Dry Deposition of Particles and Vapors

Abstract

This chapter reviews current knowledge of dry deposition of contaminants from the atmosphere. The first section introduces the topic and justifies its importance. In the second section, the dry deposition process is described in physical and mathematical terms. The physical process involves three steps: aerodynamic transport from the free atmosphere into relatively quiescent air near the surface, boundary layer transport across the quiescent region, and interactions with the surface. Mathematical relations for each step are expressed in terms of resistances to transport. The third section describes two methods by which predictions for dry deposition in the field have been developed. These include wind tunnel studies leading to development of empirical models and more detailed mathematical models based on theoretical formulations as well as field and laboratory data. In the fourth section, currently available methods for measuring dry deposition are discussed. The techniques include surface analysis methods, which involve measurement of the accumulation of contaminant on surfaces of interest, and atmospheric flux methods, which involve measurement of airborne contaminant concentrations used to infer the flux. Finally, dry deposition data from field and chamber experiments are summarized. Data are present for sulfur, nitrogen, and chloride species, as well as ozone, trace elements, and atmospheric particles. The data are compared with predicted dry deposition velocities using models presented earlier. The chapter concludes by acknowledging limitations in our understanding of dry deposition.