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

Abstract

The Arctic climate has warmed rapidly over recent decades with the combined effect of greenhouse gases and aerosols (IPCC, 2013). Sulfate is a major aerosol species offsetting the Arctic warming via scattering of solar radiation and modification of cloud microphysics (Tilgner et al., 2021;Yang et al., 2018). Quantifying the climatic effects of sulfate needs information on its sources, distribution, and formation pathways. Sulfate in the Arctic marine boundary layer (MBL) originates from sea salt, anthropogenic emissions, biogenic and volcanic sources and involves in the oxidation of SO 2 (Ghahremaninezhad et al., 2016;Jiang et al., 2021). The oxidation of SO 2 by OH in the gas phase leads to the formation of H 2 SO 4 (g), which can nucleate new particles and increase the number of cloud condensation nuclei (CCN) (Kulmala et al., 2000). In cloud droplets, the dissolved SO 2 (S(I) is easily oxidized by H 2 O 2 , O 3 , hypohalous acids (HOX = HOCl + HOBr), and transition metal ions (TMI)-catalyzed O 2 in the aqueous phase (Alexander et al., 2009;Chen et al., 2016), contributing to the growth of particles (Lelieveld & Heintzenberg, 1992). The relative importance of these mechanisms can be traced by oxygen-17 excess (Δ 17 O = δ 17 O-0.52δ 18 O) due to the Δ 17 O signature of their oxidants being transferred to sulfate during the oxidation of SO 2 and S(IV). Sulfate formed in the aqueous-phase via O 3 and H 2 O 2 oxidation leads to Δ 17 O(SO 4 2− ) = 9.8‰ and 0.7‰, respectively (Shao et al., 2019), under the assumption of Δ 17 O(O 3 ) = 26‰ (Ishino et al., 2017; Vicars & Savarino, 2014) and Δ 17 O(H 2 O 2 ) = 1.4‰ (Savarino & Thiemens, 1999). Sulfate from other processes result in Δ 17 O(SO 4 2−) = 0‰, including OH oxidation in the gas-phase, HOX and TMI-catalyzed O 2 oxidation in the aqueous phase, anthropogenic primary sulfate and sea-salt sulfate (