −H

x y z

CCl2 CClF CF2 CHCl CHF CH2

a b c d e f

Letters are in order of decreasing mass of the group substitution.

9.4. Bromofluorocarbons and Bromochlorofluorocarbons

These classes of compounds are referred to as Halons. A four digit numbering system is used where the first number is the no. of carbon atoms. The second is the no. of fluorine atoms. The third is the no. of chlorine atoms, and the fourth is the no. of bromine atoms. This shorthand Halon naming system does not distinguish between isomers. Examples include CBrF3 (Halon-1301), CBrClF2 (Halon-

Dr. Tony Cox graduated in 1966 from University of Manchester, UK, with a Ph.D. in Physical Chemistry. After postdoctoral work at NRC in Ottawa, he joined the Environmental and Medical Sciences Division at the UKAEA Harwell Laboratory in 1968. There he undertook research in many aspects of tropospheric and stratospheric chemistry, with an emphasis on laboratory studies of kinetics and photochemistry of gasphase reactions. In 1977 he was a founding member of the IUPAC Kinetics Data Evaluation Panel for Atmospheric Chemistry and was chairman from 1999 until 2008. In 1995 he joined the Chemistry Department at the University of Cambridge, starting a new research group studying kinetics of heterogeneous reactions, which he continues to apply to atmospheric chemistry.

1211), and CBrF2CBrF2 (Halon-2402).

AUTHOR INFORMATION Corresponding Author

*Phone: (303) 497-3252. E-mail: james.b.burkholder@noaa. gov. Notes

The authors declare no competing financial interest. AZ


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2007; http://www.wmo.int/pages/prog/arep/gaw/ozone_2006/ ozone_asst_report.html. (5) WMO (World Meteorological Organization). Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project-Report No. 44, Geneva, Switzerland, 1998. (6) Ravishankara, A. R.; Lovejoy, E. R. J. Chem. Soc., Faraday Trans. 1994, 90, 2159. (7) Velders, G. J. M.; Fahey, D. W.; Daniel, J. S.; McFarland, M.; Andersen, S. O. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10949. (8) (a) Wuebbles, D. J.; Patten, K. O.; Johnson, M. T.; Kotamarthi, R. J. Geophys. Res. 2001, 106, 14551. (b) Brioude, J.; Portmann, R. W.; Daniel, J. S.; Cooper, O. R.; Frost, G. J.; Rosenlof, K. H.; Granier, C.; Ravishankara, A. R.; Montzka, S. A.; Stohl, A. Geophys. Res. Lett. 2010, 37, L19804. (9) (a) Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S.; Mills, M.; Howard, C. J.; Solomon, S. Science 1994, 263, 71. (b) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J. Chem. Soc., Faraday Discuss. 1995, 100, 55. (10) Kwok, E. S. C.; Atkinson, R. Atmos. Environ. 1995, 29, 1685. (11) Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; Orkin, V. L.; Wine, P. H. