Articles | Volume 16, issue 17
Research article
14 Sep 2016
Research article |  | 14 Sep 2016

Isotopic constraints on the role of hypohalous acids in sulfate aerosol formation in the remote marine boundary layer

Qianjie Chen, Lei Geng, Johan A. Schmidt, Zhouqing Xie, Hui Kang, Jordi Dachs, Jihong Cole-Dai, Andrew J. Schauer, Madeline G. Camp, and Becky Alexander

Abstract. Sulfate is an important component of global atmospheric aerosol, and has partially compensated for greenhouse gas-induced warming during the industrial period. The magnitude of direct and indirect radiative forcing of aerosols since preindustrial times is a large uncertainty in climate models, which has been attributed largely to uncertainties in the preindustrial environment. Here, we report observations of the oxygen isotopic composition (Δ17O) of sulfate aerosol collected in the remote marine boundary layer (MBL) in spring and summer in order to evaluate sulfate production mechanisms in pristine-like environments. Model-aided analysis of the observations suggests that 33–50 % of sulfate in the MBL is formed via oxidation by hypohalous acids (HOX  =  HOBr + HOCl), a production mechanism typically excluded in large-scale models due to uncertainties in the reaction rates, which are due mainly to uncertainties in reactive halogen concentrations. Based on the estimated fraction of sulfate formed via HOX oxidation, we further estimate that daily-averaged HOX mixing ratios on the order of 0.01–0.1 parts per trillion (ppt  =  pmol/mol) in the remote MBL during spring and summer are sufficient to explain the observations.

Short summary
The formation mechanisms of sulfate in the marine boundary layer are not well understood, which could result in large uncertainties in aerosol radiative forcing. We measure the oxygen isotopic composition (Δ17O) of sulfate collected in the MBL and analyze with a global transport model. Our results suggest that 33–50 % of MBL sulfate is formed via oxidation of S(IV) by hypohalous acids HOBr / HOCl in the aqueous phase, and the daily-mean HOBr/HOCl concentrations are on the order of 0.01–0.1 ppt.
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