Cloud and Fog Formation

The ability of a given particle to become activated depends on its size and chemical composition and on the maximum supersaturation experienced by the particle. If, for example, the ambient RH does not exceed 100%, no particle will be activated and a cloud cannot be formed.2 In this section we will examine the mechanisms by which clouds are created in the atmosphere. A necessary condition for this cloud formation is the increase in the RH of an air parcel to a value exceeding 100%. This RH increase is usually the result of cooling of a moist air parcel. Even if the water mass inside the air parcel does not change, its saturation water vapor concentration decreases as its temperature decreases, and therefore its RH increases. 

FIGURE 15.9 Critical supersaturation as a function of the particle dry diameter for different contents of insoluble material. The soluble material is (NH4)2S04- 

The ability of a given particle to become activated depends on its size and chemical composition and on the maximum supersaturation experienced by the particle. If, for example, the ambient RH does not exceed 100%, no particle will be activated and a cloud cannot be formed. In this section we will examine the mechanisms by which clouds are created in 

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The second class of mists or fogs is that produced by some process in which the incipient liquid phase is introduced as a vapor and forms droplets as a result of some equilibrium condensation process, or the liquid is produced as a result of some chemical reaction. The former mechanism includes, of course, cloud and fog formations, while the latter corresponds to some chemical fogs and mists. 

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). 

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. 

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. 

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). 

The HMSA formed acts as a S(1V) reservoir protecting it from further oxidation, and its formation explains the high S(IV) concentrations that have been observed in clouds and fogs. The kinetics of the HCHO-S(IV) reaction in the pH range 0-3.5 has been shown to be slow, so that S(IV) and HCHO cannot reach the equilibrium state (Munger et al., 1984). The effectiveness of HMSA as a S(IV) reservoir depends critically on its resistivity to OH attack,... 

Of course, occasionally water molecules have sufficient energy to leave the surface, resulting in evaporation. Conversely, sometimes water molecules in the gaseous state strike the surfece of a drop of water and have insufficient energy to leave again. The result is condensation. The competing rates of evaporation and condensation lead to the formation of clouds and fog, to cloudy mirrors after a shower and iced-up windows on a winter s day. 

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982 John et al. 1990 Meng and Seinfeld 1994). 

The near pH independence can also be viewed that the pH dependences of the S(IV) solubility and of the reaction rate constant cancel each other. The reaction is very fast and indeed both field measurements (Daum et al., 1984) and theoretical studies (Pandis and Seinfeld, 1989b) have suggested that, as a result, H202(g) and 802(g) rarely coexist in clouds and fogs. The species with the lowest concentration before cloud or fog formation is the limiting reactant and is rapidly depleted inside the cloud or fog layer. 

Precipitation, clouds, and fog all result from condensation. They form as air cools when it rises or passes over cooler land or water. Their formations require a second factor, microscopic particles suspended in the air called condensation nuclei. These can be particles, such as soot and dust, or aerosols, such as sulfur dioxide and nitrogen oxide, on which water vapor condenses. In some circumstances, warm air can settle on top of cooler air, which is called a temperature inversion. Figure 12.28 shows fog trapped in a mountain valley by such an inversion. 

Formaldehyde is directly emitted into the air from vehicles. It is released in trace amounts from pressed wood products such as particleboard and plywood paneling, from old sick bnildings, and from cotton and cotton-polyester fabrics with selected crosslink finishes. Formation of formaldehyde has been observed in some frozen gadoid fish due to enzymic decomposition of the additive trimethylamine oxide (Rehbein 1985). Its concentration can build up during frozen storage of fish (Leblanc and Leblanc 1988 Reece 1985). It occurs in the upper atmosphere, cloud, and fog it also forms in photochemical smog processes. 

The high surface tension of water plays a major role in the formation of drops in clouds and fog. Surface tension is related to the cohesive properties of water, or in other words, water is attracted to other water. This property results in the growth of cloud droplets into bigger drops after collision. Water can also be attracted to other materials. This is called adhesion, a property of water which leads to capillary action, an important transport process for plants (besides transpiration). 

Surface active organic compounds, often present in atmospheric wet aerosols (i.e., clouds and fogs), alter the surface tension of the tiny liquid droplets. A large decrease of the surface tension value may change the processes of droplet nucleation and growth. As a consequence, the changes in droplet population significantly affect the cloud albedo as well as the formation of atmospheric precipitation. 

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