AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Chlorine-Initiated Photo-Oxidation of α-Pinene Under High NOx Conditions: Oxidation Pathways, Product Distribution and Partitioning Behavior
CATHERINE MASOUD, Lea Hildebrandt Ruiz, University of Texas at Austin
Abstract Number: 397 Working Group: Aerosol Chemistry
Abstract Previous work has focused on quantifying the SOA-formation potential of α-pinene when oxidized by hydroxyl (OH) radicals or ozone (O3), yet the effect of chlorine radicals (Cl) as oxidizing agents has not received much attention. Recent ambient measurements have shown that Cl can substantially affect SOA formation in continental as well as coastal regions. In this work, we focus on Cl-initiated photo-oxidation of α-pinene leading to the formation of oxidized products including SOA.
We report the SOA-formation potential of α-pinene + Cl under different experimental conditions including different NOx levels, temperatures and VOC precursor concentrations. Compared to OH-oxidation, SOA formation is much faster for Cl-initiated oxidation and SOA yields are doubled. Moreover, the mass yields of α-pinene photo-oxidation by Cl are almost doubled under low NOx conditions compared to high NOx conditions.
We use a filter inlet mounted onto a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) to monitor the gas- and particle-phase products and identify several chlorinated species. Of these, we find several chlorinated- auto-oxidation products or highly oxygenated organic molecules (Cl-HOMs) including C10H13ClO4, C10H15ClO6, C10H15ClO8. We propose mechanistic pathways for their occurrence, and we investigate their formation under different conditions. Our analysis shows that the formation of HOMs is enhanced under low NOx (consistent with the higher SOA yields observed) and high temperature conditions. We also compare the abundance of HOMs in Cl- versus OH-initiated oxidations. Finally, we use the FIGAERO-CIMS to investigate gas-particle partitioning of oxidation products under different experimental conditions.