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17; JPL Publication 10-6, Jet Propulsion Laboratory, California Institute of Technology Pasadena: CA, 2011; http://jpldataeval.jpl.nasa.gov. (12) Ammann, M.; Cox, R. A.; Crowley, J. N.; Jenkin, M. E.; Mellouki, A.; Rossi, M. J.; Troe, J.; Wallington, T. J. IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation, 2014; http://iupac.pole-ether.fr. (13) (a) Wayne, R. P. Principles and Applications of Photochemistry; University Press: Oxford, 1988. (b) Calvert, J. G.; Derwent, R. G.; Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. Mechanisms of Atmospheric Oxidation of the Alkanes; Oxford University Press: New York, 2008. (c) Finlayson-Pitts, B. J.; Pitts, J. N. J. Chemistry of the Upper and Lower Atmosphere; Academic Press: New York, 2000. (d) Atkinson, R. J. Phys. Chem. Ref. Data 1991, 20, 459. (e) Alternative Fluorocarbons Environmental Acceptability Study (AFEAS). Proceedings of the Workshop on the Atmospheric Degradation of HCFCs and HFCs, November 17−19, 1993, Boulder, CO; AFEAS: Washington, DC, 1995. (f) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, O. J.; Sehested, J.; DeBruyn, W.; Shorter, J. A. Environ. Sci. Technol. 1994, 28, 320. (14) Wu, F.; Carr, R. W. J. Phys. Chem. 1992, 96, 1743. (15) Ko, M. K. W.; Newman, P. A.; Reimann, S.; Strahan, S. E.; Plumb, R. A.; Stolarski, R. S.; Burkholder, J. B.; Mellouki, W.; Engel, A.; Atlas, E. L.; Chipperfield, M.; Liang, Q. Lifetimes of Stratospheric Ozone-Depleting Substances, Their Replacements, and Related Species. SPARC Report No. 6, WCRP-15/2013, 2013; http://www. sparc-climate.org/publications/sparc-reports/sparc-report-no6/. (16) Krubger, A. J.; Minzner, R. A. J. Geophys. Res. 1976, 81, 4477. (17) Smith, J. P.; Solomon, S.; Sanders, R. W.; Miller, H. L.; Perliski, L. M.; Keys, J. G.; Schmeltekopf, A. L. J. Geophys. Res. 1993, 98, 8983. (18) Spivakovsky, C. M.; Logan, J. A.; Montzka, S. A.; Balkanski, Y. J.; Foreman-Fowler, M.; Jones, D. B. A.; Horowitz, L. W.; Fusco, A. C.; Brenninkmeijer, C. A. M.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 2000, 105, 8931. (19) (a) Finlayson-Pitts, B. J. Anal. Chem. 2010, 82, 770. (b) Knipping, E. M.; Lakin, M. J.; Foster, K. L. Science 2000, 288, 301. (c) Laskin, A.; Wang, H.; Robertson, W. H. J. Phys. Chem. A 2006, 110, 10619. (d) Raff, J. D.; Njegic, B.; Chang, W. L.; Gordon, M. S.; Dabdub, D.; Gerber, R. B.; Finlayson-Pitts, B. J.; Halpern, J. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 13647. (20) Rudolph, J.; Koppmann, R.; Plass-Dülmer, C. Atmos. Environ. 1996, 30, 1887. (21) (a) Wingenter, O. W.; Blake, D. R.; Blake, N. J.; Sive, B. C.; Rowland, F. S. J. Geophys. Res. 1999, 104, 21819. (b) Wingenter, O. W.; Sive, B. C.; Blake, N. J.; Blake, D. R.; Rowland, F. S. J. Geophys. Res. 2005, 110, D20308.

