Atmospheric ozone: An analytic model for photochemistry in the presence of water vapor

An approximate analytic model of ozone photochemistry including reactions with hydrogen compounds is developed for the atmospheric region between 15 and 60 km. The reaction scheme is a simplified version of that used by B. G. Hunt. The model is used to study time-dependent processes, the sensitivity of the equilibrium-concentration values to uncertainties in reaction rates, and the variations in the equilibrium ozone concentration with latitude and season. There are five main results. (1) Ozone loss in the model depends on the ratio of the rate of production of O(1D) and the rate of dissociation of H2O by O(1D), but the loss rate and equilibrium ozone concentration are not very sensitive to this ratio. (2) Below 40 km the model is quite sensitive to the rates of reaction of O3 with OH and HO2; these unknown reaction rates are the weakest link in the theory. (3) Above 40 km the relevant reaction rates are comparatively certain and the ozone concentration is very likely to be controlled by reactions of atomic oxygen with OH and HO2. (4) As a consequence of (3), the ozone concentration near the stratopause is probably not sensitive to temperature, and the dynamical damping sometimes attributed to this temperature sensitivity is likely to be unimportant. (5) If the model is correct, the photochemical time scale for ozone is much less than it would be if only oxygen reactions control ozone; consequently, in low latitudes, ozone may be subject to significant photochemical influence down to as low as 15 km.