Acyclic Monoterpenes Do Not Suppress Secondary Organic Aerosol Formed via Photooxidation of Cyclic Monoterpenes

SIJIA LIU, Celia Faiola, Sergey Nizkorodov, University of California, Irvine

     Abstract Number: 111
     Working Group: Aerosol Chemistry

Abstract
The goal of this work was to investigate cross-coupling reactions occurring during photooxidation of mixtures of acyclic and cyclic monoterpenes, which are commonly co-emitted by biosphere. The hypothesis was that the presence of an acyclic monoterpene would suppress SOA formation from cyclic monoterpenes, similar to the effect previously observed in mixtures of cyclic monoterpenes and isoprene. To test this hypothesis, SOA was prepared in a smog chamber using mixtures representing a range of reactivity ratios of myrcene and d-limonene while maintaining a constant overall OH reactivity. The chemical composition of gas-phase SOA precursors was analyzed using proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) and thermal desorption–gas chromatography–mass spectrometry (TD-GC-MS). Offline ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS) was used to determine the composition of SOA particles. Viscosity and volatility of SOA were predicted using formula-based parametrizations. Our results did not confirm the initial hypothesis. Firstly, the particulate products of myrcene photooxidation exhibited less fragmentation during photooxidation compared to those from d-limonene, contrasting with a previous ozonolysis study where the opposite trend was observed. Secondly, we did not detect any significant cross-reactions between limonene and myrcene oxidation products. Indeed the observed mass spectra of SOA from the mixtures could be accurately predicted as a linear composition of SOA from individual VOCs. To further understand these unexpected results, we will use the GECKO-A model for detailed mechanism analysis. Our findings suggest that previous observations of SOA suppression by isoprene cannot be extended to larger acyclic terpenes.