Dr. Ravishankara is an atmospheric chemist. He obtained his Ph.D. from the University of Florida, Gainesville, FL, in physical chemistry. After one year of postdoctoral work at the University of Maryland, where he entered the field of atmospheric chemistry, he moved to the Georgia Institute of Technology for 8 years and then to the National Oceanic and Atmospheric Administration for 30 years. He moved to Colorado State University in 2014. He has worked over the past 38 years on the chemistry of the Earth’s atmosphere as it relates to stratospheric ozone depletion, climate change, regional air quality, and their intersections. His measurements in the laboratory and in the atmosphere have contributed to deciphering the ozone layer depletion, including the ozone hole; development of substitutes for ozone depleting substances; to quantifying the role of chemically active species on climate; and to advancing understanding of the formation, removal, and properties of pollutants. He has served or is serving on various editor boards and was an editor of Geophysical Research Letters. Of relevance to this Review, he was a member of the NASA/JPL data evaluation panel from 1982 to 2007 and is a co-Chair of the Scientific Assessment Panel of the Montreal Protocol that deals with the ozone layer and depleting substances. He has also led or authored numerous international and national assessment reports related to this Review. Over the past decade, in addition to atmospheric chemistry research, he is interested in taking scientific information to decision-makers.

ACKNOWLEDGMENTS J.B.B. was supported in part by NOAA’s Atmospheric Chemistry, Carbon Cycle, and Climate (AC4) Program and NASA’s Atmospheric Composition Program. R.A.C. thanks the UK Research Council’s SPICE project for support. A.R.R. thanks Colorado State University for support of this work. We thank the anonymous reviewers for helpful comments. REFERENCES (1) (a) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (b) Lovejoy, E. R.; Huey, L. G.; Hanson, D. R. J. Geophys. Res. 1995, 100, 18775. (2) Ravishankara, A. R.; Velders, G. J. M.; Miller, M. K.; Molina, M. J. HFCs: A Critical Link in Protecting Climate and the Ozone Layer; UNEP: 2011; 36 pp, http://www.unep.org. (3) WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring Project-Report No. 55, 416 pp, Geneva, Switzerland, 2014; http://www.wmo.int/pages/prog/arep/gaw/ozone_2014/ ozone_asst_report.html. (4) (a) Kurylo, M. J.; Orkin, V. L. Chem. Rev. 2003, 103, 5049. (b) WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project-Report No. 50, 572 pp, Geneva, Switzerland, BA


