Dry deposition of trace gases

In the dry removal of trace gases turbulent diffusion, followed by molecular diffusion in the laminar layer, plays an important role, provided that the soil, vegetation or water surface chemically adsorbs or absorbs the gas considered. 

It follows from equation [5.1] that the dry deposition of trace gases (vs = 0) may be calculated by measuring their concentration gradient and the eddy (or molecular) diffusion coefficient This procedure, called the gradient method, is widely used to determine the dTy deposition velocity. Thus, Atkins and Garland (1974) studied the dry deposition of S02 in this way. They determined the diffusion coefficient from wind profile observations. Using equation [5.3] these authors were able to calculate 

Dry deposition velocity of sulfur dioxide gas for different soil surfaces (Atkins and Garland, 1974) 

Recently, Garland (1978) has reviewed the results of measurements carried out to determine the dry deposition velocity of sulfur dioxide gas. He argues that the average removal velocity is around 0.8 cms . It is to be noted here that the dry deposition velocity of13 I gas was also found to be in agreement with the data tabulated in Table 26. Thus, Chamberlain (1960) reports values between 0.3 and 2.6 cm s 1 for this radioactive isotope. 

Unfortunately, little information is available for other gaseous sulfur and nitrogen species. As we have mentioned in Chapter 3, Judeikis and Wren (1977) published values between 0.015 and 0.28 cm s 1 for H2S and DMS, on the basis of their model experiments. They also reported indications, however, that observed deposition velocities are due primarily to reversible physical adsorption. This means that real dry removal rate of these compounds may be much smaller. 

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