Interactions between CO, OH, and CH4: Past and Future Scenarios

One key issue for atmospheric chemistry in the 21st century is to develop a better understanding of the global oxidizing capacity of the atmosphere in order to predict the concentrations of the greenhouse gases CH-4 and O3. The oxidizing capacity of the atmosphere is largely determined by the OH radical and reaction of OH with CH4 will determine future concentrations of CH4. The main sources of OH include photolysis of O3 to produce O(1D) followed by the reaction of O(1D) with H2O and reaction of HO-2 with NO. The main sinks include its reaction with CO and CH4 and, in the boundary layer, reaction with nonmethane hydrocarbons. Prediction of OH is hampered by our lack of understanding of CO and CH-4 sources. The former are especially important because the shorter lifetime of CO (about 2 months) make its concentrations vary significantly with location, so that changes in regional sources result in changes in regional OH. Thus, accurate future prediction of CH4 requires accurate knowledge of the individual sources and sinks of CO. Here, we use observed concentrations of CH4 and improved estimates for the sources and sinks of CO to estimate present OH concentrations. Comparison with observations are used to evaluate the sources and sinks of CO. Finally, we use future emissions from the new IPCC Special Report on Emissions Scenarios for CO and CH4, adjusted to account for our improved sources, to analyze future concentrations of CO, OH, and CH4. Uncertainties in estimates of future concentrations are highlighted.