Adsorption and Reaction of Trace Gas-Phase Organic Compounds on

Jan 8, 2010 - aerosols and hydrometeors (fog, mist, ice, rain, and snow) presents an important surface for the adsorption and reaction of many organic...
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Environ. Sci. Technol. 44, 865–873

Adsorption and Reaction of Trace Gas-Phase Organic Compounds on Atmospheric Water Film Surfaces: A Critical Review D . J . D O N A L D S O N † A N D K A L L I A T T . V A L S A R A J * ,‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada, and Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803

Received September 8, 2009. Revised manuscript received November 30, 2009. Accepted December 10, 2009.

The air-water interface in atmospheric water films of aerosols and hydrometeors (fog, mist, ice, rain, and snow) presents an important surface for the adsorption and reaction of many organic trace gases and gaseous reactive oxidants (hydroxyl radical (OH · ), ozone (O3), singlet oxygen (O2(1∆g)), nitrate radicals (NO3 · ), and peroxy radicals (RO2 · ). Knowledge of the air-water interface partition constant of hydrophobic organic species is necessary for elucidating the significance of the interface in atmospheric fate and transport. Various methods of assessing both experimental and theoretical values of the thermodynamic partition constant and adsorption isotherm are described in this review. Further, the reactivity of trace gases with gas-phase oxidants (ozone and singlet oxygen) at the interface is summarized. Oxidation products are likely to be more water-soluble and precursors for secondary organic aerosols in hydrometeors. Estimation of characteristic times shows that heterogeneous photooxidation in water films can compete effectively with homogeneous gas-phase reactions for molecules in the atmosphere. This provides further support to the existing thesis that reactions of organic compounds at the air-water interface should be considered in gas-phase tropospheric chemistry.

Introduction Atmospheric aerosol particles, such as fog, mist, dew, and cloud droplets, as well as organic and mineral dust particles, play an important role in climate change and chemistry, in both the troposphere and the stratosphere. They contribute to the fate and transport of trace gases in the atmosphere via their participation in heterogeneous chemistry (generally oxidative) involving these compounds. In spite of this relevance, much remains to be understood about the reactive properties of aerosol particles in the atmosphere. In particular, the differences between reactions which take place on the surface, vs within the liquid droplet, are the object of considerable current research. In the lower troposphere, water vapor is generally the trace gas at the highest concentration and so becomes an important (and often the most abundant) constituent in aerosols there. Based on spectroscopic and microscopic * Corresponding author phone: 225 578 1426; fax: 225 578 1476; e-mail: [email protected]. † University of Toronto. ‡ Louisiana State University. 10.1021/es902720s

 2010 American Chemical Society

Published on Web 01/08/2010

studies a conceptual description of an atmospheric aqueous aerosol (e.g., a sea-salt aerosol, or a mist or fog droplet) is that of a cloud condensation nucleus (CCN) core which is coated with or dissolved within patches of water and organic films (1-3). Some atmospheric aerosols are primarily organic, or are formed about an organic nucleus, but are nevertheless capable of taking up water. The water and organic films which define the surfaces of atmospheric particles largely determine their overall atmospherically relevant chemical and physical properties. CCN in aerosols typically consist of inorganic salts such as NaCl, (NH4)2SO4, NH4NO3, etc., which can hygroscopically grow into cloud and fog droplets. The organic compounds that form a typical aerosol may be water-soluble or insoluble, volatile or nonvolatile. The water-soluble organic carbon (WSOC) in aerosols is mainly comprised of dicarboxylic acids (C2-C6), dicarbonyls (e.g., glyoxal), ketoacids (C2-C5), several multifunctional compounds, polyols (C2-C7), hydroxyamines, amino acids (C2-C6), and nitrophenols. Such compounds play an important role in determining the hygroscopic properties of aerosol particles (4, 5). Waterinsoluble organic compounds can coat the surface of particles and aerosol surfaces forming surface films. Some WSOCs within a particle are surface active and so change the surface tension of the aerosol. For example, fogwater samples collected in different parts of the world show substantially reduced surface tension compared to pure water, indicative of organic partitioning to the interface (Figure 1). Typically, about 40-50% of the WSOCs in fogwater are not characterized fully and are presumed to be high molecular weight humiclike substance, HULIS (6). Water near mineral surfaces (such as those of some atmospheric aerosols) shows structural features that are different from those of bulk water. Experimental evidence shows that at a surface coverage of less than a monolayer, the water molecules participate in interactions both with the substrate and with coadsorbed water molecules. However, as the coverage increases to several monolayers the hydrogenbonded network of adsorbed molecules starts to resemble that of liquid water (7, 8). Molecular dynamics simulations on mineral oxide surfaces show that at distances of a few molecular diameters away from the surface the underlying substrate has no effect on any of the water properties (9). Thus, beyond a few monolayers of water, thin films of water have structural features similar to that of bulk water. Similar effects are seen in the “quasi-liquid layer” at the air-ice interface (10). The key difference, however, is that these water films provide very high surface areas compared to their bulk VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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processes that occur at the water film and organic surface film surfaces of aerosols.

