Atmospheric chloroform

Chloroform is one of several chlorine-containing gases in the atmosphere that by themselves are unimportant in the chlorine budget of the Earth’s atmosphere, but may be significant if taken together. We present a nine-year time series of concentrations at the surface of the polar, middle, and tropical latitudes of both hemisphere. These and related data establish its seasonal, latitudinal, and altitudinal concentrations. The average concentration observed at the surface is 18.5 pptv, but at some continental locations annual averages can reach 40 pptv. The atmospheric lifetime of chloroform is short at about 0.5 yr due to reactions with OH and may be shorter if other removal processes exist. No significant trends were observed during the last decade. Using these data we calculate that the global emission rate must be about 470 (350–600) Gg yr-1 with about 70% emitted in the northern middle and tropical latitudes.