Photooxidative Fate of Atmospheric Methylated Selenium Compounds

ERIN BOWEY, Alexa Canchola, Michael Lum, Linhui Tian, Kunpeng Chen, Ying Zhou, Don Collins, Ying-Hsuan Lin, Roya Bahreini, University of California, Riverside

     Abstract Number: 521
     Working Group: Aerosol Chemistry

Abstract
Methylated selenium (Se) compounds are subjected to oxidative conditions upon their release into the atmosphere, leading to the formation of Se-containing secondary aerosol (SA). The formation potential of this SA in terms of possible reaction pathways and product yields will largely govern its eventual fate in the environment and toxic effects on human health. Exploring factors including the speciated mass concentration and the gas-to-particle (G/P) partitioning is instrumental to understanding SA formation potential. With these goals, photooxidation experiments of the precursor species dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe) were carried out in an environmental chamber under both low- and high-NOx conditions. A mini Aerosol Mass Spectrometer (mAMS) alongside a Scanning Mobility Particle Sizer (SMPS) were used to investigate online composition and size distribution of the formed aerosol products. After the maximum experimental SA mass concentration was reached, a denuder-filter system coated with derivatizing agent O-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamine (PFBHA) was used to separately collect the gas- and particle-phase carbonyl products. These samples were then analyzed offline by GC-MS to follow G/P partitioning behavior. The average particle density for each condition was calculated from mAMS and SMPS data. Standards of two expected products, methaneseleninic acid and selenic acid, were also sampled by mAMS. Signature fragments of these species were used to quantify their contribution to the organic and inorganic fraction of the total SA formed in the chamber experiments. The methaneseleninic acid contributed a broad range of 1-80% to the total organic Se-containing SA mass concentration, depending on the precursor and NOx level, while 80±20% of the inorganic Se-containing SA was consistently attributed to selenic acid. Consolidation of these complementary measurements will refine our overall understanding of the atmospheric chemistry of methylated Se species under various oxidative conditions.