The degradation of Atrazine (ATZ) in an outdoor environment was investigated by varying the ATZ concentration and pH levels and then cross-checked with temperature and sunlight information. The overall decay rate constant of ATZ in outdoor is slower in neutral pH and faster at extreme pH levels, while parallel tests show that higher ATZ concentration leads to slower decay rate constant. Two abiotic mechanisms including direct photolysis and hydrolysis were identified and studied in the laboratory as a comparison. Hydrolysis was found to be a slow process but it is a continuous process, which is critical as the sunlight intensity is weak. Effect of temperature on the hydrolysis was also studied. A model incorporating ATZ decay rate constants, pH levels and temperatures was proposed. Photolysis, though, is a non-continuous process in the environment. It is a fast and dominant process, which contributes 82-45% (depending on pH levels) of overall ATZ decay at outdoor. In natural environment, humic acid can act as photosensitizer and enhance photolysis of ATZ at low concentration (<10 mg/L); while at high concentration of humic acid, retardation of ATZ decay was observed likely due to the scavenging of radicals and light attenuation. (C) 2011 Elsevier B.V. All rights reserved.