Atmospheric Chemical Reactivity and Ozone-Forming Potentials of

Environ. Sci. Technol. 1997, 31, 327-336

Atmospheric Chemical Reactivity and Ozone-Forming Potentials of Potential CFC Replacements GARRY D. HAYMAN* National Environmental Technology Centre, AEA Technology, E5 Culham, Abingdon, Oxfordshire, OX14 3DB U.K. RICHARD G. DERWENT Atmospheric Processes Research Branch, Meteorological Office, Bracknell, Berkshire, RG12 2SZ U.K.

The Montreal Protocol will lead to the eventual phaseout of the production of chlorofluorocarbons (CFCs) and other halogenated organic compounds that are implicated in the depletion of stratospheric ozone. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been developed as potential “in-kind” replacement compounds. To ensure that these compounds are acceptable, industry and regulatory agencies are assessing the safety, toxicology, and environmental impact of the HCFCs and HFCs before widespread production is commenced. One of the environmental issues concerns the contribution that the HCFCs and HFCs make to photochemical ozone production. Photochemical ozone creation potentials (POCPs) have been calculated using a photochemical trajectory model. The POCPs of the HCFCs and HFCs are low, indicating that these compounds do not have a large potential to contribute to ground-level ozone formation. This results from their low reactivity compared to the other, more reactive, organic compounds present during a photochemical episode. HCFCs and HFCs also contribute to stratospheric ozone depletion and climate change. While the relative importance of the compounds within these issues can be defined with some certainty, our understanding of how to weigh the relative importance of the different issues is not sufficiently well developed to assess with any degree of certainty the benefits or the harm that might result from switching to the use of HCFCs and HFCs.

Introduction Under the terms of the Montreal Protocol on Substances that Deplete the Ozone Layer, the production of chlorofluorocarbons (CFCs) such as CFC-11, -12, -113, -114, -115, and other halogenated organic compounds will be phased out. The development of environmentally-acceptable substitutes to replace these ozone-depleting substances, which have found widespread use as refrigerants, solvents, foam-blowing agents, and aerosol propellants, represents an important challenge for industry and the regulatory agencies worldwide. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), because of their similar physicochemical properties to those of the CFCs, have been considered as viable “inkind” replacement compounds. To ensure that the replacement compounds are acceptable, industry and regulatory agencies have commissioned a number of studies to assess * Corresponding author telephone: +44-1235 463108; fax: +441235 463005; e-mail: [email protected].

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the safety, toxicology, and environmental impact of the HCFCs and HFCs before commercial production was commenced. The understanding gained from the studies on the behavior and fate of the chlorinated and fluorinated derivatives of methane, ethane and propane following release into the atmosphere has expanded rapidly in recent years (1-3), and the assessments required can now be made with some confidence. CFCs have no known tropospheric sinks and are transported to the stratosphere where they are readily photolyzed by ultraviolet radiation to release chlorine atoms (4). Replacements to the CFCs were sought that ideally would have a tropospheric loss process, thereby reducing the source strength of active chlorine species in the stratosphere. Although the replacement compounds should contain fewer or no chlorine atoms, the key design feature in the HCFCs and HFCs is the presence of hydrogen atoms, which makes the compounds susceptible to reaction with OH radicals in the troposphere (1). Early developments in this area led to the substitution of CFCs in certain applications by HCFC-22 (CHF2Cl). While the first assessment of the environmental acceptability of the HCFCs and HFCs was being undertaken (1), awareness was growing of the importance of global warming and the role of trace gases other than carbon dioxide (5). The combination of a long atmospheric lifetime and strong infrared absorption bands in the atmospheric window region has led to the classification of the CFCs as important radiatively-active gases (6). The continued presence of carbon-fluorine bonds in many of the HCFCs and HFCs means that these compounds are also important radiativelyactive gases. Global warming potentials, like ozone depletion potentials, can be decreased by selecting potential replacements that have greater reactivity toward hydroxyl radicals and hence shorter atmospheric lifetimes (7). The presence of hydrogen in and hence the shorter atmospheric lifetimes of the HCFCs and HFCs mean that they may contribute to photochemical ozone formation in the boundary layer, in a similar manner to that of other volatile organic compounds (VOCs) (8). In the presence of nitrogen oxides, VOCs are oxidized in the polluted sunlit boundary layer surrounding major urban and industrial centers to generate free radical fragments and degradation products. Elevated ozone concentrations can result during such summertime photochemical episodes. The greater the rate coefficient for the reaction of the potential CFC relacement with hydroxyl radicals, the shorter the atmospheric degradation time scale will be and the greater the apparent ozone production, all other factors being equal. It has been recognized for the last 40 years that each individual hydrocarbon species may make a different quantitative impact on photochemical ozone formation (9). Early smog chamber studies developed the concept of reactivity, and various reactivity scales have been compiled for urban ozone formation (10-12). More recently, computer modeling studies have been used to define the incremental reactivities of a wide range of organic compounds on the urban scale (13). In Europe, long-range transport and multiday photochemistry are perceived to be more important than urbanscale formation, and attention has been given to a wider spectrum of reactivity (14). The aim of this study is to identify which potential CFC replacements, if any, are likely to contribute most to regional-scale boundary layer photochemical ozone formation in Europe. The photochemical ozone creation potential (POCP) (15, 16) has been used as an appropriate index of atmospheric chemical reactivity with which to assess the ability of a range of potential CFC

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replacements to form photochemical ozone. POCPs have been calculated by increasing the emission of each hydrocarbon by an additional amount in a photochemical trajectory model and comparing the amount of additional ozone formed to that formed by the same mass emission of a reference hydrocarbon, taken to be ethylene. The ultimate purpose of the POCP concept is to provide information that is relevant to the evaluation of the environmental acceptability of potential CFC replacements for industry and regulatory agencies. POCPs should help inform this policy process if they are constructed from the most upto-date information and are estimated for the most relevant environmental conditions.

