Some Comments on Modeling and Measurement of Dry Deposition

To ensure a constant-flux layer, one can simply move the measurement height closer to the surface. For the eddy correlation method, however, the response time of the instrument must be faster as the measurement height approaches the surface, because high-frequency turbulent eddies then contribute proportionally more to the concentration fluxes than at higher levels. On the other hand, fluxes measured very close to the surface may be less representative of those over the entire area for which the measurement is intended. For the gradient method, the requirement that z/zo 3 1 (based on the requirements of similarity theory) constrains the minimum measurement height. Under very stable conditions, when turbulence may be intermittent, turbulent fluxes may become very small, and the constant-flux layer may be very shallow. Under conditions such as these, it can be quite difficult to determine the aerodynamic resistance term ra. 

A complicating factor in dry deposition measurements is the presence of sources of the depositing substance in the footprint of the measurement. Whereas the flux of S02 is nearly always unidirectional (downward) and the surface is a sink for S02, gases such as H2S, NH3, and NO may have surface sources. It may be possible in some cases to specify a surface emission rate. For N02, the situation appears to be even more complex than just adding a surface emission rate to the resistance model. As much as 50% of the N02 initially removed at the surface can reappear as NO as a result of surface emissions (Meyers and Baldocchi 1988). 

Bidirectional fluxes of particles can also occur. One important example is the deposition and subsequent resuspension of automobile-emitted lead. During the decades of leaded gasoline use, deposition onto soil in urban areas exceeded resuspension and there was a net buildup of lead in surface soil. Now that leaded gasoline has been largely eliminated, resuspension exceeds deposition, and the soil surface is becoming gradually depleted in lead. Harris and Davidson (2005) have shown that the lifetime for depletion of lead in the soil is more than a century. 

The assumptions of similarity weaken as one moves from flat and uniform surfaces into hilly terrain and associated natural surface covers (Doran et al. 1989). Fluxes are likely to change substantially over rather short distances (1 km or less), and it may be extremely difficult to establish a representative flux value for more extended regions from measurements at a single site. Some measurements have been carried out over sloping 

Show the steps followed to derive (19.2). b. Show the steps followed to derive (19.7). 

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