A three‐dimensional global model study of atmospheric methyl chloride budget and distributions

Global simulations of atmospheric methyl chloride (CH 3 Cl) are conducted using the GEOS‐CHEM model in order to understand better its sources and sinks. Observations from 7 surface sites and 9 aircraft field experiments are used to evaluate the model simulations with assimilated meteorology fields for 7 years. The model simulates CH 3 Cl observations at northern mid and high latitudes reasonably well. The seasonal variation of CH 3 Cl at southern mid and high latitudes is severely overestimated, however. Simulated vertical profiles of CH 3 Cl are in general agreement with the observations in most regions; the disagreement occurs in the vicinities of major sources, principally reflecting the uncertainties in the estimated distributions of our added pseudobiogenic and the biomass burning sources. Our estimate of known sources (1.5 Tg yr −1 ) from ocean, biomass burning, incineration/industry, salt marshes, and wetlands accounts for only 34% of the total source (4.4 Tg yr −1 ). We hypothesize that the missing source of 2.9 Tg yr −1 is likely of biogenic origin. On the basis of the observed CH 3 Cl seasonality at northern mid and high latitudes, we find that this pseudobiogenic source is located at 30°N–30°S, not at mid and high latitudes. If so, the observed CH 3 Cl latitudinal distribution indicates that the annual hemispheric mean OH ratio is within the range of 0.8–1.3. The net uptake regions by ocean are located at high latitudes. A relatively small loss of 150 Gg yr −1 over these regions is critical for the model to reproduce the observed annual mean latitudinal gradient of CH 3 Cl in the southern hemisphere. The large overestimate of the seasonal variation of CH 3 Cl at southern mid and high latitudes likely implies that the seasonality of simulated oceanic uptake is incorrect as a result of defects in the parameterization of this loss in the model.