Atmospheric chloroform (CHCl3): ocean-air exchange and global mass balance

Based on the observed global concentrations of chloroform (CHCl₃) in the air and seawater we demonstrate that the tropical oceans are a source of chloroform to the atmosphere. Even though the total yearly emissions of CHCl₃, from the oceans are still uncertain, we estimate the flux to be 0.35 Tg yr^-1. Anthropogenic sources we estimate to be 0.25 Tg yr^-1. This result suggests that atmospheric CHCl₃, may be mostly natural in origin and not entirely man-made as previously thought. The global burden of CHCl₃, in the atmosphere is estimated to be about 0.33 Tg and its lifetime is about 6 months. Reaction with tropospheric OH radicals may explain most of the removal of CHCI, from the atmosphere, but the likelihood of other removal mechanisms is suggested. An average concentration of about 25 ppt was observed at ground level, declining atabout 7% km^-1 with height. The ratio of northern hemisphere to southern hemisphere burden of CHCl₃, is found to be about 1.6. A global mass balance model is used to show that these findings are mutually consistent.