93100470 Davidovits Heterogeneous reaction pathways involving aqueous droplets in clouds and fogs are important conduits for chemical transformation of atmospheric trace gases both in the troposphere and in the stratosphere. During the past eight years in a collaborative effort between the Chemistry Department at Boston College and the Center for Chemical and Environmental Physics at Aerodyne Research, Inc., supported in part by the National Science Foundation, two novel and powerful techniques have been developed for studying gas uptake processes and for measuring mass accommodation coefficients of trace atmospheric species on water and other liquids. These techniques can also yield information about the nature of chemical interactions at the gas-liquid interface. In the first method, suitable for measuring relatively large gas uptakes, the gas phase species interacts with a controllable stream of small (50 to 250 um radius) monodispered droplets of known size, temperature and chemical composition. A version of this apparatus using sulfuric acid droplets has also been built to study stratospherically important processes down to a temperature of -50oC. In the second method, used to study gases with relatively smaller uptakes, the trace gas is contained in controlled bubbles rising through the liquid of interest. During the past two NSF support periods the uptake of more than thirty gas phase species has been studied as a function of temperature over a wide range of atmospherically relevant conditions. The species studied include SO2 and reduced sulfur compounds, H2O2, N2O5, HNO3, HCl, ClNO3 and N2O5 the uptake by aqueous sulfuric acid droplets was also measured as a function of temperature down to -50oC. This work provided values for the parameters determining uptake for both tropospherically and stratospherically important species and it led to the formation of a quantitative model for the uptake of nonreactive gases. Further, important features of chemical interactions at the gas-liquid interface were noted. In some cases, such as formaldehyde and phosgene, the uptake was entirely consistent with the bulk aqueous chemistry of the species. But in other cases, such as the uptake of sulfur dioxide and acetaldehyde, results were not explainable by bulk phase chemistry. Instead, these results indicate that reactions at the gas-liquid interface dominate the gas or the liquid phase. Reactions may be orders of magnitude faster at the interface than either in the gas or in the liquid phases. It was also observed, that in the process of uptake, some species such as SO2, form a chemisorbed complex at the gas-liquid interface which is likely to participate in the heterogeneous chemistry of the species. The work plan in this grant will continue to pursue the study of atmospherically important heterogeneous processes. The research will include gas uptake and co-deposition studies for NO, NO2, NO3, N2O5, HNO3, HONO, NH3, SO2, CH2O and OH. Experiments will be constructed as a function of temperature with aqueous solutions, with sulfuric acid and with nitrosyl sulfuric acid solutions. The effect on heterogeneous processes of photochemistry, surface charge and various atmospherically relevant additives will be examined. The two major objectives of the research studies are: a) To continue the measurement of parameters required to assess the heterogeneous chemistry of species important to troposphere an stratosphere. b) To obtain a general, predictive understanding of chemistry at the gas-liquid interface.
