THE GLOBAL CYCLE OF CARBON-MONOXIDE - TRENDS AND MASS BALANCE

The annual global emissions of CO are estimated to be about 2,600 ± 600 Tg, of which about 60% are from human activities including combustion of fossil fuels and oxidation of hydrocarbons including methane. The remaining 40% of the emissions are from natural processes, mostly from the oxidation of hydrocarbons but also from plants and the oceans. Almost all the CO emitted into the atmosphere each year is removed by reactions with OH radicals (85%), by soils (10%), and by diffusion into the stratosphere. There is a small imbalance between annual emissions and removal, causing an increase of about 1% per year. It is very likely that the imbalance is due to increasing emissions from anthropogenic activities. The average concentration of CO is about 90 ppbv, which amounts to about 400 Tg in the atmosphere, and the average lifetime is about 2 months. This view of the global cycle of CO is consistent with the present estimates of average OH concentrations and the budgets of other trace gases including methane and methylchloroform. There are large remaining uncertainties that may in the future upset the apparently cohesive present budget of CO. If the present view of the global cycle of CO is correct, then it is likely that, in time, increasing levels of CO will contribute to widespread changes in atmospheric chemistry. © 1990.