Atmospheric wet and dry deposition

The element has three valence states and is found in the environment in the metallic form and in the form of various inorganic and organic complexes. The major features of the bio-geochemical cycle of mercury include degassing of mineral mercury from the lithosphere and hydrosphere, long-range transport in the atmosphere, wet and dry deposition to land and surface water, sorption to soil and sediment particulates, revolatilization from land and surface water, and bioaccumulation in both terrestrial and aquatic food chains. 

Leaf surfaces of plants are not passive receptors, but are in a dynamic relationship with the atmosphere. Wet and dry deposition of atmospheric contaminants may alter a plant s response to parasitic microorganisms in a variety of ways, depending on the sensitivity of the plant and the microorganism to the contaminant(s) in question. 

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. 

It is clear that both atmospheric and surface dusts are complex materials and not all that easy to describe. A summary is given in Fig. 2 of the sources of atmospheric and surface dusts and their inter-connection. Both natural and anthropogenic sources contribute to both dusts. The inter-connection between the two dusts is wet and dry deposition from the atmosphere to the ground, and the re-entrainment of surface dust through wind and human activity into the atmosphere. Dust is an important global component of our earth, and impinges on the wellbeing of people. 

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

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) ... 

BidlemanTF. 1988. Atmospheric processes. Wet and dry deposition of organic compounds are controlled by their vapor-particle partitioning. Environ Sci Technol 22(4) 361-367. 

Atmospheric deposition is an important source of mercury for surface waters and terrestrial environments that can be categorized into two different types, wet and dry depositions. Wet deposition during rainfall is the primary mechanism by which mercury is transported from the atmosphere to surface waters and land. Whereas the predominant form of Hg in the atmosphere is Hg° (>95%), is oxidized in the upper atmosphere to water-soluble ionic mercury, which is returned to the earth s surface in rainwater. In addition to wet deposition of Hg in precipitation, there can also be dry deposition of Hg°, particulate (HgP), and reactive gaseous mercury (RGM) to watersheds [9-11]. In fact, about 90% of the total Hg input to the aquatic environment is recycled to the atmosphere and less than 10% reaches the sediments [12]. By current consensus, it is generally accepted that sulfate-reducing bacteria (SRB)... 

In air, endrin is expected to be associated primarily with particulate matter, based on its low vapor pressure and high Koc (Kenaga 1980). However, small amounts of endrin in the atmosphere may exist in the vapor phase (Eisenreich et al. 1981). Because of its low solubility (200 pg/L, see Table 3-2), endrin would not be expected to be removed significantly from the atmosphere by wet deposition. Particle-adsorbed endrin will be removed from the atmosphere by both wet and dry deposition. In recent studies in the Great Lakes area, endrin was found in 5% of 450 wet deposition (rain/snow) samples collected between 1986-1991, at volume weighted mean concentrations ranging from 0.02 to 0.98 ng/L (ppt) (Chan et al. 1994). 

Apart from the economic significance of such loss there are potentially adverse effects on the environment arising from acidification of rain and soil. Ammonia may react with hydroxyl radicals in the atmosphere to produce NOx contributing to the acidification of rain (4). Wet and dry deposition of NH3/NH4+ inevitably contributes to soil acidification through their subsequent nitrification. This effect can be accentuated in woodland by absorption of aerosols containing NH4+ within the canopy followed by transport to the soil in stem flow (5). In more extreme cases, NH3 emission from feedlots, pig and poultry... 

DNB will exist entirely in the vapor phase in the atmosphere (Eisenreich et al. 1981). Based on a vapor pressure of 3.2x10 mm Hg at 25°C (see Table 3-2), 1,3,5-TNB is expected to exist partly in the vapor phase and partly in the particulate phase (Eisenreich et al. 1981). The transport of vapor phase 1,3-DNB from the atmosphere to the terrestrial surface is likely to occur mainly by wet deposition, while 1,3,5-TNB is likely to be transported by both wet and dry deposition (Bidelman 1988). 

Data regarding the fate and transport of thorium in the air are limited. Wet and dry deposition are expected to be mechanisms for removal of atmospheric thorium. The rate of deposition will depend... 

Nitric acid undergoes both wet and dry deposition rapidly and can be neutralized by ammonia, the major gaseous base found in the atmosphere. As discussed in Section E.2, the neutralization reaction is an equilibrium reaction so that by itself, this does not result in permanent removal from the atmosphere. However, as seen in this chapter and in Chapter 9, this acid-base reaction has some important implications for visibility in the atmosphere and for the nitrate concentrations found in respirable particles. 

Polybrominated Biphenyls. PBBs existed predominantly in the particulate phase in the atmosphere. Particulate phase PBBs are removed from the atmosphere by wet and dry deposition and should not travel long distances in the environment. In water, PBBs are expected to absorb strongly to suspended solids and sediment, and may bioconcentrate in aquatic organisms. The volatilization of PBBs from water to air is not expected to be important due to attenuation by adsorption in the water column. In soil, PBBs are adsorbed strongly and will be immobile. Volatilization of PBBs from soil to air is not important due to the low volatility of PBBs and strong adsorption of PBBs to soil. 

Polybrominated Diphenyl Ethers. In air, highly brominated PBDEs, which have low vapor pressures and exist in the particulate phase, will be removed from the atmosphere by wet and dry deposition. 

---