Fig. 2

Photogeochemical reactions, if enough information is known, can be classified using general principles of photochemistry. Examples are given for each of four categories in a simple scheme of classification based on the mechanism of reaction. Light-absorbing materials are shaded and catalysts are shown in italics. Intermediate processes in indirect reactions are indicated as separate reactions below the main reaction arrow. For additional explanation of these mechanisms, see the text and the references for specific reactions listed in Table 1