| Anthropogenic sulfur emissions greatly exceed natural sources and perturb the atmospheric sulfur cycle. Sulfate aerosols are the end product of atmospheric sulfur oxidation, and increased sulfate burdens are associated with a number of consequences. Sulfate aerosols pose a respiratory hazard, contribute to acid deposition, and disrupt the Earth's radiative balance and hydrological cycles, affecting weather and global climate. Uncertainties in the budget, chemical transformations, and the transport of species in the sulfur cycle remain. It is these latter two concerns that we aspire to elucidate with our work.; Sulfur has a sufficient number of stable isotopes to make mass-independent (MI) processes observable. This work comprises the first observation of a MI sulfur anomaly in the present atmosphere. Previous studies of meteoritic and pre-Cambrian rock samples attribute their observed MI sulfur anomalies to UV photodissociation of SO2 in a low oxygen atmosphere. Our findings are consistent with atmospheric photochemistry and support their theory. If this process is occurring in the present atmosphere, a portion of the sulfate in our samples must be formed at high altitudes where the relevant short wave UV light is available. A stratospheric source of the sulfate is supported by the small size of the aerosols containing the MI signature, the uniformity of isotopic composition regardless of sampling location, and the positive correlation of the MI signal with the activities of the cosmogenically produced radionuclides 35S and 7Be. This anomaly may be useful as a tracer of sulfate formed in the stratosphere, and may permit downward transport across the tropopause to be measured conveniently at ground level. SO2 photooxidation at slightly longer wavelengths proceeds through a different mechanism. This mechanism also produces a MI signature, but with a distinctly different relationship between the isotopes. This wavelength dependence of the effect may yield information concerning the altitude at which SO2 photochemistry is occurring. Furthermore, since the anomaly is produced in the stratosphere, volcanic eruptions preserved in the glaciological record are unambiguously classified as either stratospheric events, with large global climatological consequences, or as tropospheric events of more regional importance. |
