Cloud and fog

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

In the case of S02, oxidation in the aqueous phase, present in the atmosphere in the form of aerosol particles, clouds, and fogs, is also important. Thus S02 from the gas phase dissolves in these water droplets and may be oxidized within the droplet by such species as H202, 03, 02, and free radicals. Oxidation of S02 on the surfaces of solids either present in the air or suspended in the water droplets is also possible. On the other hand, it is believed that HN03 is formed primarily by reaction (10) in the gas phase and subsequently dissolves in droplets. 

Clearly, the concentrations of pollutants in ambient air, and hence their impacts, are determined not only by their rates of emissions but also by the nature and efficiencies of their chemical and physical sinks, e.g., chemical transformations, as well as wet and dry deposition to the earth s surface. To a large extent, these competing processes are affected not only by direct dispersion and transport but also by such meteorological factors as temperature, sunlight intensity, and the presence of temperature inversions as well as clouds and fogs. 

As discussed in Chapters 7, 8, and 9, there are a number of free radical species whose reactions in the aqueous phase drive the chemistry of clouds and fogs. These include OH, HOz, NO-, halogen radicals such as Cl2, sulfur oxide radicals, and R02. Generation of these radicals in the liquid phase for use in kinetic... 

In short, when treating the uptake of gases into particles, clouds, and fogs in the atmosphere and their reactions either at the interface or in the bulk, one must take into account all of the processes depicted in Fig. 5.12. While exact solutions for the series of coupled differential equations describing the individual steps are not always possible, approximate solutions have been derived for most situations of atmospheric interest in which the various steps can be treated as decoupled processes. In extrapolating values for the various steps derived from laboratory studies to particles in the atmosphere, one must take into account differences in conditions, including particle size. Summing up, if the fundamental parameters such as the... 

Through these studies, it was concluded that absorption of NO and N02 into the aqueous phase in the form of clouds and fogs in the atmosphere and their subsequent oxidation are not significant under typical atmospheric conditions. The major reasons for this are that NO and N02 are not highly soluble and, in addition, the reactions are kinetically rather slow due to the dependence of the rates on the square of the reactant concentration. As a result, like the oxidation of NO by 02, the reactions slow down dramatically when the... 

Numerous field studies of the rate of S02 oxidation in the troposphere have shown that the oxidation rate depends on a number of parameters. These include the presence of aqueous phase in the form of clouds and fogs, the concentration of oxidants such as H202 and... 

As we shall see in the following sections, these observations are readily understood in terms of the kinetics and mechanisms of oxidation of S02. The oxidation of S02 occurs in solution and on the surfaces of solids as well as in the gas phase. Indeed, under many conditions typical of the troposphere, oxidation in the aqueous phase provided by clouds and fogs predominates, consistent with the observed dependence on these factors. The presence of oxidizers to react with the S02 is, of course, also a requirement hence the dependence on 03 (which is a useful surrogate for other oxidants as well) and sunlight, which is needed to generate significant oxidant concentrations. 

As we have seen in Chapter 7, the oxidation of NO to HNO-, occurs to a large extent in the gas phase as well as by the hydrolysis of N2Os on surfaces, and the acid is then taken up by dissolving in clouds and fogs ... 

As a result, the initial SOz-OH reaction does not lead to the net loss of OH and a chain oxidation of S02 can result. Perhaps more important, the generation of H02 leads to increased H202 production. As discussed in Section C.3.e, this highly soluble gas is a major oxidant for S(IV) in the aqueous phase so that reaction (5) can affect not only gas-phase processes but also the oxidation in clouds and fogs (e.g., Stockwell, 1994). 

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the OH-SOz gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole. 

Although the S(IV)-aldehyde adducts are stable toward oxidation, one or more of the oxidation processes for HSOJ or S03 described below are likely to be much faster than adduct formation under typical fog and cloud conditions. For example, Fig. 8.10 shows the calculated times for complexing S(IV) with HCHO compared to the time for oxidation by H202 at different concentrations typical of various clouds and fogs as a function of pH (Rao and Collett, 1995). Even at the lowest H202 concentrations and highest HCHO concentrations, complexation only competes with oxidation at pH values above about 4.5. Thus the two processes,... 

While the volume of liquid water present is much larger in clouds and fogs than that in fine particles, the solute concentrations in the latter may be much higher, and this may serve to increase the rate of aqueous-phase oxidations. More importantly, these fine particles are believed to serve as sites for the condensation of water vapor, leading to the formation of fogs and clouds (Chapter 14.C.2). 

Much more relevant to the aqueous phase in clouds and fogs in the atmosphere is the catalyzed oxidation of S(IV) by 02. Both Fe3+ and Mn2+ catalyze the oxidation and as described in Chapter 9, both are common constituents of tropospheric aerosols even in remote... 

While the Henry s law constant for ozone is fairly small (Table 8.1), there is sufficient ozone present in the troposphere globally to dissolve in clouds and fogs, hence presenting the potential for it to act as a S(IV) oxidant. Kinetic and mechanistic studies for the 03-S(IV) reaction in aqueous solutions have been reviewed and evaluated by Hoffmann (1986), who shows that it can be treated in terms of individual reactions of the various forms of S(IV) in solution. That is, S02 H20, HSOJ, and SO2- each react with 03 by unique mechanisms and with unique rate constants, although in all cases the reactions can be considered to be a nucleophilic attack by the sulfur species on 03. 

