Identification and yields of 1,4-hydroxynitrates formed from the reactions of C8-C16 n-alkanes with OH radicals in the presence of NO(x).

Abstract

A series of C8-C16 n-alkanes were reacted with OH radicals in the presence of NOx in an environmental chamber and particulate 1,4-hydroxynitrate reaction products were collected by filtration, extracted, and analyzed by high-performance liquid chromatography with UV absorption and electron ionization mass spectrometry (HPLC/UV/MS). Observed mass spectral patterns can be explained by using proposed ion fragmentation mechanisms, permitting the identification of each hydroxynitrate isomer. Reversed-phase retention of these compounds was dictated by the length of the longer of two alkyl chains attached to the 1,4-hydroxynitrate subunit. 1,4-Hydroxynitrates were quantified in particles using an authentic analytical standard for calibration, and the results were combined with gas chromatography measurements of the n-alkanes to determine the molar yields. Yields based on analyses of particles increased with increasing carbon number from 0.00 for C8 to an average plateau value of 0.130 ± 0.008 for C14-C16, due primarily to corresponding increases in gas-to-particle partitioning. The value at the plateau, where essentially all 1,4-hydroxynitrates were in particles, was equal to the average total yield of C14-C16 1,4-hydroxynitrates. The average branching ratio for the formation of C14-C16 1,4-hydroxynitrates from the reaction of NO with the corresponding 1,4-hydroxyperoxy radicals was 0.184 ± 0.011. This value is ∼20% higher than the plateau value of 0.15 for reactions of secondary 1,2-hydroxyperoxy radicals and ∼40% lower than the plateau value of 0.29 for reactions of secondary alkyl peroxy radicals, both of which were reported previously. The branching ratios determined here were used with values reported previously to calculate the yields of C7-C18 alkyl nitrates, 1,4-hydroxynitrates, and 1,4-hydroxycarbonyls, the three products formed from the reactions of these n-alkanes.