A Review of Upper Atmospheric Photochemistry

Abstract

A review is given of the photochemical processes which determine the distribution of important minor constituents in the stratosphere and mesosphere. Although it is necessary to consider many reactions, it seems that for the first time satisfactory rate data are available for most of these processes. There is, however, a lack of knowledge about the detailed steps of the gas-phase oxidation of methane which affects theoretical results in the lower stratosphere. It is shown that rather fair agreement exists between the calculated and measured concentrations of several of the constituents. However, deviations exist concerning CH4, H2, and the NO profile which was obtained with the chemiluminescent technique.Nitrogen oxides catalyze destruction of ozone and reduce total ozone in the atmosphere considerably. The reduction of ozone by other reactions is also discussed. Some estimates are presented of the ozone reductions which could result from the operation of large fleets of supersonic aircraft due to exhaust injections of nitrogen oxides into the stratosphere.It is indicated that total ozone is rather insensitive to variations in the stratospheric water vapor content (there is a slight increase in total ozone, when the H2O content is increased). Total ozone is, however, much more sensitive to changes in the stratospheric NOx content. Variations in the stratospheric NOx content over long periods may explain those variations which have taken place in total ozone. It is essential to monitor variations in the solar output in the wavelength region 1750–2400 Å, as these may change the stratospheric NOx content and ozone production rates.The reintroduction of the ozone-destroying reaction HO2 + O3 → OH + 2O2 leads to an increase in calculated total ozone. The reason for this surprising result is given.A discussion is given of the possible role of vibrationally excited H2 and electronically excited O3 in the photochemistry of the mesosphere and thermosphere.