Deposition of Gases

Gases (as much as very small particles) are deposited mostly through turbulent diffusion and Brownian motion. Chemical absorption, impaction, photosynthesis, and other biological, chemical, and physical processes cause the deposited gases to be retained at the surface of land or water.

The transfer of gases (and also particles) between the surface and the atmosphere can be analyzed in a similar way to the electrical current flow through resistances in parallel and series. This simple analog is presented in Figure 1.

z

F

r am

Fc1

rc1

Stomata

Plant canopy

ci < s1

H H c2

Leaf surface

C 3

Soil

C3

Figure 1 A simple resistance analog of pollutant gas transfer between the atmosphere and terrestrial ecosystems with fluxes (F), resistances (r), and capacitances (c) for exchanges with stomata, leaf surfaces, and soil surface. Reproduced by permission of the Royal Society of Edinburgh from Proceedings of the Royal Society of Edinburgh, section B: Biological Sciences, Volume 97 (1991 for 1990) [Acidic Deporition: Its Nature and Impacts], pp. 35-39.

The flux of a chemical through the atmosphere Fs over homogeneous level terrain can be described by:

where Ks is the diffusion coefficient for the gas s and Ss/Sz is the vertical gradient in concentration of s.

The total resistance for the gas transfer from the atmosphere to the terrestrial surface consists of the combined total of the aerodynamic resistance for momentum transfer, the viscous sublayer resistance, and the surface or canopy resistance, as shown in Figure 1. The aerodynamic resistance for momentum transfer may be assessed directly from the measurements of wind velocity and temperature profiles over suitably uniform surfaces. The viscous sublayer resistance is regarded as an additional atmospheric boundary layer resistance. Immediately adjacent to the surface processes operating at the molecular level, it begins to prevail over aerodynamic processes. The strength of thermal rather than mechanical diffusivity can be determined. This resistance is determined largely from wind tunnel measurements. The viscous sublayer resistance is very important for gases which are deposited immediately upon transport to the surface. The canopy or surface resistance can be obtained by subtracting the atmospheric resistance and the viscous sublayer resistance from the total resistance which is a measured parameter.

A capacitance for exchanges with stomata, leaf surfaces, and soil surface is necessary to be introduced in the deposition analysis presented in Figure 1. This is because temporal changes in the concentration of gases result in changes in the equilibrium between concentrations absorbed onto, and within, leaves and those in the atmosphere.

Rates of exchange of different gases between the atmosphere and the land and water surfaces may differ by 2 orders of magnitude, particularly the rates of exchange between the atmosphere and vegetation. In general, the gases can be divided into two groups: (1) highly reactive gases for which uptake is not restricted by surface resistances: HNO3, HCL, and, for most unmanaged vegetation, NH3; and (2) gases whose exchange with terrestrial surfaces is controlled by chemical plant physiological processes, such as stomatal resistance: ozone, NO2, and to some extent SO2.

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