A strong source of methyl chloride to the atmosphere from tropical coastal land

Methyl chloride (CH₃Cl), the most abundant halocarbon in the atmosphere, has received much attention as a natural source of chlorine atoms in the stratosphere. The annual global flux of CH₃Cl has been estimated to be around 3.5 Tg on the grounds that this must balance the loss through reaction with OH radicals (which gives a lifetime for atmospheric CH₃Cl of 1.5 yr). The most likely main source of methyl chloride has been though to be oceanic emission, with biomass burning the second largest source. But recent seawater measurements indicate that oceanic fluxes cannot account for more than 12% of the estimated global flux of CH₃Cl, raising the question of where the remainder comes from. Here we report evidence of significant CH₃Cl emission from warm coastal land, particularly from tropical islands. This conclusion is based on a global monitoring study and spot measurements, which show enhancement of atmospheric CH₃Cl in the tropics, a close correlation between CH₃Cl concentrations and those of biogenic compounds emitted by terrestrial plants, and OH-linked seasonality of CH₃Cl concentrations in middle and high latitudes. A strong, equatorially located source of this nature would explain why the distribution of CH₃Cl is uniform between the Northern and Southern hemispheres, despite their differences in ocean and land area.