Water enhances the formation of fragmentation products via the cross-reactions of RO 2 and HO 2 in the photooxidation of isoprene

The photooxidation of isoprene contributes significantly to the peroxy radical (RO 2 ) pool in the atmosphere. With a widespread decreasing trend of nitrogen oxides (NO x ) emissions, the cross-reactions of isoprene-derived RO 2 with hydroperoxy radicals (HO 2 ) become increasingly important. Yet large uncertainties remain in the effect of water vapor on the products yields in these reactions. In the present study, we investigated the photooxidation of isoprene under 30 % relative 10 humidity (low) and 80 % RH (high) in which the cross-reactions of RO 2 and HO 2 were dominated. The experiments were conducted with ozone (O 3 ) photolysis as the hydroxy radical (OH) source. We found that in the first-generation reactions, the branching ratios for methacrolein (MACR) and methyl vinyl ketone (MVK) in the cross-reactions of β-isoprene hydroxy peroxy radicals (β-ISOPOO) and HO 2 under 30 % RH and 80 % RH increased to approximately three times and five times of those under dry conditions owing to a water-induced change in the complexation patterns of β-ISOPOO and HO 2 . Based on 15 the branching ratios achieved in this study, we estimated that the MACR and MVK emissions are enhanced by 4.7−12 and 18−34 Tg yr −1 while the β-isoprene hydroxy hydroperoxide (β-ISOPOOH) emission is suppressed by 39−78 Tg yr −1 on a global scale when considering the water effect. In the multi-generation reactions, the yields of formic acid (FA) and acetic acid (AA) with water vapor were raised by over fivefold than we expected, able to narrow the bias between the modeled and observed global FA productions by 20 %. and AA yields to the photo-aging process of SOA and aqueous phase oxidation of oxygenated VOCs (OVOCs) in deliquesced particles. A previous study showed that the 295 photodegradation of isoprene SOA under near-UV radiation (290−330 nm) releases FA and AA with a rate constant of 3.2 × 10 −6 and 6.5 × 10 −7 s −1 , respectively (Malecha and Nizkorodov, 2016). The aqueous-phase reactions may play a pivotal role under 80 % RH. The literature has provided evidence for the formation of FA and AA from the aqueous-phase photooxidation of dominant products from the HO 2 pathway. The aqueous-phase photodegradation of 2-methyltetrol, a major hydrolysis product of IEPOX, is a strong source of FA and AA, with a yield as high as ~78 % for FA and ~53 % for AA (Cope et al., 300 2021). The aqueous-phase photooxidation of β-IEPOX produces AA at a yield of ~14 % (Otto et al., 2019). Moreover, small multi-functional carbonyls in isoprene SOA including HACE and MGLY are potential precursors for low-molecular-weight organic acids via dark- or photo-aging process in the aqueous phase (Liu et al., 2012). The appearance of unexpected high FA and AA concentrations is prevailing in laboratory simulations of gas-phase photooxidation of isoprene (Link et al., 2020) and regions influenced by heavy biogenic emissions (Schobesberger et al., 2016; Alwe et al., 2019). Future studies may focus on 305 illustrating the definite pathways for the formation of low-molecular-weight OVOCs including FA and AA in isoprene photooxidation.