Isotopic Constraints on the Formation Mechanisms of Sulfate Aerosols in the Summer Arctic Marine Boundary Layer

Abstract

AbstractSulfate is a major aerosol species offsetting the Arctic warming. Quantifying sulfate formation mechanisms is conducive to assess its climatic effects and sulfur budget. However, relevant observations are sparse over the Arctic Ocean. Here, we present observations of surface sulfate and its Δ17O (=δ17O–0.52δ18O) over the latitude of 63.3–87.7°N in the Arctic Ocean in July–September 2012 to estimate sulfate production mechanisms. Potential contamination from ship plume on the observed sulfate was corrected based on black carbon. The corrected non‐sea‐salt sulfate (nss‐SO42−cor) varied from 0.0 to 1.3 μg m−3 with the mean of 0.3 ± 0.3(1σ) μg m−3. Samples with high contributions (≥80%) from sea‐salt sulfate and anthropogenic sulfate from ship plume possess Δ17O(SO42−) at ∼0‰. Δ17O of nss‐SO42−cor ranged from 0.0 to 0.9‰ with the mean of 0.5 ± 0.3‰. The low Δ17O values directly suggest the contribution from O3 oxidation to sulfate production is minor in summer Arctic due to this mechanism resulting in Δ17O(SO42−) = 9.8‰. Further calculations constrained by Δ17O indicate sulfate production was dominated by H2O2 oxidation with the possible contribution of 37%–66%, higher than the estimated fraction of 29% from OH oxidation. The role of HOBr oxidation is related to its abundance, which can reach a maximum contribution of 24% at assumption of [HOBr] = 1 ppt in summer Arctic. Future model studies with the constraint of sulfate and Δ17O observations in this study will further improve our knowledge of global sulfur budget and sulfate formation mechanisms in the Arctic.