Photooxidation of Dimethyl Selenide by OH and OH+NOx: Chemical Products and Bulk Aerosol Properties

ERIN BOWEY, Linhui Tian, Lillian Tran, Alexa Canchola, Michael Lum, Yumeng Cui, Ying Zhou, Don Collins, Ying-Hsuan Lin, Roya Bahreini, University of California, Riverside

     Abstract Number: 461
     Working Group: Aerosol Chemistry

Abstract
Once methylated selenium (Se) compounds, such as dimethyl selenide (DMSe), are emitted into the troposphere, they are subject to daytime photooxidation by the OH radical. It has long been predicted that the atmospheric processing of these compounds contributes to the production of oxidized Se-containing organic and inorganic gaseous pollutants and secondary aerosol (SA), similarly to dimethyl sulfide (DMS). However, the details of the involved reactions, molecular identities of the products, and properties of the resulting SA remain largely unknown. This work aims to investigate the identity of gas and particle phase products, as well as the bulk properties of the aerosol generated by OH-initiated photooxidation of DMSe in the absence or presence of added NOx. An environmental smog chamber was used for the photooxidation experiments and monitored by a suite of on- and off-line analytical techniques, including mAMS, SEMS, (I^-) ToF-CIMS with FIGAERO, and aldehyde-targeted GC-MS. Increased particle effective density (⍴_p(eff)) and volume concentration was revealed in conditions with added NOx, although the mode of the volume distributions (d_m) was not significantly distinct. Identified products included organics such as particle-phase dimethyl selenone ((CH₃)₂SeO₂) without NOx and C₂H₂N₂O₆Se with added NOx, gas-phase methaneselenonic acid (CH₄SeO₃) and CH₂SeO₄ without NOx, CHSeNO₄ with added NOx, and formaldehyde and CH₄SeO₅ under both oxidation regimes. Also, several inorganics emerged in both phases and both regimes, such as selenium dioxide (SeO₂), selenic acid (H₂SeO₄), and selenous acid (H₂SeO₃). Based on the accepted reaction products of DMS photooxidation, several Se-containing species observed in this work, such as CH₂SeO₄, CH₄SeO₅, and H₂SeO₃ were unexpected. This indicates comparable but still unique chemistry occurring in DMSe oxidation influencing the chemical composition and physical properties of the resulting gas and aerosol phase products.