Description of the Photochemical Trajectory Model In previous studies, a photochemical trajectory model has been employed to describe regional-scale ozone, PAN, and hydrogen peroxide formation over Europe (15, 17, 18). The same basic model formulation is adopted, and the chemical development in air parcels is followed as they travel across from continental Europe to the U.K. in a broadly westerly direction. The air parcel extends from the earth’s surface up to the top of boundary layer, and its horizontal dimensions are 10 km × 10 km. The depth of the model boundary layer starts off at 300 m at 0600 h and rises throughout the morning, reaching a height of 1300 m by 1400 h (19). The chemical development of the species in the air parcel is described by a series of differential equations of the form of:

dCi Ei ViCi 1 dh ) Pi + - LiCi - (Ci - Bi) dt h h h dt

(I)

where Ci is the concentration of species i in the air parcel, Pi is the instantaneous production from photochemistry, Ei is the local emission rate from pollution sources, h is the time-dependent boundary layer depth, LiCi is the instantaneous loss rate by photochemistry, Vi is the species-dependent dry deposition velocity, and Bi is the background concentration of the species aloft. The mechanism used has been compared with 25 other chemical mechanisms, available in the literature, under the hydrocarbon-limited conditions used in this study, and close agreement between the peak ozone concentrations has been recorded (20, 21). Included in this large range of mechanisms are several that have either been developed from smog chamber studies or have highly compact and parameterized representations of the chemical processes occurring. Treatment of Emissions. The model utilizes the following four coordinate systems to identify the location of the air parcel at any point in time: (1) latitude and longitude (also used to fix solar zenith angle) (2) UN ECE EMEP 150 km × 150 km coordinates (3) EC CORINAIR 50 km × 50 km coordinates (4) U.K. Ordnance Survey National Grid 10 km × 10 km eastings and northings From the location of the air parcel in each emission grid, the local instantaneous emission rates, Ei, (in molecule cm-2 s-1), were calculated for each pollutant, i, assuming that all emission rates were constant throughout the year. The details of the emission inventories employed and the pollutant species injected into the model are given in ref 22. The speciation of the total VOC emissions into 95 individual hydrocarbons (comprising 24 alkanes, 11 alkenes, 1 alkyne, 17 aromatic compounds, 8 alcohols, 7 aldehydes, 4 ketones, 3 organic acids, 2 ethers, 6 esters, 9 chlorinated hydrocarbons, and 3 other oxygenated compounds) and methane was obtained using species emission profiles for each of the nine major source categories identifed in the U.K. non-methane VOC emission inventory (23): (1) petrol-engined motor vehicle exhaust

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(2) (3) (4) (5) (6) (7) (8) (9)

diesel-engined motor vehicle exhaust petrol evaporation from motor vehicles petrol evaporation from storage distribution, and sale use of solvents chemical and industrial processes stationary combustion natural gas leakage disposal of industrial and residential waste The overall percentage emission by mass of each hydrocarbon was obtained by multiplying the species profiles by the U.K. total VOC emissions from that source category and summing overall the above nine source categories. In the absence of similar detail for other European countries, the same speciation has been adopted throughout the model domain. An additional emission term was added to eq I for isoprene to represent the natural biogenic emissions from European forests and agricultural crops. These emission estimates were prepared on a 150 km × 150 km grid by Simpson (24) using meteorological data from numerical weather prediction models to give temperatures and solar radiation on an hourly basis together with the BEIS approach (25). Background Chemical Mechanism. The chemical mechanism employed in the photochemical trajectory model describes the oxidation of methane, 95 additional hydrocarbons, carbon monoxide, sulfur dioxide, and NOx. It was written specifically to describe the contribution to ozone and PAN formation from a large number of individual hydrocarbon species and their degradation products. The mechanism comprises 515 chemical species and has a condensed version of the many thousands of chemical reactions believed to be occurring in the atmospheric boundary layer (26-28). The chemical mechanism contains a number of identifiable and separate elements: (a) The inorganic chemistry of the simple atoms and radicals derived from O, H, N, S, and CO. These comprise 48 thermal chemical reactions that have been identified in previous modeling studies as important. All rate coefficients have been taken from recent evaluations (29-31), and their full temperature and pressure dependences have been treated. (b) The photolysis reactions of the photochemically-labile species. These comprise 85 time-of-day-dependent photochemical processes involving inorganic species, aldehydes, ketones, and organic hydroperoxides. For the inorganic species and organic species containing C3 or less, quantum yield and cross-section data were taken from the recent evaluations (29-31). Additional information was obtained from several publications containing more recent data, particulary for larger molecules (32-35). Solar actinic fluxes were calculated using a two-stream multiple scattering approach from the solar spectrum at the top of the atmosphere (36). The J values for each process at each zenith angle were obtained by convoluting the solar actinic fluxes with the quantum yield and absorption cross-sections over the wavelength range from 200 to 700 nm:

J(Χ) )

∫φ(λ,X) φ(λ) I(λ) dλ

(II)

For daylight hours (i.e., for solar zenith angles less than 90°), the time-of-day dependence of the photolysis rates was described by calculating the instantaneous solar zenith angle, Χ, and using expressions of the form (eq III) below to estimate the photolysis rate, J, for a particular photochemical process:

J ) l[cos(Χ)]m exp[-n sec(Χ)]

(III)

During the night (i.e., for solar zenith angles greater than 90°), the photolysis frequencies were set to a low value, close to zero, so that these processes were effectively “switched off”.