Thermodynamics of Adsorption at the Air-Water Interface

FIGURE 1. Surface tension decrease for fog and cloudwater samples obtained from different parts of the world. The organic carbon in the atmospheric water samples was measured as dissolved organic carbon. volumes and hence surface chemistry may dominate the overall reactivity. Figure 2 shows the three regimes of water content. In Region 1, where water is present at monolayer coverage, the coadsorption of water often determines the adsorption and reactivity of trace gases on aerosols. In Region 3, where the water film thickness is sufficiently large, the bulk dissolution and diffusion of reactants determines the reactivity, although surface reactions can be also important. Region 2 is the most significant because it is in this region that the surface area is much larger compared to bulk volume, and surface chemistry often dominates the reactivity. The potential importance of surface processes is indicated by the “reactodiffusive length” parameter: δ ) (k/D)1/2, where k is the firstorder loss rate of the gas phase reactant and D represents its diffusion coefficient in aqueous solution. If δ is on the order of a few molecular diameters, surface reaction competes effectively with diffusion into the bulk; clearly, if the water layer is only this thick then surface reaction must dominate. A schematic based on the current understanding of an aqueous aerosol interacting with trace gases is shown in Figure 3. Reactions in the atmosphere are generally driven by photochemically produced gas-phase oxidants such as hydroxyl radical (OH · ), ozone (O3), singlet oxygen (O2(1∆g)), nitrate radicals (NO3 · ), and peroxy radicals (RO2 · ). These reactive oxidant species (ROS) react with atmospheric trace gases, either homogeneously in the gas phase, or on aerosols, to produce a variety of species including long-lived compounds, high molecular weight semivolatile secondary organic aerosols (SOA), and other nonvolatile oxidized species. Shown in Figure 3 are the different types of reaction sites that can be identified on aerosol droplets. Heterogeneous reaction first requires mass transfer of the trace gases (A) toward the aerosol and their subsequent adsorption at sites on the particle surface. Subsequent to the adsorption, the first type of reactive process involves the dissolution of A into the bulk region of a water film present in the surface region, followed by its oxidation by other dissolved species. A second pathway involves the adsorption of reactive oxidizing species (ROS) onto water film surfaces that contain dissolved or coadsorbed surface-active organics (A), such as humic-like substances (HULIS), followed by reaction between the coadsorbed compounds. Thermodynamics, kinetics, and transport are integral in elucidating the fate of organic and inorganic compounds that are associated with atmospheric aerosols. This review provides a brief summary of the thermodynamics of adsorption of trace gases and the various atmospheric oxidation 866

