The budgets of ethane and tetrachloroethene: Is there evidence for an impact of reactions with chlorine atoms in the troposphere?

In a number of recent papers evidence has been presented that reactions with Cl atoms substantially contribute to the turnover of organic compounds in certain regions of the marine atmosphere. However, the impact of this possible sink mechanism for organic compounds on global or hemispheric scales is still unknown. Based on the budgets of organic substances which react with Cl-atoms much faster than with OH-radicals it is possible to derive upper limits for the average tropospheric Cl-atom concentration. The rate: constants for the ethane and tetrachloroethene reaction with Cl are by a factor of 200–300 higher than those with OH-radicals, this reaction being their only significant established sink in the troposphere. From several series of measurements in the remote troposphere we derived an estimate of mean tropospheric distributions and seasonal cycles of tetrachloroethene and ethane. Together with OH-fields from model calculations we calculated the removal of ethane and tetrachloroethene by OH-radicals. Within the uncertainties the calculated removal agrees with the known emissions of these substances. In spite of the substantial uncertainties of these budgets, the relatively high reactivity of these substances towards Cl-atoms allows to estimate useful upper limits of the removal rates by Cl-atoms and thus of the average tropospheric Cl-atom concentration. For the Northern Hemisphere a plausible upper limit of less than 1000 Cl-atoms cm−3 can be derived. Due to higher uncertainties in the budgets for the Southern Hemisphere, the upper limit of the Cl-atom concentration in the Southern Hemisphere is nearly 2000 Cl-atoms cm−3. Nevertheless, these results show that on a global scale the Cl-atom-induced reactions for most organic trace gases are of minor importance.