Laboratory Studies of Tropospheric Sulfur Chemistry

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The atmospheric sulfur cycle has been the subject of intensive investigation in recent years because of the need to assess the contribution of anthropogenically produced sulfur to such problems as acid rain, visibility reduction, and climate modification. In heavily industrialized regions such as the eastern United States and western Europe, anthropogenic sulfur magnitude. On a global scale, however, natural sulfur emissions are thought approximately to equal those from anthropogenic sources. An understanding of the natural sulfur cycle is thus required in order to establish a "base line" with which anthropogenic perturbations can be compared. A substantial measurement data base for the key reduced sulfur species CS2, COS, and CH3SCH3 (DMS) has become available in recent years. Laboratory research aimed at identifying major sulfur oxidation pathways and quantifying the rates of these pathways has also progressed rapidly. Nonetheless, major uncertainties in the atmospheric chemistry of organosulfur compounds remain. This research is aimed at elucidating the kinetics and reaction mechanisms which control the atmospheric oxidation of reduced sulfur compounds, i.e. H2S, CS2, COS, mercaptans, sulfides and disulfides. Direct observation of reactive intermediates and relatively stable products in "real time" is emphasized because only through such observations can results with a unique interpretation be obtained. Some specific experimental investigations which are to be undertaken include kinetics studies of CH2S reactions with O3, HO2, and NO2, kinetics studies of IO reactions with thiols, sulfides, and disulfides, determination of the product yields of SH, CS, S2, HO2, COS, CO, and CO2 from the CS2OH+O2 reaction, determination of CH3S yields from reactions of OH and NO3 with organosulfur compounds, kinetic studies of the reactions of OH and NO3 with dimethylsulfoxide, and investigations of the mechanisms for producing the stable end products SO2, CH3SO3H, and (CH3)2SO via the OH-, NO3-, and IO-initiated oxidations of organosulfur compounds. The experimental methods to be employed involve coupling laser flash photolysis production of unstable reactants with time resolved detection of intermediates and end products by laser induced fluorescence, long path UV-visible absorption, or long path infrared absorption. Most experiments will be carried out under atmospheric conditions of temperature, pressure, and gas composition.