William L. Miller

Superoxide () is a reactive oxygen species (ROS) that is primarily produced by the one‐electron transfer of photooxidized chromophoric dissolved organic matter (CDOM) to O 2 in sunlit natural waters. Here we examine the environmental and chemical parameters (pH, ionic strength, buffer, and halides) that may influence photochemical production rates and decay pathways in natural water. Using the enzyme superoxide dismutase and H 2 O 2 measurements, we present results from an irradiated freshwater CDOM source indicating that reductive decay pathways ( P / P SOD ) dominate with increased pH and NaCl additions and maximal photoproduction rates () increase with carbonate compared to borate buffer. Over 2 h of irradiation, a significant decline in was seen for all samples along with a minor increase in oxidative pathways. These results imply shifts in decay pathways and production rates that seem to vary across natural waters and as a function of irradiation history.