The neglected autoxidation pathways for the formation of highly oxygenated organic molecules (HOMs) and the nucleation of the HOMs generated by limonene

Abstract

Recent ambient atmospheric measurements have observed highly oxygenated organic molecules (HOMs) at a number of sites. Monoterpenes, in particular, are inclined to autoxidize to form HOMs with different oxidants such as OH and O3. However, little is known about the formation and nucleation of the HOMs in the presence of water (W). In this study, we investigated oxidation mechanism of O3 addition to limonene in the gas and water phase by theoretical chemistry calculations. Besides, the mechanism of the formation of HOMs from photochemical reactions of limonene and the role of HOMs in new particle formation (NPF) with aerosol nucleation precursors, including sulfuric acid (SA), W and ions were investigated by means of molecular dynamics (MD) simulations and theoretical chemistry calculations. The rate constants of initiation reactions for O3 addition to limonene are suppressed as the temperature increases. In the process of ion-induced nucleation, HOMs can dominate the initial nucleation steps. The MD results indicated that HOMs can self-aggregate into clusters and W molecules are beneficial to promote the NPF of HOMs. In addition, the nucleation ability of HOMs have significant correlation with the oxygen-to-carbon (O/C) ratio of HOMs. The theoretical results proved the neglected autoxidation pathways including (i) the formation of an endoperoxide moiety in HOMs (ii) a CO2 elimination in presence of an unsaturated peroxy acyl radical. The current study provide valuable insight into NPF mechanism of (HOMs) (SA) and (HOMs) (ion) in the presence of W.