The indirect determination of chlorine atom concentration in the troposphere from changes in the patterns of non-methane hydrocarbons

About 200 measurements of C-2-C-9 hydrocarbons were made during a ship cruise (NATAC 91) in the western Mediterranean Sea, the eastern Mid-and North Atlantic and the North Sea in April and May 1991. The changes in the ratios of several pairs of hydrocarbons are used to investigate the possible impact of Cl-atom reactions on the atmospheric removal processes of hydrocarbons. In order to minimize the potential bias from atmospheric mixing processes, a careful selection of suitable hydrocarbon sets is essential. The NMHC mixing ratios observed during NATAC 91 are generally well above those typical for air in the remote marine background and the observed changes in the hydrocarbon patterns agree very well with those predicted from OH-radical chemistry. No significant evidence for an impact of chlorine atoms could be found in this data set. The best estimate for the average [Cl]/[OH] ratio is in the range of 10(-3). However, within the various uncertainties associated with such estimates, this value is not significantly different from zero. The upper limit is in the range of 2-7.10(-3). Based on an average OH-radical concentration of 10(6) cm(-3), this corresponds to some 10(3) Cl-atoms cm(-3). This is far below the Cl-atom concentrations found during the tropospheric ozone depletion events in arctic spring. Our values are also far below those derived in some other studies for the marine boundary layer but are still compatible with recent model calculations. Our findings support the conclusion of Rudolph and coworkers that on average, Cl-atoms are of limited importance for the chemistry of the troposphere. It is important that the major part of the uncertainty of the Cl-atom concentration estimates results from errors in the rate constants for the reactions of the hydrocarbons with OH-radicals. A reliable and consistent set of the relevant rate constants would further improve the accuracy of the Cl-atom concentration estimates or their upper limits derived from changes in the hydrocarbon patterns.