Wet, deposition

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. 

Gases and vapors in the atmosphere can be removed by dissolving into raindrops. At equilibrium, the chemical concentration in the raindrops is 

In the case of washout of sulfur dioxide (S02), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous S02 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for 

Estimate the rate (per square meter of area) of wet deposition of sulfur by washout of airborne S02 to a watershed. Assume that the S02 concentration is 20 pg/m3 (as sulfur) at the beginning of each rainstorm and that the watershed receives 1 m of precipitation per year, in 50 equals storms of 10-hr duration each, from clouds 2000 ft above the ground. 

Note that this purely empirical expression for A necessarily takes into account the net effect of all processes affecting S02 removal from air (i.e., dissolution into water droplets, hydration, oxidation, and ionization). Given that I must be expressed in millimeters per hour in Eq. [4-29]  

Wet deposition which consists of rainout (within cloud scavenging) and washout (below cloud scavenging) may be considered as an exponential decay process. Thus  

The wet deposition due to precipitation is determined by the fractional rate of removal of radionuclide by rain, X in s according to the following equation 

If T is the mean residence time of a radioactive nuclide associated with aerosol particles, in s or in d, that is the inverse of the fractional rate of removal of the radionuclide, X in s or d then Equation (3.4) becomes for the washout ratio 

By taking into consideration the mass of tropospheric air per m of earth s surface, pH = 9000 kgm , as p = 1.2 kgm the air density and H = 7500 m the vertical extent of the plume, the precipitation rate p = 3.6x 10 kgm s = 3.6 kgm d over the northern hemisphere (Garland and Playford, 1991), the mean residence time of tropospheric aerosol particles associated with a radionuclide t = 8 d = 6.9 x 10 s (Papastefanou and Bondietti, 1991 Papastefanou and loannidou, 1995), and the fractional rate of removal of a radionucUde 

ApSimon et al. (1989) used for the fractional rate of removal of a radionuclide by rain. A., the following formula  

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Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Pesticides can be transported away from the site of appHcation either in the atmosphere or in water. The process of volatili2ation that transfers the pesticide from the site of appHcation to the atmosphere has been discussed in detail (46). The off-site transport and deposition can be at scales ranging from local to global. Once the pesticide is in the atmosphere, it is subject to chemical and photochemical processes, wet deposition in rain or fog, and dry deposition. 

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. 

Figure 2 Comparison of measured wet deposition of ammonium at Rothamsted, England with model estimates by Asman et al for regions whieh assume ehanges in emissions are only due to differenees in animal numbers. (Taken from Sutton et al ).

The measurement data for reduced atmospheric nitrogen in the UK are of variable quality, largely because monitoring ambient NHj is subject to very large variations." However, the wet deposition monitoring data provide a good... 

Acid deposition refers to the transport of acid constituents from the atmosphere to the earth s surface. This process includes dry deposition of SO2, NO2, HNO3, and particulate sulfate matter and wet deposition ("acid rain") to surfaces. This process is widespread and alters distribution of plant and aquatic species, soil composition, pH of water, and nutrient content, depending on the circumstances. 

Fig. 10-12. Atmospheric processes involved in acidic deposition, The two principal deposition pathways are dry deposition (nonrain events) and wet deposition (rain events).

The two components of acidic deposition described in Chapter 10 are wet deposition and dry deposition. The collection and subsequent analysis... 

Except for fine particulate matter (0.2 /xm or less), which may remain airborne for long periods of time, and gases such as carbon monoxide, which do not react readily, most airborne pollutants are eventually removed from the atmosphere by sedimentation, reaction, or dry or wet deposition. 

Uniform mixing in the vertical to 1000 m and uniform concentrations across each puff as it expands with the square root of travel time are assumed. A 0.01 h transformation rate from SO2 to sulfate and 0.029 and 0.007 h" dry deposition rates for SO2 and sulfate, respectively, are used. Wet deposition is dependent on the rainfall rate determined from the surface obser% ation network every 6 h, with the rate assumed to be uniform over each 6-h period. Concentrations for each cell are determined by averaging the concentrations of each time step for the cell, and deposition is determined by totaling all depositions over the period. 

The EURMAP model has been useful in estimating the contribution to the concentrations and deposition on eveiy European nation from every other European nation. Contributions of a nation to itself range as foUows SO2 wet deposition, 25-91% SO2 dry deposition, 31-91% sulfate wet deposition, 2-46% sulfate dry deposition, 4-57%. 

