UC Irvine

With a reasonably complete and up-to-date photochemical model of the stratosphere, we find that the calculated stratospheric ozone-column response to chlorine injections is highly nonlinear. The model calculations assume that the background inorganic (or odd) chlorine, ClX, is due to CH3Cl and CCl4. Additional ClX is added to the stratosphere by varying input fluxes of CCl2F2 and CCl3 F. The sensitivity, ΔO3/ΔClX, of the stratospheric O3 column to added ClX is relatively small for ClX ≤3 ppb or ΔClX ≤2 ppb; slight ozone increases with ClX are possible over a limited range of ClX if the formation of chlorine nitrate proceeds rapidly. This may have important implications for total ozone-column trend assessment. As ClX increases beyond 3 ppb, the stratospheric O3 column decreases with ClX increasinly rapidly. This marked departure from the linearity calculated in past years is largely due to presently accepted faster rates of reaction of OH with HNO3, HNO4, HO2, and H2O2. If stratospheric ClX increases to about 9 ppb due to continued usage of CCl2F2, CCl3F, and CH3CCl3, the stratospheric O3 column depletion is calculated to be 6.7-9.0%. Principal uncertainties in these calculations, including the rate of formation of chlorine nitrate, the products of its photolysis, and the present day mixing ratio of ClX are discussed. Calculated ozone decreases due to increased N2O concentrations are also presented.