Chamber Study Exploring Aerosol Formation from NO3 Oxidation of α-pinene and Δ-carene under Varying HO2/RO2/NO3 Regimes

Tuesday, 16 December 2014
Hyungu Kang1, Benjamin R Ayres1, Juliane Fry1, Steven S Brown2, Douglas A Day3,4, Samantha Thompson3,4, Weiwei Hu3,4, Pedro Campuzano Jost3,4, Harald Stark3,5, Jose L Jimenez3,4, April Ranney6 and Paul J Ziemann3,4, (1)Reed College, Dept. of Chemistry, Portland, OR, United States, (2)NOAA Earth System Research Lab, Chemical Sciences Division, Boulder, CO, United States, (3)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (4)University of Colorado at Boulder, Dept. of Chemistry and Biochemistry, Boulder, CO, United States, (5)Aerodyne Research Inc., Billerica, MA, United States, (6)University of California Riverside, Environmental Toxicology Graduate Program, Riverside, CA, United States
Although monoterpenes are pervasive in wooded environments, their reactions with nitrate radicals (NO3, a potent nighttime oxidant downwind of combustion sources) and the resulting secondary aerosol formation are not well characterized. To better understand these reactions, environmental chamber experiments have often been conducted at elevated terpene concentrations and HO2/RO2/NO3 ratios that are not representative of the real atmosphere, resulting in a range of yields.

To elucidate the reasons for these varying yields, a new series of experiments were conducted with varying concentration ratios of α-pinene or Δ-carene with N2O5 (source of NO3 radical) and with/without formaldehyde in a 8000 L Teflon chamber. Formaldehyde served as a precursor for HO2 to bias the system towards HO2-RO2 reactions, elevated N2O5 caused NO3-RO2 reactions to dominate, and elevated monoterpene concentrations (but not amount reacted) favored RO2-RO2 reactions. The chamber products in the gaseous and aerosol phase were characterized using an NO3/N2O5 Cavity Ringdown Spectrometer (CRDS), an Aerodyne High-Resolution Aerosol Mass Spectrometer (AMS), a Scanning Mobility Particle Sizer (SMPS), an Ultrafine Condensation Particle Counter (UCPC), an Aerodyne High-Resolution Chemical Ionization Mass Spectrometer using Iodide ion chemistry (I- CIMS), and a chemiluminescence NOx detector. The mechanistic reasons for the starkly different SOA yield from the NO3 + α-pinene vs. NO3 + Δ-carene systems were explored in addition to differences in gas and aerosol-phase composition and yields under the varying conditions of the primary terpene RO2 radical fate.