A quantitative assessment of uncertainties affecting estimates of global mean OH derived from methyl chloroform observations

Abstract

We estimated the global abundance of OH for the years 1988-1994 by interpreting observations of methyl chloroform (MCF) from two networks using an inverse technique and a 3-D chemical transport model driven by assimilated meteorology. Our inversion approach optimized both the emissions of MCF and the abundance of OH. Because of an a priori overestimate of the latitudinal gradient of MCF concentration by the model in the standard setup, the inversion lowers global emissions and the global sink due to OH. Optimized emissions are about 10% lower than published inventories on average between 1988 and 1994, and the decrease in the sink suggested by the inversion implies an average lifetime for MCF (with respect to tropospheric OH) of about 6.9 years, 11-21% longer than the 5.7-6.2 years reported in previous optimization studies. Our results are driven by the need to match the observed latitudinal gradient of MCF while balancing the MCF budget. We find that these results depend on the a priori constraint placed on MCF emissions, the rate of interhemispheric mixing in the model, the interhemispheric distribution of OH assumed, and the model simulation of pollution events. Since these factors are highly uncertain, we believe that the level of understanding on global lifetimes of pollutants removed by OH is lower than might be implied by the narrow range of estimates for MCF lifetime in the literature. Copyright 2008 by the American Geophysical Union.