Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today's value.
Butler, James H.; Battle, Mark; Bender, Michael L.; Montzka, Stephen A.; Clarke, Andrew D.; Saltzman, Eric S.; Sucher, Cara M.; Severinghaus, Jeffrey P.; and Elkins, James W., "A record of atmospheric halocarbons during the twentieth century from polar firn air" (1999). Physics Faculty Publications. 23.