Sulfur Isotopic Fractionation in the Gas-Phase Oxidation of Sulfur Dioxide Initiated by Hydroxyl Radicals

The sulfur isotopic signature of atmospheric sulfate aerosol reflects not only the chemical and isotopic composition of its precursors, but also their oxidation pathways. Thus, to trace back the sources of sulfate, a quantitative assessment of sulfur isotopes fractionation in major atmospheric processes is required. In this paper, we evaluate S-isotope fractionation ratios f = (k_(OH+^(34)SO_2))/(k_OH+^(32)SO_2)) for the gas-phase oxidation of SO_2 by OH-radicals, using RRKM transition-state theory. Calculations were constrained by reliable rates for the HO + ^(32)SO_2 + M = HO^(32)SO_2 + M reaction, an ab initio transition-state structure, and actual spectroscopic data for the sulfur isotopomers of the hydroxysulfonyl HOSO_2 radical. By assuming plausible Lennard-Jones parameters for HOSO_2 collisions with N_2 as bath gas, which are consistent with the experimental values of 〈ΔE_(down)〉 ∼ 200 cm^(-1) and a collisional efficiency of β_c ∼ 0.2 in air, we derive f > 1 values at the temperatures and pressures prevalent in the terrestrial atmosphere below 30 km. Present results rationalize the evolution of stratospheric aerosol sulfur isotopic composition after volcanic SO_2 injections above 15 km and the ^(34)S enrichment of tropospheric sulfate aerosol during the summer months.