Ozone perturbations by enhanced levels of CFCs, N2O, and CH4: A two‐dimensional diabatic circulation study including uncertainty estimates

Abstract

Ozone perturbation estimates are performed in a two‐dimensional diabatic circulation model of the stratosphere and troposphere. Increased future release rates of N2O, CH4, and chlorofluorocarbons (CFCs) are considered in separate as well as in combined scenarios. As there are clear observational indications that the atmospheric concentrations of CH4 and N2O are increasing, in addition to the well‐established fact that CFCs are increasing, model studies of future ozone perturbations should focus on the combined effects of these compounds and their chemical interaction, which we find play a significant role. With our “best estimate” of combined scenarios of N2O, CH4, and CFCs (CFCl3 + CF2Cl2), leading to N2O increases of 40% and CH4 increases of 100% above their 1980 concentrations and CFC levels corresponding to steady state with 1980 releases as representative for the year 2050, the calculations show an average total global ozone depletion of 2.6%. The latitudinal gradients are particularly large. Total ozone depletion ranges from less than 2% at low latitudes to larger than 10% at high latitudes. The transport scheme adopted in our model (diabatic circulation with low eddy diffusivity in the stratosphere) is important for the obtained pattern in distribution of species and ozone depletion. Furthermore, increases in methane contribute strongly to the large variations we estimate for the ozone perturbation with height and latitude, making methane particularly important from a climatic point of view. Because of the strong coupling of the NOx and the Clx chemistry, future ozone perturbations by CFCs, including the nonlinear response in ozone depletion to chlorine increases, depend on levels of odd nitrogen in the stratosphere. The uncertainties connected with present stratospheric NOx measurements and also with key chemical reactions (for example, N2O + O(1D) → NO + NO) are shown to introduce large uncertainties in ozone perturbation calculations. Significant uncertainties may also be introduced by inadequate knowledge of the chlorine chemistry (for example, reactions converting Cl + ClO to HCl) in future high‐chlorine situations.