Bhavadharini_2026_Environ.Sci.Process.Impacts__

The reaction of pesticides with atmospheric oxidants, such as reactive oxygen species and reactive nitrogen species is a crucial factor in assessing their environmental persistence, long-range transport, degradation and impact. This study investigates the gas phase oxidation of fenthion, a widely used organothiophosphate pesticide initiated by hydroxyl radical (OH) and ozone (O(3)) using density functional theory (DFT) calculations at M06-2X/6-311++G(d,p) level of theory. For the OH initiated reactions 20 reaction pathways were studied and the OH addition to the phosphorus atom of fenthion is found to be the dominant reaction. The overall rate constant for the degradation of fenthion by OH is found to be 2.29 x 10(-9) cm(3) per molecule per s at 298 K, leading to a short atmospheric lifetime of -4 minutes. Subsequent oxidation of the key intermediates formed from the reaction of fenthion with OH leads to the formation of toxic products, notably fenthion oxon (P5 and P6) which is an acetylcholinesterase inhibitor, and O,O-dimethyl phosphorothioic acid (P3) known for its estrogenic activity. The results obtained from time-dependent DFT calculations show that neither fenthion nor its products undergo photolysis under tropospheric sunlight. For the O(3)-initiated degradation, 25 reaction pathways were explored, including addition of O(3) to the aromatic ring and the PS bond. Although O(3)-initiated reactions are exothermic, their high energy barrier limits their atmospheric significance. Overall, this study highlights the high reactivity of fenthion with OH and the significant toxicity of its degradation products, emphasizing the need for evaluating both the atmospheric fate and environmental toxicity of organophosphorus pesticides.