Dry versus Wet Deposition

One can often be misled by the relative slow removal of material by dry deposition and rapid removal by wet deposition into underestimating the significance of the dry removal pathway that operates continuously. 

Note that impaction of cloud or fog droplets can lead locally to significant fluxes. Lovett et al. (1982) reported impaction fluxes at a high elevation forest of 280, 160, and 240 mmol m-2 yr-1 for sulfate, nitrate, and H+, respectively. 

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To determine the contribution of dry versus wet deposition, Goodwin initiated laboratory tests using synthetic rainfall of different pH values. Dry deposition was attained by setting panels in a louvered box outdoors. Rain intensities between 0 and 12 mm/h were used. This technique made it possible to vary sulfate concentration independently of pH. Rolled pure zinc panels were used for all tests. Atmospheric corrosion rates of zinc is Scandinavia, when only dry deposition was allowed, were measured to be 12 g/m / year based on an exposure period of 2.5 months. Exposure of these dry panels to artificial rainfalls with pH values of 4.5 and 3.5 decreased their corrosion rates. Only when rainfall pH was lowered to 2.5 did zinc corrosion rate increase over that seen with solely dry deposition (Fig. 2.7). 

Gatz and Chu (1986) reported that the great majority of lead in rainwater samples is in soluble form, about 83%, an observation of importance to the subsequent environmental fate of lead in receiving environmental compartments. Hicks (1986) reported that the amounts of lead removed by dry versus wet deposition phenomena differed greatly, ranging in ratio from 0.4 to 1.8. 

The relative importance of dry versus wet precipitation is essentially dependent upon the distance from the source at which acid pollutants are deposited. The closer to the source the more likely dry deposition is to take place. 

Environmental Fate. CDDs are subject to atmospheric transport and both wet and dry deposition (Kieatiwong et al. 1990). They are partitioned to air, water, sediment, and soil, and they accumulate in both aquatic and terrestrial biota. CDDs can volatilize to the atmosphere from water and soil surfaces. They adsorb strongly to soils and are not likely to leach into groundwater (Eduljee 1987b). In the aquatic environment, CDDs partition to sediment or suspended particulates. TCDD, HpCDD, and OCDD are subject to photolysis in air, water, and soil (Plimmer et al. 1973). 2,3,7,8-TCDD is biodegraded very slowly in soil and thus is likely to persist in the soil. A better understanding of environmental behavior of CDDs is needed with respect to the importance of vapor-phase versus particulate transport, the... 

Figure 3.6 Wet versus dry deposition of oxidized nitrogen in the larger North Atlantic...

At our Laboratory, a series of experiments were carried out to empirically evaluate our dry fallout data. For 13 consecutive months in 1974-1975> wet, dry and bulk collections from New York City were analyzed for Sr. The results are shown in Fig. 4 the sum of the wet plus dry data are plotted versus the bulk or total collection. The straight line is a least squares fit. Clearly, the relationship is linear with a high (0.98) correlation coefficient. These results, plus the information derived from the worldwide deposition studies, strongly indicate that the bucket used in our wet/dry instrument quantitatively collects the dry Sr deposit. There is an increasing body of evidence that this conclusion holds equally well for aerosols of like and even smaller particle size. 

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