Abstract. Alkenes generally react rapidly by addition of OH and subsequently O2 to form beta hydroxy peroxy radicals. These peroxy radicals react with NO to form beta hydroxy nitrates with a branching ratio α. We quantify α for C2–C8 alkenes at 296 K ±3 and 993 hPa. The branching ratio can be expressed as α = (0.042 ± 0.008) × N − (0.11 ± 0.04) where N is the number of heavy atoms (excluding the peroxy moiety), and listed errors are 2σ. These branching ratios are larger than previously reported and are similar to those for peroxy radicals formed from H abstraction from alkanes. We find the isomer distributions of beta hydroxy nitrates formed under NO-dominated peroxy radical chemistry to be similar to the isomer distribution of hydroxy hydroperoxides produced under HO2-dominated peroxy radical chemistry. With the assumption of unity yield for the hydroperoxides, this implies that the branching ratio to form beta hydroxy nitrates from primary, secondary, and tertiary RO2 are similar. Deuterium substitution enhances the branching ratio to form hydroxy nitrates in both propene and isoprene by a factor of ~1.5. These observations provide further evidence for importance of the ROONO lifetime in determining the branching ratio to form alkyl nitrates. We use these measurements to re-evaluate the role of alkene chemistry in the Houston region. We find that small alkenes play a larger role in oxidant formation than previously recognized.
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