The isotopic composition of chlorine sampled from the atmosphere has been measured for the first time. Eight aerosol samples collected on bulk filters at Bermuda are enriched in 37Cl relative to sea water. Stable chlorine isotopic compositions are reported in units of per mil as Δ37Cl which is defined as 1000¿[(Rsample/Rsea water)-1> where R is the ratio of 37Cl to 35Cl. The Δ37Cl of the aerosols range from +0.42 to +2.53 per mil and strongly correlate with chlorine deficit (based upon sea salt sodium) and non-sea salt sulfate. The data trend is consistent with the fractional distillation (loss) of HCl from an acidified aerosol governed by a factionation factor of 0.9972. Subsequent experiments support the hypothesis that HCl evaporation was most likely the primary mechanism of chlorine volatilization in the sampled aerosols. Furthermore, the data indicate that marine aerosols perturbed by anthropogeni NOx and SO2 are an important source of isotopically light chlorine in the free troposphere. ¿ American Geophysical Union 1994