Preprints
https://doi.org/10.5194/acp-2022-444
https://doi.org/10.5194/acp-2022-444
 
25 Jul 2022
25 Jul 2022
Status: this preprint is currently under review for the journal ACP.

Water enhances the formation of fragmentation products via the cross-reactions of RO2 and HO2 in the photooxidation of isoprene

Jiayun Xu, Zhongming Chen, Xuan Qin, and Ping Dong Jiayun Xu et al.
  • State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China

Abstract. The photooxidation of isoprene contributes significantly to the peroxy radical (RO2) pool in the atmosphere. With a widespread decreasing trend of nitrogen oxides (NOx) emissions, the cross-reactions of isoprene-derived RO2 with hydroperoxy radicals (HO2) 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 humidity and 80 % RH in which the cross-reactions of RO2 and HO2 were dominated. The experiments were conducted with ozone 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 HO2 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 HO2. Based on 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 %. Since β-ISOPOOH and MACR, as well as FA and AA, play pivotal roles in aerosol formation and growth, a better interpretation of their yields helps understand the fate of isoprene in the atmosphere and improve the effect of the simulations of isoprene-derived aerosol burdens and chemical compositions.

Jiayun Xu et al.

Status: open (until 05 Sep 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Jiayun Xu et al.

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Short summary
We investigated the photooxidation of isoprene in the presence of water. We found that water enhanced the formation of methacrolein and methyl vinyl ketone in the first-generation reactions by changing the structures of the reaction intermediates and raised formic acid and acetic acid yields considerably in the multi-generation reactions. The results of this study help understand the fate of isoprene in the atmosphere and improve the effect of the atmospheric simulations.
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