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Conceptual Approaches. A number of organic and inorganic compounds exist in the atmosphere as trace gases. Examples of the former include alkanes, alcohols, amines, ketones, aromatic compounds, esters, and acids; as well, oxides of nitrogen and sulfur are important atmospheric players. Some of these species are hydrophobic, some are quite soluble and still others are only somewhat soluble. Many of these species also display surfactant properties; that is, they show greater affinity toward the air-water interface than the bulk. Molecular dynamics calculations of the potential of mean force (free energy) for moving various molecules across the air-water interface are shown in Figure 4a-d. Several different cases are shown. Panel (a) shows the chemical potential profile for OH radical entering the water layer. In panel b the free energy for transferring nonpolar gas molecules (Ar, CO2, N2) is shown. In panels c and d, the free energy for transferring aromatic hydrocarbons (benzene and phenanthrene) are shown. For species such as Ar, CO2, and N2 there is a considerable increase in the free energy as they enter the liquid phase. For OH radical and aromatic hydrocarbons, there is a decrease in the overall free energy. Ab-initio calculations provide further evidence for the unique bonding characteristics at the air-water interface (11). The common feature of all these profiles is that there is a substantial minimum in the free energy for the surface adsorbed state. The idea that soluble gases could adsorb onto a water-air interface was first proposed in 1928 by Rice (12), who measured the surface tension vs concentration for solutions of ammonia. This idea received little attention until fairly recently. The propensity for an organic (or any other) compound to adsorb at the air-aqueous interface is indicated by a negative free energy of transfer of the compound from a bulk phase (either atmosphere or solution) to the surface. Thermochemical measurements relating to this transfer are most often determined at equilibrium, yielding a standard free energy of adsorption, ∆G0. Usually, this quantity has been determined by measurements of equilibrium constant for the partitioning of the compound of interest to the interface, measured chromatographically in the case of adsorption from the gas phase, or via an adsorption isotherm yielding the surface concentration as a function of the dissolved solute concentration. Many atmospheric gases that adsorb at the air-water interface are also at least somewhat soluble in aqueous solution. Donaldson (13) treated the general case of the threephase (gas, solution, and interface) adsorption thermodynamics of (semi)volatile, (semi)soluble species onto the water surface. The free energy for transferring one mole of species “i” from bulk phase X (either gas or solution) to the surface is given by: ∆XfσG ) µiσ - µiX ) (µiσ,0 - µiX,0) + RTln

[

( )/

γσ

π

π0

( )] aX

aX0

(1)

where the µi represent the chemical potentials, a0 gives the standard activity (1 mol kg-1 in solution; 1 atm in the gas phase), the solution and gas phase activities are ai ) γi mi, and ai ) γi pi, respectively. The γi are concentrationdependent activity coefficients and π represents the film pressure, given by the difference between the surface tension (surface free energy per unit area) of the air-aqueous interface prior to adsorption and that following adsorption.

FIGURE 2. Atmospheric water regimes on aerosols. Region 1 involves molecular adsorption of water on surface and competitive adsorption of trace gases. Molecular clusters can be formed between water and trace gases. Region 2 is one in which the thin film of water exists on surface and processes are dominated by adsorption/reaction on the water surface. Region 3 is where the bulk volume processes dominate absorption and reactions. Note that the scale varies from nm to mm in film thickness as one goes from Region 1 to 3.

FIGURE 3. A conceptual schematic of the typical aqueous aerosol droplet interacting with trace gases (≡A) and gas-phase reactive oxidant species (≡ROS). There are two pathways indicated: (1) uptake and reaction on bare water film surface, and (2) uptake and reaction on the surface active HULIS-covered surface of the water film. Shown also are the other operative transport mechanisms such as diffusion of both species in the gas phase and the aqueous phase and reactions in the bulk aqueous phase as well as desorption of materials from the surface. The standard state of the adsorbed species is conveniently taken to be that proposed by Kemball and Rideal (14): a film with the same number density as would be present in an ideal gas at 1 atm in a container of thickness 6 Å. In terms of film pressure this choice of standard state gives π0 ) 0.06084 dyn cm-1. This choice of standard states is arbitrary,

