| Summary: | Four trihalomethane (THM; CHCl 3 , CHBrCl 2 , CHBr 2 Cl and CHBr 3 ) and two dihalomethane (DHM; CH 2 BrCl and CH 2 Br 2 ) trace gases have been measured in air extracted from polar firn collected at the North Greenland Icecore Project (NGRIP) site. CHCl 3 was also measured in firn air from Devon Island (DI), Canada, Dronning Maud Land (DML), Antarctica and Dome Concordia (Dome C), Antarctica. All of these species are believed to be almost entirely of natural origin except for CHCl 3 where anthropogenic sources have been reported to contribute ~10% to the global burden. A 2-D atmospheric model was run for CHCl 3 using reported emission estimates to produce historical atmospheric trends at the firn sites, which were then input into a firn diffusion model to produce concentration depth profiles that were compared against the measurements. The anthropogenic emissions were modified in order to give the best model fit to the firn data at NGRIP, Dome C and DML. As a result, the contribution of CHCl 3 from anthropogenic sources, mainly from pulp and paper manufacture, to the total chloroform budget appears to have been considerably underestimated and was likely to have been close to ~50% at the maximum in atmospheric CHCl 3 concentrations around 1990, declining to ~29% at the beginning of the 21st century. We also show that the atmospheric burden of the brominated THM's in the Northern Hemisphere have increased over the 20th century while CH 2 Br 2 has remained constant over time implying that it is entirely of natural origin.
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