COMPARISON OF DRY DEPOSITION PREDICTED FROM MODELS AND MEASURED WITH A WATER-SURFACE SAMPLER

Atmospheric deposition, which is commonly classified as either dry or wet, has received a great deal of study over the past decade due to co ncerns about the effect of deposited material on the environment. Atmo spheric deposition is an important mechanism controlling the fate of a irborne toxics and their transfer from the atmosphere to natural surfa ces. In this study, a circular water surface sampler was developed to measure the dry deposition flux of atmospheric gases and particles. Th e sampler consists of a sharp-edged, acrylic plate, filled with water (37 cm diameter and 0.5 cm deep) that is continuously replenished from a reservoir by a pump that maintains a constant water depth. To evalu ate the water surface sampler, the sulfate flux was measured in Chicag o, IL. Sulfate was selected as the test compound because it is deposit ed to water as both a particulate (SO42-) and a gas (SO2) (SO2 is quic kly hydrolyzed and oxidized to SO42- in water). The SO2 fluxes measure d directly with a water surface sampler were found to agree well with those predicted with hive models. One model was an empirical model tha t accounts for both natural and forced evaporation, and the other mode l was a resistance model developed by analogy to electrical or heat Ro w through a series of resistances.