Observations of Gas and Particle Phase Composition of a-Thujene Ozonolysis Products

MICHELIA DAM, Adam Thomas, James Smith, University of California, Irvine

     Abstract Number: 356
     Working Group: Aerosol Chemistry

Abstract
Ozonolysis (O3) of monoterpene (MT) compounds is a known major contributor to secondary organic aerosol (SOA) formation. Some of these MT + O3 systems have been widely studied (a-pinene, b-pinene, etc.), but a-thujene is a MT compound with unique structural features that we hypothesize will lead to mechanistically specific reactivity in the gas phase, forming oxidized reaction products that influence particle formation and growth. The explicit composition of a-thujene ozonolysis products has never been observed, to our knowledge. Therefore, we conducted laboratory chamber experiments to measure the gas-phase and particle-phase composition of oxidized organics from the a-thujene ozonolysis system with various concentrations of gas precursors. Gas-phase composition was measured online using high resolution time of flight chemical ionization mass spectrometry (HR-TOF-CIMS) with NO3- and I- reagent ions to probe less and more highly oxidized products, respectively. Offline particle composition was measured with an ultrahigh resolution mass spectrometry UHPLC-HRMS system with an Orbitrap mass analyzer. Additionally, particle size distributions were measured (SMPS) and new particle formation was observed under all experimental conditions except the lowest MT concentration condition. The three methods of analysis captured very different product distributions from various stages of oxidation, with C10H14O5 (I-CIMS), C5H6O6 (NO3-CIMS) and C9H14O4 (Orbitrap) identified as the most abundant species. The contribution of individual species to the overall measured signal was observed as the MT concentration was modulated. In general, increasing MT concentration resulted in an increase of observed particle concentration and highly oxidized products measured with I-CIMS and in the particle phase, but a decrease was observed with NO3-CIMS. Understanding the molecular composition of these reaction products will allow us to assess the importance of the a-thujene ozonolysis system relative to other MT systems in both ambient and relevant specific conditions, such as indoors, when high a-thujene concentrations can be observed.