Multiple oxygen and sulfur isotope compositions of atmospheric sulfate in Baton Rouge, LA, USA

Abstract

Secondary atmospheric sulfates (SAS) is the ultimate oxidation product and sink for sulfur gases of biological, volcanic, and anthropogenic origins on Earth. Their presence in the atmosphere as aqueous or solid phases contributes to acid rain and climate change, thus, understanding SAS formation pathways is pertinent. There has been extensive measurement of δ 34S values for SAS, which mainly aimed at source identification. Relatively fewer oxygen isotope compositions ( δ 18O, Δ 17O), which are most useful for resolving competing oxidation pathways, were available, however. This study represents the first effort to characterize the Δ 17O, δ 18O, and δ 34S simultaneously for SAS in a tropospheric air shed. We measured a total of 20 samples collected in Baton Rouge (LA, USA) during a 600-day period. The isotope compositions for atmospheric sulfate range from +0.25‰ to +1.43‰ for Δ 17O, +11.8‰ to +19.3‰ for δ 18O, and −1.4‰ to +3.8‰ for δ 34S. No apparent correlation is found among Δ 17O, δ 18O, or δ 34S values. The Δ 17O has no seasonal variation and its values are consistent with an oxidation pathway dominated by aqueous H 2O 2. The δ 18O and δ 34S are within the range of those observed in other sites around the world and are not characteristic for Baton Rouge. Despite the huge variability in atmospheric condition among mid-latitude sites, the long-term average Δ 17O value for SAS appears to fall within a fairly narrow range from +0.6‰ to +0.8‰, which is ∼1‰ to 2‰ lower than those in polar sites.