J. Rudolph et al., THE BUDGETS OF ETHANE AND TETRACHLOROETHENE - IS THERE EVIDENCE FOR AN IMPACT OF REACTIONS WITH CHLORINE ATOMS IN THE TROPOSPHERE, Atmospheric environment, 30(10-11), 1996, pp. 1887-1894
In a number of recent papers evidence has been presented that reaction
s with Cl atoms substantially contribute to the turnover of organic co
mpounds in certain regions of the marine atmosphere. However, the impa
ct of this possible sink mechanism for organic compounds on global or
hemispheric scales is still unknown. Based on the budgets of organic s
ubstances 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 tetrachloroe
thene reaction with Cl are by a factor of 200-300 higher than those wi
th OH-radicals, this reaction being their only significant established
sink in the troposphere. From several series of measurements in the r
emote troposphere we derived an estimate of mean tropospheric distribu
tions and seasonal cycles of tetrachloroethene and ethane. Together wi
th OH-fields from model calculations we calculated the removal of etha
ne 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 rela
tively 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 H
emisphere a plausible upper limit of less than 1000 Cl-atoms cm(-3) ca
n be derived. Due to higher uncertainties in the budgets for the South
ern Hemisphere, the upper limit of the Cl-atom concentration in the So
uthern 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.