Understanding the Atmospheric Chemistry and Physicochemical Properties of Secondary Aerosol Formation from Gas- and Aqueous-phase Oxidation of Methylated Selenium Species

NINGJIN XU, Yumeng Cui, Michael Lum, Ying Zhou, Roya Bahreini, Ying-Hsuan Lin, Don Collins, University of California, Riverside

     Abstract Number: 486
     Working Group: Aerosol Chemistry

Abstract
Aerosols have a wide variety of impacts on air quality, global climate, and human health, with the magnitude of those effects dependent on the composition of the aerosol. There has been only limited research on the sources of trace toxic components of aerosols in agricultural environments. Selenium (Se) is an essential trace element present in various forms in the environment and is cycled through biogeochemical processes. Among the most abundant species, the gaseous methylated selenium compounds, such as dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe), are the major volatile organoselenium compounds and are emitted from both natural and anthropogenic sources. Atmospheric oxidation of those and other organoselenium species can result in formation of secondary aerosol, though the pathways and products of the chemistry remain largely unknown. Recent chamber studies have reported the aerosol formation yield, composition, and health-related characteristics of the products of gas-phase oxidation of DMSe. However, little is known about how the composition and Se oxidation state evolve with continued atmospheric processing through both gas- and aqueous-phase chemistry and how those changes affect physicochemical properties such as solubility, mobility, and toxicity.

In this work, systematic studies of gas- and aqueous-phase oxidation of several methylated selenium compounds have been carried out in a modified oxidation flow reactor under variable precursor concentration, seed type, oxidant type (OH and O3), and oxidation time scale. The results are used to develop an improved understanding of the atmospheric fate of these compounds and the potential impact of the oxidation products on health. We will present initial results of the secondary aerosol formation yield as a function of OH exposure for gas-phase oxidation and for gas+aqueous-phase oxidation in the presence of aqueous seed particles or cloud droplets. The composition of the Se-containing secondary aerosol is measured using a mini time-of-flight aerosol mass spectrometer (mAMS) and a chemical ionization time-of-flight mass spectrometer (CIMS).