Stable chlorine isotope variations in the atmosphere

A high-precision, high-sensitivity method for determining natural variations of stable chlorine isotope ratios has been developed. Sample sizes on the order of 2 micrograms are analyzed via thermal ionization mass spectrometry. The method precision (1σ is −0.25 per mil) allows for the exploration into the stable chlorine isotope composition of the atmosphere for the first time.; My investigations have revealed that sea salt aerosol particles are often chemically and isotopically altered. In my initial study, sea salt aerosol particles collected at Bermuda were found to be deficient in chlorine. Measured δ 37Cl values were all enriched in 37Cl relative to sea water (+0.42 to +2.53 per mil) where δ37Cl strongly correlated with Cl deficit and non-sea salt (Nss) sulfate plus nitrate excess (r = 0.88 and 0.9, respectively). This fractionation trend was reproduced experimentally by allowing acidified seawater to evaporate to dryness. Therefore, I conclude that HCl volatilization was responsible for the observed chlorine deficits and isotope fractionation.; Subsequent investigation of size-aggregated sea salt aerosol particles collected from the central equatorial Pacific Ocean also revealed chlorine deficits. The fine particle fraction had a significant excess of Nss-sulfate plus nitrate, and were enriched (δ37Cl ranged from +0.29 to +2.39 per mil) with Cl deficit. However, the coarse particle fraction (>1.5 micrometers) lacked sufficient Nss-sulfate plus nitrate to account for their Cl deficit. Furthermore, isotopic analysis found that in contrast to Bermuda, these particles were systematically depleted in 37Cl, where δ 37Cl ranged from 0 to −1 per mil. This finding indicates that these particles may have undergone an alternative Cl volatilization process.; Most recently, the stable chlorine isotopic composition of tropospheric methyl chloride and stratospheric aerosol chlorine have been measured for the first time. The methyl chloride sample was depleted relative to sea water (−6 per mil). Three stratospheric aerosol samples, collected by aircraft, had δ37Cl values decreasing from −11.2 to −14.1 per mil with altitude. These findings are consistent with a hypothesis that isotopic fractionation may occur during the photolysis of CFCs.; The body of data established here supports the argument that exploration into stable chlorine isotope systematics of the atmosphere should continue.