The potential impact on atmospheric ozone and temperature of increasing trace gas concentrations

The response of the atmosphere to emissions of chlorofluorocarbons (CFCs) and other chlorocarbons and to increasing concentrations of other radiatively active trace gases such as CO 2 , CH 4 , and N 2 O is calculated by means of a coupled chemical‐radiative‐transport one‐dimensional model. It is shown that significant reductions in the ozone concentration and in the temperature should be expected in the upper stratosphere as a result essentially of increasing concentrations of active chlorine produced by photo decomposition of the CFCs. For a constant emission of chlorofluorocarbons‐11 and ‐12 at approximately the 1980 level (309 kT/yr for F‐11 and 433 kT/yr for F‐12) the calculated decrease (assuming no other changes) in the ozone concentration relative to the preindustrial atmosphere is approximately 60–70% at 40 km, and the reduction in the ozone column is 8.7% when temperature feedback is included in the model and 5.5% when it is omitted. The model also shows that a doubling of CO 2 leads to a 1.8% increase in the ozone column abundance and a 2 K increase in the surface temperature. At 40 km the ozone density is enhanced by 15% and the temperature reduced by 8 K. A doubling of methane produces a 2.4% increase in the ozone column. A 20% enhancement in the nitrous oxide content leads to an ozone depletion of 1.4%. Time‐dependent calculations of ozone and temperature show that if the production of F‐11 and F‐12 increases by 3%/yr until it reaches a capacity cap equal to 1.5 times the 1985 world production level, the maximum ozone depletion should be of the order of 4–5%, assuming a growth in the concentration of CO 2 , CH 4 , and N 2 O of about 0.5, 1.0, and 0.25%/yr, respectively. For this scenario the maximum ozone depletion is found to occur in year 2070. The slight increase appearing after 2070 is directly dependent on the future growth in the methane concentration. If the production of F–11 and F–12 increases continuously by 3%/yr, without capacity cap, and if the changes in the concentration of other trace gases are ignored, the ozone column abundance is predicted to be reduced by more than 10% after year 2050. The model shows an almost linear relation between the ozone depletion and the chlorine content as long as the mixing ratio of active chlorine remains smaller than that of active nitrogen. The O 3 /Cl x sensitivity, however, is a strong function of NO y content. For amounts of chlorine comparable or larger than those of active nitrogen the ozone depletion increases rapidly with the amount of chlorine present in the atmosphere.