The Stratosphere as a Chemical Sink for Carbon Monoxide

Abstract

Past and recent data on atmospheric CO levels indicate no significant increase of average concentrations- despite the increasing anthropogenic emission rate-so that one or more natural sinks for CO seem to be operative. It is shown that the stratosphere provides a sink for CO on account of the reaction OH + CO ? CO2 + H. The radical OH is produced photochemically in a moist ozonosphere with a time constant several orders of magnitude less than that of the CO oxidation reaction, so that almost all the CO entering the stratosphere will be destroyed. The rate limiting factor is the transport of CO rich air through the tropopause. To determine the significance of the stratosphere sink, two theoretical models for the tropospheric CO behavior are considered. Although the theory provides several constraints for the rate of CO removal from the atmosphere, it is found that the uncertainties concerning measurements and source functions preclude the derivation of a reliable value for the total CO removal rate. An estimate indicates that the stratosphere sink contributes significantly, but only partially, to the overall removal of CO from the atmosphere.