Methyl bromide, other brominated methanes, and methyl iodide in polar firnair

We report measurements of brominated, bromochlorinated, and iodinated metha nes in air extracted from deep firn at three polar locations (two Antarctic and one Arctic). Using a firn diffusion model, we are able to reconstruct a consistent temporal trend for methyl bromide from the two Antarctic sites . This indicates a steady increase by about 2 ppt from the midtwentieth cen tury to 8 ppt today. The Arctic firn, however, contained extremely high lev els of methyl bromide as well as numerous other organic gases, which are ev idently produced in situ. The other brominated species (dibromomethane, bro mochloromethane, bromodichloromethane, dibromochloromethane, and bromoform) showed Little or no long-term trend in Antarctic firn and therefore are ev idently of entirely natural origin in the Southern Hemisphere. A clear seas onal trend was observed in the upper firn for the shortest-lived halocarbon s (notably bromoform and methyl iodide). The same species were present at l ower abundance at the higher altitude and more inland Antarctic site, possi bly due to their origin from more distant oceanic sources.