Rainout process

In addition to aerosol age, phosphorus aerosol speciation is also affected by the humidity of the ambient environment (Van Voris et al. 1987). Washout and rainout processes transport both the reaction products of vapor phase phosphorus and unreacted particles of phosphorus to water and land (Berkowitz et al. 1981). Because of its lower water solubility, physical state (gas), and slower reactivity, phosphine formed during the combustion of white phosphorus or released to the atmosphere from other media persists in the atmosphere longer than other reaction products. 

Many studies indicate that aerosols tend to be more readily scavenged by rainout processes such as condensation than by the washout process, such as capture by falling droplets. Thus sulphate and nitrate aerosols are thought by some to act as cloud condensation nuclei. In addition particulate material containing sulphate and nitrate are captured by cloud droplets via impaction, interception and Brownian diffusion. 

The chlorofluorocarbons (CFCs) have very long lifetimes in the troposphere. This is a consequence of the fact that they do not absorb light of wavelengths above 290 nm and do not react at significant rates with 03, OH, or N03. In addition to the lack of chemical sinks, there do not appear to be substantial physical sinks thus they are not very soluble in water and hence are not removed rapidly by rainout. While laboratory studies have shown that some of the CFCs decompose on exposure to visible and near-UV present in the troposphere when the compounds are adsorbed on siliceous materials such as sand (Ausloos et al., 1977 Gab et al., 1977, 1978), the lifetimes for CFC-11 and CFC-12 with respect to these processes have been estimated to be 540 and 1800 years, respectively (National Research Council, 1979). Similarly, an observed thermal decomposition when adsorbed on sand appears to be an insignificant loss process under atmospheric conditions. 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

Precipitation. Meteoric waters acquire different isotopic compositions by a variety of processes, the most important of which are temperature during condensation and degree of rainout in the air mass (17, 18). On the basis of hundreds of analyses, Craig (17) noted a linear relation between the 8D and 8lsO values of most meteoric waters specifically,... 

It is thought that rainout and washout may also be important removal mechanisms for soluble tropospheric gases with photochemical lifetimes longer than a few days. From the data for aerosols and fission debris, we can estimate a lifetime of at least one week, and possibly longer. It would also appear that this process is significant in only the first 5 km of the troposphere [Junge (128)]. 

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). 

There is a clear difference in needs for the on-line coupling of chemistry transport models and the off-line coupling. For the off-line coupled models it would advantageous to improve the quality of meteorological outputs, especially for fair weather conditions (including calm conditions and a focus on extreme situations). Parameters that can be used directly in off-line coupled models and that are important for processes like rainout should also be readily available. (It is important to remember that the couplings can be done in two directions not only from NWP to atmospheric CTM, but also from atmospheric CTM to NWP, see above). 

For the treatment of spread and rainout of the particles we must in this study assume similar behavior as during normal atmospheric conditions. This is probably not valid, as much sunlight absorption would take place in the atmosphere and not at the earth s surface. This would lead to profound and global changes in many meteorological processes. For this study, however, it is impossible to treat these problems satisfactorily and we must rely on a simple model, based on current experience. 

A passing cold front is heralded by clouds, a drop in temperature, and precipitation cooler air and clear skies occur behind the cold front (see Fig. 4-15b). The slower moving warm front is characterized by a more gradual lowering of cloud heights, followed by rain or snow (Fig. 4-15a). As air masses pass, so do their burdens of airborne chemicals. The clouds and precipitation formed along the front act as sinks for certain atmospheric chemicals because of rainout and washout processes. These processes, which remove particles, gases, and dissolved chemicals from the atmosphere and deposit them on Earth s surface, are discussed in Section 4.5. 

Wet deposition refers to processes in which atmospheric chemicals are accumulated in rain, snow, or fog droplets and are subsequently deposited onto Earth s surface. Wet deposition removes from the atmosphere many chemicals, including gases, whose rates of gravitational settling, impaction, or absorption are slow or even zero. When incorporation of chemicals into water droplets occurs within a cloud, the process is called rainout. When incorporation occurs beneath a cloud, as precipitation falls through the air toward Earth s surface, the process is called washout. 

FIGURE 4-39 The acid deposition process. Acid precursors, notably oxides of nitrogen and sulfur, are emitted to the atmosphere, primarily by fuel-burning equipment. Acid precursors are oxidized in the atmosphere to nitric and sulfuric acids by a variety of homogeneous and heterogeneous reactions. The acids are deposited by precipitation-related processes such as washout and rainout, by sorption of nitric acid vapor, and by dry deposition of acidic particulate material such as ammonium sulfate aerosol. (Stern et ah, 1984.)... 

H2S04 has an average daily rate of approximately 0.01/hr, whereas heterogeneous oxidation, in the presence of clouds, fog, or sea salt aerosols, can proceed at a rate greater than 0.3/hr (Luria and Sievering, 1991). Both rainout and washout processes contribute to the incorporation of H2S04 into precipitation. 

Figure 5.63. Schematic diagram of the physical and chemical processes related to the formation, growth, transport, and destruction of atmospheric aerosols C = coagulation, Ch = chemistry, D = diffusion, E = evaporation, Em = emission, G = condensation and growth, I = injection, N = nucleation, P = photolysis, S = sedimentation, W = washout and rainout. From Turco et al. (1979).

The process is more important in the troposphere since in that region H202 can dissolve in cloud water and be lost by rainout. 

Hydrometeors acquire trace components by scavenging processes occurring within clouds as well as below clouds. In the older literature, these are distinguished as rainout and washout, respectively. The term washout, however, has also been used to describe precipitation scavenging more... 

The above equations may be used to derive the residence time of a water-soluble gaseous constituent in the troposphere due to rainout. For simplicity we assume that the cloud-water concentration cs is approximately preserved during precipitation—that is, we ignore below-cloud processes such as the adjustment of temperature in and evaporation of water from falling rain drops. The average mass flux for the substance being removed by precipitation is... 

This time constant is 30 times greater than that associated with rainout of NH3. As a sink process, the reaction with OH is relatively unimportant. The absolute sink strength can be estimated from the global mass of gaseous NH3 in Table 9-2. A rate of 1 Tg N/yr is obtained. 

Wet deposition refers to the natural processes by which material is scavenged by atmospheric hydrometeors (cloud and fog drops, rain, snow) and is consequently delivered to the Earth s surface. A number of different terms are used more or less synonymously with wet deposition including precipitation scavenging, wet removal, washout, and rainout. Rainout usually refers to in-cloud scavenging and washout, to below-cloud scavenging by falling rain, snow, and so on. 

TREATABILITY/REMOVABILITY Process, Removable Range (%), Avg. Achievable Cone. (pg/L)) Gravity oil separation, not available, 150 Filtration, 0, negative removal Sedimentation, >57->97, <5 Granular activated carbon adsorption, >80, <0.02 Powdered activated carbon adsorption (based on synthetic wastewater), 97, 0.013 chlorination, 6 mg/L chlorine for 6 hr, initial concentration 68.74 ppb 56% reduction atmospheric losses are caused by gravitational settling and rainout... 

Because mercury adsorbs strongly onto surfaces, the equilibrium conditions between atmospheric and particulate matter are analyzed next. This is the most important process for the removal of mercury from the gaseous phase, but the limited information available cannot provide a strong basis for relationships which have been observed. Once dust settles by dry fallout or rainout, mercury goes back to soil or is washed into water bodies. 

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