Experimental and theoretical study on the impact of a nitrate group on the chemistry of alkoxy radicals.

The chemistry of nitrated alkoxy radicals, and its impact on RO2 measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study. Quantum chemical and theoretical kinetic calculations show that the decomposition of β-nitrate-alkoxy radicals is much slower than β-OH-substituted alkoxy radicals, and that the spontaneous fragmentation of the α-nitrate-alkyl radical product to a carbonyl product + NO2 prevents other β-substituents from efficiently reducing the energy barrier. The systematic series of calculations is summarized as an update to the structure-activity relationship (SAR) by Vereecken and Peeters (2009), and shows increasing decomposition rates with higher degrees of substitution, as in the series ethene to 2,3-dimethyl-butene, and dominant H-migration for sufficiently large alkoxy radicals such as those formed from 1-pentene or longer alkenes. The slow decomposition allows other reactions to become competitive, including epoxidation in unsaturated nitrate-alkoxy radicals; the decomposition SAR is likewise updated for β-epoxy substituents. A set of experiments investigating the NO3-initiated oxidation of ethene, propene, cis-2-butene, 2,3-dimethyl-butene, 1-pentene, and trans-2-hexene, were performed in the atmospheric simulation chamber SAPHIR with measurements of HO2 and RO2 radicals performed with a LIF instrument. Comparisons between modelled and measured HO2 radicals in all experiments, performed in excess of carbon monoxide to avoid OH radical chemistry, suggest that the reaction of HO2 with β-nitrate alkylperoxy radicals has a channel forming OH and an alkoxy radical in yields of 15-65%, compatible with earlier literature data on nitrated isoprene and α-pinene radicals. Model concentrations of RO2 radicals when including the results of the theoretical calculations described here, agreed within 10% with the measured RO2 radicals for all species investigated when the alkene oxidation is dominated by NO3 radicals. The formation of NO2 in the decomposition of β-nitrate alkoxy radicals prevents detection of the parent RO2 radical in a LIF instrument, as it relies on formation of HO2. The implications for measurements of RO2 in ambient and experimental conditions, such as for the NO3-dominated chemistry during nighttime, is discussed. The current results appear in disagreement with an earlier indirect experimental study by Yeh et al. on pentadecene.