FIGURE 1. General reaction scheme to describe the atmospheric degradation of the HCFCs and HFCs. X, Y, and Z represent a combination of the substituents, H, Cl, or F. The species surrounded by boxes in the schemes are the stable products formed in the degradation while short-lived radical species are shown enclosed by ellipses. The more important degradation pathways are indicated by the thick arrows. At point 1, this reaction only occurs if one of the three X values in CX3O is a Cl atom. CF3O does not undergo a unimolecular decomposition reaction to give COF2 and F. The coefficients l, m, and n were determined for each process by fitting the J values calculated using the two-stream scattering model of Hough (36) to the functional form given in eq III. The difference between the fitted and the calculated photolysis frequencies was less than 0.5% for solar zenith angles close to 30° but increased to 5% for a solar zenith angle of 70°. The difference was significantly larger for those photolysis processes having a strong dependence on the solar zenith angle. In these cases however, the photolysis frequencies calculated for solar zenith angles close to 90° were many orders of magnitude smaller than those calculated for

smaller zenith angles. With such low photolysis frequencies, these processes will make a negligible contribution to the overall photochemistry. (c) The atmospheric degradation of methane and 95 additional hydrocarbons. These comprise 771 thermal chemical reactions following the photolysis or attack by hydroxyl (OH) or ozone on the emitted organic species or their degradation products. The rates of these initiation reactions were either based on laboratory measurements, as reviewed by Atkinson (37-39), or calculated using structurereactivity methods developed by Atkinson (8, 40).


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Mechanism simplification in this study has been achieved in several ways, for example, by neglecting reaction intermediates that are redundant, since their fates are independent of chemical development. The most significant simplification, however, concerns the chemistry included for the peroxy radical intermediates that, in addition to reacting with NOx, may also react with HO2 and other peroxy radicals (R′O2). In this mechanism, only the reactions of peroxy radicals with NO and CH3O2 were included. This was justified since (i) The unstable peroxynitrates formed by the reaction of peroxy radicals with NO2 have short lifetimes (typically minutes) for the temperatures, close to 288 K, that are observed in the boundary layer. They rapidly decompose to give back the original reactants. However, the reactions of acyl peroxy radicals with NO2 to give peroxy acyl nitrates, which have longer atmospheric lifetimes, were included. (ii) Due to the extremely large number of possible reactions of peroxy radicals with HO2 and other R′O2 species, the inclusion of this chemistry would pose a computational problem in explicit mechanisms (28). These reactions were not thus explicitly considered but were effectively treated through the inclusion of the chain-propagating reaction of the peroxy radicals with CH3O2. CH3O is formed, which, through its reaction with O2, acts as a source of HO2 that can then react with CH3O2 to form CH3OOH. The net effect of the reaction of RO2 radicals with HO2 on the chemistry of the atmosphere depends on the fate of hydroperoxide (i.e., CH3OOH), which is reaction with OH, photolysis, or physical deposition to the ground. These processes can represent interconversion of HOx or net removal of HOx and/or ROx radicals from the atmosphere, thereby reducing the ozone formed. The base case experiment was set up to be in the hydrocarbon-limited regime for each of the 5 days of simulation. The NOx emissions were sufficient to ensure that almost all of the peroxy radicals reacted with NO. In this way, substantial concentrations of ozone built up, and the impact of mechanism simplification was minimized. No attempt was made to characterize the reactivity of hydrocarbons under low NOx conditions where competing reactions of peroxy radicals and acyl peroxy radicals with other peroxy radicals might compete or dominate over reactions with NO. Mechanisms To Describe the Atmospheric Degradation of HCFCs and HFCs. In the original AFEAS assessment (1), processes involved in the degradation of the HCFCs and HFCs in the atmosphere were identified by analogy with those observed for hydrocarbons and simple halogenated organic compounds [as described in Lightfoot et al. (41)]. Figure 1 shows the generic mechanism developed to describe the gasphase degradation of the HCFCs and HFCs in the troposphere following reaction with OH radicals. The characterization of the processes involved was identified as one of the major uncertainties in assessing the environmental impact arising from the use of these compounds. Schemes were formulated from the generic mechanism to describe the processes involved in the boundary layer chemistry of HCFC-22, -123, -124, -141b, -142b, -225ca, and -225cb, HFC-23, -32, -125, -134a, -143a, -152a, and -227ea, and methyl chloroform. As for other hydrocarbons, the limited time period (5 days) considered in the trajectory model allowed a number of strategic simplifications to be made to the oxidation mechanisms. In general, the generic scheme could be reduced to the following three reactions:

OH + RH f RO2

(RH ) HCFC or HFC)

