Atmospheric rainout

Retallack [2] computed relative acidification for the Brownie Butte boundary bed by using the impact bed as the parent material, and obtained a value of 0.054 meq cm. Since typical late Cretaceous/early Paleocene paleosols have acid consmnption rates of 0.01-0.02 meq cm yr" , this is evidence for enhanced leaching from the boundary bed relative to the impact bed. Because the boundary bed was emplaced -minutes to hours after tlie impact [33] and the bulk of the impact bed (including shocked quartz) was emplaced -hours to days after tlie impact bed, the boundary bed may have experienced somewhat greater acid deposition. This was true only if significant acid deposition occurred in tlie interval between boundary and impact bed emplacement. In the normal atmosphere rainout of acid in the troposphere occurs on timescales of days in the post-impact atmosphere rainout may have occurred soon after the unpack once the atmosphere cooled. Because of the uncertainties in such timescales and the possibility of different parent material compositions for the impact and boundary beds, we do not consider tlie relative acidification of the two beds further. 

Deposition. The products of the various chemical and physical reactions in the atmosphere are eventually returned to the earth s surface. Usually, a useful distinction is made here between wet and dry deposition. Wet deposition, ie, rainout and washout, includes the flux of all those components that are carried to the earth s surface by rain or snow, that is, those dissolved and particulate substances contained in rain or snow. Dry deposition is the flux of particles and gases, especially SO2, FINO, and NFl, to the receptor surface during the absence of rain or snow. Deposition can also occur through fog, aerosols and droplets which can be deposited on trees, plants, or the ground. With forests, approximately half of the deposition of SO(, NH+,andH+ occurs as dry deposition. 

Settling and rainout are important mechanisms of contaminant transfer from the atmospheric media to both surface soils and surface waters. Rates of contaminant transfer caused by these mechanisms are difficult to assess qualitatively however, they increase with increasing soil adsorption coefficients, solubility (for particulate contaminants or those adsorbed to particles), particle size, and precipitation frequency. 

Areas affected by significant atmospheric concentrations of contaminants exhibiting the foregoing physical and chemical properties should also be considered as potentially affected by contaminant rainout and settling to surface media. Contaminants dissolved in rainwater may percolate to ground water, run off or fall directly into surface waters, and adsorb to... 

Examples of the need for multimedia models are found in contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted into the atmosphere as trace impurities with the products of coal combustion. The organics have low vapor pressure and partially condense on emitted particulates in a stack plume. The particulates are transferred to the soil by dry deposition, rainout or washout. The metals manifest... 

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. 

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. 

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. 

Typical values of scavenging ratio lie within the range 300-2000. Scavenging ratios are rather variable, dependent upon the ehemieal nature of the trace substance (particle or gas, soluble or insoluble, etc) and the type of atmospheric precipitation. Incorporation of gases and particles into rain can occur both by in-cloud scavenging (also termed rainout) and below-cloud scavenging (termed washout). 

The source of HNO is the photochemical oxidation of NC, and the sinks are rainout and dry deposition of HN03 and its related nitrate salts. While HNO has been observed in a smog environment [Price and Stephens (197)], it has not been found in the normal atmosphere. Levy (153,154) has predicted that HNO should exist in the troposphere, with a number density as high as 30 ppb. Eriksson (58) has determined the latitude dependence of nitrate precipitation to some extent, it follows the behavior suggested by Robinson and Robbins (216) for NO... 

Using Eriksson s (58) estimates of rainout and adding 25% for dry deposition, we find a residence time of approximately one month for atmospheric NH-j. The inclusion of gaseous deposition as a loss mechanism [Robinson and Robbins (216)] gives a residence time of one week. It is believed that the seasonal and altitude variations in the NH number densities are due to variations in its biological source at the ground and its short residence time in the troposphere. 

As was learned in Section II.D, not much is known quantitatively about either the global circulation of sulfur compounds or their atmospheric photochemistry. It is assumed that atmospheric sulfides and oxides are produced at the surface and then converted to sulfuric acid and sulfate salts, which are removed from the atmosphere by rainout and dry deposition. 

The tropospheric sulfur chemistry is different. Unlike the nitrogen and carbon chemistry, where combustion is an insignificant source, the combustion source of SO2 appears to be very important. While OH reactions can be shown to convert sulfides to SO2, it is not clear that normal atmospheric chemistry is important in the next step—the conversion of S02 to H2SO, which is then removed from the atmosphere by rainout. It has also been suggested that a large amount of SO2 is removed directly by rainout. Unfortunately we have the fewest data, both kinetic and atmospheric, on sulfur compounds. Most of the kinetic data we do have are at high temperatures, and most of the atmospheric data are for polluted environments. 

In order to determine the importance of rainout, measurements of n(HN03) in the surface atmosphere are necessary. 

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). 

To calculate the rainout rate of H2CO, the total atmospheric burden or column density (molecules cm ) must be known. The H2CO column density as a function of H2, CO2, and solar UV flux levels is summarized in Table 4. The surface production and loss rates, including the loss due to rainout of H2CO as a function of H2, CO2, and solar UV levels, are summarized in Table 5. The last entries in this table summarize the rainout fluxes of H2CO (molecules cm sec ). The rainout flux is the product... 

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. 

For different rainout rates we have calculated widely different transmission of solar radiation through the atmosphere. For slow rainout rates, the transmission may remain below 10% for a whole month (see Figure 2), which means that sunlight transmission would be less than 1%, if twice the amount of aerosol were to be dumped in the atmosphere. Because of the uncertain analysis of the amounts of fuels burnt and particulate matter formed from different materials, and considering also the simplicity of the model adopted in this study, this possibility may not be discounted. A prolonged stay of aerosol particles in the atmosphere would occur if much particulate matter rapidly reached the stratosphere because of the intense solar heating of the smoke clouds. 

---