Mid Pacific determinations of atmospheric 14CO and implications for the oxidizing capacity of the atmosphere.

The principal sink for atmospheric ¹⁴CO is oxidation to ¹⁴CO_2 by the hydroxyl radical (OH), with a relatively short chemical lifetime of about 1--1.5 months in the tropics. OH is produced throughout the atmosphere by the reaction of O(1D) with water vapor, and has a maximum in the troposphere where lower photolytic production of O(1D) is more than compensated for by high water vapor levels. Because this production rate is dependent on the photolysis rate of O(1D) generation, it varies seasonally and is expected to be the principal driver of seasonal cycles and inter-hemispheric differences in the abundance of atmospheric ¹⁴CO. Hence, assuming that ¹⁴CO transport and production processes are well known, ¹⁴CO data may be used to infer atmospheric OH distributions. In this work we present new ¹⁴CO data from air samples collected aboard container ships on eight voyages between New Zealand and the US west coast. The data span a 4-year period and because each voyage lasts only 3 weeks provide snapshot determinations of the ¹⁴CO inter-hemispheric gradient covering various seasons. The limited dataset shows symmetrical ¹⁴CO distributions in each hemisphere that anti-correlate with expected abundances for OH. Systematic ¹⁴CO minima occur within 5 degrees of latitude on either side of the intertropical convergence zone. We discuss these data in terms of cross tropopause transport and expected OH distributions.