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General Schemes of Tropospheric Chemistry

Ozone is present in the natural, unpolluted troposphere, and its tropospheric column density is approximately 10% of the total atmospheric (troposphere + stratosphere) ozone column density. (Logan, 1985; Brühl and Crutzen, 1989; Fishman et al., 1990). The ozone present in the stratosphere absorbs short-wavelength radiation (image nm (nanometers or 10-9 meters)) from the sun and allows only those wavelengths image nm to penetrate into the troposphere (Peterson, 1976; Demerjian et al., 1980). The sources of ozone in the natural troposphere are downward transport from the stratosphere and in situ photochemical production. Losses result from photochemical processes and from deposition and destruction at the earth's surface. The rates of downward transport, production, and losses are estimated to be of the same order of magnitude (Logan, 1985). The ozone present in the troposphere is important in the atmospheric chemistry because the OH radical is generated from the photolysis of ozone at wavelengths <319 nm (Levy, 1971; DeMore et al., 1990). The formation of OH radicals leads to cycles of reactions that result in the photochemical degradation of organic compounds of anthropogenic and biogenic origin, the enhanced formation of ozone, and the atmospheric formation of acidic compounds (see, for example, Heicklen et al., 1969; Stedman et al., 1970; Finlayson-Pitts and Pitts, 1986; WMO, 1986). The generation of the OH radical from ozone is shown in the following reactions:

image

Energy from solar radiation is represented by hv, the product of Planck's constant, h, and the frequency, v, of the electromagnetic wave of solar radiation. O(1D) is an excited oxygen atom, and M is an inert compound, such as N2 or O2. O(3P) is a ground state oxygen atom. The chemistry of the clean, unpolluted troposphere is dominated by the chemistry of methane (CH4) and



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