Composition of Aerosols Formed by OH, Cl, or Ozone-initiated Oxidation of Limonene

ANITA AVERY, Mitchell Alton, Manjula Canagaratna, Andrew Lambe, Aerodyne Research, Inc.

     Abstract Number: 267
     Working Group: Aerosol Chemistry

Abstract
Reactions of volatile organic compounds (VOCs) with various radicals generate oxygenated VOCS (OVOCs) and secondary organic aerosol (SOA) species. However, the variability and availability of these oxidants changes drastically within the atmospheric lifetime of many VOCs, so that a different oxidant will dominate the initial reaction and resulting products. Here we use limonene as a ubiquitous VOC to compare oxidation by chlorine radicals (Cl), hydroxyl radicals (OH), and ozone (O₃), using an oxidation flow reactor (OFR). The yield and chemical composition of OVOCs/SOA generated from these reactions were characterized with several online measurements, including an aerosol mass spectrometer (AMS), an extractive electrospray chemical ionization mass spectrometer (EESI-CIMS), and a scanning mobility particle sizer (SMPS). We observe that in addition to Cl being highly efficient at oxidizing organic molecules, it also promotes the formation of condensable vapors compared to OH and O₃ - maximum SOA yields were highest with limonene/Cl, followed by OH and O₃. Resulting aerosols were small, with a mode number size distribution generally below 50 nm. Low-volatility limonene oxidation products such as C₁₀H₁₈O₅ were detected by the EESI to provide such molecular characterization of products to compliment the bulk organic aerosol measured by the AMS. Cl oxidation produces large molecular weight ions with small mass defects, indicating Cl addition to the limonene backbone is a contributor to the total aerosol mass. OH and O₃ oxidation produce smaller molecular weight compounds, but with different characteristic ions that can be used in future work to differentiate between oxidation pathways.