Atmospheric histories of halocarbons from analysis of Antarctic firn air: Methyl bromide, methyl chloride, chloroform, and dichloromethane

We reconstruct atmospheric levels of methyl bromide (CH 3 Br), methyl chloride (CH 3 Cl), chloroform (CHCl 3 ), and dichloromethane (CH 2 Cl 2 ) back to before 1940 using measurements of air extracted from firn on Law Dome in Antarctica. The firn air at this site has a relatively narrow age spread, giving high time resolution reconstructions. The CH 3 Br reconstructions confirm previously measured firn records but with more temporal structure. Our CH 3 Cl reconstruction is slightly different from previous reconstructions, raising some questions about CH 3 Cl in the firn. Our reconstructions for CHCl 3 and CH 2 Cl 2 are the first published records of concentration prior to direct atmospheric measurements. A two‐box atmospheric model is used to investigate the budgets of these gases. Much of the variation in CH 3 Cl can be explained by biomass burning emissions that increase up to 1980 and then are relatively stable apart from some high burning years such as 1997–1998. The CHCl 3 firn reconstruction suggests that the anthropogenic source for CHCl 3 is greater than previously thought, with human influence on the soil source a possible important contributor here. The CH 2 Cl 2 firn reconstruction is consistent with industrial emission estimates based on audited sales data but suggests that the ocean source of CH 2 Cl 2 is less than previously estimated.