Theoretical investigation on the atmospheric degradation mechanism, kinetics, and fate of hydroxymethyl nitrate initiated by ˙OH radicals

Abstract

Organic nitrate (RONO₂) is an important component of atmospheric particulate matter, and an in-depth understanding of its degradation mechanism is of great significance to prevent and control the atmospheric pollution. The degradation mechanism, kinetic properties, isotopic effects, and subsequent transformation of hydroxymethyl nitrate (HOCH₂ONO₂) initiated by ˙OH radicals were investigated using density functional theory. Three types of mechanisms including H-abstraction, substitution, and ˙OH-addition are mainly considered; the results showed that H-abstraction was the main reaction mechanism. The rate constants and branching ratios of the different mechanisms were calculated through the canonical variational transition state theory with small curvature tunneling effect correction. The rate constants were also corrected with the multistructural torsional anharmonicity effects. The total rate constant of HOCH₂ONO₂ + ˙OH reaction was 1.55 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ at 298 K. The atmospheric lifetime of HOCH₂ONO₂ was predicted theoretically from 4.98 to 112.00 days. At the same time, the isotopic effects on the target reaction are in the normal influence range within 250–500 K. The process of peroxyl radical IM1* opening the C–O bond, accompanied by a hydrogen transfer reaction, producing O₂NOCHO and HO₂˙ radical into the surroundings was the major pathway. Since there is currently no experimental research on the ˙OH-initiated oxidation of HOCH₂ONO₂, the obtained kinetic values are important for the further systematic development or guidance of experimental datasets in the future.