Abstract
Chemical reactions in the atmosphere result largely from the absorption of solar radiation in the visible and ultraviolet spectral regions provided the incoming photons (light quanta) carry sufficient energy for chemical processes to be initiated. Photochemical reactions always occur in two steps: the first is the excitation of a molecule by the absorption of radiation; the second is a reaction of the excited molecule. In atmospheric chemistry, the most important process to be considered is dissociation of the excited molecule. Other modes of energy dissipation, such as fluorescence or collisional energy transfer, usually do not lead to chemical changes so that they are of lesser interest. Any quantitative assessment of the photochemical activity of an atmospheric constituent requires knowledge of (a) the photon flux of solar radiation, (b) the absorption cross section of the species under consideration, and (c) the primary quantum yield of the photo-dissociation process. These parameters depend on the wavelength of radiation; absorption cross sections and quantum yields may additionally depend on temperature and/or pressure.
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