Tetrachloroethylene as an indicator of low Cl atom concentrations in the troposphere

Tetrachloroethylene (C2Cl4), a largely man-made chemical pollutant, is known to react with Cl atoms at a rate that is some 300 times faster than with OH radicals (kCl/kOH = 365 at 275 K). Analysis of C2Cl4 data, with the help of a global 2-D model and in conjunction with the OH field derived from CH3CCl3 observations, has been used to provide a sensitive means for evaluating Cl atom abundance in the troposphere. In the “mean case” scenario, that employs best available measurements, emissions and kinetic parameters, it is found that OH oxidation is adequate to balance the C2Cl4 budget and significant removal by Cl is not indicated (Cl ≤10² molec. cm−3). An “upper limit” analysis that takes into account possible uncertainties in measurements, source emissions (man-made and natural), and reaction rates, is performed to estimate that annually averaged Cl atom concentrations in the troposphere are <5–10×10² molec. cm−3. If we assume that nearly all (80–100%) of the Cl atoms reside in the marine boundary layer (MBL), mean MBL concentrations are estimated to be <5–15×10³ molec. cm−3. This analysis implies that mean Cl concentrations in the MBL are below or near the lower end of the values inferred in recent studies (104-106 molec. cm−3). We conclude that despite their high reactivity, Cl atoms are too few to compete with OH radicals (≈ 106 molec. cm−3) in influencing the oxidizing capacity of the global troposphere.

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