Cloud droplets

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

Fenner (11) has pointed out that short-lifetime constituents of the atmosphere such as nitrogen oxides, carbon monoxide, and nonmethane hydrocarbons may also play roles related to global warming because of their chemical relations to the longer-lived greenhouse gases. Also, SO, with a very short life interacts with ozone and other constituents to be converted to particulate sulfate, which has effects on cloud droplet formation. 

Scavenging of particles or gases may take place in clouds (rainout) by cloud droplets or below clouds(washout) by precipitation. A scavenging ratio or washout ratio W can be defined as... 

Atmospheric dust of desert origin can have important effects on the pH of rainfall. In a study in Northern Israel, cloud droplets with pH as low as 2.5 changed in pH as they grew by condensation into raindrops to pH as high as 8.2 (26). The condensing raindrops had apparently scavenged dust... 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

As mentioned after Equation (10), the equilibrium constant may be expressed when the reactants are in several phases. As an example, the equilibrium between ammonia in a large cloud droplet and in the gas phase, NH3(aq) and NH3(g), is described by the equilibrium constant expression... 

Cloud nucleation also has chemical consequences. The soluble material of the CCN introduces solute into cloud droplets which, in many instances, is a major and even dominant ingredient of cloud and rainwater. A simple but useful expression for the amount of solute from CCN is... 

The key features of soot are its chemical inertness, its physical and chemical adsorption properties, and its light absorption. The large surface area coupled with the presence of various organic functional groups allow the adsorption of many different materials onto the surfaces of the particles. This type of sorption occurs both in the aerosol phase and in the aqueous phase once particles are captured by cloud droplets. As a result, complex chemical processes occur on the surface of soot particles, and otherwise volatile species may be scavenged by the soot particles. 

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

SO2 oxidation to H2SO4 on aerosols, in cloud droplets, and by gas phase reactions following attack by OH. 

In its pure form, nitric acid is a liquid with a high vapor pressure (47.6 torr at 20°C), so that in the lower atmosphere HNO3 exists as a gas, in an aerosol or in a cloud droplet. When nitric acid reacts with a base a nitrate salt is produced, if... 

Twohy, C. H., Austin, P. H., and Charlson, R. J. (1989). Chemical consequences of the initial diffusional growth of cloud droplets a marine case. Tellus 4 (B), 51-50. 

Both models apply the same chemical scheme of mercury transformations. It is assumed that mercury occurs in the atmosphere in two gaseous forms—gaseous elemental HgO, gaseous oxidized Hg(II) particulate oxidized Hgpart, and four aqueous forms—elemental dissolved HgO dis, mercury ion Hg2+, sulphite complex Hg(S03)2, and aggregate chloride complexes HgnClm. Physical and chemical transformations include dissolution of HgO in cloud droplets, gas-phase and aqueous-phase oxidation by ozone and chlorine, aqueous-phase formation of chloride complexes, reactions of Hg2+ reduction through the decomposition of sulphite complex, and adsorption by soot particles in droplet water. 

One final interesting aspect of clouds and actinic fluxes is that inside the cloud droplets themselves, an... 

Gill, P. S., T. E. Graedel, and C. J. Weschler, Organic Films on Atmospheric Aerosol Particles, Fog Droplets, Cloud Droplets, Raindrops, and Snowflakes, Rev. Geophys. Space Phys., 21, 903-920 (1983). 

In addition to gas-phase chemistry, aqueous-phase chemistry discussed in Chapter 8.C.3 taking place in clouds can also be important in remote regions. For example, modeling studies by Lelieveld and Crutzen (1990) suggest that clouds may decrease the net production of 03 by uptake of H02, dissociation to H+ + 02, and reaction of 03 with 02 in cloud droplets. 

The chemical composition of fogs, clouds, and particles (see Chapter 9) varies as a function of particle size. For example, Figure 8.19 shows the concentrations of the major cations and anions measured in small and large cloud droplets at La Jolla peak in southern California (Collett et al., 1994, 1999). The large drops are enriched in soil and sea salt derived species such as Mg2+, Ca2+, Na+, and Cl whereas the smaller particles contain higher concentrations of sulfate and H+,... 

Gurciullo, C. S., and S. N. Pandis, Effect of Composition Variations in Cloud Droplet Populations on Aqueous-Phase Chemistry, J. Geophys. Res., 102, 9375-9385 (1997). 

Hering and Friedlander (1982) made similar observations for particle sulfate and attributed the smallest mode (referred to as the condensation mode) to formation from gas-phase S02 oxidation and the larger modes (the droplet mode) to oxidation in the condensed phase. Meng and Seinfeld (1994) have shown that the droplet mode particles cannot arise from growth of the smaller, condensation mode particles and propose that the condensation mode particles are activated to form fog or cloud droplets, followed by chemical reactions and subsequent evaporation to form the droplet particles. 

Particles in the accumulation range tend to represent only a small portion of the total particle number (e.g., 5%) but a significant portion (e.g., 50%) of the aerosol mass. Because they are too small to settle out rapidly (see later), they are removed by incorporation into cloud droplets followed by rainout, or by washout during precipitation. Alternatively, they may be carried to surfaces by eddy diffusion and advection and undergo dry deposition. As a result, they have much longer lifetimes than coarse particles. This long lifetime, combined with their effects on visibility, cloud formation, and health, makes them of great importance in atmospheric chemistry. 

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