Hydroxy nitrate production in the OH-initiated oxidation of alkenes

Alkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O-2 leading to the formation of beta-hydroxy peroxy radicals. These peroxy radicals react with NO to form beta-hydroxy nitrates with a branching ratio alpha. We quantify alpha for C-2-C-8 alkenes at 295K +/- 3 and 993 hPa. The branching ratio can be expressed as alpha = (0.045 +/- 0.016) x N - (0.11 +/- 0.05) where N is the number of heavy atoms (excluding the peroxy moiety), and listed errors are 2 sigma. 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 different than the isomer distribution of hydroxy hydroperoxides produced under HO2-dominated peroxy radical chemistry. Assuming unity yield for the hydroperoxides implies that the branching ratio to form beta-hydroxy nitrates increases with substitution of RO2. Deuterium substitution enhances the branching ratio to form hydroxy nitrates in both propene and isoprene by a factor of similar to 1.5. The role of alkene chemistry in the Houston region is re-evaluated using the RONO2 branching ratios reported here. Small alkenes are found to play a significant role in present-day oxidant formation more than a decade (2013) after the 2000 Texas Air Quality Study identified these compounds as major contributors to photochemical smog in Houston.