Abstract
Abstract. Alkenes are oxidized rapidly in the atmosphere by addition of OH and subsequently O2 leading to the formation of β-hydroxy peroxy radicals. These peroxy radicals react with NO to form β-hydroxy nitrates with a branching ratio α. We quantify α for C2–C8 alkenes at 295 K ± 3 and 993 hPa. The branching ratio can be expressed as α = (0.045 ± 0.016) × N − (0.11 ± 0.05) 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 β-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 β-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 ~ 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.
Highlights
The formation of alkyl nitrates is an important process controlling tropospheric oxidants and the lifetime of NOx
The fate of alkyl nitrates is thought to be determined by either (1) deposition leading to loss of atmospheric NOx or (2) further reactions that lead to recycling of NOx or conversion of the organic nitrates to HNO3
Knowledge of the branching ratio of RO2 + nitric oxide (NO) to form alkyl nitrates from RO2 derived from specific volatile organic compounds (VOCs) is important for diagnosing the role of individual VOCs in ozone and aerosol formation
Summary
Knowledge of the branching ratio of RO2 + NO to form alkyl nitrates from RO2 derived from specific volatile organic compounds (VOCs) is important for diagnosing the role of individual VOCs in ozone and aerosol formation. Previous studies have suggested that the branching ratio to form β-hydroxy nitrates from reaction of β-hydroxy peroxy radicals with NO is lower than for peroxy radicals produced from reactions of alkanes of the same carbon number with OH (O’Brien et al, 1998). Additional studies conducted on alkenes using long-path FT-IR have determined total alkyl nitrate yields similar to those determined for n-alkanes These studies provide only upper bounds for branching ratios to RONO2 due to the possible formation of organic nitrate from RO + NO2 chemistry (Atkinson et al, 1985; Tuazon et al, 1998; Aschmann et al, 2010). The TD-LIF instrument provides independent confirmation that the observed signals are alkyl nitrates and enables secondary calibration of CF3O− CIMS sensitivity by the TD-LIF for individual β-hydroxy nitrates
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