The atmospheric¹²CO and¹⁴CO budget

The budget of CO in the atmosphere is discussed. For 12CO the known sources are anthropogenic emission and production by the oceans. The magnitude of the latter source is derived for the northern hemisphere from measurements of CO concentrations over the Atlantic and Pacific oceans. The destruction of 12CO occurs in the stratosphere and by a tropospheric sink of unknown nature. The anthropogenic and the maritime CO sources do not contribute significantly to the production of 14CO that occurs mainly by cosmic radiation. A model is presented for the budgets of 14CO and 12CO in the atmosphere by considering the stratospheric and tropospheric domains separately. On the basis of this model the flux of 14CO across the tropopause can be calculated. The results provide the magnitude of the tropospheric sink as a function of the tropospheric 14CO concentration and enable a comparison of the 14CO and 12CO residence times in the troposphere. If the tropospheric 14CO value is that derived from the measurements of McKay et al. (1963), the residence times of 14CO and 12CO are approximately the same and are about 0.2 yr. It is argued, however, that the 12CO residence time is probably about 1 yr. In that instance the 14CO and 12CO residence times may differ appreciably depending on several parameters involved.