20th century trends and budget implications of chloroform and related tri-and dihalomethanes inferred from firn air

[摘要] Four trihalomethane (THM; CHCl₃, CHBrCl₂, CHBr₂Cl andCHBr₃) and two dihalomethane (DHM; CH₂BrCl and CH₂Br₂)trace gases have been measured in air extracted from polar firn collected atthe North Greenland Icecore Project (NGRIP) site. CHCl₃ was alsomeasured in firn air from Devon Island (DI), Canada, Dronning Maud Land(DML), Antarctica and Dome Concordia (Dome C), Antarctica. All of thesespecies are believed to be almost entirely of natural origin except forCHCl₃ where anthropogenic sources have been reported to contribute~10% to the global burden. A 2-D atmospheric model was run forCHCl₃ using reported emission estimates to produce historicalatmospheric trends at the firn sites, which were then input into a firndiffusion model to produce concentration depth profiles that were comparedagainst the measurements. The anthropogenic emissions were modified in orderto give the best model fit to the firn data at NGRIP, Dome C and DML. As aresult, the contribution of CHCl₃ from anthropogenic sources, mainlyfrom pulp and paper manufacture, to the total chloroform budget appears tohave been considerably underestimated and was likely to have been close to~50% at the maximum in atmospheric CHCl₃ concentrations around1990, declining to ~29% at the beginning of the 21st century.We also show that the atmospheric burden of the brominated THM's in theNorthern Hemisphere have increased over the 20th century whileCH₂Br₂ has remained constant over time implying that it isentirely of natural origin.

[学科分类] 大气科学