Dry deposition of sulfate onto surrogate surfaces

Abstract

Measurements of SO₄²⁻ fluxes to surrogate surfaces have been conducted as part of the Dry Deposition Intercomparison Study in Champaign, Illinois. Data for Teflon plates without rims and for polycarbonate petri dishes with 1‐cm rims have been obtained and compared with data for Teflon sheets and polyethylene dustfall buckets used by other investigators. Results show that deposition velocities for the Teflon plates are in the range of 0.17–0.42 cm/s, while corresponding values for the petri dishes are in the range of 0.18–0.61 cm/s. Comparisons among the four types of surrogate surfaces show deposition velocities which increase in the order Teflon sheets ∼ Teflon plates < petri dishes < dustfall buckets. Differences in collector geometry as well as composition are responsible for these trends. Airborne size distribution data obtained during the study show that much of the SO₄²⁻ mass is associated with particles of 0.5–1‐μm aerodynamic diameter. However, using the size distributions as inputs to dry deposition models in the literature suggests that larger particles are responsible for most of the SO₄²⁻ mass deposition onto the surrogate surfaces. The calculated values for natural grass surfaces are in reasonable agreement with measured deposition velocities onto the petri dishes. Overall, the results of this study suggest that inertial impaction and sedimentation of supermicron airborne particles are more important than deposition of submicron material in influencing total SO₄²⁻ mass deposition onto surrogate surfaces and onto natural vegetation at this site.