Rapid formation of secondary aerosol precursors from the autoxidation of C₅–C₈ n-aldehydes

Abstract. Long chain aldehydes are common atmospheric constituents, and their gas-phase oxidation form low volatility condensable products leading to secondary organic aerosol. Although the oxidation of n-aldehydes initiated by OH radicals is dominated by aldehydic hydrogen abstraction, the non-aldehydic hydrogen abstractions tend to become competitive with the increase of aldehyde carbon chain length. Here, we experimentally investigated the oxidation of C₅–C₈ n-aldehydes in variable reaction times (1–13 s) in a flow tube reactor coupled to a nitrate ion time-of-flight chemical ionization mass spectrometer (NO₃^–-ToF-CIMS). Octanal produced highly oxygenated organic molecules (HOMs – low volatility products) with up to 7 O atoms within 1.0 s while the same level of oxygenation was acquired by pentanal within 2.3 s. In long reaction time (11–13 s) experiments, we observed HOMs with progressively more O atoms and higher intensity product signals with the increase of carbon atoms in the precursor aldehydes. Our experiments in the presence of high NO concentrations (2 ppb to 1 ppm) showed the formation of prominent highly oxygenated organonitrates along with the suppression of HOM accretion products. However, some enhancement in the monomeric HOMs even with 6 O atoms were seen under variable NO conditions. Results from hydrogen to deuterium (H/D) exchange experiments showed that the studied n-aldehydes undergo similar autoxidation mechanisms, but the reactivity and HOM formation potential increase with increasing carbon chain length.