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

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 δ34S values for SAS, which mainly aimed at source identification. Relatively fewer oxygen isotope compositions (δ18O, Δ17O), which are most useful for resolving competing oxidation pathways, were available, however. This study represents the first effort to characterize the Δ17O, δ18O, and δ34S 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 Δ17O, +11.8‰ to +19.3‰ for δ18O, and −1.4‰ to +3.8‰ for δ34S. No apparent correlation is found among Δ17O, δ18O, or δ34S values. The Δ17O has no seasonal variation and its values are consistent with an oxidation pathway dominated by aqueous H2O2. The δ18O and δ34S 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 Δ17O 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.