THE CHLORINE BUDGET OF THE PRESENT-DAY ATMOSPHERE - A MODELING STUDY

We present results from a time-dependent model simulation of the atmospheric total chlorine and show the distribution of source gases and of inorganic chlorine (ClY) derived from them as functions of latitude and altitude. Source gases modeled include CFC-11, CFC-12, CCl4, CFC-113, HCFC-22, CH3CCl3, and CH3Cl. Results are presented for the 1985 atmosphere using historical emissions; steady state results are also shown for comparison. The relationship between the upper stratospheric concentration of ClY derived from a specific source gas and the surface concentration of the source gas reflects both the removal rate of the gas and whether the gas is in steady state with the emissions. Observations show that ozone changes in the past decade have been dominated by losses in the mid-latitude lower stratosphere and the polar regions (WMO, 1988). The ClY concentration in these regions is derived from source gases in the following proportions for the 1985 model-calculated atmosphere: CFC-11 24%, CH3Cl 21%, CCl4 19%, CH3CCl3 18%, CFC-12 15%, CFC-113 2%, and HCFC-22 1%. CFC-11, CCl4, and CH3CCl3 play a larger role in the lower stratosphere than the corresponding steady state estimates using surface concentration would suggest.