Insights into the atmospheric persistence of α-endosulfan

Endosulfan, an organochlorine insecticide, poses serious risks to human health, ecosystems, and the environment. Understanding its chemical transformation and atmospheric persistence is therefore critical for environmental protection, risk assessment, and regulatory decision-making. To resolve the atmospheric persistence of α-endosulfan (the dominant isomer of endosulfan) and reveal chemical insights into its transformation, we combine accurate quantum calculations and the master equation model to construct a detailed kinetic model for the α-endosulfan + OH reaction. Matching the previous kinetic measurements reported by P. C. Alarcon, et al., Environ. Sci. Technol., 2023, 57(42), 15999-16005 at 348-395 K and 750 Torr, the model confidently reveals the increasing trend of its persistence with temperature, which contradicts the previous extrapolated data below 348 K. Also, the persistence is found to be further amplified by increasing pressure. Such new observations are due to a highly stable pre-complex and its transformation, which significantly speeds up the reaction. Through product branching analysis, the H-abstraction at the -CH₂- moiety in the α-endosulfan ring is found to be dominant in the wide T & P range. The calculated atmospheric pseudo-unimolecular half-life (at 200-400 K and 76-760 Torr) ranges from 1.1 to 295.4 hours, suggesting α-endosulfan could be a persistent organic pollutant. Photolysis potential calculations indicate that neither α-endosulfan nor its primary products are readily degraded by sunlight, highlighting the importance of removal pathways via hydroxylation. Furthermore, the reported similar reactivity of its minor isomer, β-endosulfan, in which the OH addition at the S atom plays an important role at low temperatures (e.g., T ≤ 298 K), provides an overall picture of the atmospheric conversion of the technical endosulfan.