MEASUREMENT OF (CO)-C-13 AND (CO)-O-18 IN THE FREE TROPOSPHERE

We present delta 13C, delta 18O, and concentration measurements of atm ospheric 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 differ ent seasons, between 1990 and 1993. Seasonally dependent spatial trend s for all three species are observed in different regions, and reflect both the changing relative strengths of CO sources and the strength o f the loss mechanism of CO+OH. For example, CO enriched in O-18 result ing from fossil fuel combustion is clearly observed during winter in t he northern hemisphere, in both remote and urban areas, whereas sample s collected near Antarctica during the summer have very low concentrat ions of CO and depleted values of O-18. 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 d uring GO-OH oxidation and by changing sources; the combination of isot opic values and concentration data suggests there may be a CO sink in addition to oxidation by reaction with OH. Relatively small changes we re observed in the tropical Pacific between summer and winter months.