Aqueous-Phase Atmospheric Chemistry

Relative to the levels of the species we have been considering, water vapor is at a high concentration in the atmosphere. Liquid water, in the form of clouds and fog, is frequently present. Small water droplets can themselves be viewed as microscopic chemical reactors where gaseous species are absorbed, reactions take place, and species evaporate back to the gas phase. Droplets themselves do not always leave the atmosphere as precipitation more often than not, in fact, cloud droplets evaporate before coalescing to a point where precipitation can occur. In terms of atmospheric chemistry, droplets can both alter the course of gas-phase chemistry through the uptake of vapor species and act as a medium for production of species that otherwise would not be produced in the gas phase or would be produced by different paths at a lower rate in the gas phase (Fig. 10). Concentrations of dissolved species in cloud, fog, and rain droplets are in the micromolar range, and therefore one usually assumes that the atmospheric aqueous phase behaves as an ideal solution. 

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 chemistry that occurs in cloud and fog droplets in the atmosphere has been shown, in the last decade or so, to be highly complex. Most atmospheric species are soluble to some extent, and the liquid-phase reactions that are possible lead to a diverse spectrum of products. The aspect of atmospheric aqueous-phase chemistry that has received the most attention is that involving dissolved S02. Sulfur dioxide is not particularly soluble in pure water, but the presence of other dissolved species such as H202 or 03 displaces the dissolution equilibrium for S02, effectively 

The liquid water content of the atmosphere, wL, is usually expressed either in grams of water per cubic meter of air, or as a dimensionless volume fraction L (e.g., cubic meters of liquid water per cubic meter of air). Typical liquid water content values are 0.1 to 1 g m-3 (L = 10 7-10 6) for clouds, 0.05 to 0.5 g m-3 (L = 5 X 10 7-5 X 10-6) for fogs, and only 10-5 to 10 4 g m-3 (L = 10-1I-10 I°) for aerosols. 

Henry s Law Coefficients of Some Atmospheric Gases Dissolving in Liquid Water 

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Pandis, S. N., and J. H. Seinfeld, Sensitivity Analysis of a Chemical Mechanism for Aqueous-Phase Atmospheric Chemistry, 94, 1105-1126 (1989b). 

Pandis, S.N., Seinfeld, J.H. Sensitivity analysis of a chemical mechanism for aqueous-phase atmospheric chemistry. J. Geophys. Res. 94, 1105-1126 (1989)... 

Initiation with X = OH has been discussed earlier. Table 8.11 summarizes some of the aqueous-phase HO, chemistry in which OH is generated and reacts in the atmosphere. (Note that the rate constants for some of the aqueous phase reactions shown in Tables 8.10-8.16 depend on such factors as ionic strength see Chapter 5.D.) Involved with this chemistry is that of bicarbonate/carbonate, since OH reacts with these species as well (Table 8.12). It is interesting that, in contrast to the high reactivity of OH toward S(IV) in aqueous solutions, direct reactions of H02/02 with S(IV) do not appear to be important (Sedlak and Hoigne, 1994 Yermakov et al., 1995). 

Herrmann, H., A. Tilgner, P. Barzaghi, Z. Majdik, S. Gligorovski, L. Poulain and A. Monod (2005) Towards a more detailed description of tropospheric aqueous phase organic chemistry CAPRAM 3.0. Atmospheric Environment 39, 4351-4363... 

Although this area of reactions at interfaces is relatively new and not well understood, it may potentially be more significant than previously recognized. Because of the unique characteristics of such processes both kinetically and mechanistically compared to bulk aqueous-phase or gas-phase reactions, we suggest the term fourth phase be used to describe this chemistry at gas-liquid interfaces in the atmosphere. 

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. 

Huie, R. E., Free Radical Chemistry of the Atmospheric Aqueous Phase, in Progress and Problems in Atmospheric Chemistry (J. R. Barker, Ed.), pp. 374-419, World Scientific, Singapore, 1995. 

Peroxy radicals are intermediates in the atmospheric oxidation of virtually all organic compounds. HO is soluble in aqueous aerosols (21) and can participate in a number of oxidation reactions in the aerosols. The overall importance of the aqueous-phase processes compared to the gas-phase chemistry is uncertain. 

Hoffmann, M. R., and Calvert, J. G., Chemical Transformation Modules for Eulerian Acid Deposition Models. Volume II The Aqueous-Phase Chemistry. National Center for Atmospheric Research, Boulder, Colo., 1985. 

Kok, G. L. Heikes B. G. Lazrus, A. L. Gas and aqueous phase measurements of hydrogen peroxide. Symposium on Acid Rain I. Sources and Atmospheric Processes, Division of Petroleum Chemistry, Inc. American Chemical Society, preprints, 1986, Vol. 31, No. 2, pp. 541-544. 

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.,... 

Lee, Y.-N. Atmospheric aqueous-phase reactions of nitrogen species Kinetics of some aqueous-phase reactions of peroxy-acetyl nitrate. Conference on Gas-Liquid Chemistry of Natural Waters Brookhaven National Laboratory, 1984 BNL 51757,... 

Describe a cloud chemistry simulation facility to emulate atmospheric aqueous phase interactions among gases, particles, and liquid water droplets. 

Absence of nitro derivatives was also observed upon irradiation of nitro-phenols and nitrate. Also in this case, the electron-withdrawing character of the nitro group can account for the inhibition of nitration [109]. The difficulty to nitrate nitrophenols to dinitrophenols is widely recognised [125] and also constitutes a problem in environmental chemistry, since field data seem, in contrast, to indicate that the nitration of 2-nitrophenol to 2,4-dinitrophenol in the atmospheric aqueous phase (e.g. cloud water) is an important process [126]. In fact, aqueous-phase nitration might be a relevant sink for 2-nitrophenol and possibly the main source of the dinitro compound, which is a powerful phytotoxic agent [127,128]. In the presence of nitrate under irradiation the main transformation intermediates of nitrophenols are the hydroxyl derivatives, while other compounds may derive from the direct photolysis of the substrates (catechol and 2-nitrosophenol from 2-nitrophenol hydroquinone, benzoquinone, hydroxybenzoquinone and 4-nitrosophenol from 4-nitrophenol) [109]. 

Gas-phase HO- is generated by photolysis of HOOH and by cosmic radiation and solar radiation of O2/H2O in the atmosphere. The latter process is an important contributor to the atmospheric chemistry associated with organic pollutants. The gas-phase reactions of HO- with organic molecules have been exhaustively reviewed and summarized in a recent compendium. In aqueous solutions, HO- is produced by radiolysis (continuous and pulsed) (equation 87), ... 

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). 

Yes there are reactions involving dissolved ozone, even photochemical reactions which can produce free radicals. However given the atmospheric abundance of ozone of about 20-40 ppbv and its very low solubility, it can be easily shown that the ozone concentration in cloudwater is low — about 10 moles per liter — and hterefore that homogeneous aqueous-phase ozone reactions have a negligible effect on cloud and precipitation chemistry. 

Lin C. J. and Pehkonen S. O. (1999) Aqueous phase reactions of mercury with free radicals and chlorine implications for atmospheric mercury chemistry. Chemosphere 38(6), 1253-1263. 

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