Aqueous-phase reactions, clouds

Schwartz, S. E., Mass-Transport Considerations Pertinent to Aqueous Phase Reactions of Gases in Liquid-Water Clouds, NATO AS1 Series, G6, 416-471 (1986), and in Chemistry of Multiphase Atmospheric Systems (W. Jaeschke, Ed.), pp. 415-471, Springer-Verlag, New York, 1986. 

It was assumed that there were no limitations on the rates of oxidation due to mass transport as discussed in detail by Schwartz and Freiberg (1981), this assumption is justified except for very large droplets (> 10 yarn) and high pollutant concentrations (e.g., 03 at 0.5 ppm) where the aqueous-phase reactions are very fast. It was also assumed that the aqueous phase present in the atmosphere was a cloud with a liquid water content (V) of 1 g m-3 of air. As seen earlier, the latter factor is important in the aqueous-phase rates of conversion of S(IV) thus the actual concentrations of iron, manganese, and so on in the liquid phase and hence the kinetics of the reactions depend on the liquid water content. 

FIGURE 9.10 Modified particle modes and growth processes for sulfate particles involving aqueous-phase reactions in low altitude fogs and in higher altitude clouds upon advection of boundary-layer air upwards. (Adapted with permission from Ondov and Wexler, 1998. Copyright 1998 American Chemical Society.)... 

Atmospheric Transformations. S02 can be oxidized to form H2S0A in either the gas phase through photochemical reactions, or in the aqueous phase in cloud water (16). Clouds can then evaporate, leaving behind sulfate particles. The photochemical reactions are fairly slow, 1-3%/hr, but operate during all daylight hours. 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

The rate of aqueous-phase in-cloud reaction can be evaluated, for specified reagent concentrations (including pH) and physical conditions, by means of the following assumptions ... 

Nitrogen Oxide Reactions. Examination of possible aqueous-phase reactions of nitrogen dioxide and peroxyacetyl nitrate has revealed no reactions of importance to cloud chemistry (21,22). This situation is a consequence of the low solubilities and/or low reactivities of these gases with substances expected to be present in cloudwater, although studies with actual precipitation samples would be valuable in confirming this supposition. NO2 has been shown (23) to react with dissolved S(IV), but the details of the mechanism and rate of this reaction remain to be elucidated. 

Techniques are at hand to evaluate the rates of aqueous-phase acid formation reactions in clouds. Such evaluations indicate that oxidation of SO2 by H2O2 and O3 can be important in-cloud reactions for assumed representative reagent concentrations and other conditions. Rapid aqueous-phase reactions do not appear to be indicated for oxidation of nitrogen oxides to nitric acid. 

Schwartz, S. E. Mass-transport considerations pertinent to aqueous-phase reactions of gases in liquid-water clouds. In Chemistry of Multiphase Atmospheric Systems Jaeschke, W.,... 

GEM-AQ only has a simplified aqueous phase reaction module for oxidation of SO2 to sulphate. Thus, for the gas phase species, wet deposition processes are treated in a simphfied way. Only below-cloud scavenging of gas phase species is considered in the model. The efficiency of the rainout is assumed to be proportional to the precipitation rate and a species-specific scavenging coefficient. The coefficients apphed are the same as those used in the MATCH model (Multiscale Atmospheric Transport and Chemistry Model) used by the Swedish Meteorological and Hydrological Institute (SMHl) (Langner et al. 1998). 

Extrapolation to the K/T boundary requires consideration of the time scales of acid deposition. Nitric acid formation occurs rapidly by aqueous phase reaction of NO and NO2 with liquid water produced by tlie incident K/T bolide on both impact and infall of ejecta. For tlie quantities of NO produced by the K/T impact ( 10 5 moles), conversion to HNO3 occurred wiUiin days, assuming sufficient liquid water was available in the posl-K/T atmosphere. The nitric acid will form an acid rain of pH 0 for a liquid water content of 1 g/m (typical of tropospheric clouds) but will contain enough protons to weather only 3 x 10 moles of Sr, for Sr/(Ta -0.003 in soil and bedrock minerals. Sulfuric acid formation occurred on a time scale of years [7] due to the slow rate of gas phase SO2 oxidation. Spread evenly over 10 years, 10 moles of SO2 produced a global acid rain of pH —4, and released —3 x 10 moles of Sr. 

