On Bounding Dry Deposition Rates in Complex Terrain

The formulations used to quantify dry deposition rates from air chemistry and related meteorological information are based on the results of experimental programs conducted over flat and homogeneous surfaces. For trace gases, the relevant mechanism for deposition is vertical diffusion, as is represented by the concept of a "deposition velocity." In complex terrain, horizontal advection and filtration through the canopy might add substantially to the vertical diffusion component. The extent to which advection/filtration adds to the total dry deposition rate cannot yet be accurately quantified, however it is clear that relatively simple arguments can be used to bound the total deposition rate between extremes corresponding to flat earth and exceedingly complex terrain. Two separate kinds of terrain complexity are addressed -- (a) a horizontal landscape with patches of forest, and (b) a mountainous area. Extending the arguments presented here to more representative landscapes will require either measurements that cannot yet be made or supreme confidence in models that cannot yet be verified. Nevertheless, it is argued that dry deposition rates derived from classical applications of deposition velocities are likely underestimates. These studies started under the EPA/NOAA collaboration during the acid rain era of the 1980s, and have continued to this day -- a direct indication of the difficulty of the problems being addressed.