RO2 + NO f RO + NO2

is the fate of the oxy radical, RO. Three processes are in principle feasible (see Figure 1). These are (a) reaction of the oxy radical with O2 to give HO2 and an halogenated carbonyl containing the same number of carbon atoms as the RO radical (b) carbon-carbon bond fission to give a halogenated carbonyl and a radical species, which both contain fewer carbon atoms than the RO radical (c) carbon-chlorine bond fission to give a chlorine atom and a halogenated carbonyl containing the same number of carbon atoms as the RO radical The fate of the oxy radical formed from the HCFCs and HFCs is very dependent on its structure. Only one, or possibly two, of the processes given above occur and these are shown below for the HCFCs and HFCs considered: HCFC/HFC

oxy radical

fate of oxy radical

HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HCFC-225ca HCFC-225cb HFC-32 HFC-125 HFC-134a

CF2ClO CF3CCl2O CF3CFClO CFCl2CH2O CF2ClCH2O CF3CF2CCl2O CF3ClCF2CFClO CHF2O CF3CF2O CF3CHFO

HFC-143a HFC-152a HFC-227ea methyl chloroform

CF3CH2O CH3CF2O (CF3)2CF(O) CCl3CH2O

C-Cl bond fission C-Cl bond fission C-Cl bond fission reaction with O2 reaction with O2 C-Cl bond fission C-Cl bond fission reaction with O2 C-C bond fission C-C bond fission/ reaction with O2 reaction with O2 C-C bond fission C-C bond fission reaction with O2

The oxy radical, CF3O, formed in the oxidation of a number of the HFCs does not undergo any of the processes given above but instead can abstract hydrogen atoms or react with a number of the trace gases found in the lower atmosphere (e.g., O3, NO, NO2, ...). The reactions leading to the production and loss of CF3O are listed in the chemical mechanism given in Table 1. The reactions of CF3O with H2O, CH4, and other hydrocarbons lead to the formation of CF3OH and can cause further ozone production. The oxidation of products formed in the degradation that had lifetimes much longer than the time period considered were neglected. Thus, only the aldehydes, which are formed in the oxidation of compounds with the formula CH3CX3 (X ) Cl, F, or a combination of Cl and F), had short enough lifetimes to allow significant further oxidation to occur. The oxidation of the aldehydes, RCHO, was represented by

OH/NO3 + RCHO f RCO + H2O/HNO3 RCHO + hν (+ 2 O2) f RO2 + CO + HO2 RCO + O2 f RCOO2 RCO f R + CO RCOO2 + NO2 f RCOO2NO2 RCOO2NO2 f RCOO2 + NO2 RCOO2 + NO f RO2 + NO2 + CO2 + O2 RCOO2 + CH3O2 f RO2 + CH3O + CO2 + O2 RO2 + NO f RO + NO2 RO2 + CH3O2 f RO + CH3O

RO2 + CH3O2 f RO + CH3O A key stage in the oxidation process that influences the amount of ozone that can be generated by the HCFC or HFC

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Many of the oxidation products formed in the degradation of the HCFCs and HFCs in the atmosphere are soluble in the aqueous phase and can be removed by washout (e.g.,

TABLE 1. Reactions Involving or Initiated by CF3O Radicals reactions of CF3Oa

rate coefficient, k(T)/(cm3 molecule-1 s-1)

CF3O + O3 CF3O + NO (+ CH4, O2) CF3O + CH4 (+ O2) CF3O + C2H6 (+ O2) CF3O + C3H8 (+ O2)

f f f f f

CF3O + CO (+ O2)

f

CF3O2 [+ O2] NO + CH3O2 [+ COF2] CH3O2 + CF3OH C2H5O2 + CF3OH CF3OH + 0.67secC3H7O2 + 0.33n-C3H7O2 CF3O2 [+ CO2]

CF3O + HO2 CF3O + H2O2 CF3O + H2O CF3O2 + O3

f f f f

CF3OH [+ O2] CF3OH + HO2 CF3OH + OH CF3O [+ 2O2]

2.0 × 10-12 exp(-1300/T) 3.7 × 10-11 exp(110/T) 2.5 × 10-12 exp(-1420/T) 4.7 × 10-12 exp(-400/T) 4.7 × 10-12

k0 ) 2.5 × 10-31 (T/300)-3.8 [M]; k∞ ) 6.8 × 10-14 (T/300)-1.2; Fc ) 0.6 3.0 × 10-11 7.0 × 10-12 2.0 × 10-17 3.0 × 10-15

ref 31 31b 31 31 47 31 48 48 31 31

a The reactions given are composite reactions involving a number of elementary reactions. The species given in brackets (round and square) have been included for clarity to represent those compounds whose concentration were either constant during the simulation (e.g., O2) or played no further part in the chemistry (e.g., COF2). b The reaction gives COF2 and FNO. FNO is photolyzed to give F + NO with F then reacting with CH4.

hydroperoxides such as CF3OOH or peroxynitrates such as CF3O2NO2). Other products (e.g., carbonyl halides such as CF3COCl, CF3COF, HCOF, COF2, COFCl, COCl2, and the alcohol CF3OH) are irreversibly lost by rapid hydrolysis on uptake into the aqueous phase to form organic acids or CO2 together with HF or HCl:

CF3COCl(g) f CF3COCl(aq) f CF3COOH(aq) + HCl(aq) The photochemical trajectory model also does not have an explicit formulation of liquid-phase chemistry to treat the uptake and possible transformations of soluble intermediate products. However, the neglect of these processes does not have a significant effect since the following: (a) The water vapor content is likely to be low under the meteorological conditions leading to photochemical pollution episodes, thus reducing the significance of any liquid-phase process. (b) The low reactivity of the parent HCFC or HFC means that the products formed will not build up appreciably, and thus these compounds do not represent a significant new source of or loss process for active radical species. All the HCFCs considered result in the generation of Cl atoms. Reactions of Cl and ClO (formed by the reaction of Cl with O3) have been included, and detailed schemes were developed for the Cl atom-initiated oxidation of CO and C2H4. Reactions of Cl atoms were included for those hydrocarbons likely to represent the most significant loss processes for Cl atoms either because of the high reactivity of the hydrocarbon or because it has an elevated concentration for the conditions considered. The yields of the different peroxy radicals formed when Cl-atoms react with propane, n-butane, and isobutane were calculated using the structure-activity relationships proposed by Aschmann and Atkinson (42). The reactions leading to the production and loss of Cl atoms are listed in the chemical mechanism given in Table 2. The chemical schemes developed for HCFC-123 and HFC134a are given in Table 3. Similar schemes were developed for the other HCFCs and HFCs treated, but these are not listed in the interest of brevity. The schemes were incorporated for use in the photochemical trajectory model. The data needed to characterize the chemical and photochemical processes involved (i.e., rate coefficients, branching ratios, UV absorption cross-sections, and quantum yields) were taken from experimental measurements where available. The OH rate coefficients used have been updated in accordance with the recommendations given in the latest NASA Evaluation (29). For those processes that have not been characterized experimentally, the parameters needed were based on those of related compounds (CF3O2, CF2ClO2, and CFCl2O2; C2H5O2 and CH3COO2). A total of 61 new species and a further 134

thermal and photochemical reactions were added to the original chemical mechanism. Emission Rates of the HCFCs and HFCs. The emission rates of a number of chlorinated hydrocarbons already considered in the model, the HCFCs and HFCs, were initially set to 1 kt/yr emitted into each 50 × 50 km2 grid cell. However, calculations using these emission rates gave negligible increases in the ozone concentration over the base run. In order to give observable changes in the ozone concentration and hence to improve the numerical precision of the POCP calculations, the emission rates were increased by a factor of 10 to 10 kt/yr into each 50 × 50 km2 grid cell. The emission rate of ethylene for the POCP calculations however was kept at 1 kt/yr into each 50 × 50 km2 grid cell. Numerical Methods. The resulting system of 576 simultaneous stiff differential equations was integrated with a variable order Gear’s method (FACSIMILE (43)) on a desktop microcomputer. The initial concentrations for most species were set to zero. For a small number of species, initial concentrations were set at realistic tropospheric baseline levels for a polluted boundary layer situation. These were as follows: NO 2 ppb, NO2 6 ppb, SO2 5 ppb, CO 120 ppb, methane 1700 ppb, formaldehyde 2 ppb, ozone 50 ppb, and hydrogen 550 ppb. Aloft concentrations for each species, Bi, were set at the levels reached in the model during the previous early evening and held constant during the intervening night. Aloft concentrations for the first day were taken to be the initial model concentrations.

Results and Discussion Base Case Model Experiment. In the base case model experiment, the highly idealized anticyclonic meteorological situation of easterly winds, leading to a broad westerly air flow carrying photochemically-aged polluted air masses out of Europe toward the British Isles, was adopted. Wind speeds and directions of 4 m s-1 and 100° (from grid north) were taken across the entire model domain. Air parcels arriving in the southwest of the British Isles on the afternoon of the fifth day of photochemistry had started off on the first day in Austria. By the mid-afternoon of the second day, they had passed over southeast Germany, northwest Germany on the third day, Belgium and northern France on the fourth, and over southeast England on the fifth day. In contrast, air parcels arriving in central Scotland on the afternoon of the fifth day had started off in Poland and traveled over the Baltic and North Seas and over Denmark. This highly idealized meteorological situation is a broad generalization of the Germany-Ireland emission scenario case that has been employed in previous studies (15, 18). This base case model experiment does not represent the conditions prevailing during a particular photochemical


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TABLE 2. Reactions Involving or Initiated by Cl or ClO reactionsa


Reactions Involving Cl Atoms ClO [+ O2] 2.9 × 10-11 exp(-260/T) HCl + CH3O2 1.1 × 10-11 exp(-1400/T) HCl + C2H5O2 7.7 × 10-11 exp(-90/T) HCl + 0.386n-C3H7O2 + 0.614sec-C3H7O2 1.4 × 10-10 exp(30/T) HCl + 0.285n-C4H9O2 + 0.715sec-C4H9O2 2.14 × 10-10 exp(12/T) HCl + 0.564iso-C4H9O2 + 0.436tert-C4H9O2 1.41 × 10-10 HCl + HO2 + CO 8.1 × 10-11 exp(-30/T) HCl + HO2 1.1 × 10-11 exp(-980/T) HCl + H 3.7 × 10-11 exp(-2300/T) ClCHO + CO + HO2 k0 ) 5.4 × 10-30 (T/300)-2.1 [M]; k∞ ) 2.1 × 10-10 (T/300)-1.0; Fc ) 0.6

ref

Cl + O3 Cl + CH4 (+ O2) Cl + C2H6 (+ O2) Cl + C3H8 (+ O2) Cl + n-C4H10 (+ O2) Cl + iso-C4H10 (+ O2) Cl + HCHO (+ O2) Cl + H2O2 Cl + H2 Cl + C2H2 (+ 2O2)

f f f f f f f f f f

Cl + C2H4 (+ O2)

