SULFATE DRY DEPOSITION MEASURED WITH A WATER-SURFACE SAMPLER - A COMPARISON TO MODELED RESULTS

Sulfate can be deposited to water from the atmosphere as both a partic ulate (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 t o measure the dry deposition flux of atmospheric SO2 and sulfate conta ining particles in Chicago, Illinois, during the spring, summer, and f all of 1994 and 1995. A sharp-edged, greased surrogate surface was use d to simultaneously measure the dry deposition flux of atmospheric sul fate containing particles. The total sulfate flux to the WSS was consi stently higher than the flux to the greased surface due to gas phase d eposition. When the dry deposition of SO2 gas to the WSS was calculate d by subtracting the greased surface flux (particulate sulfate flux) f rom the WSS flux (particulate sulfate plus SO2 flux), it was found to contribute significantly more (70-96%) than the particulate phase to o verall 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 mo dels and the measured ambient SO2 concentration. The predicted SO2 flu xes calculated from both models were highly correlated with the measur ed SO2 flux obtained with the WSS. The resistance model predictions we re statistically the same as the measured values; however the evaporat ion model significantly underpredicted the deposition of compounds tha t exist in both the gas and particulate phases.