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Products and Mechanism of the Reaction of 1-Pentadecene with NO3 Radicals and the Effect of a -ONO2 Group on Alkoxy Radical Decomposition.

The linear C15 alkene, 1-pentadecene, was reacted with NO3 radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO3 to the C═C double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxypentadecane and 1-hydroxy-2-nitrooxypentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C14 and C1 products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284 and should be similar for other linear 1-alkenes ≥ C6 whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously, and they differ significantly from those estimated using structure-activity relationships, which predict >99% isomerization. It appears that the presence of a -ONO2 group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a -OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure-activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging.