Dry deposition of atmospheric contaminants: The relative importance of aerodynamic, boundary layer and, surface resistances

Measurements of dry deposition were conducted to potted plants at Claremont, CA, during the Southern California Air Quality Study and to patches on an equestrian statue at Gettysburg National Military Park, PA. Simultaneous dry deposition measurements were also conducted on aerodynamically designed surrogate surfaces (symmetric airfoils) with predictable boundary layer characteristics at both locations. Species of interest include gaseous SO₂ and particulate SO²⁻₄, NO₃₋Ca and, Pb. The importance of each transport step in the deposition process was evaluated by combining the data from several types of surfaces with different surface characteristics. The significance of particle resuspension was determined by simultaneous sampling with different exposure times. The results suggest that the relative importance of aerodynamic, boundary layer and, surface resistances can vary greatly, depending on the depositing chemical species and the surface characteristics. Of particular interest is the highly variable surface resistance which is often the rate-limiting step in the overall process. Rates of resuspension for several surfaces including natural vegetation are in the range of 10⁻⁶ to 10⁻⁵ sec⁻¹ for the aerosol chemical species considered. This implies that net dry deposition rates depend on the exposure times, which are not often considered in most ambient dry deposition studies.