but affects the numerical values of the resulting thermochemical parameters. At phase equilibrium ∆XfσG ) 0 and so ∆XfσG0 ) -RT ln[(γσ((π)/(π0)))/(((aX)/(aX0 )))]eq. By plotting the quantity -RT ln [(γσπ/π0)/(aX/aX0)] against the activity of the bulk phase and extrapolating to zero bulk phase concentration, “ideal gas” surface adsorption standard free energies may be obtained. Several organic surface-active solutes have been treated this way at several temperatures, yielding values of ∆XfσG0, as well as ∆XfσH0, and ∆XfσS0 (from the T-dependences) for adsorption to the air-water interface. A dependence of ∆XfσG0 on bulk phase concentration indicates that the activity coefficients are dependent on concentration, suggestive of different (at least in magnitude) intermolecular interactions operative at the interface than in the bulk. The gas-surface partition coefficient may be obtained from the ∆XfσG0, recognizing that the choice of standard states for the gas and surface phases will affect the numerical value. The standard enthalpy of adsorption is not too sensitive to the choice of standard state (at least in the low-coverage regime), so comparisons of this parameter may be readily made without conversion. The values of ∆XfσH0 obtained in this way are generally proportional to the standard enthalpies of solvation, consistent with the “surface solvation” model of Davidovits and co-workers (15). There is generally good agreement among various groups on the enthalpies of adsorption of alkane and aromatic hydrocarbons. These tend to increase with molecular size, yet are smaller than the corresponding vaporization enthalpies. For compounds that are expected to be better solvated by water, the standard enthalpies of adsorption are again different from those of vaporization, but are related to the infinite-dilution solvation enthalpies. These observations also suggest that adsorption of gases to the water surface involves specific interactions there, rather than the surface merely providing a site for condensation. A similar approach was taken by Raja et al. (16). The relationship between the adsorbed concentration (Γi/ mol · m-2) and the gas phase concentration (Ci/mol · m-3) is VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Molecular dynamics simulations of the potential of mean force (free energy) for transfer of solutes from the air to the bulk water: (A) OH radical (81); (B) CO2, N2, Ar, and CH3CN (82 with permission); (C) benzene (30 with permission); (D) phenanthrene (30 with permission). obtained from the standard chemical potential difference between the two phases ∆afσµi0 ) µiσ - µia ) -RTln

(

Γi(Ci) 1 · Ci δ0

)

(2)

where δ0 is the standard state thickness of the adsorbed phase given by the Kemball-Rideal standard state. In this expression, Γi(Ci) represents the adsorption isotherm, which, for submonolayer adsorbates on water surfaces is generally well described by a Langmuir adsorption isotherm: Γi(Ci) )

ΓmaxCi C1/2 + Ci

(3)

where Γmax is the maximum monolayer concentration and C1/2 is the gas-phase concentration at one-half of the saturated monolayer concentration. Statistical mechanical considerations (17) suggest that Γmax ) (1)/(Am) and C1/2 ) (exp (βEB))/ (Vm), where Am is the molecular surface area of the adsorbed solute, EB is the binding energy (negative for stable binding) of the solute to the interface, and Vm is the molecular volume of the adsorbate. The parameter β is 1/kT with k being the Boltzmann’s constant. For many solutes at dilute concentrations, the value of C1/2 . Ci and hence we can use the linear portion of the isotherm Γi(Ci) )

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( )

Γmax · Ci ) KσACi C1/2

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(4)

In the above equation, KσA is the partition constant for the solute between the gas and the adsorbed phase, as discussed above. Experimental Adsorption Data. Published data on the adsorption of volatile and semivolatile organic compounds on water surfaces exist. In general, measurements are difficult for water surfaces due to the vaporization of water, small surface concentrations, and irreproducibility of ultraclean water surfaces. The relatively low vapor pressures of most organic compounds also affect the ability to make quantitative measurements of their adsorptive properties. In the early 1950s, Ottewill and co-workers (18-20) studied the adsorption of volatile aromatic hydrocarbon vapors at the air-water interface using a conventional Wilhelmy plate technique for surface tension measurements. Adsorption isotherms for various hydrocarbons (benzene, toluene, o-xylene, chlorobenzene) and alcohols were obtained. Further work expanded the database to low molecular weight aliphatic hydrocarbons (C5-C16 alkanes) (21-23). Axi-symmetric pendant drop shape analysis, the capillary rise method, and the DuNuoy tensiometer methods have been employed to obtain the adsorption isotherms for several atmospherically relevant gases (NH3, methylamines, benzene, toluene, C1-C4 alcohols, and acids) on water surfaces (13, 24, 25). The above methods are only reliable for compounds with substantial partial pressures above the water surface, so that measurable surface tension changes can be observed, but are difficult for those atmospherically relevant compounds which have very low vapor pressures. Indirect methods such as inverse

