Functionalization vs. fragmentation: n-aldehydeoxidation mechanisms and secondary organic aerosol formation

Abstract

Because of their relatively well-understood chemistry and atmospheric relevance, aldehydes represent a good model system for carbon–carbon fragmentation reactions in organic-aerosol aging mechanisms. Small aldehydes such as ethanal and propanal react with OH radicals under high NO_x conditions to form formaldehyde and ethanal, respectively, with nearly unit yield. CO₂ is formed as a coproduct. This path implies the formation of the C_n−1 aldehyde, or an aldehyde with one fewer methylene group than the parent. However, as the carbon number of the n-aldehyde increases, reaction with the carbon backbone becomes more likely and the C_n−1 formation path becomes less important. In this work we oxidized n-pentanal, n-octanal, n-undecanal and n-tridecanal with OH radicals at high NO_x. The C_n−1 aldehyde molar yields after the peroxyl radical + NO reaction were 69 ± 15, 36 ± 10, 16 ± 5 and 4 ± 1%, respectively. Complementary structure–activity relationship calculations of important rate constants enable estimates of branching ratios between several intermediates of the C_nn-aldehyde reaction with OH: C_n peroxyacyl nitrate versus C_n alkoxyacyl radical formation, C_n−1 alkyl nitrate versus C_n−1 alkoxy radical, and C_n−1 aldehyde formation versus isomerization products. We also measured SOA mass yields, which we compare with analogous n-alkanes to understand the effect of fragmentation on organic-aerosol formation.