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

• Published in: Geophysical research letters Vol. 23; no. 12; pp. 1529 - 1532
• Main Authors: Singh, H B, Thakur, AN, Chen, YE, Kanakidou, M
• Format: Journal Article
• Language: English
• Published: 01.01.1996
• ISSN: 0094-8276

Tetrachloroethylene (C sub(2) Cl sub(4) ), 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 (k sub(C) sub(l) /k sub(O) sub(H) =365 at 275 K). Analysis of C sub(2) Cl sub(4) data, with the help of a global 2-D model and in conjunction with the OH field derived from CH sub(3) CCl sub(3) 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 C sub(2) Cl sub(4) budget and significant removal by Cl is not indicated (Cl less than or equal to 10 super(2) molec. cm super(-) super(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 x 10 super(2) molec. cm super(-) super(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 x 10 super(3) molec. cm super(-) super(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 (10 super(4) -10 super(6) molec. cm super(-) super(3) ). We conclude that despite their high reactivity, Cl atoms are too few to compete with OH radicals ( identical with 10 super(6) molec. cm super(-) super(3) ) in influencing the oxidizing capacity of the global troposphere.