Dry deposition process

Sulfur oxides (SO,) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. It is a colorless gas that can be detected by taste and smell in the range of 1, (X)0 to 3,000 uglm. At concentrations of 10,000 uglm , it has a pungent, unpleasant odor. Sulfur dioxide dissolves readily in water present in the atmosphere to form sulfurous acid (H SOj). About 30% of the sulfur dioxide in the atmosphere is converted to sulfate aerosol (acid aerosol), which is removed through wet or dry deposition processes. Sulfur trioxide (SO3), another oxide of sulfur, is either emitted directly into the atmosphere or produced from sulfur dioxide and is readily converted to sulfuric acid (H2SO4). 

Two observationally constrained box-models, based on the Master Chemical Mechanism and with different levels of chemical complexity, have been used to study the HOx radical chemistry during the SOAPEX-2 campaign, which took place during the austral summer of 1999 (January-February) at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. The box-models were constrained to the measured values of long lived species and photolysis rates and physical parameters (NO, NO2, O3, HCHO, j(01D), j(N02), H2O and temperature). In addition the detailed model was constrained to the measured concentration of CO, CH4 and 17 NMHCs, while the simple model was additionally constrained only to CO and CH4. The models were updated to the latest available kinetic data and completed with a simple description of the heterogeneous uptake and dry deposition processes. 

Processes (a) to (d) are classified as dry deposition, process (e) as wet deposition. Occult precipitation, which is the impaction of cloud or fog droplets onto vegetation, may be classified either way. 

Airborne particles may be delivered to surfaces by wet and dry deposition. Several transport mechanisms, such as turbulent diffusion, precipitation, sedimentation, Brownian diffusion, interception, and inertial migration, influence the dry deposition process of airborne particles. Large particles (dNIOAm) are transported mainly by sedimentation hence, large particulate PAHs tend to be deposited nearer the sources of emission Small particles (dblAm), which behave like gases, are often transported and deposited far from where they originated (Baek et al., 1991 Wu et al., 2005). 

Wet removal processes are further controlled by precipitation types and rates. Dry deposition processes on surfaces are affected by atmospheric transport rates that mix fresh pollutant into the surface boundary layers and by the physical properties of particles. For the Eastern U.S., the approximate annual deposition rates of sulfate can be compared as follows (Table III), considering that deposition flux is the product of a concentration and a velocity of deposition (Vd) (20) ... 

An important parameter used to quantify dry deposition processes is the velocity of deposition (Kg) (Chamberlain, 1953). Vg is defined as the downward flux of aerosol or gas to a vegetation or soil surface, normalised to the ambient atmospheric gas or aerosol concentration above that surface. In the case of radionuclide deposition processes flux and concentration are, respectively, measured in units of radioactivity per unit area and volume, hence... 

Deposition of PCBs from the atmosphere includes wet and dry depositional processes. Estimates of these processes can be obtained from measured air concentrations in the vapour and particulate phases which are applied to mass transfer equations, or they can be measured directly. 

In August, 1983, members of the Research Staff of Ford Motor Company carried out a field experiment at two rural sites in southwestern Pennsylvania involving various aspects of the acid deposition phenomenon. This presentation will focus on the wet (rain) deposition during the experiment, as well as the relative importance of wet and dry deposition processes for nitrate and sulfate at the sites. Other aspects of the experiment have been discussed elsewhere the chemistry of dew and its role in acid deposition (1), the dry deposition of HNO3 and SO2 to surrogate surfaces (2), and the role of elemental carbon in light absorption and of the latter in visibility degradation (3). 

Dry deposition processes are best understood by considering a resistance analogue. In direct analogy with electrical resistance theory, the major resistances to deposition are represented by three resistors in series. Considering the resistances in sequence, starting well above the ground, these are as follows ... 

It includes wet and dry deposition processes and a simplified photochemical smog mechanism that is only applicable to regional and urban pollution. 

Fig. 4. The dry deposition process A free atmosphere B laminar boundary layer C plant surface.

If released to air, haloperidol will exist primarily in the particulate phase physical removal from air will occur through wet and dry deposition processes. 

Crutzen 1983 Dawson 1977 Galbally and Roy 1983 Moller and Schieferdecker 1985). The reaction of atmospheric ammonia with acidic substances in the air results in the formation of ammonium aerosols that can subsequently be removed from the atmosphere by dry or wet deposition. In general, dry deposition processes predominate where there are high amounts of NH3 emissions where NH3 emissions are lower, wet deposition of particulate NH/ predominates (Asman et al. 1998). 

