ABSTRACTS

Chemical substances are removed from the atmosphere through homogeneous processes, for example, gas-phase reactions with OH radicals and photolysis by sunlight, and heterogeneous processes in which liquid such as rain and solid such as soils contribute to removal of chemical substances. In recent years, the role of the heterogeneous
processes has been evaluated in relation to acid precipitation and Antarctic stratospheric ozone hole events, which leads to remarkable development of experimental techniques and theories for uptake coefficients and reaction probabilities. While relatively unstable compounds have been studied in these investigations, heterogeneous reactions on dusts of more stable compounds such as chlorofluorocarbons have been studied in relation to their tropospheric lifetime or role in Cl carriers to stratosphere. In this study, heterogeneous reactions of some relatively stable volatile halocarbons have been examined using a closed-circulation reactor in relation to their tropospheric lifetime. The following four kinds of mechanisms have been proposed for heterogeneous reactions on solid particles such as dusts that serve as a potential tropospheric sink of halocarbons;

(1) photocatalytic actions of components such as metal oxides

(2) reactions on acid sites that can be formed by removal of surface water

(3) redshift of UV absorption of halocarbons by the effect of the surface such as electric fields

(4) reactions with degradation products such as Cl.

In chapter 1 , tropospheric removal processes and recent laboratory studies on them are overviewed. The reactions examined in this study are summarized.

In chapter 2, the heterogeneous reactions of trichlorofluoromethane on various particles are described. We found that trichlorofluoromethane was decomposed or disproportionated into tetrachloromethane and dichlorodifluoromethane at ambient temperature by the reaction on solid particles when particles had been heated in a vacuum. The reaction was sensitively inhibited by the presence of water. Implication of this reaction was considered.

In chapter 3, the insufficient points in the above experiments and the improvements of the closed-circulation reactor
are described.

In chapter 4, the heterogeneous photoreactions of methyl perfluoroalkyl ethers on titanium dioxide particles are described. Their degradation products were determined by using a long-path cell and an FTIR spectrometer. By measuring both the amount adsorbed and the photolysis rate in the identical experimental run, we found that the photolysis rate was proportional to square root of their amount adsorbed and/or gas-phase partial pressure. Based on the obtained relation, implication for a possible sink through the reaction on titanium oxides was considered.

In chapter 5, the heterogeneous reactions of methyl chloroform (1,1,1-trichloroethane), which is used as a standard for estimation of atmospheric concentration of OH radicals, on clay particles are described. We found that dehydrochlorination of methyl chloroform proceeded to produce l ,1-dichloroethene on the allophane or the halloysite
particles at 313 K. The dependence of the reaction rate on the pretreatment temperature, the relative humidity and the reaction temperature suggests that the reaction could proceed even under environmental conditions. By using the obtained reaction probability and the reported atmospheric concentration of clays, the atmospheric lifetime through the reaction was estimated, which suggests that the reaction could serve as a tropospheric sink and that atmospheric OH concentration may be reevaluated.

In chapter 6, the heterogeneous photoreactions of tetrachloroethene, which causes a groundwater pollution, are described. We found that chloroethenes such as tetrachloroethene were heterogeneously photodecomposed on halloysite particles. The reaction was accelerated by Cl atoms, which were degradation products, after an induction period. The reaction was presumed to begin with absorption of light by a charge transfer complex between chloroethene and oxygen molecules. Contribution of the reaction could be significant even if the reaction with Cl atoms is suppressed by coexisting compounds in the environment.

In chapter 7, four types of mechanisms are proposed so that the reaction on solid particles such as dusts could serve as a tropospheric sink of halocarbons. The subjects in the future research are described.

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