On Estimating Dry Deposition Rates in Complex Terrain

Abstract

In complex terrain, horizontal advection and filtration through a canopy can add substantially to the vertical diffusion component assumed to be the dominant transfer mechanism in conventional deposition velocity formulations. To illustrate this, three separate kinds of terrain complexity are addressed here: 1) a horizontal landscape with patches of forest, 2) a uniformly vegetated gentle hill, and 3) a mountainous area. In flat areas with plots of trees, the elevation of the standard area-weighted dry deposition velocity will likely depend on the product hn1/2, where h is the tree height and n is the number of plots per unit area. For the second case, it is proposed that the standard ?flat earth? deposition velocity might need to be increased by a factor like [1 + Ra/(Rb + Rc)]1/2. For mountainous ecosystems, where no precise estimate of local dry deposition appears attainable, the actual dry deposition rate is probably bounded by the extremes associated with 1) the flat earth assumption involving aerodynamic, quasi-boundary layer, and canopy resistances as in conventional formulations, and 2) an alternative assumption that the aerodynamic resistance is zero. Such issues are of particular importance in the context of atmospheric loadings to sensitive ecosystems, where the concepts of critical loads and deposition forecasting are now of increasing relevance. They are probably of less importance if the emphasis is on air quality alone, because air quality responds slowly to changes in deposition rates. The issues addressed here are mainly appropriate in the context of air surface exchange that is not controlled by surface resistance (e.g., for deposition of easily captured chemicals such as nitric acid vapor, and perhaps for atmospheric momentum) and for chemicals that have no local sources. It is argued that dry deposition rates derived from classical applications of deposition velocities are often underestimates.