f ClCH2CH2O2

31

ClCH2CH2O2 + NO OH + CH2ClCHO (+ O2) NO3 + CH2ClCHO (+ O2) CH2ClCOO2 + NO (+ O2) CH2ClCOO2 + NO2

f f f f f

c 29 d e e

CH2ClCOO2NO2

f

CH2ClCH2O2 + CH3O2 (+ O2) f CH2ClCOO2 + CH3O2 (+ O2) f CH2ClCHO + hν (+ 2O2) f

Cl Atom-Initiated Oxidation of C2H4 k0 ) 1.6 × 10-29 (T/300)-2.1 [M]; k∞ ) 3.1 × 10-10 (T/300)-1.0; Fc ) 0.6 CH2ClCHO + NO2 + HO2 4.2 × 10-11 exp(180/T) CH2ClCOO2 [+ H2O] 3.0 × 10-12 CH2ClCOO2 + HNO3 1.4 × 10-12 exp (-1860/T) CH2ClO2 + NO2 [+ CO2 + O2] 2.0 × 10-11 CH2ClCOO2NO2 k0 ) 2.7 × 10-28 (T/300)-7.1 [M]; k∞ ) 1.2 × 10-11 (T/300)-0.9; Fc ) 0.3 CH2ClCOO2 + NO2 k0 ) 4.9 × 10-3 exp(-12100/T) [M]; k∞ ) 4.0 × 1016 exp(-13604/T); Fc ) 0.3 CH2ClCHO + HO2 + CH3O [+ O2] 2.5 × 10-14 CH2ClO2 + CH3O [+ CO2 + O2] 3.0 × 10-12 CH2ClO2 + HO2 + CO photolysis rate assumed to be that of CH3CHO

ClCO + O2 ClCOO2 + NO ClCOO2 + CH3O2

Cl Atom Initiated Oxidation of CO 1.3 × 10-33 (T/300)-3.8 [M] use rate coefficient for forward reaction and equilibrium constant, K, where K ) 1.6 × 10-25 exp(4000/T) f ClCOO2 5.0 × 10-12, estimated f Cl + NO2 [+ CO2] 2.0 × 10-11 f Cl + CH3O [+ CO2 + O2] 3.0 × 10-12

ClO + ClO

f Cl2O2

Cl2O2

f ClO + ClO

ClO + ClO

f Cl + Cl [+ O2]

ClO + ClO

f Cl + Cl [+ O2]

ClO + ClO

f Cl + ClO + O

ClO + HO2 ClO + OH ClO + OH ClO + O ClO + NO ClO + NO2

f f f f f f

HOCl [+ O2] Cl + HO2 HCl [+ O2] Cl [+ O2] Cl + NO2 ClONO2

ClONO2 ClONO2 + OH HOCl + OH HCl + OH Cl2O2 + hν

f f f f f

ClO + NO2 HOCl + NO3 ClO [+ H2O] Cl [+ H2O] Cl + Cl [+ O2]

HOCl + hν

f OH + Cl

ClONO2 + hν

f 0.67Cl + 0.67NO3 + 0.33ClO + 0.33NO2

Cl + CO ClCO

f ClCO f Cl + CO

Reactions Involving ClO

k0 ) 2.2 × 10-32 (T/300)-3.1 [M]; k∞ ) 3.5 × 10-12 (T/300)-1.0; Fc ) 0.6 use rate coefficient for forward reaction and equilibrium constant, K, where K ) 1.3 × 10-27 exp(8744/T) 1.0 × 10-12 exp(-1590/T), channel giving Cl2 + O2 with Cl2 photolyzed 3.0 × 10-11 exp(-2450/T), channel giving Cl + ClOO with ClOO decomposition 3.5 × 10-13 exp(-1370/T), channel giving Cl + OClO with OClO photolyzed 4.8 × 10-13 exp(700/T) 0.85 × 1.1 × 10-11 exp(120/T) 0.15 × 1.1 × 10-11 exp(120/T) 3.0 × 10-11 exp(70/T) 6.4 × 10-12 exp(290/T) k0 ) 1.8 × 10-31 (T/300)-3.4 [M]; k∞ ) 1.5 × 10-11 (T/300)-1.9; Fc ) 0.6 6.9 × 10-7 exp(-10909/T) [M] (in s-1) 1.2 × 10-12 exp(-330/T) 3.0 × 10-12 exp(-500/T) 2.6 × 10-12 exp(-350/T) calculated as described in text; l ) 1.893 × 10-3, m ) 0.6984, n ) 0.1839 calculated as described in text; l ) 3.476 × 10-4, m ) 0.6253, n ) 0.2230 calculated as described in text; l ) 6.748 × 10-5, m ) 0.7050, n )0.1636

31 31 31 29b 29b 29b 31 31 31 31

e f g

31 31

e g 31 31 31 31 31 31 31 31 31 31 31 49 31 31 31 h

h h

a The reactions given are composite reactions involving a number of elementary reactions. The species given in brackets (round and square) have been included for clarity to represent those compounds whose concentration were either constant during the simulation (e.g., O2) or played no further part in the chemistry (e.g., COF2). (b) Yields of peroxy radicals calculated using structure-activity rules developed for reactions of Cl with n-alkanes by Aschmann and Atkinson (42). c Use the rate coefficient derived for the rate coefficient for CH3O2 + NO (29). d Use the rate coefficient derived for the reaction NO3 + CH3CHO (29). e Use the rate coefficient expression derived for the analogous reaction of CH3COO2 (29). f Based on the estimate derived for the rate coefficient of the reaction of CH3O2 with a secondary alkyl peroxy radical (28). g Based on the estimate derived for CH3COO2 + CH3O2 by Madronich and Calvert (28). h Uses UV absorption cross sections taken from NASA Evaluation 11 (31) and the solar irradiances given in WMO Report 16 (50).

