Wet, occult and dry deposition of pollutants on forests

Abstract

Mechanisms by which air pollutants are transferred to forests by wet and dry deposition and in wind-driven cloud water (occult deposition) are reviewed. New methods of making deposition estimates are described. In upland forests, orographic cloud can enhance the amount and ionic concentration of precipitation so that wet deposition may be 4–5 times larger than in nearby valleys. Recent work supports the use of a simple aerodynamic relation to estimate rates of occult deposition. Occult deposition rates to forests in identical weather conditions may vary by an order of magnitude depending on forest structure. Evaporation of deposited water may cause the solute concentration of water on foliage to be substantially larger than in cloud. Dry deposition of HNO 3 and HCl is controlled by aerodynamic properties of the canopy. Several kg N ha −1 year −1 are deposited to forests by this path. Transfer of SO 2, H 2O 2 and O 3 is restricted by surface and internal resistances of foliage. Controls of dry deposition of ammonia and nitrogen oxides and of particles are not well known. A canopy transfer model “Maestro” was modified to include gaseous pollutants, and agrees well with measured SO 2 fluxes to a forest. Estimates of wet, occult and dry deposition on Keilder Forest, England, and Whitetop Mountain, U.S.A., show substantial differences in occult deposition which are only partly explained by differences in cloud frequency. Regional cloud chemistry probably differs between the sites, and different models of occult deposition also disagree.