How humidity makes HONO the dominant sink of alkyl substituted Criegee intermediates and a key nocturnal source of OH radicals

Abstract. Criegee intermediates (CIs) play a key role in the production of OH radicals in the nocturnal environment. It is known that only the unimolecular decomposition of CIs leads to the formation of OH radicals, whereas bimolecular reactions are key for the formation of secondary organic aerosols. In the present work, we have investigated the reactions of CIs (CH₂OO, (CH₃)₂COO, anti-CH₃CHOO, and syn-CH₃CHOO) with HONO in the presence of water using quantum chemical calculations and kinetic modelling. The investigation reveals that H₂O catalyzed CI + HONO reactions become a major atmospheric sink for methyl substituted CIs and a prominent source of OH radicals. In the presence of water, CIs loss via CI + HONO reaction is found to be several orders of magnitude higher compared to other traditional sinks such as water and SO₂. For (CH₃)₂COO, the H₂O catalysed CI + HONO reaction was found to be ∼ 7 orders of magnitude faster than the H₂O/(H₂O)₂/SO₂ reactions. Similarly, for syn-CH₃CHOO, the H₂O catalyzed CI + HONO reaction was found to be ∼ 8 orders of magnitude faster than the corresponding H₂O/(H₂O)₂/SO₂ reactions. The present study reveals that, in the presence of humidity, CI + HONO can control the fate of CIs and act as an efficient route for converting HONO into OH radicals in the absence of light. Incorporation of the kinetics into a global chemical transport model indicates that the water-catalyzed CI + HONO reaction constitutes a major sink (HONO) for Criegee intermediates (CIs), accounting for ∼ 60 – 95 % of CI loss depending on atmospheric conditions. Globally, this reaction contributes ∼ 60 % of CH₃CHOO removal, while in the Antarctic winter it dominates CI loss, ∼ 95 % consumption. In addition, this reaction acts as a source of OH radicals, leading to enhancements of ∼ 10 % under nocturnal conditions and a global mean increase by ∼ 1.6 % in OH concentrations.