Exploring Spontaneous Oxidation Chemistry at the Air-Water Interface of Microdroplets Using a Suite of Mass Analyzers and Ionization Sources

PAUL TUMMINELLO, Nathan Bays, David Schafer, Leonid Sheps, Jared Echternach, Ryan Davis, Sandia National Laboratories

     Abstract Number: 38
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

Abstract
Aerosol particles, which are ubiquitous in Earth’s atmosphere, play a key role in atmospheric processes such as cloud seeding, pollution transport, and heterogenous reaction mechanisms. Aerosol interfacial chemistry, specifically the air-water-interface (AWI), has recently become a topic of interest in the literature. For example, some studies utilizing pure water microdroplets have demonstrated the spontaneous formation of hydrogen peroxide, suggesting the spontaneous formation of hydroxyl radicals at the AWI. Validation of these results and further elucidation of the mechanistic pathways for these observations occurring on microdroplets may revolutionize our understanding of the atmospheric oxidant budget, having far reaching implications in pollution transport timescales and atmospheric lifetimes.

In this work, microdroplets were generated by a custom-built sprayer apparatus which allowed for online, continuous sampling of both aerosol and gas phase reaction products as well as collection of larger, impacted aerosol particles for offline analysis. Online sampling utilized Orbitrap mass spectrometry coupled with an open-port sampling manifold to record real-time, high-resolution composition information. A triple quadrupole liquid chromatography mass spectrometer was employed to characterize the particles collected via impaction. Gaseous byproducts were analyzed with a photoionization time-of-flight mass spectrometer and a cryo-focusing gas chromatography mass spectrometer. Gaseous volatile organic carbon (VOC) compounds intermixed with the carrier gas (UHP O2/N2 mixtures) prior to microdroplet generation provided the maximum equilibration time possible between a given VOC and water microdroplets. Preliminary results of microdroplet generation, wherein average particle diameter centered ~20 µm, in the presence of various mixtures of carrier gases and VOCs (e.g., methane, ethane, butene, isoprene), will be presented. Oxidation products were observed, although in far lower concentrations than previously reported in the literature. Potential reasons for these observations will be discussed.