The Elements of Stratospheric and Tropospheric Chemistry

Abstract

The present chapter addresses some specific topics in the chemistry of the stratosphere and the troposphere. In particular, we shall consider in some detail the processes that generate and destroy ozone in the two regions (Johnston, 1992; Wayne, 1991). Ozone is a critically important component in the Earth’s atmosphere, because the existence of the stratospheric ozone layer permits the survival of life on the surface of the planet. Some ozone from the stratosphere can be transported across the tropopause to enter the troposphere, but there are also local sources of ozone within the troposphere itself. In this lower atmospheric region, ozone plays a key rôle in oxidation processes, both by day and by night, although the ozone is most important as a precursor of radicals, such as the hydroxyl radical (OH) or the nitrate radical (NO₃), rather than as a direct oxidant. A comparison of tropospheric and stratospheric chemistry is complex, especially since the oxides of nitrogen (NO and NO₂, here collectively referred to as NO_х) are implicated both in the production of ozone at low altitudes, and its destruction higher up. One factor that determines the different behaviour at different altitudes is the concentration of atomic oxygen, which is greater at higher altitudes both because its photochemical production is favoured and because the loss in recombination with O₂, whose rate is proportional to the square of the atmospheric pressure, increases rapidly with decreasing altitude. Reduced organic species are much more abundant and varsed in the lower atmosphere, and tropospheric chemistry is thus intimately bound up with the oxidation of these compounds.