Rapid Atmospheric Reactions between Criegee Intermediates and Hypochlorous Acid

HOCl plays a key role in atmospheric processes, including tropospheric oxidation and ozone depletion. Its removal is primarily through photolysis and OH reactions during the day. These mechanisms are less active at night, leaving HOCl to persist. Understanding nighttime removal pathways is essential for predicting atmospheric behavior. Previous studies have focused on daytime reactions and photolysis. The role of Criegee intermediates in HOCl removal remains unclear. This gap motivated investigations into nighttime chemical interactions. This study addresses the lack of data on HOCl removal via Criegee intermediates.

This study aims to evaluate the reactivity of Criegee intermediates with HOCl during nighttime conditions. The goal is to determine how these reactions contribute to HOCl removal. The researchers used quantum chemical methods to model these interactions. They sought to understand the impact of molecular structure on reaction rates. The study also aimed to assess the influence of anharmonicity on reaction dynamics. A dual-level strategy was employed to compute accurate rate constants. The focus was on CH₂OO and CH₃CHOO isomers with HOCl. The results provide insights into nighttime atmospheric chemistry.

The study used high-level quantum chemical calculations as a benchmark. These calculations were based on CCSDT(Q)/CBS methods, known for high accuracy. Rate constants were calculated using a dual-level strategy. Transition state theory was applied at the benchmark level. Variational transition state theory with tunneling effects was also used. A validated density functional method supported the calculations. The researchers examined both CH₂OO and CH₃CHOO isomers. The focus was on their reactions with HOCl under atmospheric conditions.

The rate constants for CH₂OO + HOCl increased by a factor of 18-5 due to anharmonicity. In contrast, anharmonicity had a minor effect on CH₃CHOO + HOCl reactions. The loose transition state in anti-CH₃CHOO + HOCl was identified as rate-determining. These reactions were studied at temperatures between 190-350 K. The findings suggest that Criegee intermediates contribute to HOCl removal at night. The methyl group significantly influenced reaction behavior. The dual-level strategy provided accurate kinetic data. These results align with theoretical predictions and experimental trends.

The study concludes that Criegee intermediates react rapidly with HOCl at night. These reactions provide a significant pathway for HOCl removal in the atmosphere. The researchers found that anharmonicity strongly affects CH₂OO reactivity. The methyl group in CH₃CHOO reduces the impact of anharmonicity. The loose transition state in anti-CH₃CHOO reactions is rate-limiting. These findings support the importance of Criegee intermediates in nighttime chemistry. The results are consistent with the authors' computational models. The study highlights the need to consider molecular structure in atmospheric models.