Congratulations to Paul Heine, PhD student (Nadine Borduas-Dedekind group), who has received 3rd place in the AMS 23rd Conference student poster presentation competition for his presentation "The Oxidative Fate of Organic Selenium over Marine Environments".
Selenium (Se) is an essential element for human health and has an important marine component to its biogeochemical cycle. Recent studies estimate 29–36 Gg of Se cycling through the atmosphere per year1.
Inorganic, oxidized Se emitted to the oceans is assimilated by phytoplankton with subsequent bio-reduction leading to the incorporation into selenoproteins and other organo-Se species. Further transformations of these Se containing biochemicals result in the production of volatile organic selenium (VOSe) species such as dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe). Our research focuses on the evaluation of the spatiotemporal and physiochemical fate of VOSe in the gas phase related to marine environments.
In this work, we use proton transfer reaction time-of-flight (PTR-TOF) mass spectrometry (MS), Vocus (TOFWERK, Aerodyne, Inc.) in conjunction with online gas chromatography (GC) (Aerodyne, Inc.) and 77Se NMR spectroscopy to study the fate of the oxidative transformation of DMSe and DMDSe. We first investigated the rate constants of VOSe species with ozone (O3) since this oxidant is most relevant for remote marine environments. DMSe reacted with an excess of O3 under pseudo first order reaction conditions with a rate constant of 5.8 x 10-17 molecules · s-1 · cm-3. The reaction of DMDSe with an excess of O3 resulted in a slower rate constant of 3.0 x 10-17 molecules · s-1 · cm-3. Overall, the lifetimes of DMSe and DMSe against O3 are 10.2 hours and 19.7 hours, respectively, at 20 ppbv of O3. Furthermore, our untargeted analysis of high time resolution mass spectrometry data identified C2H6SeO and
CH4SeO2 as the major products of the ozonolysis of DMSe and DMDSe, respectively. These obtained molecular formulas are assignable to the chemical species dimethyl selenoxide (DMSeO) and methylselenic acid (MeSeOOH). Interestingly, these findings are consistent with liquid phase studies3 of the same reactions. Considering the low volatility of DMSeO and MeSeOOH and wet deposition as the predominant removal mechanism of particulate Se4, our preliminary results imply a rapid atmospheric turnover by returning VOSe back to its source. Future efforts are targeted towards synthesizing DMSeO and MeSeOOH and subsequent analysis using 77Se NMR to verify the reaction products.
These findings aid in the elucidation of the spatiotemporal and physiochemical fate of VOSe emitted by marine environments. Ultimately, this research provides insight into currently unknown aspects of marine gas exchange of selenium leading to an improved understanding of sea-air interactions.