Deposition Velocity Estimates

The great diversity of airborne trace chemical properties, surface conditions, and environmental conditions prevents the generation of universally applicable dry deposition parameterization schemes. Studies have tended to focus on substances important in atmospheric chemistry or likely to be harmful to human health, biota, and man-made materials. The types of surface most often chosen for investigation are ones that are fairly common in a given region, because the total amount of substances removed from the atmosphere depends on the amount of areal exposure to the surfaces. For example, the deposition of 03 to agricultural areas and forests in the eastern half of the United States has been studied fairly frequently. Figure 2 shows the results of some studies conducted by Argonne National Laboratory in the 1990s. As can be seen, the deposition velocity to soybean fields tends to be larger than for maize, a pattern related to the structures and physiological properties of the plants. Cloudy conditions and the resulting reduction in solar radiation for the central day in Figure 2 caused leaf stomata to be partially closed and the value of Rc to be fairly large. At night, the stomata were closed for all three canopies, leaving open only the deposition pathway to the outer surfaces of the vegetation and the soil surface beneath. Deposition to the tallgrass prairie tended to be suppressed in general because of the effects of fairly dry soil and the tendency of the grass to increase stomatal resistance in such conditions to reduce transpiration of moisture.

Ozone is taken up through plant stomata because destruction of 03 occurs fairly rapidly in the substomatal cavities. The oxidization by 03 of some organic substances in solution in the film of water that envelops the mesophyll cells is the primary reason for the very small mesophyll resistance. Ozone also reacts strongly with many surface materials, although the waxy outer coating of leaves is an effective barrier. Because 03 is poorly soluble in water, flux pathways are insignificant to water that is free from significant amounts of substances with which 03 reacts. In

Hour (Local Standard Time) Figure 2 Observations of the dry deposition velocity for ozone over three types of surfaces.

general, measures of the oxidizing capacity of various substances provide a means of evaluating their ability to be destroyed at surfaces of various types.

Many studies have shown that S02 is also taken up by vegetation with practically no mesophyll resistance. The primary factor that affects S02 removal is its fairly large effective solubility in water. Usually the amount of exposure to water is a major factor in assessing the deposition velocity of substances that dissolve and dissociate rapidly in water. Table 1 shows deposition velocity values for S02, 03, and N02 for various surfaces, based on experimental observations and resistance models. As can be seen, the deposition velocities for N02 tend to be smaller than for S02 and 03, mainly because the water solubility of N02 is small and its ability to oxidize surface materials is weak compared to that of 03. In general, measures of the effective solubility and oxidizing capacity of gases provide a means of estimating their deposition velocities relative to those seen experimentally for S02 and 03.

TABLE 1 Typical Deposition Velocities (cm/s1) for S02, 03, and NOz at Height of 10 m°

Grassland,

Deciduous

Coniferous

Soybeans

Maize

Forest

Forest

Substance

1 2

1 2

1 2

1 2

Midsummer with Lush Vegetation

S02

1.4

0.4

0.8

0.3

0.9

0.1

0.6

0.1

03

1.0

0.2

0.7

0.2

0.8

0.1

0.5

0.1

no2

0.8

0.1

0.4

0.05

0.7

0.03

0.4

0.03

Autumn with Unharvested Cropland S02 0.4 0.2 0.4

Late Autumn after Frost, No Snow S02 0.5 0.2 0.2

Autumn with Unharvested Cropland S02 0.4 0.2 0.4

Late Autumn after Frost, No Snow S02 0.5 0.2 0.2

Winter, Snow on Ground and Near Freezing

Transitional Spring with Partially Green Short Annuals

0.03

0.01

"Cases 1 and 2 for each surface type correspond to solar irradiances of 500 and 0 W/m2. respectively. Dry surfaces and moderate wind speeds are assumed.

The deposition of nonpolar, nonreactive gases such as some organic compounds is usually assumed to be small, although solubility in lipids in vegetation might slightly enhance deposition. Studies have shown that this pathway is measurable but very small. Deposition velocities of less than 0.1 cm/s are likely.

Particle deposition velocities can be strongly dependent on particle size. For particles smaller than 0.1 to 0.2 ^im in diameter, deposition by transport through the quasilaminar sublayer can be fairly strong; the extremely fine particles diffuse through air similarly to molecules of gas. Particles larger than 1 to 2 ^im are deposited mainly by gravitational settling, for which the associated deposition velocities can be several centimeters per second. For the so-called accumulation size mode, in which particle diameters are larger than 0.1 to 0.2 ^im and smaller than 1 to 2\im, mechanisms of deposition are often thought to be ineffective. Some field studies have shown, however, that processes of interception and impaction in gusty wind conditions can enhance deposition velocities substantially, to values exceeding 0.5 cm/s during daytime conditions over typical terrestrial surfaces. Such deposition velocities have been seen over grass for sulfate, which usually exists primarily in the accumulation size mode; values exceeding 1.0 cm/s for sulfate and nitrate have been seen over partially wetted coniferous forests.

4 MODELS OF DEPOSITION VELOCITY

Models have become significantly more sophisticated during the past two decades and are becoming more effective tools for the environmental worker who must make estimates of deposition rates of trace chemicals. "Big-leaf models" that use Eq. (2) with little breakdown of Rc into component resistances have been supplanted to some extent by multilayer canopy models for vegetated surfaces at specific sites where local conditions are observed directly (e.g., Meyers and Baldocchi, 1988; Meyers et al., 1998). Variations of big-leaf models in which Rc is represented by several possible flux pathways have been used extensively in dry deposition modules intended for regional- and large-scale numerical models of atmospheric chemistry (e.g., Pleim et al., 1984; Wesely, 1989; Padro and Edwards, 1991; Benkovitz et al., 1994; Ganzeveld and Lelieveld, 1995).

The potential is high for advancing the accuracy of dry deposition estimates by using advanced atmospheric models with notably improved descriptions of the surface conditions that affect dry deposition. Third-generation models are expected to have capabilities that will reduce the dependency on empirically derived resistance values and provide a means of coupling deposition and emission more closely (Peters et al., 1995). Third-generation models are also likely to incorporate better simulations of the structure of the planetary boundary layer, to provide estimates of soil moisture content and évapotranspiration that can be valuable inputs to dry deposition modules, and to allow the use of parameterizations of vegetative processes that are based on physiological processes, such as processes that control photosynthesis and uptake of carbon dioxide.

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