FIGURE 5. Reaction of gas phase A and Ox at the surface and the bulk aqueous film. The incomplete “cage” effect at the surface and larger surface concentration leads to enhanced reactivity at the film surface, whereas the complete “solvation cage” and slower bulk diffusion leads to smaller reaction rates in the bulk phase. gas chromatography (IGC) and thin film flow reactor studies are useful for obtaining surface adsorption parameters for such compounds. Inverse gas chromatography involves the measurement of retention times for gas phase species. A known thickness of a water film-coated chromatographic column is used to retain a gaseous organic compound via simultaneous and independent bulk water dissolution and adsorption on the water surface. The following equation can be derived for the retention of solute on the GC column (26)

(

tR -1 tm

)( ) ( )

VM Aw ) KσA + KWA Vw Vw

(5)

where Aw and Vw are, respectively, the air-water interfacial area and water volume on the column. VM is the volume of gas-phase per gram of adsorbent. The above definition is strictly applicable to low surface concentrations (i.e., the limit of zero surface coverage). tR and tM are, respectively, the retention time of the solute and that of a nonadsorbing solute (methane). By plotting the group of terms on the left-hand side of the equation versus Aw/VW, we can obtain the values of KσA (from the slope) and KWA (from the intercept). Note that experiments repeated for different GC column temperatures can give the temperature dependent partition constants. The same data can also be used to obtain the entropy and enthalpy of adsorption at the air-water interface. Several investigators (26-28) obtained the gas-water partition constants for several low-molecular weight organic solutes (C5-C8 alkanes, cycloalkanes, chloroalkanes, aromatic hydrocarbons, and esters) using IGC. Raja et al. (16) used IGC to obtain the partition constants for three specific polycyclic aromatic hydrocarbons (benzene, naphthalene, phenanthrene), which are atmospherically important and obtained the temperature dependent partition constants. More recently, Roth et al. (2002) provided a comprehensive set of data at 298 K for 61 specific organic molecules using the IGC method. Gas partitioning on a thin film of water in a cylindrical flow reactor can be used to obtain the interface partition constant (29). In this method a thin film of water is coated on a glass plate and placed inside a cylindrical reactor and exposed to a stream of a PAH vapor. GC/MS is used to detect the compound at the outlet of the reactor. The total equilibrium partitioning is given by the following (29, 30) K *WA ) KWA +

KσA δ

(6)

where δ is the water film thickness (m). The measured value of the overall partition constant is plotted against the inverse of the film thickness to obtain the slope KσA. There are several correlations between KσA and other physicochemical properties of the compounds that have been

proposed based on experimental data. The first such correlation was suggested by Valsaraj (31, 32) for log KσA versus log Kow, molecular surface area, molar volume, and other descriptors such as molecular connectivity index. However, this was based on a limited data set and included only nonpolar compounds. There are two linear free energy correlations prescribed in the literature for KσA at 288 K based on the comprehensive data sets given above. The first correlation developed (33) used only 28 compounds and related KσA to the subcooled liquid vapor pressures (Pl*) of compounds: logKσA ) -0.615lnP *L + 7.66β - 10.41 - [385lnP *L 1 1 6037β - 6611] T 323

(

)

(7)

where R and β are, respectively, the solute electron acceptor (H-bonding acidity) and electron donor (H-bonding basicity) of the molecule. Roth et al. (34) used a more comprehensive set of 61 molecules to obtain a better correlation with the solute’s hexadecane/air partition constants, KHA at 288 K: logKσA ) 0.635logKHA + 3.60R + 5.11β - 8.47

(8)