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TABLE 3. Schemes Used To Describe Atmospheric Degradation of HCFCs and HFCs in Photochemical Trajectory Model reactions used to describe the degradation mechanism of HCFC-123a CF3CHCl2 + OH (+ O2) CF3CCl2O2 + NO CF3CCl2O2 + CH3O2

f f f

CF3CCl2O2 [+ H2O] Cl + NO2 [+ CF3COCl] Cl + CH3O [+ CF3COCl + O2]

reactions used to describe the degradation mechanism of HFC-134aa

OH + CF3CH2F (+ O2) CF3CHFO2 + NO CF3CHFO (+ O2) CF3CHFO + O2 CF3CHFO2 + CH3O2 CF3O2 + NO CF3O2 + CH3O2

f f f f f f f

CF3CHFO2 [+ H2O] CF3CHFO + NO2 CF3O2 [+ HCOF] HO2 [+ CF3COF] CF3CHFO + CH3O [+ O2] CF3O + NO2 CF3O + CH3O [+ O2]


ref

7.0 × 10-13 exp(-900/T) 1.5 × 10-11 (T/300)-1.3 2.5 × 10-14

31 b c


ref

1.5 × exp(-175)/T) 1.28 × 10-11 (T/300)-1.2 7.4 × 1011 exp(-472)/T) 6.0 × 10-14 exp(-925/T) 2.5 × 10-14 1.6 × 10-11 (T/300)-1.2 2.5 × 10-14

31 31d 51 e c 29 c

10-12

a The reactions given are composite reactions involving a number of elementary reactions. The species given in brackets (round and square) have been included for clarity to represent those compounds whose concentration were either constant during the simulation (e.g., O2) or played no further part in the chemistry (e.g., COF2, CF3COCl). b Use the rate coefficient expression derived for CFCl2O2 + NO (29). c Based on the estimate derived for the rate coefficient of the reaction of CH3O2 with a secondary alkyl peroxy radical (28). d Use the rate coefficient derived at room temperature (52) with the temperature dependence derived for the rate coefficient of the reaction CF3O2 + NO (29). e Assumed to be the same as that for the reaction C2H5O + O2 (29).

FIGURE 2. Evolution of the ozone concentration over the 5-day trajectory. The broken line (- - -) represents the base case run; the full line (s) represents the run with HFC-152a added, and the dotted line (‚‚‚) represents the run with C2H4 added. Note that the runs undertaken with HFC-152a or C2H4 added are almost superimposed. episode. Such an exercise would require the detailed preparation of emissions and meteorological fields, which is far beyond our present capabilities. Some of the difficulties experienced in formulating episode-specific data for this photochemical trajectory model have been described elsewhere (44). The emission scenarios have been carefully chosen to reflect some form of worst case situation. The trajectory path was selected from the review of meteorological analyses prepared for the PHOXA study covering the period 1980-1985 (45, 46). Such trajectory paths are frequently associated with elevated ozone concentrations over The Netherlands and the United Kingdom. The scenario is not the worst ever case but is meant to be relevant for the study of regional-scale ozone formation and its control.

One particular trajectory was selected for further study to determine the contribution to the photochemical ozone production from the oxidation of each hydrocarbon. The trajectory chosen was the one that gave the maximum ozone concentration of 118 ppb, as shown in Figure 2. This particular trajectory started on the first day in the vicinity of Vienna, Austria, and in the middle of the following days, passed over the areas of Plzen, Nurnberg, Bonn, Brussels, and the Home Counties of England, Oxford, and ultimately arriving in South Wales. The time-integrated NOx and hydrocarbon emissions amounted to 103 and 115 kg km-2 respectively, in this base case calculation. Photochemical Ozone Creation Potentials. The POCPs for a particular HCFC or HFC were calculated from the results


9

333

TABLE 4. Photochemical Ozone Creation Potentials for Selected Hydrochlorofluorocarbons and Hydrofluorocarbonsa species ethylene HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HCFC-225ca HCFC-225cb HFC-23 HFC-32 HFC-125 HFC-134a HFC-143a HFC-152a HFC-227ea methyl chloroform methyl chloride methylene chloride tetrachloroethylene trichloroethylene vinyl chloride 1,1-dichloroethylene

a

integrated POCP C2H4 CHF2Cl CF3CHCl2 CF3CHFCl CH3CFCl2 CH3CF2Cl CF3CF2CHCl2 CF2ClCF2CHFCl CHF3 CH2F2 CF3CHF2 CF3CH2F CH3CF3 CH3CHF2 CF3CHFCF3 CH3CCl3 CH3Cl CH2Cl2 C2Cl4 C2HCl3 CH2CHCl CCl2CH2

100.0 0.1 0.3 0.1 0.1 0.1 0.2 0.1 0.0 0.2 0.0 0.1 0.0 1.0 0.0 0.2 1.1 1.6 1.0 9.0 25.6 17.3

b

a The POCPs have systematic errors of up to (0.1 due to numerical precision. Values of POCPs less than 0.1 are less reliable because of footnote a.

