Articles | Volume 18, issue 7
Research article
06 Apr 2018
Research article |  | 06 Apr 2018

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

Felix A. Mackenzie-Rae, Helen J. Wallis, Andrew R. Rickard, Kelly L. Pereira, Sandra M. Saunders, Xinming Wang, and Jacqueline F. Hamilton

Abstract. The molecular composition of the water-soluble fraction of secondary organic aerosol (SOA) generated from the ozonolysis of α-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 α-phellandrene SOA leads to a greater understanding of the atmospheric processes and implications of monoterpene emissions and SCIs, especially around eucalypt forests where α-phellandrene is primarily emitted.

Short summary
Native to Australasia, the remarkable adaptability, rapid growth rates and high quality wood of eucalypt trees has led to them the most widely planted hardwood forest trees in the world. In contrast to boreal and tropical forests, there has been little study of aerosol formation in these regions. Here, we study the secondary organic aerosol formation from the very fast reaction of α-phellandrene, emitted from eucalypts, and identify key products and reaction pathways.
Final-revised paper