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Introduction
Ultraviolet-B radiation (280-315 nm) is sufficiently energetic to cause photolysis of atmospheric trace gases such as ozone (O3), nitrogen dioxide (NO2), hydrogen peroxide (H2O2), formaldehyde (HCHO), and nitric acid (HNO3) (e.g.: Finlayson-Pitts and Pitts, 1986; Graedel and Crutzen, 1993). The products of photolysis: O, NO, OH, H, HCO, and eventually HO2 and organic peroxy radicals, are highly reactive. The oxidizing capacity of the troposphere is controlled by these photolytic products, especially hydroxyl radicals (OH), which originate from the photolysis of ozone in the presence of water vapor:

O3 + hn (l £330nm) ® O(1D) + O2

O(1D) + H2O ® OH + OH

Ozone photolysis in the troposphere is strongly dependent on the available UV-B radiation, and therefore is sensitive to absorption by stratospheric ozone. Figure 6.1 shows the rate coefficients for this process, measured at Mauna Loa (Hawaii), as a function of the amount of ozone in the path of the solar beam.
 
Fig. 6.1. Dependence of the rate coefficient JO3 for the reaction O3 + hn (330 nm) ® O(1D) + O2 on the amount of ozone in the path of the solar beam (the slant ozone column). The points give over 33,000 direct measurements of JO3 obtained with a chemical actinometer at Mauna Loa Hawaii, during 1991-1992 by Shetter et al. (1996).

Important atmospheric trace gases, such as CH4 and other hydrocarbons, several halocarbons, NO2, as well as sulfur containing species are primarily removed by OH. In addition, OH plays an important role in the production of tropospheric ozone. UV-B radiation is a key environmental factor controlling tropospheric chemistry. Stratospheric O3 reduction and its consequence of increased surfaced UV-B radiation on a globe scale have been confirmed (WMO, 1994;1998). A general increase in tropospheric photochemistry and perturbations to concentrations of O3 and OH radicals are expected. These changes may affect tropospheric composition and air quality. The magnitude of these effects is uncertain because of the complexity and non-linear nature of tropospheric chemistry and is the subject of active research. An assessment of the state of knowledge is presented below.

    Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), are substitutes for ozone-depleting substances. HCFCs and HFCs are degraded in the atmosphere, and some of these compounds produce trifluoroacetic acid (TFA), which has no known significant atmospheric degradation mechanism. TFA formed in the atmosphere is expected to enter the hydrosphere (mostly via precipitation), and may have detrimental effects on biota if accumulated in sufficient quantities. Production of TFA during the atmospheric degradation of HFCs and HCFCs is assessed below.


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