Low molecular weight dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls as ozonolysis products of isoprene: Implication for the gaseous-phase formation of secondary organic aerosols

Oxidation of isoprene, a major biogenic volatile organic compound emitted from forest canopies, is a potential source of oxalic acid; the dominant species in organic aerosols. We evaluated here ozonolysis of isoprene in dry darkness as a source of oxalic (C-2), malonic (C-3) and succinic (C-4) acids. We found that oxalic acid and methylglyoxal are dominant products within 10 min of reaction followed by glyoxylic, malonic or succinic adds. Interestingly, molecular distributions of oxidation products from early reactions (9-29 min) were characterized by the predominance of methylglyoxal followed by C-2, which became dominant after 30 min. The isoprene-derived secondary organic aerosols (SOAs) showed chemical evolution with reaction time towards the molecular characteristics of dicarboxylic acids similar to those of ambient aerosols (C-2>C-3 >= C-4). The carbon-based relative abundances of methylglyoxal decreased steadily (40%-> 30%), while those of C-2 increased with reaction time (15%-> 25%), but no such variations persisted for glyoxal (6-10%). This finding means that methylglyoxal is more important intermediate of oxalic acid than glyoxal. In contrast, smaller variability and lower concentrations of pyruvic and glyoxylic acids than other intermediates indicate that oxalic add formation under dry conditions follows a different pathway than in aqueous-phase heterogeneous chemistry usually invoked for cloud/fog/atmospheric waters. Here, we propose new reaction schemes for high levels of methylglyoxal and oxalic add via gas-phase chemical reactions with ozone and OH radicals to better interpret the ambient SOA composition. Furthermore, the relative abundances of C-2 exhibit small variability from 1 to 8 h, suggesting its stable character towards the oxidation by hydroxyl radicals. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Oxidation of isoprene can lead to the production of oxalic acid, with methylglyoxal identified as an important intermediate. The relative abundances of methylglyoxal and oxalic acid fluctuate significantly during the reaction, suggesting a complex pathway of oxalic acid formation under dry conditions. The study proposes new reaction schemes for interpreting the ambient SOA composition and highlights the stable character of C-2 towards oxidation by hydroxyl radicals.