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 series of global measurements taken over the last 16 yr at seven locations distributed among the polar, middle, and tropical latitudes of both hemispheres (1981–1997). Measurements were also taken at 20 more sites between 1987 and 1989. The vertical distribution was measured during campaign experiments in the Arctic, Western Atlantic, and over Brazil. Small, mostly decreasing trends are observed, showing that on average, there was 4% less methyl chloride during the last three years (1994–1996) than there was in the first three years (1985–1987) of the experiment. The latitudinal variation is marked by highest concentrations in the tropics and lowest in the polar regions. Sites representing inland locations show higher concentrations, suggesting 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 calculated at the polar, middle, and tropical latitudes represented by the six long-term primary sites. Using a photochemical model of OH, we estimate that a global source of about 3.7 Tg yr -1 of methyl chloride is needed to explain the observed concentrations. Other removal processes have been identified that may add to this estimate of the global annual emissions. The results further establish that some 85% of the emissions must come from the half of the earth’s surface between 30°S and 30°N, representing tropical and sub-tropical latitudes. Small emissions are estimated for the middle latitudes, and no emissions are expected from the polar regions.