Predicting photoreaction rates in surface waters

Predicting the rates of photolysis of organic compounds in natural waters is limited by several factors. For direct photolysis of dilute solutions of organic compounds in surface waters, the product of the quantum yield (efficiency) and light absorption rate are needed, but can rarely be calculated from structure activity relations (SARs); direct kinetic constants can be estimated reliably using measured quantum yields and solar models or simple spread sheet data bases to calculate rates as a function of latitude and season. Indirect photoreaction constants can be calculated from measured or estimated rate constants and the average transient photooxidant concentration. SARs for photooxidant rate constants are available for some classes of compounds. The overall indirect photolysis rate for a specific compound is the sum of all significant oxidation reactions for the compound. Quantitation of the rate process requires that a reaction profile be developed for each compound.