A preliminary three-dimensional global model study of atmospheric methyl chloride distributions

A global three-dimensional atmospheric model of methyl chloride (CH₃Cl) is presented. When incorporating known terrestrial and oceanic source terms, the tropospheric budget of CH₃Cl is unbalanced. We show that a reduction in the atmospheric CH₃Cl loss rate could account for the net budget discrepancy but fails to reproduce the observed latitudinal distribution. We find that observed mixing ratios and latitudinal distributions can be reproduced by addition of a tropical terrestrial CH₃Cl source of 2330-2430 Gg yr^-1 combined with a 50% reduction in the southeastern Asian biomass burning contribution. This is equivalent to a net source of 3800-3900 Gg yr^-1, slightly higher than previously estimated. The magnitude of additional emissions required to match observations is sensitive to their latitudinal distribution. We successfully simulate tropical land-based observations best when the added source is increased at the coasts relative to inland areas. Mixing ratios at remote sites are relatively insensitive to the finer details of the source parameterization.