August 2007
A Quantitative Assessment of Uncertainties Affecting Estimates of Global Mean OH Derived from Methyl Chloroform Observations

We estimated the global abundance of OH by interpreting observations of methyl chloroform (MCF) from two networks using an inverse technique and the 3-D GEOS-Chem model. Our inversion approach optimized both the emissions of MCF and the abundance of 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 studies.

The figure shows latitudinal profiles of annual mean MCF concentrations, for the standard inversion setup. a) 1988 (GAGE). b) 1992 (ESRL, without South Pole site). Black squares = observations, red cross = model a priori, green plus = model a posteriori. The a priori interhemispheric gradient is too large, with concentrations too high in the NH and too low in the SH. To reduce the gradient, the inversion reduces emissions, lowering MCF at the NH sites, and reduces OH, increasing MCF at the SH sites (the SH is not influenced much by local emissions); the lowering of OH also maintains a global mass balance for MCF.

We find that our 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?s 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.

For more information see Wang et al [2007].