Ozonolysis of α-phellandrene - Part 2: Compositional analysis of secondary organic aerosol highlights the role of stabilised Criegee intermediates

The molecular composition of the water-soluble fraction of secondary organic aerosol (SOA) generated from the ozonolysis of alpha-phellandrene is investigated for the first time using high-pressure liquid chromatography coupled to high-resolution quadrupole-Orbitrap tandem mass spectrometry. In total, 21 prominent products or isomeric product groups were identified using both positive and negative ionisation modes, with potential formation mechanisms discussed. The aerosol was found to be composed primarily of polyfunctional first-and second-generation species containing one or more carbonyl, acid, alcohol and hydroperoxide functionalities, with the products significantly more complex than those proposed from basic gas-phase chemistry in the companion paper (Mackenzie-Rae et al., 2017). Mass spectra show a large number of dimeric products are also formed. Both direct scavenging evidence using formic acid and indirect evidence from double bond equivalency factors suggest the dominant oligomerisation mechanism is the bimolecular reaction of stabilised Criegee intermediates (SCIs) with non-radical ozonolysis products. Saturation vapour concentration estimates suggest monomeric species cannot explain the rapid nucleation burst of fresh aerosol observed in chamber experiments; hence, dimeric species are believed to be responsible for new particle formation, with detected first-and second-generation products driving further particle growth in the system. Ultimately, identification of the major constituents and formation pathways of alpha-phellandrene SOA leads to a greater understanding of the atmospheric processes and implications of monoterpene emissions and SCIs, especially around eucalypt forests where alpha-phellandrene is primarily emitted.