Formation and temperature dependence of Highly Oxygenated Organic Molecules (HOM) from Δ³-carene ozonolysis

Abstract. ∆3-carene is a prominent monoterpene in the atmosphere, contributing significantly to secondary organic aerosol (SOA) formation. However, knowledge about ∆3-carene oxidation pathways, particularly regarding its ability to form highly oxygenated organic molecules (HOM), is still limited. In this study, we present HOM measurements during ∆3-carene ozonolysis under various conditions in two simulation chambers. We identified numerous HOM (monomers: C7-10H10-18O6-14, dimers: C17-20H24-34O6-18) using a chemical ionization mass spectrometer (CIMS). ∆3-carene ozonolysis yielded higher HOM concentrations than α-pinene, with a distinct distribution, indicating differences in formation pathways. All HOM signals decreased considerably at lower temperatures, reducing the estimated molar HOM yield from ~3 % at 20 °C to ~0.5 % at 0 °C. Interestingly, temperature change altered the HOM distribution, increasing the observed dimer-to-monomer ratios from roughly 0.8 at 20 °C to 1.5 at 0 °C. HOM monomers with 6 or 7 O-atoms condensed more efficiently onto particles at colder temperatures, while monomers with nine or more O-atoms and all dimers condensed irreversibly even at 20 °C. Using the gas- and particle-phase chemistry kinetic multilayer model ADCHAM, we were also able to reproduce the experimentally observed HOM composition, yields and temperature dependence.