Photochemical oxidation of dissolved elemental mercury, Hg(0), affects mercury chemical speciation and its transfer at the water-air interface in the aquatic environment. The mechanisms and factors that control Hg(0) photooxidation, however, are not completely understood, especially concerning the role of dissolved organic matter (DOM) and carbonate (CO32-) in natural freshwaters. Here, we evaluate Hg(0) photooxidation rates affected by reactive ionic species (e.g., DOM, CO32-, and NO3-) and free radicals in creek water and a phosphate buffer solution (pH 8) under simulated solar irradiation. The Hg(0) photooxidation rate (k = 1.44 h(-1)) is much higher in the presence of both CO32- and NO3- than in the presence of CO32-, NO3-, or DOM alone (k = 0.1-0.17 h(-1)). Using scavengers and enhancers for singlet oxygen (O-1(2)) and hydroxyl (HO center dot) radicals, as well as electron paramagnetic resonance spectroscopy, we found that carbonate radicals (CO3 center dot-) primarily drive Hg(0) photooxidation. The addition of DOM to the solution of CO32- and NO3- decreased the oxidation rate by half. This study identifies an unrecognized pathway of Hg(0) photooxidation by CO3 center dot- radicals and the inhibitory effect of DOM, which could be important in assessing Hg transformation and the fate of Hg in water containing carbonate such as hard water and seawater.