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(22) Riedel, T. P.; Bertram, T. H.; Crisp, T. A.; Williams, E. J.; Lerner, B. M.; Vlasenko, A.; Li, S.-M.; Gilman, J.; de Gouw, J.; Bon, D. M.; Wagner, N. L.; Brown, S. S.; Thornton, J. A. Environ. Sci. Technol. 2012, 46, 10463. (23) (a) Riedel, T. P.; Wagner, N. L.; Dube, W. P.; Middlebrook, A. M.; Young, C. J.; Ozturk, F.; Bahreini, R.; VandenBoer, T. C.; Wolfe, D. E.; Williams, E. J.; Roberts, J. M.; Brown, S. S.; Thornton, J. A. J. Geophys. Res. 2013, 118, 8702. (b) Tanaka, P. L.; Riemer, D. D.; Chang, S.; Yarwood, G.; McDonald-Buller, E. C.; Apel, E. C.; Orlando, J. J.; Neece, D. J.; Mullins, C. B.; Allen, D. T. Atmos. Environ. 2003, 37, 1393. (24) (a) Jobson, B. T.; Niki, H.; Yokouchi, Y. J. Geophys. Res. 1994, 99, 25355. (b) Wingenter, O. W.; Blake, D. R.; Blake, N. J. J. Geophys. Res. 1999, 104, 21819. (c) Wingenter, O. W.; Sive, B. C.; Blake, N. J. J. Geophys. Res. 2005, 110, D20308. (d) Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.; Plastridge, R. A.; Hubbe, J. M.; Fast, J. D.; Berkowitz, C. M. Nature 1998, 394, 353. (25) Feierabend, K. J.; Papanastasiou, D. K.; Burkholder, J. B. J. Phys. Chem. A 2010, 114, 12052. (26) Baasandorj, M.; Hall, B. D.; Burkholder, J. B. Atmos. Chem. Phys. 2012, 12, 11753. (27) Brown, S. S.; Stutz, J. Chem. Soc. Rev. 2012, 41, 6405. (28) (a) Wayne, R. P.; Barnes, I.; Biggs, P.; Burrows, J. P.; Canosamas, C. E.; Hjorth, J.; Lebras, G.; Moortgat, G. K.; Perner, D.; Poulet, G.; Restelli, G.; Sidebottom, H. Atmos. Environ. 1991, 25, 1. (b) Kerdouci, J.; Picquet-Varrault, B.; Doussin, J.-F. ChemPhysChem 2010, 11, 3909. (c) Marston, G.; Monks, P. S.; Canosa-Mas, C. E.; Wayne, R. P. J. Chem. Soc., Faraday Trans. 1993, 89, 3899. (29) Acerboni, G.; Jensen, N. R.; Rindone, B.; Hjorth, J. Chem. Phys. Lett. 1999, 309, 364. (30) Martinez, E.; Cabañas, B.; Aranda, A.; Martin, P.; Salgado, S. Int. J. Chem. Kinet. 1997, 29, 927. (31) Papadimitriou, V. C.; Lazarou, Y. G.; Talukdar, R. K.; Burkholder, J. B. J. Phys. Chem. A 2011, 115, 167. (32) Acerboni, G.; Beukes, J. A.; Jensen, N. R.; Hjorth, J.; Myhre, G.; Nielsen, C. J.; Sundet, J. K. Atmos. Environ. 2001, 35, 4113. (33) Madronich, S. Tropospheric Ultraviolet-Visible Model (TUV), 2013; http://www.acd.ucar.edu/TUV/. (34) (a) Ausloos, P. J. Res. Natl. Bur. Stand. 1977, 82, 1. (b) Wylie, D. J.; Cooney, R. P.; Seakins, J. M.; Millar, G. J. Vib. Spectrosc. 1995, 9, 245. (35) (a) Schwartz, S. E. Chemistry of Multiphase Atmospheric Systems; Springer-Verlag: Berlin, 1986. (b) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. (c) Hanson, D. R.; Ravishankara, A. R.; Solomon, S. J. Geophys. Res. 1994, 99, 3615. (d) Ravishankara, A. R. Science 1997, 276, 1058. (36) Davidovits, P.; Hu, J. H.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. Faraday Discuss. 1995, 100, 65. (37) Tang, M. J.; Cox, R. A.; Kalberer, M. Atmos. Chem. Phys. 2014, 14, 9233. (38) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill, Inc.: New York, 1987. (39) McNeill, V. F.; Loerting, T.; Geiger, F. M.; Trout, B. L.; Molina, M. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 9422. (40) (a) Ullerstam, M.; Abbatt, J. P. D. Phys. Chem. Chem. Phys. 2005, 7, 3596. (b) Kärcher, B.; Abbatt, J. P. D.; Cox, R. A.; Popp, P. J.; Voigt, C. J. Geophys. Res. 2009, 114, D13306. (41) Carslaw, K. S.; Clegg, S. L.; Brimblecombe, P. J. Phys. Chem. 1995, 99, 11557. (42) Ammann, M.; Cox, R. A.; Crowley, J. N.; Jenkin, M. E.; Mellouki, A.; Rossi, M. J.; Troe, J.; Wallington, T. J. Atmos. Chem. Phys. 2013, 13, 8045. (43) Shi, Q.; Jayne, J. T.; Kolb, C. E.; Worsnop, D. R.; Davidovits, P. J. Geophys. Res. 2001, 106, 24259. (44) Williams, L. R.; Long, F. S. J. Phys. Chem. 1995, 99, 3748. (45) Kolb, C. E.; Cox, R. A.; Abbatt, J. P. D.; Ammann, M.; Davis, E. J.; Donaldson, D. J.; Garrett, B. C.; George, C.; Griffiths, P. T.; Hanson, D. R.; Kulmala, M.; McFiggans, G.; Poschl, U.; Riipinen, I.;

