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

摘要

Sulfate is an important component of global atmospheric aerosol, and haspartially compensated for greenhouse gas-induced warming during theindustrial period. The magnitude of direct and indirect radiative forcing ofaerosols since preindustrial times is a large uncertainty in climate models,which has been attributed largely to uncertainties in the preindustrialenvironment. Here, we report observations of the oxygen isotopic composition(ΔO) of sulfate aerosol collected in the remote marine boundarylayer (MBL) in spring and summer in order to evaluate sulfate productionmechanisms in pristine-like environments. Model-aided analysis of theobservations suggests that 33–50 % of sulfate in the MBL is formed viaoxidation by hypohalous acids (HOX  =  HOBr + HOCl), a productionmechanism typically excluded in large-scale models due to uncertainties inthe reaction rates, which are due mainly to uncertainties in reactive halogenconcentrations. Based on the estimated fraction of sulfate formed via HOXoxidation, we further estimate that daily-averaged HOX mixing ratios on theorder of 0.01–0.1 parts per trillion (ppt  =  pmol/mol) in the remote MBLduring spring and summer are sufficient to explain the observations.