Atmospheric Photochemistry

Abstract

The main photochemical processes occurring in the Earth’s atmosphere and their effects on its chemistry and structure are described. The solar flux and its interaction with the components of air are discussed in Sect. 5.1. Of these, O₂ is the most photochemically active, UV absorption causing photodecomposition into ground state and excited state O atoms (Sect. 5.2). This causes differential heating of the atmosphere as the solar flux passes through. Without this, the air in thermal equilibrium cools with increasing altitude due to the effect of gravity. Combining the two effects creates the distinct layers (Sect. 5.3) known as the tropo-, strato-, meso- and thermo-spheres. Other designations e.g. the high altitude (D–F) regions containing high charge density are due to photo-ionisation. The detailed photochemistry of each region is discussed in Sects. 5.4–5.7, dominated at high altitude by oxygen atom and ozone reactions, which culminates in the stratospheric ozone layer. Ozone depletion due to the photochemistry involving chlorofluorocarbons is discussed. Comparatively little UV penetrates through to the troposphere, except enough to induce the formation of OH. It is the secondary reactions of OH which set off the oxidative chain reactions which dominate low altitude chemistry and initiates ground level ozone production. The combination of strong sunlight and automobile emissions causes photochemical smog. Aerosols play a vital role in dissolving the soluble reactants and oxidised hydrocarbons formed, thus removing them from the atmosphere by deposition as rain, and completing the cycle of pollutant emission and removal from the atmosphere.