Atmospheric methyl chloride

There are about 5 Tg of methyl chloride in the Earth's atmosphere making it one of the largest reservoirs of gas-phase chlorine. We discuss the time s eries of global measurements taken over the last 16 yr at seven locations d istributed among the polar, middle, and tropical latitudes of both hemisphe res (1981-1997). Measurements were also taken at 20 more sites between 1987 and 1989. The vertical distribution was measured during campaign experimen ts in the Arctic, Western Atlantic, and over Brazil. Small, mostly decreasi ng trends are observed, showing that on average, there was 4% less methyl c hloride during the last three years (1994-1996) than there was in the first three years (1985-1987) of the experiment. The latitudinal variation is ma rked by highest concentrations in the tropics and lowest in the polar regio ns. Sites representing inland locations show higher concentrations, suggest ing continental sources, mostly confined to the tropics. There are seasonal variations at various latitudes that can be explained mostly by the cycles of OH radicals, which are the dominant removal process for methyl chloride in the atmosphere. Based on these data, the expected emissions can be calc ulated at the polar, middle, and tropical latitudes represented by the six long-term primary sites. Using a photochemical model of OH, we estimate tha t a global source of about 3.7 Tg yr(-1) of methyl chloride is needed to ex plain the observed concentrations. Other removal processes have been identi fied that may add to this estimate of the global annual emissions. The resu lts further establish that some 85% of the emissions must come from the hal f of the earth's surface between 30 degrees S and 30 degrees N, representin g tropical and sub-tropical latitudes. Small emissions are estimated for th e middle latitudes, and no emissions are expected from the polar regions. ( C) 1999 Elsevier Science Ltd. All rights reserved.