Kelly et al. (35) provided a detailed analysis of the linear free energy relationship (LFER) for air-water interface partition constants using a basis set of 85 molecules and showed that very good predictions of the free energy of solvation at the air-water interface are possible using the atomic surface tensions developed from solvent descriptors. Thus, at present correlations of the thermodynamic partition constants at the air-water interface are available. Molecular dynamics (MD) simulations can accurately predict the interfacial adsorption (free energy minimum) of aromatic molecules (30, 36). This is directly reflected in the large surface excess observed for the molecule. For example, in Figure 4d we show the theoretically obtained free energy profile for a molecule such as phenanthrene. The free energy difference for adsorption from the gas phase is -35 kJ · mol-1 with a hydration free energy of -17 kJ · mol-1. In other words, there is a significant free energy barrier to desorption of phenanthrene from the air-water interface to the bulk water phase (full solvation), whereas the adsorption (partial solvation) at the interface from the gas phase is quite favorable. Similar simulations for other PAHs also showed deep surface energy minima. The free energy of adsorption varied in proportion to the molecular surface area. Mmereki et al. (37) measured the uptake of two PAHs (anthracene and pyrene) from the gas phase to the air-water interface by monitoring their surface concentrations using glancing-angle laser-induced fluorescence. Consistent with their similar molecular surface areas, similar uptake coefficients were noted for the two compounds. By making the water surface seem progressively more “organic” in nature (varying the coverage of octanol from submonolayer to a few monolayer), the propensity for uptake was increased significantly over that seen on bare water. A similar increase in the surface partitioning of pyrene from aqueous solution was noted previously (38). These results highlight the strong sensitivity of the uptake process to the specific chemical nature of the interface.

Heterogeneous Reactions of Adsorbed Species at the Air-Water Interface Surface versus Bulk Reaction in Thin Water Films. Reactions in thin water films occur via two simultaneous mechanisms: (i) a heterogeneous reaction that occurs strictly on the surface and, (ii) a homogeneous reaction that occurs in the bulk of the aqueous film. The relative rates of these two processes can be vastly different depending on several factors, primarily the surface area and bulk volume. The availability of oxidants on the surface will depend on the preference of ROS to adsorb VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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while its availability in the bulk phase will depend on its rate of solvation and bulk diffusion. On the surface, one might expect that partial (incomplete) solvation of both trace gas and ROS species would facilitate reaction. In the bulk phase, since the ROS and trace gas species are completely solvated, a “cage effect” will limit their diffusion and encounter within the bulk phase. Hence, one should expect slower reaction in the bulk compared to the surface (Figure 5). This aspect has been investigated in the literature. A number of specific cases where surface reactions have shown a major role are summarized in Table 1. For example, Benner et al. (39) determined that fractional conversion of SO2 to SO42- by NH3 is vastly increased when conducted in a thin film of water than in bulk solution. Further, they also observed an increase in the fractional conversion with increasing specific surface area of the water film. In another case, involving the reaction of HNO3(g) with a thin film of water on NaCl(s) the surface reaction rate was observed to increase dramatically as the surface coverage of water increased, although this effect is partially due to an increased ion mobility and nonpassivation of the surface (40). More recently, two cases of UV photooxidation by O2(1∆g) of PAHs (naphthalene, phenanthrene) were studied in a water film (29, 41). The reaction was observed to increase substantially as δ decreased indicating the predominance of surface reaction over the bulk reaction. The rate of disappearance of the adsorbed PAH was described by

kPAH ) khomo +

( )

khetero Kσa · δ Kwa

(9)

where khomo and khetero are, respectively, the homogeneous and heterogeneous rate constants, and δ represents the water film thickness. Similarly, the photooxidation reactions of naphthalene and anthracene are both enhanced by close to an order of magnitude in the quasi-liquid layer on ice surfaces, compared to the reaction in solution (42). The uptake of ROS such as gaseous O3 on aqueous films containing an appropriate scavenger (e.g., fulvic acid or HULIS) also showed the importance of surface over bulk reactions in a thin film of water. For a given water film thickness, the rate constant for the uptake of Ox was shown to vary with scavenger (HULIS) in the following manner (43, 44) kOx ) khomo[HULIS]1/2 + khet[HULIS]

(10)

Thus, for processes dominated by a surface reaction, the rate constant showed a distinct linear relationship with the scavenger concentration and not the square root dependence as for the bulk phase reaction. Similar observations of surface dominance in thin films and droplet surfaces were also described by other investigators (45-53). Direct Measurements of Reactions at Water Surfaces. The studies mentioned above determined a surface component to the heterogeneous reaction via kinetic analysis, which

TABLE 1. Influence of Water Surface Film Thickness (Area) on Surface Reaction Ratesa (39-41, 79, 80) hν