Rossi, M. J.; Rudich, Y.; Wagner, P. E.; Winkler, P. M.; Worsnop, D. R.; Dowd, C. D. O. Atmos. Chem. Phys. 2010, 10, 10561. (46) (a) Nienow, A. M.; Roberts, J. T. Annu. Rev. Phys. Chem. 2006, 57, 105. (b) Andreae, M. O.; Rosenfeld, D. Earth-Sci. Rev. 2008, 89, 13. (47) Clegg, S. L.; Brimblecombe, P.; Wexler, A. S. J. Phys. Chem. A 1998, 102, 2155. (48) Peter, T.; Luo, B. P.; Wirth, M.; Kiemle, C.; Flentje, H.; Yushkov, V. A.; Khattatov, V.; Rudakov, V.; Thomas, A.; Borrmann, S. Atmos. Chem. Phys. 2003, 3, 1083. (49) IUPAC. Compendium of Chemical Terminology, 2nd ed.; Compiled by A. D. McNaught and A. Wilkinson. XML on-line corrected version: http://goldbook.iupac.org (2006) created by M. Nic, J. Jirat, and B. Kosata; updates compiled by A. Jenkins.; Blackwell Scientific Publications: Oxford, 1997. (50) (a) Poeschl, U.; Rudich, Y.; Ammann, M. Atmos. Chem. Phys. 2007, 7, 5989. (b) Crowley, J. N.; Ammann, M.; Cox, R. A.; Hynes, R. G.; Jenkin, M. E.; Mellouki, A.; Rossi, M. J.; Troe, J.; Wallington, T. J. Atmos. Chem. Phys. 2010, 10, 9059. (51) (a) Okake, H. Photochemistry of Small Molecules; Wiley: New York, 1978. (b) Good, D. A.; Francisco, J. S. Chem. Rev. 2003, 103, 4999. (52) Papadimitriou, V. C.; McGillen, M. R.; Smith, S. C.; Jubb, A. M.; Portmann, R. W.; Hall, B. D.; Fleming, E. L.; Jackman, C. H.; Burkholder, J. B. J. Phys. Chem. A 2013, 117, 11049. (53) Köppenkastrop, D.; Zabel, F. Int. J. Chem. Kinet. 1991, 23, 1. (54) Ravishankara, A. R.; Solomon, S.; Turnipseed, A. A.; Warren, R. F. Science 1993, 259, 194. (55) Laube, J. C.; Newland, M. J.; Hogan, C.; Brenninkmeijer, C. A. M.; Fraser, P. J.; Martinerie, P.; Oram, D. E.; Reeves, C. E.; Röckmann, T.; Schwander, J.; Witrant, E.; Sturges, W. T. Nat. Geosci. 2014. (56) (a) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 2011, 507, 19. (b) Jia, X.; Chen, L.; Mizukado, J.; Kutsuna, S.; Tokuhashi, K. Chem. Phys. Lett. 2013, 572, 21. (57) Lin, K.-C.; Tsai, P.-Y. Phys. Chem. Chem. Phys. 2014, 16, 7184. (58) Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. Chem. Rev. 2003, 103, 4657. (59) Bhatnagar, A.; Carr, R. W. Chem. Phys. Lett. 1996, 258, 651. (60) Tuazon, E. C.; Atkinson, R. J. Atmos. Chem. 1993, 17, 179. (61) Mörs, V.; Hoffmann, A.; Malms, W.; Zellner, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 540. (62) Orkin, V. L.; Khamaganov, V. G. J. Atmos. Chem. 1993, 16, 157. (63) Somnitz, H.; Zellner, R. Phys. Chem. Chem. Phys. 2001, 3, 2352. (64) Iyer, R. S.; Chen, C.-Y.; Rowland, F. S. J. Phys. Chem. 1985, 89, 2042. (65) Dong, Z. F.; Schneider, M.; Wolfrum, J. Int. J. Chem. Kinet. 1989, 21, 387. (66) Albaladejo, J.; Notario, A.; Cuevas, C. A.; Ballesteros, B.; Martínez, E. J. Atmos. Chem. 2003, 45, 35. (67) Sulbaek Andersen, M. P.; Nielsen, O. J.; Toft, A.; Nakayama, T.; Matsumi, Y.; Waterland, R. L.; Buck, R. C.; Hurley, M. D.; Wallington, T. J. J. Photochem. Photobiol., A: Chem. 2005, 176, 124. (68) Kaiser, E. W.; Wallington, T. J. J. Phys. Chem. A 2012, 116, 5958. (69) Hurley, M. D.; Wallington, T. J.; Javadi, M. S.; Nielsen, O. J. Chem. Phys. Lett. 2008, 450, 263. (70) Søndergaard, R.; Nielsen, O. J.; Hurley, M. D.; Wallington, T. J.; Singh, R. Chem. Phys. Lett. 2007, 443, 199. (71) Hurley, M. D.; Ball, J. C.; Wallington, T. J. J. Phys. Chem. A 2007, 111, 9789. (72) Mashino, M.; Ninomiya, Y.; Kawasaki, M.; Wallington, T. J.; Hurley, M. D. J. Phys. Chem. A 2000, 104, 7255. (73) Baasandorj, M.; Ravishankara, A. R.; Burkholder, J. B. J. Phys. Chem. A 2011, 115, 10539. (74) Ljubić, I.; Sabljić, A. J. Phys. Chem. A 2002, 106, 4745. (75) Nielsen, O. J.; Javadi, M. S.; Sulbaek Andersen, M. P.; Hurley, M. D.; Wallington, T. J.; Singh, R. Chem. Phys. Lett. 2007, 439, 18. (76) Nilsson, E. J. K.; Nielsen, O. J.; Johnson, M. S.; Hurley, M. D.; Wallington, T. J. Chem. Phys. Lett. 2009, 473, 233. (77) Avzianova, E. V.; Ariya, P. A. Int. J. Chem. Kinet. 2002, 34, 678. BB


