Decrease of the carbon monoxide mixing ratio at the tropopause

Atmospheric CO was measured with the aid of instrumented aircraft operating in the vicinity of the tropopause and in the lower stratosphere. In confirmation of previous measurements the CO mixing ratio was found to decrease above the tropopause. From the observed gradients the size of the stratospheric CO sink can be calculated. Above the mixing region adjacent to the tropopause the CO mixing ratio assumes a new constant value of about 0.04 ppm. The stratospheric CO value is interpreted as the balance between the production by oxidation of hydrocarbons (e.g., methane) and the consumption by reaction with OH radicals. The steady-state CO mixing ratio and the OH radical concentration required to provide the observed stratospheric CO sink are in satisfactory agreement with a recent model by McConnell et al. (1971).