NAPH 98 COU

δ/µm k/min-1

1250 3.7 × 10-4

450 3.3 × 10-4

22 4.8 × 10-4

hν

PHE 98 BenzCOU

δ/µm k/min-1

505 4 × 10-5

200 1 × 10-4

150 2.2 × 10-4

22 6.7 × 10-4

3 0.35-0.45

5 0.45

O2

SO2 98 SO42-

As/x/-

1 0.15-0.22

2 0.3-0.4 H2O

HNO3(g) + NaCl(s) 98 HCl(g) + NaNO3(s)

θw/k/s-1

0.0 0.5

0.2 2.5

0.5 4.5-6.5

0.75 5.5-9.0

aqueous

2NO2(aq) + H2O(L) 98 HONO(aq) + H+(aq) + NO3-(aq)

Nw/k/min-1

2 0.15 ( 0.05

4 0.30 ( 0.10

12 1.30 ( 0.30

a NAPH - naphthalene; PHEN - phenanthrene; COU- coumarin; Benz-COU - benzocoumarin. k: first order reaction rate constant on water film. δ: water film thickness on etched glass. As: normalized water film surface area. x: fractional conversion of SO2. θw: surface coverage of water on solid NaCl. Nw: number of water layers on borosilicate glass.

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TABLE 2. Reaction Rate Constants for Gaseous PAHs with Gaseous Oxidants (Ozone and Singlet Oxygen) at the Air-Water Interfacea Langmuir-Hinshelwood rate parameters for oxidation of anthracene with gaseous ozone on different air-water interfaces(56) surface

kmax/s-1

B/molecule · m-3

water 1-octanol on water octanoic acid on water hexanoic acid on water

(2.55 ( 0.17) × 10-3 (2.54 ( 0.14) × 10-3 (1.11 ( 0.14) × 10-3 (0.48 ( 0.07) × 10-3

(21.43 ( 0.41) × 10-20 (5.08 ( 0.88) × 10-20 (6.81 ( 2.91) × 10-20 (11.8 ( 3.6) × 10-20

Overall first order reaction rate constant for photochemical reaction of gaseous PAHs with singlet oxygen on different water surfaces (29, 42) surface water (bulk) water (ice) water (22 µm film)

PAH

ks/s-1

naphthalene anthracene naphthalene anthracene naphthalene phenanthrene

(2.4 ( 0.1) × 10-5 (1.7 ( 0.5) × 10-4 (2.2 ( 0.5) × 10-4 (1.06 ( 0.05) × 10-3 2.9 × 10-4 6.0 × 10-5

a Note: Langmuir-Hinshelwood overall first order kinetic rate constant is given by (55). kLH )(kmax[Ox]g)/(B + [Ox]g), where [Ox]g is the gaseous oxidant (ozone) concentration expressed in molecule · m-3.

indicated that reaction in the bulk was not the only reactive loss process for the trace gas of interest. Another approach monitors surface reactions directly, when there exists a suitable surface-sensitive probe. Glancing-angle laser-induced fluorescence has been used in this manner to determine the reaction kinetics of gas phase ozone with several PAHs (54-56), chlorophyll (57), and bromide anions (58) present at the air-water interface. Here, the reagents are all at least somewhat surface active, ensuring a higher concentration at the interface than in the bulk. Their concentrations could be monitored as a function of time, giving the kinetics from the perspective of the adsorbed species. In the case of the reaction with chlorophyll (57), a complementary study in a thin water film was interpreted using the analog of eq 10, above. This is the first report that matched up the heterogeneous kinetics from the points of view of both the gas phase and surface-adsorbed species. Sumfrequency generation (59), laser tweezers-Raman spectroscopy (60), and surface tension measurements (61) have also been used as direct probes of reactions of ozone with organics at aqueous surfaces, although not over a range of oxidant concentrations. To date, all reactions of species adsorbed at the water surface with gas phase ozone show a Langmuir-Hinshelwood kinetic mechanism, indicating that the reaction takes place on the surface, following ozone adsorption there. Table 2 lists the literature-available Langmuir-Hinshelwood parameters for the reaction of gaseous anthracene with gaseous ozone at the air-water interface. Table 2 also lists the literature-available overall first order rate constant for the singlet oxygen O2(1∆g) photochemical oxidation of three PAHs on different air-water interfaces. A significant dependence of the reaction kinetics on the presence of an organic coating (even in monolayer or below amounts) is seen. This dependence is due to two factors: an enhancement of the adsorption of ozone to organic-coated water surfaces and some (as yet unidentified) influence of the organic coating on the two-dimensional rate coefficient. The water surface may itself act as a catalyst or explicit reagent, for example in hydrolysis reactions. The hydrolysis of key atmospheric trace gases, such as NO2 and N2O5, as well as the uptake of SO2, CO2, and small acids such as HCl and HNO3, has been the object of much laboratory and theoretical study (see, for example 62-73). These reactions often have very small reacto-diffusive lengths, so take place at, or very close to (i.e., within a few molecular diameters), the air-water interface (63, 74, 75). Water may act as a bridging species, “shuttling” protons or hydrogen atoms via