Chemical Reviews

Review

(78) Zhang, J.; Hatakeyama, S.; Akimoto, H. Int. J. Chem. Kinet. 1983, 15, 655. (79) Blume, C. W.; Hisatsune, I. C.; Heicklen, J. Int. J. Chem. Kinet. 1976, 8, 235. (80) Sulbaek Andersen, M. P.; Nilsson, E. J. K.; Nielsen, O. J.; Johnson, M. S.; Hurley, M. D.; Wallington, T. J. J. Photochem. Photobiol., A: Chem. 2008, 199, 92. (81) Mashino, M.; Ninomiya, Y.; Kawasaki, M.; Wallington, T. J.; Hurley, M. D. J. Phys. Chem. A 2000, 104, 7255. (82) NIST Chemical Kinetics Database, 2014; webbook.nist.gov/ kinetics. (83) Abbatt, J. P. D.; Anderson, J. G. J. Phys. Chem. 1991, 95, 2382. (84) Gierczak, T.; Baasandorj, M.; Burkholder, J. B. Phys. Chem. Chem. Phys. 2014, 118, 11015. (85) Orkin, V. L. J. Phys. Chem. A 2014, 118, 5263. (86) Orkin, V. L.; Louis, F.; Huie, R. E.; Kurylo, M. J. J. Phys. Chem. A 2002, 106, 10195. (87) Sulbaek Andersen, M. P.; Hurley, M. D.; Wallington, T. J. Chem. Phys. Lett. 2009, 482, 20. (88) Calvert, J. G.; Mellouki, A.; Orlando, J. J.; Pilling, M. J.; Wallington, T. J. Mechanisms of Atmospheric Oxidation of the Oxygenates; Oxford University Press: New York, 2011. (89) Kutsuna, S.; Chen, L.; Abe, T.; Mizukado, J.; Uchimaru, T.; Tokuhashi, K.; Sekiya, A. Atmos. Environ. 2005, 39, 5884. (90) Taniguchi, N.; Wallington, T. J.; Hurley, M. D.; Guschin, A. G.; Molina, L. T.; Molina, M. J. J. Phys. Chem. A 2003, 107, 2674. (91) Takahashi, K.; Nakayama, T.; Matsumi, Y.; Solomon, S.; Gejo, T.; Shigemasa, E.; Wallington, T. J. . Geophys. Res. Lett. 2002, 29. (92) Papadimitriou, V. C.; McGillen, M. R.; Fleming, E. L.; Jackman, C. H.; Burkholder, J. B. Geophys. Res. Lett. 2013, 40, 1. (93) Mühle, J.; Huang, J.; Weiss, R. F.; Prinn, R. G.; Miller, B. R.; Salameh, P. K.; Harth, C. M.; Fraser, P. J.; Porter, L. W.; Greally, B. R.; O’Doherty, S.; Simmonds, P. G. J. Geophys. Res. 2009, 114, D05306. (94) Papadimitriou, V. C.; Portmann, R. W.; Fahey, D. W.; Mühle, J.; Weiss, R. F.; Burkholder, J. B. J. Phys. Chem. A 2008, 112, 12657. (95) Lesclaux, R.; Dognon, A. M.; Caralp, F. J. Photochem. Photobiol. 1987, 41, 1. (96) Rontu Carlon, N.; Papanastasiou, D. K.; Fleming, E. L.; Jackman, C. H.; Newman, P. A.; Burkholder, J. B. Atmos. Chem. Phys. 2010, 10, 6137. (97) WMO (World Meteorological Organization). Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project-Report No. 47, 516 pp, Geneva, Switzerland, 2003; http://www.wmo.int/pages/prog/arep/gaw/ozone_2002/ ozone_2002.html. (98) Orlando, J. J.; Tyndall, G. S.; Wallington, T. J.; Dill, M. Int. J. Chem. Kinet. 1996, 28, 433. (99) Kamboures, M. A.; Hansen, J. C.; Francisco, J. S. Chem. Phys. Lett. 2002, 353, 335. (100) Papanastasiou, D. K.; McKeen, S. A.; Burkholder, J. B. Atmos. Chem. Phys. 2014, 14, 3017. (101) Bilde, M.; Sehested, J.; Wallington, T. J.; Nielsen, O. J. J. Phys. Chem. A 1997, 101, 5477. (102) Bilde, M.; Wallington, T. J.; Ferronato, C.; Orlando, J. J.; Tyndall, G. S.; Estupiñan, E.; Haberkorn, S. J. Phys. Chem. A 1998, 102, 1976. (103) Gilles, M. K.; Burkholder, J. B.; Gierczak, T.; Marshall, P.; Ravishankara, A. R. J. Phys. Chem. A 2002, 106, 5358. (104) Solomon, S.; Burkholder, J. B.; Ravishankara, A. R.; Garcia, R. R. Geophys. Res. Lett. 1994, 99, 20929. (105) Kwok, E. S. C.; Atkinson, R. Atmos. Environ. 1995, 29, 1685. (106) Orlando, J. J.; Piety, C. A.; Nicovich, J. M.; McKee, M. L.; Wine, P. H. J. Phys. Chem. A 2005, 109, 6659. (107) Cotter, E. S. N.; Booth, N. J.; Canosa-Mas, C. E.; Wayne, R. P. Atmos. Environ. 2001, 35, 2169. (108) Pope, F. D.; Hansen, J. C.; Bayes, K. D.; Friedl, R. R.; Sander, S. P. J. Phys. Chem. A 2007, 111, 4322. (109) Papanastasiou, D. K.; Papadimitriou, V. C.; Fahey, D. W.; Burkholder, J. B. J. Phys. Chem. A 2009, 113, 13711.

