OH radical formation from the gas-phase reaction of ozone with terminal alkenes and the relationship between structure and mechanism

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals in quantities that vary from 7 to 100% de pending on the structure of the alkene. OH radicals are the most important oxidizing species in the lower atmosphere, and the OH-alkene reaction is a large source of new OH radicals, important in urban and rural air during bo th day and night. Evidence for OH formation comes both from low-pressure di rect measurements and from tracer experiments at high pressure, With the go al of measuring OH formation yields with good precision, a small-ratio rela tive rate technique was developed. This method uses small amounts of fast-r eacting aromatics and aliphatic ethers to trace OH formation yields. Here, we report OH formation yields for a series of terminal alkenes reacting wit h ozone. Measured OH yields were 0.29 +/- 0.05, 0.24 +/- 0.05, 0.18 +/- 0.0 4, and 0.10 +/- 0.03 for 1-butene, 1-pentene, 1-hexene, and 1-octene, respe ctively. For the methyl-substituted terminal alkenes methyl propene and 2-m ethyl-1-butene, OH yields were 0.72 +/- 0.12 and 0.67 +/- 0.12, respectivel y. The results are discussed both in terms of their atmospheric implication s and the relationship between structure and OH formation.