Use of an Oxidation Flow Reactor to Study Secondary Aerosol Formation from Methylated Selenium Compounds
YING ZHOU, Linhui Tian, Erin Bowey, Michael Lum, Ying-Hsuan Lin, Roya Bahreini, Don Collins, University of California, Riverside
Abstract Number: 239
Working Group: Aerosol Chemistry
Abstract
Selenium (Se) is an essential dietary trace element impacting human health. It is emitted into the atmosphere from a variety of natural and anthropogenic sources. Some volatile methylated selenium compounds, such as dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe), exist widely in the atmosphere. However, their potential to form secondary aerosol and their role in the atmospheric cycling of Se are still poorly understood. Here we describe the results from the use of an oxidation flow reactor (OFR) to study secondary aerosol formation from DMSe and DMDSe when dry seed aerosols, aqueous seed aerosols, or droplets were present in the reactor. The gas composition of the flow exiting the OFR was measured by a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS), while the size distribution and composition of the exiting particles were measured by a Scanning Mobility Particle Sizer (SMPS) and a mini-Aerosol Mass Spectrometer (mAMS), respectively. For OFR operation resulting in an equivalent photochemical age of 7-8 days, the formation of secondary aerosol from DMSe, was 25% higher when aqueous seed particles were present in the reactor than when dry seed particles were, but was 57% lower when ~3.5 micrometer cloud droplets were present. Secondary aerosol formation from DMDSe for a photochemical age of 8-9 days was 35% and 7% higher when aqueous seed particles and cloud droplets were present, respectively, compared to that with dry seed particles. Additionally, we identified various gas-phase oxidation products initiated by OH radicals, including selenium dioxide (SeO2), selenious acid (H2SeO3), and selenic acid (H2SeO4). These results provide evidence of the varied secondary aerosol formation potential of DMSe and DMDSe from gas- and aqueous-phase oxidation and reveal several gas-phase oxidation products that will be helpful for future studies aimed at understanding the dominant oxidation mechanisms.