Heterogeneous Photocatalytic Reactions in the Troposphere

Photocatalysts, however, can absorb much smaller light quanta, from the visible. 

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Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

The relatively low temperatures and partial pressures of most of the reagents, low catalyst concentration, and the intensity of the solar light flux do not favor high rates of heterogeneous catalytic and photocatalytic reactions in the atmosphere. However, due to the enourmous total volume of the atmosphere, even very slow reactions can result in chemical transformations of huge amounts of some atmospheric components. Note that most heterogeneous reactions are expected to occur in the lowest layer of the atmosphere, i.e. the troposphere, since the concentration of aerosols and their surface area are much higher here than in the upper layers of the atmosphere. 

At present it is not possible to suggest more precise estimates than those made in this article, of the actual role of particular photocatalytic reactions in the atmosphere. To improve present knowledge, it is necessary to study in laboratories the quantitative characteristics of heterogeneous photocatalysis and thermal catalysis over natural aerosols, and under conditions that would be more close to those in the atmosphere. The most important characteristic to be measured is the quantum yield of photocatalytic reactions of atmospheric components on atmospheric aerosols containing Fe203, Ti02, and ZnO, since these are the most plausible candidates for the role of photocatalysts due to their appropriate photochemical properties and rather high concentration in the troposphere. 

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