AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Effects of Acidity on the Chemical Composition of Secondary Aerosol from the Isoprene/NOx Photooxidation: Measurements Using an Aerosol Mass Spectrometer
KEI SATO, Akinori Takami, Takashi Imamura, Hong Li, Xuezhong Wang, National Institute for Environmental Studies
Abstract Number: 636 Working Group: Aerosol Chemistry
Abstract In order to study the effects of acidity on the chemical composition of isoprene secondary organic aerosol (SOA), we conducted a series of laboratory chamber experiments. In a typical experiment, we injected gaseous SO2 into a laboratory chamber after a substantial amount of SOA is produced by the isoprene/NOx photooxidation. The initial concentrations of isoprene and NO were 2 ppm and 0.5 ppm, respectively. The concentration of SO2 added was 0.2 ppm. When SO2 was injected into the chamber, the concentrations of organic aerosol (OA), nitrate aerosol (NO3), and sulfate aerosol (SO4), measured using an Aerodyne high-resolution aerosol mass spectrometer (AMS), were 38 μg/m3, 4.7 μg/m3, and 0.19 μg/m3, respectively. In one hour after the SO2 addition, the concentrations of OA, NO3, and SO4 increased up to 282 μg/m3, 23 μg/m3, and 47 μg/m3, respectively. Increase in OA concentration would result from acid-catalyzed oligomerization of organic compounds. The OA oxygen-to-carbon (O/C) ratio decreased from 0.58 to 0.49 after the SO2 injection. NO3 observed in this study is predominantly attributed to organonitrates formed from the gas-phase oxidation of isoprene in the presence of NOx. SO4 observed in this study is attributed to a mixture of organosulfates and sulfuric acid. Sulfuric acid is formed from the oxidation of SO2. Organosulfates will then be produced by the acid-catalyzed decomposition of organonitrates. The NO3/OA ratio decreased from 0.13 to 0.08 after the SO2 injection. In contrast, the concentration of gaseous nitric acid, measured using FT-IR, increased from 11 ppb to 16 ppb after the SO2 injection. Sulfuric acid aerosol will catalyze the decomposition of organonitrates on the surface of particles or in the condensed phase to form gaseous nitric acid.