Because the processes by which a molecule, or particle, is transported to the surface of the earth and absorbed by the surface are quite complex, the dry deposition process is represented by an overall transfer coefficient. It is generally assumed that the dry deposition flux is proportional to the local pollutant concentration [at a known reference height (zr), typically 10 m], resulting in the expression F = — vdC, where F represents the dry deposition flux (the amount of pollutant depositing to a unit surface area per unit time) and C is the local pollutant concentration at the reference height. The proportionality constant, vd, has units of length per unit time and is known as the deposition velocity. 

Hg has an average residence time in the atmosphere of between 6 months and 2 years, and will thus be distributed fairly evenly in the troposphere. Oxidized mercury (Hg(II)) may be deposited relatively quickly by wet and dry deposition processes, leading to a residence time of hours to months (Lindqvist and Rodhe 1985). However, the atmospheric residence time for some... 

The advantage of the deposition velocity representation is that all the complexities of the dry deposition process are bundled in a single parameter, vd. The disadvantage is that, because vd contains a variety of physical and chemical processes, it may be difficult to specify properly. The flux F is assumed to be constant up to the reference height at which C is specified. Equation (19.1) can be readily adapted in atmospheric models to account for dry deposition and is usually incorporated as a surface boundary condition to the atmospheric diffusion equation. 

The final step in the dry deposition process is actual uptake of the vapor molecules or particles by the surface. Gaseous species may absorb irreversibly or reversibly into the surface particles simply adhere. The amount of moisture on the surface and its stickiness are important factors at this step. For moderately soluble gases, such as S02 and O3, the presence of surface moisture can have a marked effect on whether the molecule is actually removed. For highly soluble and chemically reactive gases, such as HNO3, deposition is rapid and irreversible on almost any surface. Solid particles may bounce off a smooth surface liquid particles are more likely to adhere upon contact. 

Pollutants are removed from the atmosphere by rain and snow and are transferred to soils, natural waters, and vegetation by wet and dry deposition processes. Through these processes, plants are exposed periodically to substances dissolved in atmospheric precipitation and to gaseous pollutants. The major soluble constituents in rain and snow in the eastern U.S. are hydrogen, sulfate, and nitrate ions and there is concern over the environmental influence of these substances, particularly acidity (Jacobson et al., 1976). Knowledge of plant nutrition and response to pollutants raises the question of whether it is necessary to consider the supply of nitrate and sulfate in rain and the concentration of ozone in the atmosphere when determinations are made of the effects of acidic precipitation on vegetation of the eastern U.S. 

Acid rain The deposition of strongly acidic pollutants, especially sulfuric and nitric acids, from the atmosphere to the earth s surface. The term strictly refers only to wet deposition (in rain, snow, etc.) but is often used to include dry deposition processes (i.e., due to settling or turbulent transfer). 

There has been considerable debate over the linearity or otherwise of relationships between nitrate and sulfate and their respective precursor gases, NOx and sulfur dioxide. Currently, the majority of data support the idea of a more or less linear reduction in sulfate with reductions in SO2 emissions, which have been quite marked in most developed countries. Reductions in NOx, taking account of all sources, have been very much less, and in Western Europe the inorganic particles are changing from an aerosol dominated by sulfate to one dominated by nitrate. In North America, nitrate dominates the aerosol on the West Coast due to the predominant influence of road traffic emissions within this region, while on the East Coast, sulfate tends to predominate, in part due to the very low ammonia levels, which preclude the formation of substantial ammonium nitrate concentrations, leaving the nitrate in the vapor phase as nitric acid, which is subject to relatively rapid wet and dry deposition processes. 

Pollutants particularly suited to transport over long distances within the atmosphere are those that are removed only ineffectively by chemical reactions or wet and dry deposition processes. Examples include fine particulate matter (PM2 5) and ozone. In the case of fine particles, the deposition processes are inefficient and the particles may remain airborne for many days. In the case of ozone, dry deposition is relatively efficient but operates mainly dtrring daytime, when photochemistry rapidly replaces depleted ozone. At nighttime, dry deposition occurs from within a shallow, stable layer of air in contact with the grotmd, while above that layer the ozone remains imaffectedby dry deposition and is depleted by only rather slow chemical reaction processes. Once stmshine mixes the atmosphere... 

Clearly, several uncertainties affect these estimates. The masses of the released pollutants and the release duration are uncertain. The transformation and dry deposition processes were not modeled in detail. The dispersion computations do not include the changes of meteorological conditions with time and location along each trajectory. 

There are two very important reasons for studying lead in air. Firstly, inhaled lead may pass via the respiratory system to the bloodstream and hence contribute to the lead exposure of the population. Secondly, airborne lead is progressively removed from the atmosphere by wet and dry deposition processes causing contamination of other environmental media. 

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