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Formation of secondary organic aerosol marker compounds from the photooxidation of isoprene and isoprene-derived alkene diols under low-NO(x) conditions.

In the present work, we have evaluated whether isomeric C5-alkene diols (1,2-dihydroxy-2-methyl-3-butene, 1,2-dihydroxy-3-methyl-3-butene, and 1,4-dihydroxy-2-methyl-2-butene (cis + trans)), which have first been detected upon photooxidation of isoprene in the absence of NO and are known to be formed in the ambient atmosphere, can serve as precursors for the 2-methyltetrols, C5-alkene triols, and 2-methylglyceric acid under low-NO(x) conditions. The C5-alkene diols were prepared following published synthesis procedures. It is shown that under the applied chamber conditions the isomeric C5-alkene diols give rise to 2-methyltetrols with different threo/erythro abundance ratios and that certain diols produce 2-methylglyceric acid, but that they do not form C5-alkene triols. Furthermore, it is shown that the photooxidation of isoprene under the applied chamber conditions employing photolysis of H2O2 under dry conditions yields relatively small amounts of C5-alkene triols compared to those of the 2-methyltetrols, unlike under ambient conditions. It is argued that the chamber conditions are not optimal for the formation of C5-epoxydiols, which serve as gas-phase precursors for the C5-alkene triols, and likely as in some previous studies favor the formation of C5-alkene diols as a result of RO2 + RO2 reactions.