hydrogen exchange processes in 5-7 membered rings. Such molecular structures typically lower activation barriers considerably, allowing these reactions to take place at atmospheric temperatures. In the above cases, we have considered only the adsorption and oxidation of trace gas-phase organic species from the gas phase on the water surface. One should also consider the possibility that adsorbed trace gases can react with oxidants generated in the aqueous film in so-called “bottom-up” reactions. This was first demonstrated by the reaction of photogenerated Cl atoms with ethanol at the water surface (76); more recently it has been observed in the oxidation of a phospholipid by OH (77) and the reaction of chlorophyll with halogen atoms (78), all at the water surface. Environmental Implications. In atmospheric water films of fogs and aerosols, reactions can be expected to occur at rates different from bulk phases. Of the various gas-phase trace species of interest, we shall consider two PAHs, viz., naphthalene and phenanthrene, for which we have previously obtained the thermodynamic and kinetic rate parameters for adsorption and UV photooxidation on thin water films (29, 41). The competing reaction pathways that we will compare are (i) homogeneous gas-phase OH oxidation and, (ii) heterogeneous UV-mediated oxidation in water films. The characteristic time constants for the two processes are given in eqs 11 and 12 below (29). Homogenous gas-phase OH oxidation of PAH: τ1 )

1 kOH[OH]

(11)

Heterogeneous photo-oxidation of PAH on thin water films: τ2 )

(

khomo

1 khetero KσA + · KWA · δ KWA

)

(12)

where kOH is the second order gas-phase rate constant for the PAH, [OH] is the gas phase hydroxyl radical concentration, khomo and khetero are, respectively, the overall first order homogeneous and heterogeneous reaction rate constants of the PAH on the water films. A water film of 15 µm thickness is assumed for the calculation. A typical atmospheric [OH] ) 1.4 × 1012 molecule · m-3 is used. Table 3 lists the various parameters and the characteristic lifetimes for the two PAHs. It is obvious from the example calculations above that UVVOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Characteristic Reaction Lifetimes for Naphthalene and Phenanthrene in the Gas Phase (29, 41) parameter

naphthalene

phenanthrene

kOH/m3 · molecule-1 · s-1 KσA/µm KWA/[-] khomo/min-1 khetero/min-1 τ1 τ2

2.2 × 10-17 21 86 3.7 × 10-4 9.9 × 10-3 9h 22 min

1.3 × 10-17 33,000 1019 1.1 × 10-3 3.6 × 10-3 5.6 h 0.11 min

initiated photo-oxidation on water films can compete favorably with the homogeneous gas-phase reaction loss; this may be of significance in atmospheric dispersions (e.g., fog, cloud, mist). The photoreactions in thin surface films will become even more significant as the film thickness becomes smaller. The products of heterogeneous oxidation on water surfaces are often oxygenated compounds that have much lower vapor pressure and greater aqueous solubility than the parent gas-phase organic compound. Hence, these materials are likely to serve as precursors for secondary organic aerosol (SOA) in the aqueous phase. This can increase the SOA burden in the atmosphere through surface and liquid-phase reactions on water thin films in hydrometeors. Therefore, a complete understanding of atmospheric oxidation processes demands a full treatment of the adsorption and oxidation reactions of gas-phase organic molecules at the air-water interface of water films.

Acknowledgments This material is based upon work supported at LSU by the National Science Foundation under Grant AGS-0907261 and in Toronto by NSERC and by CFCAS. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funding agencies.

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