Quantitative analysis of different reaction pathways for the transformation of aquated sulfur dioxide in atmospheric droplet systems has been a major objective of the research conducted in the principal investigator s laboratory for the last four years. Available thermodynamic and kinetic data for the aqueous-phase reactions of SO2 have been incorporated into a dynamic model of the chemistry of urban fog that has been developed by Jacob and Hoffmann (23) and Hoffmann and Calvert (39). The fog and cloud water models developed by them are hybrid kinetic and equilibrium models that consider the major chemical reactions likely to take place in atmospheric water droplets. Model results have verified that... 

Pedersen and Sehested (2002) showed that the aqueous-phase reaction of isoprene with ozone was insignificant for the processing of isoprene in the atmosphere. They estimated the overall and individual lifetimes of isoprene due to reactions with ozone and the hydroxyl radical, at 25 "C and typical in-cloud conditions. The results (Table 3) indicate that clouds generally should not contribute much to the processing of isoprene in the atmosphere. Only in the aqueous phase, were the lifetimes of isoprene due to reactions with ozone and with OH radicals comparable. Similar conclusions were drawn for methyl vinyl ketone, while for methacrolein the clouds could reduce the overall atmospheric lifetime by 50 %. 

Cloud processes have been predicted to have a significant effect on the chemistry of the clean troposphere (Lelieveld and Crutzen, 1990, 1991 Warneck, 1991, 1992). For example, the uptake of HCHO, HOz radicals, and N2Os into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of HOz radicals, and N205 into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of H02 radicals [reactions (33)—(36)1, and consequent conversion of NO to N02. Also, aqueous-phase reactions of H2C(OH)2, the hydrated form of HCHO, lead to the formation of 02, which can react with dissolved 03 to enhance the rate of transfer of 03 to the liquid phase over that based solely on physical solubility. Absorption of N2Os into... 

The consumption rate of methanol in reaction (143) is typically around 0.1% CHjOH(g) per hour, and only a negligibly small decrease of the gas-phase methanol concentration is expected because of its cloud chemistry. Similarly, aqueous-phase reactions of CH3OOH, CH302, and CH3C(0)00H are too slow to influence either the corresponding gas-phase concentrations or the aqueous-phase reaction system. 

The OH radical participates in a number of aqueous-phase reactions at least 16 reactions producing OH and 19 consuming it have been proposed by various investigators. The principal sink for dissolved OH under conditions typical of remote continental clouds is reaction (142). Other sinks are the reactions with hydrogen peroxide, formic acid, and S(IV). The main aqueous-phase sources of OH are reaction (146) and the photolysis of dissolved hydrogen peroxide. Secondary sources are the photolysis of NO and the oxidation of S(IV) by HOz. 

The notion that chemical reactions in the aqueous phase may be important to atmospheric chemistry dates back at least 30 yr, when Junge and Ryan (1958) called attention to the great potential of cloud water for the oxidation of dissolved S02 by heavy-metal catalysis. At that time the process appeared to be the only viable oxidation mechanism for atmospheric S02. Later, when the concept of OH radical reactions gained ground, the gas-phase oxidation of S02 by OH was recognized to be equally important. The recent revival of interest in aqueous phase reactions is connected with efforts to achieve a better understanding of the origins of rainwater acidity. An oxidation of N02 to nitric acid also takes place in cloud water. Contrary to previous ideas, however, this process was recently shown to have little influence on atmospheric reactions of N02. 

