Abstract
Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.