Chlorine isotopic composition of marine aerosols: Implications for the release of reactive chlorine and HCl cycling rates

We have determined the elemental and the stable chlorine isotopic composition of size‐segregated aerosols collected in the marine boundary layer. All size fractions have chlorine deficiencies. Mid‐size aerosols are isotopically enriched in δ 37 Cl and contain significant quantities of non‐sea‐salt‐SO 4 2− and NO 3 − . This is consistent with the acid displacement of HCl from the aerosols. Coarse aerosols tend to be isotopically depleted and have Cl deficiencies that are not charge balanced by MSA − , SO 4 2− and NO 3 − with Cl volatilization by a different mechanism. The isotope systematics imply a fractionation factor, α, of 1.00306±0.0005 if the volatilization mechanism is first order with respect to Cl or 1.00280±0.0005 if the volatilization mechanism is zeroth order with respect to Cl. Our data are consistent with the efficient recycling of aerosol Cl through the gas phase.