Comparison of Dry Deposition Predicted from Models and Measured with a Water Surface Sampler

Abstract

Atmospheric deposition, which is commonly classified as either dry or wet, has received a great deal of study over the past decade due to concerns about the effect of deposited material on the environment. Atmospheric deposi tion is an important mechanism controlling the fate of airborne toxics and their transfer from the atmosphere to natural surfaces. In this study, a circular water surface sampler was developed to measure the dry deposition flux of atmospheric gases and particles. The 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 evaluate the water surface sampler, the sulfate flux was measured in Chicago, IL. Sulfate was selected as the test compound because it is deposited to water as both a particulate (SO42-) and a gas (SO2) (SO2 is quickly hydrolyzed and oxidized to SO42- in water). The SO2 fluxes measured directly with a water surface sampler were found to agree well with those predicted with two models. One model was an empirical model that accounts for both natural and forced evaporation, and the other model was a resistance model developed by analogy to electrical or heat flow through a series of resistances.