Removal From The Atmosphere Wet And Dry Deposition

While the central thrust of this book involves the chemical transformations occurring in air, it is clear

Local time

FIGURE 2.20 Schematic of mixing processes in atmosphere close to the earth's surface as a function of time of day. (Adapted, with kind permission from Kluwer Academic Publishers, from R. B. Stull, 1988, An Introduction to Boundary Layer Meteorology, Fig. 1.7. © 1988 by Kluwer Academic Publishers.)

Local time

FIGURE 2.20 Schematic of mixing processes in atmosphere close to the earth's surface as a function of time of day. (Adapted, with kind permission from Kluwer Academic Publishers, from R. B. Stull, 1988, An Introduction to Boundary Layer Meteorology, Fig. 1.7. © 1988 by Kluwer Academic Publishers.)

that physical removal at the earth's surface is important for many primary and secondary pollutants.

Meteorology, however, also plays important roles other than mere transport of pollutants as they react. As we have seen, the formation of acids from their precursors involves reactions not only in the gas phase but also in the liquid phase (e.g., in clouds, fogs, and dews) and perhaps on the surfaces of solids as well. Both particles and gases can be deposited at the earth's surface in two ways, termed dry deposition or wet deposition, depending on the phase in which a species strikes the earth's surface and is taken up. Thus pollutants may be dissolved in clouds, fog, rain, or snow; when these water droplets impact the earth's surface (which includes not only soil but grass, trees, buildings, etc.), it is termed wet deposition. However, pollutants in the form of either gases or small particles can also be transported to ground level and absorbed and/or adsorbed by materials there without first being dissolved in atmospheric water droplets; this is called dry deposition. It should be noted that the surface itself may be wet or dry; the term dry deposition only refers to the mechanism of transport to the surface, not to the nature of the surface itself.

Because of the highly variable nature of precipitation events, quantitatively estimating wet deposition of pollutants is difficult. In addition to meteorological factors, parameters such as the solubility of the pollutant in ice, snow, and rain and how this varies with temperature and pH, the size of the water droplets, and the number present must also be considered; for example, snow may scavenge some species more efficiently than rain.

As an approximation, the rate of wet deposition of a pollutant is sometimes taken as AC, where C is the pollutant concentration and A is known as a washout coefficient which is proportional to the precipitation intensity (Shaw, 1984).

Dry deposition can also be a very important mechanism for removing pollutants from the atmosphere in the absence of precipitation. Indeed, even in such places as eastern England, the ratio of dry to wet removal of S02 has been estimated to be ~ 2:1 (Davies and Mitchell, 1983). If this is the case, then in arid and semiarid regions such as much of the western United States dry deposition is clearly important.

Dry deposition is usually characterized by a deposition velocity, V. The net flux (/•') of a species to the surface is proportional to the concentration of that species in air, [S]; i.e., F a [S], The deposition velocity is just the proportionality constant relating flux and concentration, i.e.,

where [S] is the concentration at some reference height z. By convention, the deposition velocity is a positive number, but fluxes toward the surface are taken as negative; hence the negative sign in (S). The amount of the species deposited per unit area per second in a geographical location, that is, the flux, can be calculated if the deposition velocity and the pollutant concentration are known. The deposition velocity is also frequently related to a resistance r:

By analogy to electrical systems, the resistance r can be thought of as consisting of several components. For convenience, three such components are often defined, a surface resistance (rsurf), which depends on the affinity of the surface for the species, a boundary layer resistance (rhoun), which depends on the molecular diffusiv-ity of the gas in air, and a gas-phase resistance (r ), which depends on the micrometeorology that transports the gas to the surface:

The gas-phase resistance depends on the height (z) above the earth's surface, as does the concentration of the pollutant; as a result the deposition velocity is also a function of height.

Figure 2.21 schematically depicts the dry deposition of a pollutant to a typical surface in the form of resistances (Lovett, 1994; Wesely and Hicks, 1999). In this case, the surface resistance rsurl has been broken down even further into a combination of parallel and series resistances (rs, rm, rKt, rsoil, rwalcr, etc.). Since leaves may absorb pollutants either through stomata or through the cuticles, the absorption into the leaf is represented by two parallel resistances, rcl for the cuticular resistance and rs for the stomatal resistance, which is in series with a mesophyllic resistance rm. Also shown are resistances for uptake into the lower part of the plant canopy and into water, soil, or other surfaces.

The relative importance of gas-phase and surface resistances depends on the nature of the pollutant and the surface as well as the meteorology (Shaw, 1984; Unsworth et al., 1984; Chameides, 1987; Wesely and Hicks, 1999). The gas-phase resistance (rgas) is determined by the vertical eddy diffusivity, which depends on the evenness of the surface and the meteorology, for example, wind speed, solar surface heating, and so on. The surface resistance (rsurl) depends on the detailed characteristics of the surface (e.g., type, whether

Atmospheric Source Ca

Aerodynamic Gas Resistance (rgas)

Stagnant film of air

Boundary Layer (rb0

Stomatal (rs) Mesophyll (rm)

" Cuticular (rct)

Aerodynamic r to lower canopy

Lower canopy vWWV^-®

Aerodynamic r to ground --

Soil

(rsoil)

(rwater)

Cc (r0ther)

FIGURE 2.21 Schematic diagram of dry deposition of a pollutant onto the surface of a leaf, using the concept of resistances (r) for deposition (adapted from Weseley and Hicks, 1999).

leaf, building soil, snow, wetness, etc.; see Dasch, 1985; Vandenberg and Knoerr, 1985) as well as on the nature of the pollutant being deposited. The resistance rh()un in the film of air immediately adjacent to the surface depends on the shape of the surface and the molecular diffusivity of the depositing species, whereas the resistances rct and rs reflect the resistance to adsorption by the surface, which depends both on the nature of the surface itself and on the depositing species. For highly reactive gases, the surface resistance may be sufficiently small so that transport to the surface becomes rate limiting; for example, the surface resistance for deposition of HN03 on grass during the day has been shown to be approximately zero (Huebert and Robert, 1985). Similarly, Chameides (1987) has shown that the dynamical resistance determines essentially entirely the uptake into dew of highly soluble species such as HN03, whereas for less soluble compounds such as S02 and 03, the surface resistance plays an important role.

Because of the effects of physical, chemical, biological, and meteorological parameters on the "resistances," the deposition velocity (V) defined by Eq. (T) also depends on these. As a result, deposition velocities reported for various pollutants show a wide range, from several hundredths to several cm s-1, depending on the conditions during the measurement. For example, the mean value of V for S02 over grass in one study was 0.56 cms"1 for dry grass but 0.93 cms"1 for wet grass (Davies and Mitchell, 1983). A diurnal variation in deposition velocities has been reported, with daytime values being greater than those at night (Hicks et al., 1983); presumably both rgas and rsur, increase under nighttime conditions. For example, peak values of 03 deposition of up to 1 cm s"1 during midday but less than 0.1 cm s"1 in the evening have been reported (Droppo, 1985).

For particles, V depends on the particle size; thus a minimum in V is generally observed at particle diameters around 0.5 yu,m, ~ 0.01 cm s_1 being a typical

TABLE 2.4 Typical Values of Dry Deposition Velocities (Vg)a

Surface Typical range for K,'1

TABLE 2.4 Typical Values of Dry Deposition Velocities (Vg)a

Surface Typical range for K,'1

Larger particles

Exterior surfaces

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