Atmosphere water droplets

FIGURE 8.11 Schematic of steps involved in the transfer of S02 from the gas phase to the aqueous phase of an atmospheric water droplet and its oxidation in the liquid phase. S02(i) = S02 at the water-gas interface. 

Behra, P., and L. Sigg, Evidence for Redox Cycling of Iron in Atmospheric Water Droplets, Nature, 344, 419-421 (1990). 

Homogeneous oxidation of these compounds by H202 may not be an important source of acidity in cloudwater and atmospheric water droplets. Oxidation of CS2 and OCS is preceded by the rate-controlling hydrolysis... 

It should be mentioned, that solar photolysis of ferrioxalate is of importance within atmospheric water droplets (cf. Sigg and Stumm, 1996). Potassium ferrioxalate... 

In this chapter we will deal with some important reactions at the gas-water interface and discuss above all the partitioning of molecules between the gas phase and the water phase (Henry s law). We will also explain the processes that influence wet and diy deposition and the composition of atmospheric water droplets (clouds, fog, rain, snow, dew) and illustrate how pollutants relee.sed into the atmosphere are transferred back to the land. Attention will be paid to the disturbance of the proton balance by the oxides of C, N, and S, antliro-pogenically released into the atmosphere, and how this disturbance is transferred from the atmosphere to the terrestrial and aquatic ecosystems. 

The adducts formed are relatively stable. The kinetics of the formation of these adducts depends on various factors the rates are often slow in acid solutions. The S(IV) aldehyde compounds, especially hydroxymethanesulfo-nate (CH20HS03 ), can make up a large fraction of dissolved S(IV) in atmospheric water droplets. Thus the adduct formation may enhance the solubility of S(IV), especially in the low pH range. These adducts are less reactive with regard to oxidation by oxidants. 

More detailed analysis on the enhancement of CO2 transfer has been made by Emerson (1975) and Schwartz (1984) for the transfer of reactive gases in atmospheric water droplets. 

This type of reaction plays an important role in sunlit surface waters and in atmospheric water droplets. 

Important photooxidants that are formed in the atmosphere may become ab-sori)ed into atmospheric water droplets (clouds, fog, dew) and in surface waters. Most important emissions are HO2 and O3. Assuming a dry deposition of 0.1 mg O3 h, a water film of 0.1-mm depth would receive a mean dose rate... 

Such processes play an important role not only in surface waters but also in atmospheric water droplets. Both Fe and oxalate are present in cloud and fog water oxalate is an intermediate in the oxidation of atmospheric organic pollutants. Iron is introduced into the atmosphere from dust it is present in these mostly slightly acid water droplets as in dissolved or colloidal form as Fe(II) and Fe(III) (Behra and Sigg, 1990). H2O2 formed by reactions such as 19-21 can oxidize SO2 in the aqueous atmosphere (see Example 9.3) (Faust, 1994 Hoigne et al., 1994 Kotronarou and Sigg, 1993 Sedlak and Hoigne, 1993, 1994). 

The iron cycle shown in Figure 12.11 illustrates some of the redox processes typically observed in soils, sediments, waters, and atmospheric water droplets, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(III) (hydr)oxides by organic ligands, which may also be photo-catalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III) (hydr)oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr)oxides is accompanied by the release of these substances into the water column. 

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... 

At the National Institute of Chemistry (NIC), in the frame of CMD subproject of EUROTRAC-2, experimental studies of the role of soluble constituents of atmospheric aerosols in the aqueous-phase autoxidation mechanisms of S(IV) was studied. The research focused on atmospheric water droplets (clouds, fog), where soluble constituents of atmospheric particles may be important in aqueous SO2 oxidation under non-photochemical conditions. In the frame of CMD project laboratory experiments in a semi-batch continuous stirred tank reactor under controlled conditions (T, air flow rate, stirring), were made in order to study the autoxidation of S(IV)-oxides catalyzed by transition metal ions (Fe(III), Fe(II), Co(II), Cu(II), Ni(II), Mn(II)). These studies were carried out at the National Institute of Chemistry. 

Munger, J. W., C. Tiller, and M. R. Hoffmann (1986), Identification and Quantification of Hydroxymethanesulfonic Acid in Atmospheric Water Droplets, Science 231, 247-249. 

In short, although photochemistry in bulk seawater has been extensively studied, very little is known about the micro layer. Even less is known of the photochemical reactions that may be occurring in aerosol particles. Since marine aerosol particles may undergo wet deposition as rain, fog droplets, or cloud waters, photochemical studies of these atmospheric waters over the ocean and in coastal areas can shed some light on potential aerosol processes. Since hydrogen peroxide is formed from the photolysis of DOM and nitrates in aqueous solutions, its presence may serve as a potential marker for the occurrence of these photoreactions within aerosol particles or atmospheric water droplets. Peroxides also play significant roles in oxidation reactions and OH production in atmospheric waters. 

Olson, T. M. and M. R. Hoffmann (1989) Hydroxyalkylsulfonate formation Its role as a S(IV) reservoir in atmospheric water droplets. Atmospheric Environment 23, 985-997 Oltmans, S. J. and H. Levy II (1994) Surface ozone measurements from a global network. Atmospheric Environment 28, 9-24... 

Wet deposition is linked with atmospheric water droplets and combines the effects of rain and snow as well as the impaction of wind-blown contaminated cloud or fog droplets on to vegetation when the cloud or fog extends down to the surface. The removal by rain or snow is particularly efficient just 1 mm of rain can remove more material than can be deposited by dry deposition operating over 24 h. 

The sulfuric acid is generated in atmospheric water droplets and remains in solution, undergoing advective transport with the droplets on air currents. Eventually, some of the acidified water droplets fall to the earth as acid rain. In the United States, coal-bnrning power plants in the Midwest have been a source of acid rain in the East, and this was a major impetns behind passage of the Clean Air Act in 1990. 

Acid-base reactions occur between acidic and basic species in the atmosphere. The atmosphere is normally at least slightly acidic because of the presence of a low level of carbon dioxide, which dissolves in atmospheric water droplets and dissociates slightly ... 

Ammonia is particularly important as a base in the air because it is the only water-soluble base present at significant levels in the atmosphere. When it is dissolved in atmospheric water droplets, ammonia plays a strong role in neutralizing atmospheric acids, as shown by the following reactions ... 

Absorption, Dissociation, and Aqueous-Phase Chemical Reactions The diffusive penetration of gases or gas mixtures into a condensed phase (e g., droplet) is called absorption. In equilibrium, the absorbed gas is dissolved at a certain concentration inside the droplet and the equilibrium vapor pressure over the droplet surface is proportional to the concentration at the droplet surface (Henry s law). The concentration inside the droplet itself can be influenced by dissociation or chemical reactions (sulfur production by oxidation of dissolved SO2 to SOt ). If these processes represent a sink for the solute, the concentration inside the droplet and, consequently, the vapor pressure at the droplet surface is decreased (i.e., mass transfer is enhanced). Typical gases that dissolve into atmospheric water droplets are CO2, SO2, NH3, H2O2, and O3. 

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