Atmospheric components, photocatalytic

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. 

Figure 3. Basic scheme of the possible photocatalytic action of an atmospheric dust particle with semiconductor properties in which the particle is considered to be covered with a layer of adsorbed atmospheric water. A and B are atmospheric components undergoing, respectively, oxidation and reduction by light-generated holes 0 and electrons Aa and Ba are forms of A and B that have been adsorbed on the particle surface (or absorbed by the water layer). shows the direction of energy change for electrons in the semiconductor particle g is the width of the forbidden band. For simplicity, possible interactions of electrons and holes with water are not shown.

Experimental Data on Photocatalytic Reactions of Atmospheric Components... 

To be converted catalytically or photocatalytically involving a solid aerosol that is not covered by a water layer, atmospheric components have to be adsorbed onto their surface. Experimental data on adsorption for real atmospheric aerosols, or their models under natural conditions, are still very scarce. However, as suggested by the estimates made in Ref. 2 with CO2 and H2S as particular examples, even at a very low pressures of a trace component (e.g. 10" bar for H2S) they still may be adsorbed on a solid aerosol with a high enough coverage 0. 

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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