(110) Chen, H.-Y.; Lien, C.-Y.; Lin, W.-Y.; Lee, Y. T.; Lin, J. J. Science 2009, 324, 781. (111) Lien, C.-Y.; Lin, W.-Y.; Chen, H.-Y.; Huang, W.-T.; Jin, B.; Chen, I. C.; Lin, J. J. J. Chem. Phys. 2009, 131, 174301. (112) Jin, B.; Chen, I. C.; Huang, W. T.; Lien, C. Y.; Guchhait, N.; Lin, J. J. J. Phys. Chem. A 2010, 114, 4791. (113) Wilmouth, D. M.; Hanisco, T. F.; Stimpfle, R. M.; Anderson, J. G. J. Phys. Chem. A 2009, 113, 14099. (114) (a) Young, I. A. K.; Murray, C.; Blaum, C. M.; Cox, R. A.; Jones, R. L.; Pope, F. D. Phys. Chem. Chem. Phys. 2011, 13, 15318. (b) Young, I. A. K.; Jones, R. L.; Pope, F. D. Geophys. Res. Lett. 2014, 41, 1781. (115) Cox, R. A.; Hayman, G. D. Nature 1988, 332, 796. (116) Gómez Martín, J. C.; Ashworth, S. H.; Mahajan, A. S.; Plane, J. M. C. Geophys. Res. Lett. 2009, 36, L09802. (117) Butz, A.; Bosch, H.; Camy-Peyret, C.; Chipperfield, M. P.; Dorf, M.; Kreycy, S.; Kritten, L.; Prados-Roman, C.; Scwartzle, J.; Pfeilsticker, K. Atmos. Chem. Phys. 2009, 9, 7229. (118) (a) Bobrowski, N.; von Glasow, R.; Aiuppa, A.; Inguaggiato, S.; Louban, I.; Ibrahim, O. W.; Platt, U. J. Geophys. Res. 2007, 112, D06311. (b) Sander, R.; Keen, W. C.; Pszenny, A. A. P.; Arimoto, R.; Ayers, G. P.; Baboukas, E.; Cainey, J. M.; Crutzen, P. J.; Duce, R. A.; Hönninger, G.; Huebert, B. J.; Maenhaut, W.; Mihalopoulos, N.; Turekian, V. C.; Van Dingenen, R. Atmos. Chem. Phys. 2003, 3, 1301. (119) Joseph, D. M.; Ashworth, S. H.; Plane, J. M. C. Phys. Chem. Chem. Phys. 2007, 9, 5599. (120) Moessinger, J. C.; Rowley, D. M.; Cox, R. A. Atmos. Chem. Phys. 2002, 2, 227. (121) Ghosh, B.; Papanastasiou, D. K.; Burkholder, J. B. J. Chem. Phys. 2012, 137, 164315. (122) Molina, L. T.; Molina, M. J. Geophys. Res. Lett. 1996, 23, 563. (123) Wallington, T. J.; Schneider, W. F. Environ. Sci. Technol. 1994, 28, 1198. (124) WMO (World Meteorological Organization). Scientific Assessment for Decision-Makers: Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring ProjectReport No. 56, 88 pp, Geneva, Switzerland, 2014; http://www.wmo. int/pages/prog/arep/gaw/ozone_2014/ozone_asst_report.html. (125) De Bruyn, W. J.; Shorter, J. A.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. Environ. Sci. Technol. 1995, 29, 1179. (126) Kutsuna, S.; Chen, L.; Ohno, K.; Tokuhashi, K.; Sekiya, A. Atmos. Environ. 2004, 38, 725. (127) (a) Behnke, W.; Elend, M.; Krüger, H.-U.; Zetzsch, C. Proceedings of the AFEAS Workshop; Brussels, Belgium, 1992; p 68. (b) Behnke, W.; Elend, M.; Krüger, H.-U.; Zetzsch, C. Proceedings of the STEP-HALOCSIDE/AFEAS Workshop; University College: Dublin, Ireland, 1993. (128) (a) George, C.; Lagrange, J.; Lagrange, P.; Mirabel, P.; Pallares, C.; Ponche, J. L. J. Geophys. Res. 1994, 99, 1255. (b) George, C.; Saison, J. Y.; Ponche, J. L.; Mirabel, P. J. Phys. Chem. 1994, 98, 10857. (c) George, C.; Ponche, J. L.; Mirabel, P. Proceedings of the AFEAS Workshop; Brussels, Belgium, 1992; p 18. (d) George, C.; Ponche, J. L.; Mirabel, P. STEP-HALOCSIDE/AFEAS Workshop; University College: Dublin, Ireland, 1993. (129) Abbatt, J. P. D. Geophys. Res. Lett. 1997, 24, 1479. (130) (a) Huthwelker, T.; Malmstrom, M. E.; Helleis, F.; Moortgat, G. K.; Peter, T. J. Phys. Chem. A 2004, 108, 6302. (b) Cox, R. A.; Fernandez, M. A.; Symington, A.; Ullerstam, M.; Abbatt, J. P. D. Phys. Chem. Chem. Phys. 2005, 7, 3434. (131) (a) Hynes, R. G.; Fernandez, M. A.; Cox, R. A. J. Geophys. Res. 2002, 107, 4797. (b) Sokolov, O.; Abbatt, J. P. D. Geophys. Res. Lett. 2002, 29, 1851. (132) (a) Symington, A.; Cox, R. A.; Fernandez, M. A. Z. Phys. Chem. 2010, 224, 1219. (b) von Hessberg, P.; Pouvesle, N.; Winkler, A. K.; Schuster, G.; Crowley, J. N. Phys. Chem. Chem. Phys. 2008, 10, 2345. (133) Sokolov, O.; Abbatt, J. J. Phys. Chem. A 2002, 106, 775. (134) Symington, A.; Leow, L. M.; Grifiths, P. T.; Cox, R. A. J. Phys. Chem. A 2012, 116, 5990. BC


