Effects of Diurnal Variations and Scattering on Ozone in the Stratosphere for Present-Day and Predicted Future Chlorine Concentrations

Abstract

The effect on ozone of diurnal variations in the concentrations of chlorine, nitrogen, hydrogen and oxygen compounds was studied with a diurnal integration of the photochemical equations at 30° latitude. The vertical distribution of the sum of the nitrogen oxides was specified and ozone was assumed to be in photochemical equilibrium. Calculations were performed for three different distributions of ClX (ClX = Cl + ClO + HCl + ClONO2), in which the maximum ClX mixing ratios were 1.5, 7.5 and 15 ppb. The results were compared with approximate calculations employing average daytime dissociation rates and 24-hour averaged dissociation rates. It was found that the methods employing daytime-averaged dissociation coefficients yielded more accurate O3 concentrations at all attitudes and, more importantly, were more reliable indicators of changes in total ozone amount resulting from chlorofluoromethane pollution. Several useful approximations are developed to improve the accuracy of photochemical models designed to study the effects of chlorine compounds on ozone. The effects of scattering on dissociation at wavelengths > 290 nm were included in the model with the parameterization of Lacis and Hansen. It was found that upward reflection of sunlight from clouds was quite important and that NO, N2O3, ClONO2, HCl, OH, O(1D) and O(3P) were significantly affected. A simple technique for including the effects of scattering and reflection in photochemical calculations is presented.