Sulfate dry deposition measured with a water surface sampler: A comparison to modeled results

Abstract

Sulfate can be deposited to water from the atmosphere as both a particulate (SO42-) and a gas (SO2) (SO2 is quickly hydrolyzed and oxidized to SO42- in water). To determine the relative importance of these two types of deposition, a circular water surface sampler (WSS) was used to measure the dry deposition flux of atmospheric SO2 and sulfate containing particles in Chicago, Illinois, during the spring, summer, and fall of 1994 and 1995. A sharpedged, greased surrogate surface was used to simultaneously measure the dry deposition flux of atmospheric sulfate containing particles. The total sulfate flux to the WSS was consistently higher than the flux to the greased surface due to gas phase deposition. When the dry deposition of SO2 gas to the WSS was calculated by subtracting the greased surface flux (particulate sulfate flux) from the WSS flux (particulate sulfate plus SO2 flux), it was found to contribute significantly more (70-96%) than the particulate phase to overall dry deposition of sulfate to the water surface. For each sample the SO2 flux was also calculated as the product of the individual SO2 gas phase mass transfer coefficient calculated using two different models and the measured ambient SO2 concentration. The predicted SO2 fluxes calculated from both models were highly correlated with the measured SO2 flux obtained with the WSS. The resistance model predictions were statistically the same as the measured values; however the evaporation model significantly underpredicted the deposition of compounds that exist in both the gas and particulate phases.