Newly observed several peroxides from the gas phase ozonolysis of isoprene using a flow tube reactor and the water vapor effect on their formation and decomposition

In recent years, one has been paying more and more attention to the formation of hydrogen peroxide and organic peroxides in the oxidation of volatile organic compounds (VOCs) because peroxides play important roles, such as reservoir of OH, HO2 and RO2 radicals, intermediate of Criegee radical chemistry and contributor to secondary organic aerosol. However, to the best of our knowledge, in the reaction of ozone with VOCs, only several small peroxides such as hydrogen peroxide (H2O2), hydroxymethyl hydroperoxide (HMHP), and methyl hydroperoxide (MHP) were separately identified, and their yields varied widely between different studies. Moreover, the information on the formation mechanism of peroxides in the ozonolysis of VOCs was mostly from a speculation rather than experimental evidence. Notably, a static chamber was employed in most of the previous studies, potentially resulting in the decomposition and heterogeneous reaction of peroxides on the chamber walls within an experiment time of tens of minutes to several hours, and possibly missing the details about the generation of peroxides. In the present study, we have used a flow quartz tube reactor to investigate the formation of peroxides in the ozonolysis of isoprene at various relative humidities (RH). A variety of peroxides have been detected on the tens of seconds of time scale using an online high performance liquid chromatography coupled with post-column derivatization using p-hydroxyphenylacetic acid and fluorescence detection. Our experimental results show that in addition to the three peroxides mentioned previously, more four ones, those are peroxyacetic acid (PAA) and three unknown peroxides, have been found. Furthermore, the total yield of the three small peroxides (H2O2, HMHP and MHP) is found to be similar to the result of literature; while for PAA and three unknown peroxides, they highlight a combined molar yield, for example, ~ 40% at 5% RH, much higher than that of the three small peroxides. Opposite to the previous conclusion that the peroxide yield would be positively correlated with RH, the yields of PAA and three unknown peroxides detected in the present study decreased with the RH increase. We tentatively assign these unknown peroxides to be hydroxyl hydroperoxides, which are produced by the reaction of different Criegee radicals with water. We used a box model coupled with the MCM v3.2 mechanism to simulate the reaction processes of the ozone-initiated oxidation of isoprene, adding the reaction between the gaseous water (and water dimer) and Criegee radicals and the decomposition of water-assisted hydroxyl hydroperoxides. We find that this modified mechanism would better explain the variation of peroxides with the RH increase, implying that molecular water and water cluster should be involved in the production and removal of peroxides in the future model.