Numerical simulation of the transport and chemistry of CH 4 and co in the troposphere

Abstract

A model for the methane-carbon monoxide system which considers the physico-chemical behavior of these trace gases in the troposphere is discussed. Source-sink descriptions are incorporated into the model based on the best available information relative to the major anthropogenic and natural contributions. The distributions of methane and carbon monoxide in the atmosphere are interrelated by the chemical reactions in which they participate; and a photochemical model, based on the pseudo-steady state approximation for the intermediate species and for inclusion in the species continuity equations, accounts for these reactions. Two 20-day dynamic simulations are described, both of which use January climatological data for meteorological input to numerically solve the conservation of mass equations for CO and CH 4. The models employed, respectively, current estimates of the anthropogenic CO sources, and five times that value. Based on the results, residence times of 0.21 yr for CO and 2.7–4.0 yr for CH 4 were calculated. Furthermore, the CO mixing ratio over the Pacific Ocean was predicted to be reasonably symmetric, indicating longitudinal CO concentration variations and the lack of widespread north-south asymmetry as has been observed over the Atlantic Ocean. This is consistent with a relatively short CO residence time and the model prediction that substantial anthropogenic CO is not transported from North America to Europe. Increasing the anthropogenic CO production rate substantially increased the CO concentrations near the sources, and this effect was more pronounced over Europe than over Japan or North America. Detailed comparisons of the model predictions with large-scale features suggested by other studies, such as oceanic source strength and north-south interhemispheric exchange, were calculated and generally show good agreement.