A product study of the OH radical-initiated oxidation of isoprene: formation of C5-unsaturated diols

The product formation from the OH-initiated oxidation of isoprene has been investigated in the presence as well as in the absence of NO sub x. The experiments were carried out in a 1080 l static quartz reactor under atmospheric relevant conditions (room temperature, 900 mbar). Irradiation of isoprene/H sub2 O sub 2/air mixtures was used to study the reaction of isoprene with OH radicals in the absence of NO sub x. Products were monitored in situ by means of FT-IR-spectroscopy and analysed by GC-FID and GC-MS. Two C sub 5-unsaturated dihydroxy compounds, 2- and 3-methyl-3-butene-1,2-diol have been identified by comparison with authentic standards; the yields of these products were (4.7 plus/minus 1.4) % and (2.4 plus/minus 0.9) %, respectively. The identification of these diols is the first experimental evidence that the self- and cross-reactions of peroxy radicals derived from the isoprene plus OH reaction follow the general peroxy radical reaction scheme. An estimate of the potential relevance of these products has been made. The molar yields measured for the well-known main products methacrolein, methyl vinyl ketone and formaldehyde confirm the results from earlier studies. In addition, methanol and methyl hydroperoxide have been observed as minor products in the absence of NO sub x. New mechanistic information on the OH-initiated oxidation of isoprene has been derived from this study, which helps to improve the understanding of tropospheric isoprene degradation mechanisms.