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

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 NOx conditions to form formaldehyde and ethanal, respectively, with nearly unit yield. CO2 is formed as a coproduct. This path implies the formation of the Cn−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 Cn−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 NOx. The Cn−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 Cnn-aldehyde reaction with OH: Cn peroxyacyl nitrate versus Cn alkoxyacyl radical formation, Cn−1 alkyl nitrate versus Cn−1 alkoxy radical, and Cn−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.