Comment on acp-2022-376

Abstract

Hydroperoxide esters, formed in the reactions of carbonyl oxides (also called Criegee intermediates, CIs) with formic acid, play a crucial role in the formation of secondary organic aerosol (SOA) in the atmosphere. However, the transformation mechanism of hydroperoxide esters in the presence of stabilized Criegee intermediates (SCIs) is not well understood. Herein, the oligomerization reaction mechanisms and kinetics of distinct SCIs (CH2OO, syn-CH3CHOO, anti-CH3CHOO and (CH3)2COO) reactions with their respective hydroperoxide esters as well as with hydroperoxymethyl formate (HPMF) are investigated in the gas phase using quantum chemical and kinetics modeling methods. The calculations show that the addition reactions of SCIs with hydroperoxide esters proceed through successive insertion of SCIs into hydroperoxide ester to form oligomers that involve SCIs as the repeating unit. The exothermicity of oligomerization reactions significantly decreases when the number of methyl substituents increases, and the exothermicity of anti-methyl substituted carbonyl oxides is obviously higher than that of syn-methyl substituted carbonyl oxides. The –OOH insertion reaction is energetically more feasible than the –CH insertion pathway in the SCIs oligomerization reactions, and the barrier heights increase with increasing the number of SCIs except syn-CH3CHOO. For the reactions of distinct SCIs with HPMF, the barrier of –OOH insertion pathway shows a dramatic decrease when a methyl substituent occurs at the anti-position, while it reveals a significant increase when a methyl group is introduced at the syn-position and dimethyl substitutions. Compared with the rate coefficients of the CH2OO + HPMF reaction, the rate coefficients increase by about one order of magnitude when a methyl substituent occurs at the anti-position, whereas the rate coefficients decrease by 1–2 orders of magnitude when a methyl group is introduced at the syn-position. These new findings advance our current understanding on the influence of Criegee-chemistry on the formation processes and chemical compositions of SOA.