A record of atmospheric halocarbons during the twentieth century from polar firn air

Measurements of trace gases in air trapped in polar firn (unconsolidated sn ow) demonstrate that natural sources of chlorofluorocarbons, halons, persis tent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are m inimal or non-existent. Atmospheric concentrations of these gases, reconstr ucted back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric l ifetimes. The measurements confirm the predominance of human activity in th e atmospheric budget of organic chlorine, and allow the estimation of atmos pheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to t hat at present, while the burden of methyl bromide was probably over half o f today's value.