Fractionation of sulfur isotopes during heterogeneous oxidation of SO 2 on sea salt aerosol: A new tool to investigate non-sea salt sulfate production in the marine boundary layer

Abstract

The oxidation of SO 2 to sulfate on sea salt aerosols in the marine environment is highly important because of its effect on the size distribution of sulfate and the potential for new particle nucleation from H2SO4 (g). However, models of the sulfur cycle are not currently able to account for the complex relationship between particle size, alkalinity, oxidation pathway and rate-which is critical as SO 2 oxidation by O 3 and Cl catalysis are limited by aerosol alkalinity, whereas oxidation by hypohalous acids and transition metal ions can continue at low pH once alkalinity is titrated. We have measured 34S/32S fractionation factors for SO 2 oxidation in sea salt, pure water and NaOCl aerosol, as well as the pH dependency of fractionation. Oxidation of SO 2 by NaOCl aerosol was extremely efficient, with a reactive uptake coefficient of ≈0.5, and produced sulfate that was enriched in 32S with αOCl Combining double low line 0.9882±0.0036 at 19 °C. Oxidation on sea salt aerosol was much less efficient than on NaOCl aerosol, suggesting alkalinity was already exhausted on the short timescale of the experiments. Measurements at pH Combining double low line 2.1 and 7.2 were used to calculate fractionation factors for each step from SO 2(g) → multiple steps → SOOCl 2-3. Oxidation on sea salt aerosol resulted in a lower fractionation factor than expected for oxidation of SO 3-2 by O 3 (αseasalt Combining double low line 1.0124±0.0017 at 19 °C). Comparison of the lower fractionation during oxidation on sea salt aerosol to the fractionation factor for high pH oxidation shows HOCl contributed 29% of S(IV) oxidation on sea salt in the short experimental timescale, highlighting the potential importance of hypohalous acids in the marine environment. The sulfur isotope fractionation factors measured in this study allow differentiation between the alkalinity-limited pathways-oxidation by O 3 and by Cl catalysis (α34 Combining double low line 1.0163±0.0018 at 19 °C in pure water or 1.0199±0.0024 at pH Combining double low line 7.2)-which favour the heavy isotope, and the alkalinity non-limited pathways-oxidation by transition metal catalysis (α34 Combining double low line 0.9905±0.0031 at 19 °C, Harris et al., 2012a) and by hypohalites (α34 Combining double low line 0.9882±0.0036 at 19 °C)-which favour the light isotope. In combination with field measurements of the oxygen and sulfur isotopic composition of SO 2 and sulfate, the fractionation factors presented in this paper may be capable of constraining the relative importance of different oxidation pathways in the marine boundary layer. © 2012 Author(s).