of a separate model experiment, each a variant on the base case scenario. In each separate model experiment, an emission term of 10 kt per 50 × 50 km2/yr was employed for each HCFC or HFC across the entire model domain; the emissions of the other HCFCs or HFCs being set to zero. The HCFC or HFC emission stimulated additional ozone formation over the base case, and this incremental ozone amount could be defined for a particular point along the trajectory or integrated over the entire trajectory length. These ozone increments were compared with the corresponding increments for a reference hydrocarbon, taken to be ethylene (ethene). The POCP for a particular hydrocarbon, i, was accordingly defined in eq IV as

POCPi ) ozone increment with the ith hydrocarbon × 100 (IV) ozone increment with ethylene for the same incremental integrated emission. Ethylene is assigned a POCP of 100. The values obtained for the POCP index are given in Table 4 and are discussed in some detail below.

Discussion and Implications Value of the POCP Concept. Derwent et al. (22) have analyzed several approaches by which the production of ozone resulting from the oxidation of hydrocarbons and other organic compounds and the relative contributions made by these compounds can be considered. Whereas some compounds are oxidized rapidly and lead to significant ozone formation comparatively close to the point of emission, other more slowly reacting compounds may ultimately be responsible for a greater quantity of ozone production. It is therefore a complex and challenging problem to define an index that is capable of realistically indicating the relative impact of a wide variety of compounds on a regional scale. The “total ozone formation” approach considers the total quantity of ozone formed as a byproduct from the complete oxidation to form CO2 and H2O, whereas the “OH reactivity” scale uses the reactivity of the compound with OH as an indication of how quickly it can form ozone in the boundary

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FIGURE 3. (a) Plot of the photochemical ozone creation potential against the logarithm of the ratio of the OH rate coefficient at 298 K to the relative molecular mass of the HCFC or HFC. (b). Log-log plot of the photochemical ozone creation potential against the ratio of the OH rate coefficient at 298 K to the relative molecular mass of the HCFC or HFC. layer. It is not surprising to find that the two approaches gave different rankings as they only consider part of the overall oxidation processes. The total ozone formation approach takes account of how ozone formation is influenced by the structure and precise oxidation mechanism of the VOC, but does not consider where or how quickly ozone is formed. On the other hand, the OH reactivity approach identifies the time scales over which initial ozone formation occurs, but does not consider the impact of intermediate oxidation products and the subsequent degradation chemistry. The POCP approach, used in this study, contains elements of both approaches by incorporating the detailed chemical degradation schemes and the rates of key reactions in a photochemical trajectory model. Ozone formation is calculated over a realistic time scale under simulated conditions that are reasonably representative of the planetary boundary layer. Derwent et al. (22) concluded that the POCP values are influenced by the trend of reactivity of the compounds but clearly also reflect the total ozone formation potential. The POCP index thus provides a realistic measure of the relative ozone-producing abilities of organic compounds on a regional scale. POCPs of the HCFCs and HFCs. The POCPs calculated for the HCFCs and HFCs, as expected from the reactivity of these compounds with the OH radical, are very low and in some cases close to 0, as shown in Table 4. The numerical precision of the calculations and the method used to calculate the POCPs suggest that the POCPs are susceptible to a

TABLE 5. Comparison of Parameters for HCFCs and HFCs rate coefficienta (cm3 molecule-1 s-1)

GWPe

species

k(T)

k(298 K)

lifetime/yrb

HCFC-22 (CHF2Cl) HFC-23 (CHF3) HFC-32 (CH2F2) HCFC-123 (CF3CHCl2) HCFC-124 (CF3CHFCl) HCFC-141b (CH3CFCl2) HCFC-142b (CH3CF2Cl) HFC-125 (CF3CHF2) HFC-134a (CF3CH2F) HFC-143a (CH3CF3) HFC-152a (CH3CHF2) HCFC-225ca (CF3CF2CHCl2) HCFC-225cb (CF2ClCF2CHFCl) HFC-227ea (CF3CHFCF3)

1.0 × 10-12 exp(-1600/T) 1.0 × 10-12 (-2440/T) 1.9 × 10-12 (-1550/T) 7.0 × 10-13 (-900/T) 8.0 × 10-13 (-1350/T) 1.7 × 10-12 (-1700/T) 1.3 × 10-12 (-1800/T) 5.6 × 10-13 (-1700/T) 1.5 × 10-12 (-1750/T) 1.6 × 10-12 (-2100/T) 2.4 × 10-12 (-1260/T) 1.0 × 10-12 (-1100/T) 5.5 × 10-13 (-1250/T) 5.0 × 10-13 (-1700/T)

4.7 × 10-15 2.8 × 10-16 1.0 × 10-14 3.4 × 10-14 8.6 × 10-15 5.7 × 10-15 3.1 × 10-15 1.9 × 10-15 4.2 × 10-15 1.4 × 10-15 3.5 × 10-14 2.5 × 10-14 8.3 × 10-15 1.7 × 10-15

14.2 295.3 6.3 1.6 7.2 12.0 22.5 36.5 16.3 54.1 1.7 2.3 7.3 40.8

a Taken from NASA Evaluation 11 (31). with OH. Lifetimes calculated using

b

POCPc 0.1 0.0 0.2 0.3 0.1 0.1 0.1 0.0 0.1 0.0 1.0 0.2 0.1 0.0

ODPd 0.04

Atmospheric Chemical Reactivity and Ozone-Forming Potentials of

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