Chemical Reviews

Review

(135) Moreno, E.; Aranda, A.; Díaz-deMera, Y.; Notario, A.; Rodríguez, D.; Bravo, I. Atmos. Environ. 2012, 60, 76. (136) Pouvesle, N.; Kippenberger, M.; Schuster, G.; Crowley, J. N. Phys. Chem. Chem. Phys. 2010, 12, 15544. (137) Moreno, E.; Aranda, A.; Diaz-de-Mera, Y.; Martinez, E.; Bravo, I.; Rodriguez, D. Phys. Chem. Chem. Phys. 2012, 14, 4425. (138) (a) Solomon, S.; Borrmann, S.; Garcia, R. R.; Portmann, R.; Thomason, L.; Poole, L. R.; Winker, D.; McCormick, M. P. J. Geophys. Res. 1997, 102, 21411. (b) Scheuer, E.; Dibb, J.; Twohy, C.; Rogers, D. D.; Heymsfield, A. J.; Bansemer, A. J. Geophys. Res. 2010, 115, D00J03. (139) Morozov, I.; Gligorovski, S.; Barzaghi, P.; Hoffmann, D.; Lazarou, Y. G.; Vasiliev, E.; Herrmann, H. Int. J. Chem. Kinet. 2008, 40, 174. (140) Lawrence, M. G.; Crutzen, P. J. Tellus, Ser. B 1988, 50B, 263. (141) Sulbaek Andersen, M. P.; Toft, A.; Nielsen, O. J.; Hurley, M. D.; Wallington, T. J.; Chishima, H.; Tonokura, K.; Mabury, S. A.; Martin, J. W.; Ellis, D. A. J. Phys. Chem. A 2006, 110, 9854. (142) Molina, M. J.; Molina, L. T.; Zhang, R.; Meads, R. F.; Spencer, D. D. Geophys. Res. Lett. 1997, 24, 1619. (143) Thornton, J. A.; Kercher, J. P.; Riedel, T. P.; Wagner, N. L.; Cozic, J.; Holloway, J. S.; Dube, W. P.; Wolfe, G. M.; Quinn, P. K.; Middlebrook, A. M.; Alexander, B.; Brown, S. S. Nature 2010, 464, 271. (144) Roberts, J. M.; Osthoff, H. D.; Brown, S. S.; Ravishankara, A. R. Science 2008, 321, 1059. (145) Robinson, G. N.; Worsnop, D. R.; Jayne, J. T.; Kolb, C. E.; Davidovits, P. J. Geophys. Res. 1997, 102, 3583. (146) Talukdar, R. K.; Burkholder, J. B.; Roberts, J. M.; Portmann, R. W.; Ravishankara, A. R. J. Phys. Chem. A 2012, 116, 6003. (147) Lopez-Hilficker, F. D.; Constantin, K.; Kercher, J. P.; Thornton, J. A. Atmos. Chem. Phys. 2012, 12, 5237. (148) Wachsmuth, M.; Gaggeler, H. W.; von Glasow, R.; Ammann, M. Atmos. Chem. Phys. 2002, 2, 121. (149) Eigen, M.; Kustin, K. J. Am. Chem. Soc. 1962, 84, 1355. (150) Hanson, D. R. J. Geophys. Res. 2003, 108, 4239. (151) Roberts, T. J.; Jourdain, L.; Griffiths, P. T.; Pirre, M. Atmos. Chem. Phys. 2014, 14, 11185. (152) (a) Abbatt, J. P. D. J. Geophys. Res. 1995, 100, 14009. (b) Waschewsky, G. C. G.; Abbatt, J. P. D. J. Phys. Chem. A 1999, 103, 5312. (153) Hanson, D. R. J. Geophys. Res. 2003, 108, 4239. (154) Chameides, W. L.; Davis, D. D. J. Geophys. Res. 1980, 85, 7383.

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