Reaction between linear perfluoroaldehydes and hydroperoxy radical in the atmosphere: Reaction mechanisms, reaction kinetics modelling, and atmospheric implications

Abstract

Linear perfluoroaldehydes are important products formed in the atmospheric oxidation of industrial fluorinated compounds. However, their atmospheric lifetimes are incompletely known. Here, we employ high level quantum chemistry methods and a dual-level strategy for kinetics to investigate the reactions of C2F5CHO and C3F7CHO with HO2. Our calculated results unveil almost equal activation enthalpies at 0 K for linear perfluoroaldehydes reaction with HO2, indicating that the carbon chain length negligibly influences reaction thermodynamics. The calculated kinetics reveal that vibrational anharmonicity enhances rate constants by a factor of 3-10, while torsional anharmonicity reduces rate constants by 34 %-55 %. Additionally, we also find that the reaction of C3F7CHO with HO2 exhibits significant pressure dependence, with transition pressures ranging from 0.026 to 2.3 bar across a temperature range of 190-350 K. Furthermore, atmospheric lifetimes of C2F5CHO and C3F7CHO are discussed based on the homogenous and heterogeneous processes. Our findings also reveal that the reactions of C2F5CHO and C3F7CHO with HO2 radicals dominate over those with OH radicals in Russia, Malaysia, and parts of Africa by the calculated results in combination with data based on global atmospheric chemical model simulations. Under nighttime conditions, HO2-initiated degradation represents a major atmospheric sink, comparable in magnitude to photolysis and Cl-initiated oxidation in gas phase, whereas hydrolysis at the air-water interface plays a critical role in the sink of linear perfluoroaldehydes. These findings establish chain-length-dependent pressure effects and conformational sampling as critical, previously unrecognized factors in kinetics calculations, providing a framework for modelling complex fluorotelomer transformations and guiding emission mitigation strategies.