FIGURE 7.16 Rate of aqueous-phase oxidation of S(IV) by ozone (30 ppb) and hydrogen peroxide (I ppb), as a function of solution pH at 298 K. Gas-aqueous equilibria aie assumed for all reagents. R/ so. represents the aqueous phase reaction rate per ppb of gas-phase SOj. R/L represents rate of reaction referred to gas-phase SO2 pressure per (gm 3) of cloud liquid water content. 

Free radicals, such as OH and H02, can be scavenged heterogeneously from the gas phase by cloud droplets or produced in the aqueous phase. More than 30 aqueous-phase reactions involving OH and H02 have been proposed (Graedel and Weschler 1981 Chameides and Davis 1982 Graedel and Goldberg 1983 Schwartz 1984 Jacob 1986 Pandis and Seinfeld 1989a). 

Wine, R H., Tang, Y., Thom, R. P., Wells, J. R., and Davis D. D. (1989) Kinetics of aqueous phase reactions of the SO4 radical with potential importance in cloud chemistry, J. Geophys. Res. 94, 1085-1094. 

Dissolution of atmospheric species into cloud droplets followed by aqueous-phase reactions involves the following series of steps ... 

The reactants for aqueous-phase atmospheric reactions are transferred to the interior of cloud droplets from the gas phase by a series of mass transport processes. We would like to compare the rates of mass transport in the gas phase, at the gas-water interface, and in the aqueous phase in an effort to quantify the mass transport effects on the rates of aqueous-phase reactions. If there are no mass transport limitations, the gas and aqueous phases will remain at Henry s law equilibrium at all times. Our objective will be to identify cases where mass transport limits the aqueous-phase reaction rates and then to develop approaches to quantify these effects. 

In Section 12.2.4 we showed that after time z,ja = RjJit2Daq the concentration profile inside a cloud droplet becomes uniform. The characteristic time for aqueous-phase reaction was found to be equal to xra = [A]// aq. If the characteristic aqueous-phase diffusion time is much less than the characteristic reaction time, then aqueous-phase diffusion will be able to maintain a uniform concentration profile inside the droplet. In this case, there will be no concentration gradients inside the drop and therefore no aqueous-phase mass transport limitations on the aqueous-phase kinetics. This criterion can be written as... 

We would like to use the tools developed above to determine whether aqueous-phase reaction between ozone and S(IV) is limited by mass transport in a typical cloud. The subsequent analysis was first presented by Schwartz (1988). Typical ambient conditions that need to be examined include droplet diameters in the 10-30 pm range, cloud pH values in the 2-6 range, and cloud temperatures varying from 0°C to 25°C. The reaction rate for the S(IV)-03 reaction is given by... 

The equations above have been the basis of most atmospheric aqueous-phase chemistry models that include mass transport limitations [e.g., Pandis and Seinfeld (1989)]. These equations simply state that the partial pressure of a species in the cloud interstitial air changes due to mass transport to and from the cloud droplets (incorporating both gas and interfacial mass transport limitations). The aqueous-phase concentrations are changing also due to aqueous-phase reactions that may be limited by aqueous-phase diffusion included in the factor Q. 

Aerosol particles in the atmosphere contain a variety of volatile compounds (ammonium, nitrate, chloride, volatile organic compounds) that can exist either in the particulate or in the gas phase. We estimate in this section the timescales for achieving thermodynamic equilibrium between these two phases and apply them to typical atmospheric conditions. The problem is rather different compared to the equilibration between the gas and aqueous phases in a cloud discussed in the previous section. Aerosol particles are solid or concentrated aqueous solutions (cloud droplets are dilute aqueous solutions), they are relatively small, and aqueous-phase reactions in the aerosol phase can be neglected to a first approximation because of the small liquid water content. 

For less soluble gases like S02, uptake is a complex function not only of species properties and cloud droplet distribution, but also of other species that may participate in aqueous-phase reactions (e.g., H202) or control the cloud droplet pH (e.g., NH3). 

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