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

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 time 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. Based on the estimated fraction of sulfate formed via HOX oxidation, we further estimate that daily-averaged HOX concentrations 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.