THE INDIRECT DETERMINATION OF CHLORINE ATOM CONCENTRATION IN THE TROPOSPHERE FROM CHANGES IN THE PATTERNS OF NONMETHANE 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-a nd North Atlantic and the North Sea in April and May 1991. The changes in the ratios of several pairs of hydrocarbons are used to investigat e the possible impact of Cl-atom reactions on the atmospheric removal processes of hydrocarbons. In order to minimize the potential bias fro m atmospheric mixing processes, a careful selection of suitable hydroc arbon 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 signific ant evidence for an impact of chlorine atoms could be found in this da ta set. The best estimate for the average [Cl]/[OH] ratio is in the ra nge of 10(-3). However, within the various uncertainties associated wi th 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 O H-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 v alues are also far below those derived in some other studies for the m arine boundary layer but are still compatible with recent model calcul ations. Our findings support the conclusion of Rudolph and coworkers t hat on average, Cl-atoms are of limited importance for the chemistry o f the troposphere. It is important that the major part of the uncertai nty 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 fu rther improve the accuracy of the Cl-atom concentration estimates or t heir upper limits derived from changes in the hydrocarbon patterns.