Measurement of ¹³CO and C¹⁸O in the free troposphere

We present δ¹³C, δ¹⁸O, and concentration measurements of atmospheric CO from several sampling trips over a broad range of latitude. More than 130 samples were collected from the middle troposphere and the boundary layer, from Antarctica to northern Sweden, during different seasons, between 1990 and 1993. Seasonally dependent spatial trends for all three species are observed in different regions, and reflect both the changing relative strengths of CO sources and the strength of the loss mechanism of CO+OH. For example, CO enriched in ¹⁸O resulting from fossil fuel combustion is clearly observed during winter in the northern hemisphere, in both remote and urban areas, whereas samples collected near Antarctica during the summer have very low concentrations of CO and depleted values of ¹⁸O. Of particular interest is the data set from the middle and high latitude southern hemisphere during the summer months, which is influenced both by fractionation effects during CO-OH oxidation and by changing sources; the combination of isotopic values and concentration data suggests there may be a CO sink in addition to oxidation by reaction with OH. Relatively small changes were observed in the tropical Pacific between summer and winter months.