Reply on RC3

Y Huang

doi:10.5194/acp-2021-890-ac3

Abstract

Abstract. Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs remain incompletely understood. Herein, the quantum chemical and kinetics modeling methods are applied to explore the mechanisms of the OH-initiated oxidation of the distinct HHPs (HOCH2OOH, HOCH(CH3)OOH, and HOC(CH3)2OOH) formed from the reactions of CH2OO, anti-CH3CHOO, and (CH3)2COO with water vapor. The calculations show that the dominant pathway is H-abstraction from the -OOH group in the initiation reactions of the OH radical with HOCH2OOH and HOC(CH3)2OOH. H-abstraction from the -CH group is competitive with that from the -OOH group in the reaction of the OH radical with HOCH(CH3)OOH. The barrier of H-abstraction from the -OOH group slightly increases when the number of methyl groups increase. In pristine environments, the self-reaction of the RO2 radical initially produces a tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes into propagation and termination products through asymmetric two-step OâO bond scission, in which the rate-limiting step is the first OâO bond cleavage. The barrier height of the reactions of distinct RO2 radicals with the HO2 radical is not affected by the number of methyl substitutions. In urban environments, the reaction with O2 to form formic acid and the HO2 radical is the dominant removal pathway for the HOCH2O radical formed from the reaction of the HOCH2OO radical with NO. The Î²-site CâC bond scission is the dominant pathway in the dissociation of the HOCH(CH3)O and HOC(CH3)2O radicals formed from the reactions of NO with HOCH(CH3)OO and HOC(CH3)2OO radicals. These new findings deepen our understanding of the photochemical oxidation of hydroperoxides under realistic atmospheric conditions.

Highlights

Hydroxyalkyl hydroperoxides (HHPs), generated via the reactions of Criegee intermediates (CIs) with water vapour, play important roles in the formation of secondary organic aerosol (SOA) (Qiu et al, 2019; Kumar et al, 2014)
We mainly investigate the detailed mechanisms and kinetic properties of distinct HHPs oxidation initiated by OH radical by employing quantum chemical and kinetics modeling methods
Conclusion are summarized as follows: (a) The H-abstraction by OH radical from the -OOH group of distinct HHPs leading to the formation of RO2 radicals is preferable, and the barrier of dominant pathway is increased as the number of methyl group is increased

Summary

Introduction

Hydroxyalkyl hydroperoxides (HHPs), generated via the reactions of Criegee intermediates (CIs) with water vapour, play important roles in the formation of secondary organic aerosol (SOA) (Qiu et al, 2019; Kumar et al, 2014). The CIs formed from the ozonolysis of alkenes are characterized by high reactivity and excess energies, which can proceed either prompt unimolecular decay to OH radical or, after collisional stabilization, bimolecular reactions with various trance gases like SO2, NO2 and H2O to produce sulfate, nitrate and SOA, thereby influencing air quality and human health (Lester and Klippenstein, 2018; Chen et al, 2017, 2019; Liu et al, 2019; Chhantyal-Pun et al, 2017; Anglada and Solé, 2016; Gong and Chen, 2021). While the product of syn-CH3CHOO reaction with water is not taken into consideration because it mainly proceeds thermal unimolecular decay to OH radical, rather than reaction with water (Zhou et al, 2019)

Electronic structure and energy calculations

Kinetics calculations

Initiation reaction of HHPs with OH radical

Subsequent reactions of H-abstraction products RO2 radicals in pristine environment

Reactions mechanism for the self-reaction of RO2 radicals

Reactions mechanism for the reaction of RO2 with HO2 radical

Reactions mechanism for the autoxidation of RO2 radicals

Subsequent reactions of H-abstraction products RO2 radicals in urban environments

Conclusions