It is interesting, however, that the HONO-HSO-,-reaction has been shown to form a nitrene (HON ), which Mendiara and co-workers (1992) suggest could contribute to free radical formation in clouds and fogs. 

As expected based on our knowledge of gas-phase chemistry, in addition to the Fenton type chemistry involving iron, photolysis of Os, H202, HONO, and HNO-, are all potential OH sources in clouds and fogs. In addition, the photolysis of nitrite, nitrate, and HOJ in aqueous solutions can also form OH. In short, there are many potential sources of OH in clouds and fogs. 

The reason that the aqueous-phase concentrations in fogs can be so high is related in part to the liquid water content (LWC), which is a major difference between clouds and fogs. The liquid water content for fogs is typically of the order of 0.1 g m-3 of air, whereas that for clouds is about an order of magnitude higher. This small LWC in fogs corresponds to increased solute concentrations. 

The estimates in Fig. 8.21 show that H202 is expected to be the most important oxidant for S(IV) in clouds and fogs at pH <4.5. At higher pH values, both 03 and the iron-catalyzed 02 oxidation can compete. 

While the emphasis has been on oxidation of DMS and other reduced sulfur compounds in the gas phase, there is some indication that oxidation in the aqueous phase in clouds and fogs should also be considered. For example, Lee and Zhou (1994) have shown that DMS reacts with 03 in aqueous solutions quite rapidly, with a rate constant at 288 K of 4 X 108 L mol-1 s-1. They estimate that at 30 ppb 03, a level found globally, the lifetime for in-cloud oxidation of DMS is about 3 days, of the same order of magnitude as that for the gas-phase oxidation by OH (see Table 8.17). Given the moderately high solubility of not only DMS but other sulfur compounds as well (see Henry s law constants in Table 8.1), this is clearly an area that warrants further research. 

Faust, B. C., and J. M. Allen, Aqueous-Phase Photochemical Sources of Peroxy Radicals and Singlet Molecular Oxygen in Clouds and Fog, J. Geophys. Res., 97, 129I3-I2926 (1992). 

Faust, B. C., C. Anastasio, J. M. Allen, and T. Arakaki, Aqueous-Phase Photochemical Formation of Peroxides in Authentic Cloud and Fog Waters, Science, 260, 73-75 (1993). 

We have seen in Chapter 8 that reactions in the aqueous phase present in the atmosphere in the form of clouds and fogs play a central role in the formation of sulfuric acid. Thus, an additional mechanism of particle formation and growth involves the oxidation of SOz (and other species as well) in such airborne aqueous media, followed by evaporation of the water to leave a suspended particle. 

There are a variety of methods for collecting and measuring H202 and organic peroxides in air. H202 is especially water soluble and hence partitions between the gas phase and clouds and fogs (e.g., Macdonald et al., 1995). While the collection techniques for air versus clouds and fogs are different, the analytical techniques are the same. 

The CF3C(0)C1 hydrolyzes in clouds and fogs to form CF3COOH, trifluoroacetic acid. 

Effect of aerosol particles on cloud drop number concentrations and size distributions Clouds and fogs are characterized by their droplet size distribution as well as their liquid water content. Fog droplets typically have radii in the range from a few /an to 30-40 /an and liquid water contents in the range of 0.05-0.1 g m" Clouds generally have droplet radii from 5 /an up to 100 /im, with typical liquid water contents of 0.05-2.5 gin"5 (e.g., see Stephens, 1978, 1979). For a description of cloud types, mechanisms of formation, and characteristics, see Wallace and Hobbs (1977), Pruppacher (1986), Cotton and Anthes (1989), Heyms-field (1993), and Pruppacher and Klett (1997). 

This has important implications for nucleation in the atmosphere. Condensation of a vapor such as water to form a liquid starts when a small number of water molecules form a cluster upon which other gaseous molecules can condense. However, the size of this initial cluster is very small, and from the Kelvin equation, the vapor pressure over the cluster would be so large that it would essentially immediately evaporate at the relatively small supersaturations found in the atmosphere, up to 2% (Prup-pacher and Klett, 1997). As a result, clouds and fogs would not form unless there was a preexisting particle upon which the water could initially condense. Such particles are known as cloud condensation nuclei, or CCN. 

In addition to wet and dry deposition, many high-elevation sites may receive substantial inputs of N from clouds or fog (15-17). Few quantitative estimates of cloud deposition are available, but results from one site on Whiteface Mountain in the Adirondacks indicate that clouds and fog can contribute up to 40% of total deposition (18). Rates of wet and dry deposition at Whiteface Mountain were comparable to the Adirondack values given in Table I ( 740 equiv/ha), but total deposition rates (including cloud and fog deposition) averaged 1170 equiv/ha. 

Zuo, Y. (1995) Kinetics of photochemical/chemical cycling of iron coupled with organic substances in cloud and fog droplets. Geochim. Cosmochim. Acta, 59, 3123-3130. 

Heterogeneous reactions involving water droplets in clouds and fogs are important mechanisms for the chemical transformation of atmospheric trace gases. The principal factors affecting the uptake of trace gases by liquid droplets are the mass accommodation coefficient of the trace gas, the gas phase diffusion of the species to the droplet surface and Heniy s Law saturation of the liquid. The saturation process in turn involves liquid phase diffusion and chemical reactions within the liquid droplet. The individual processes are discussed quantitatively and are illustrated by the results of experiments which measure the uptake of SOj by water droplets. 

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