Dry deposition, although not as efficient as the wet removal process, is continuous while wet deposition occurs only during... 

Deposition is the atmospheric removal process by which gaseous and particulate contaminants are transferred from the atmosphere to surface receptors - soil, vegetation, and surface waters (22,27,28, 32). This process has been conveniently separated into two categories dry and wet deposition. Dry deposition is a direct transfer process that removes contaminants from the atmosphere without the intervention of precipitation, and therefore may occur continuously. Wet deposition involves the removal of contaminants from the atmosphere in an aqueous form and is therefore dependent on the precipitation events of rain, snow, or fog. 

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. 

Fig. 4-13 Calculated and observed annual wet deposition of sulfur in mgS/m per year. (Reprinted from "Atmospheric Environment," Volume 30, Feichter, J., Kjellstrom, E., Rodhe, H., Dentener, F., Lelieveld, and Roelofs, G.-J., Simulation of the tropospheric sulfur cycle in a global climate model, pp. 1693-1707, Copyright 1996, with permission from Elsevier Science.)...

The flux of particles in the other direction, deposition on the ocean surface, occurs intermittently in precipitation (wet deposition) and more continuously as a direct uptake by the surface (dry deposition). These flux densities may be represented by a product of the concen-... 

We begin our analysis by comparing the surface fluxes. According to the indicated partitioning factors, 74% of the 11 Mg DMS-S/m /h emitted from the ocean surface should be returned as nss-SO in rain. This leads to a predicted wet deposition flux of nss-SO of 8.1 Mg S/(m /h), which is 37% lower than the measured flux of 13 Mg S/(m /h). Since the estimated accuracy of the DMS emission flux is 50% (Andreae, 1986), this is about as good agreement as can be expected. It indicates that our "closed system" assumption is at least a reasonable first approximation. (A more sophisticated treatment would consider sulfur oxida-... 

Impurities travel from atmosphere to ice sheet surface either attached to snowflakes or as independent aerosols. These two modes are called wet and dry deposition, respectively. The simplest plausible model for impurity deposition describes the net flux of impurity to ice sheet (which is directly calculated from ice cores as the product of impurity concentration in the ice, Ci, and accumulation rate, a) as the sum of dry and wet deposition fluxes which are both linear functions of atmospheric impurity concentration Ca (Legrand, 1987) ... 

Methyl parathion can be released to surface waters by storm runoff from sprayed fields atmospheric deposition following aerial application (wet deposition from rain and fog water) waste water releases from formulation, manufacturing, or processing facilities and spills. 

The methyl parathion released to the atmosphere can be transported back to surface water and soil by wet deposition. Methyl parathion that is released to the atmosphere can also be transformed by indirect photolysis to its oxygen analog, methyl paraoxon, by oxidation with photochemically produced oxygen radicals. However, methyl parathion is not expected to undergo significant transformation to methyl paraoxon. 

In the same study, at the agricultural site in Mississippi, the total wet deposition of methyl parathion during the 6-month study was 1,740 pg/m (89% of the total wet depositional loading at that site), greater than the totals for each of the other 46 compounds monitored in the study (Majewski et al. 2000). Methyl parathion was not detected in the wet deposition at the Iowa site and was detected only once at each of the two Minnesota sites (Majewski et al. 2000). 

Trichloroethylene has been detected in a number of rainwater samples collected in the United States and elsewhere (see Section 5.4.2). It is moderately soluble in water, and experimental data have shown that scavenging by rainwater occurs rapidly (Jung et al. 1992). Trichloroethylene can, however, be expected to revolatilize back to the atmosphere after being deposited by wet deposition. Evaporation from dry surfaces can also be predicted from the high vapor pressure. 

The total wet deposition flux consists of 2 contributory factors. The first derives from the continuous transfer of Hg to cloud water, described by chemistry models. There are 2 limiting factors 1) the uptake of gas phase Hg(0), which is regulated by the Hemy s corrstant and 2) the subsequent oxidation of Hg(0) to Hg(ll), which is governed by reaction rate constants and the irritial concentratiorrs of the oxidant species. The total flirx depends on the hquid water content of the cloud and the percentage of the droplets in the cloud that reach the Earth s surface. 

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