A perspective on the reactions of organic peroxy radicals with HO2

Abstract

The chemistry of organic peroxy radicals (RO₂) is crucial for ozone and secondary organic aerosol formation in the troposphere. The level of nitrogen monoxide (NO) exerts a major control on further reactions of peroxy radicals. The research on these reactions in the absence of NO has been receiving increasing attention recently. The current studies under these conditions, typically associated with pristine environments, are focused on understanding the formation of highly oxygenated organic molecules (HOMs) via autoxidation and generation of accretion products, which supposedly result from peroxy radical permutation reactions (RO₂ + RO₂). Apart from the potential OH production from some oxygenated peroxy radicals, there is less research activity on the reactions of peroxy radicals with HO₂. This article reviews the existing literature data available on RO₂ + HO₂ reactions and highlights the gaps where future research is required. To date, limited information has been provided on the reactions of HO₂ with functionalized RO₂, particularly for β-hydroxyalkyl peroxy radicals, carbonyl-substituted peroxy radicals other than acyl peroxy, and peroxy radicals containing at least two functionalities. In addition, the temperature dependence of product branching ratios is not well established. Future studies targeting the influence of RO₂ + HO₂ on the tropospheric HO_x (OH + HO₂) budget should ideally enlarge the dataset of OH yields from various peroxy radical structures. This also highlights the need to broaden the investigations on the formed hydroperoxides, whose gas-phase chemistry is not well known.