CTM study of changes in tropospheric hydroxyl distribution 1990-2001 and its impact on methane

Impacts of emission changes on hydroxyl (OH) and methane lifetime in the troposphere are studied using an emission inventory for the period 1990-2001 as input to a global Chemical Transport Model (CTM) run with repeated year 2000 meteorology. Global OH is estimated to increase, in average by 0.08% per year, with exception of 1997 and 1998 when deviations from the trend result from unusually large biomass burning emissions. The global OH evolution is highly dependent on the CO to NOx emission ratio, and a statistically significant linear relation between this ratio and the OH concentration is found. However, we find large regional variations of OH distributions and methane loss. One third of a calculated increase in global average methane loss (0.50% per year) can be explained by OH changes resulting from emission changes of CO, NOx and NMVOCs, while two third is due to increases in methane itself.