Multiphase Control of Complex Kinetics in Aqueous Microdroplets
ALEXANDRA DEAL, Franky Bernal, Alexander Prophet, Richard Saykally, Kevin R. Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 529
Working Group: Aerosol Chemistry
Abstract
Understanding reaction kinetics in aqueous microdroplets including aerosols and cloud droplets is challenging due to the multiphase nature of these environments. Recent efforts in atmospheric chemistry have led to enhanced efforts in this area, but a complete model of dynamics and kinetics in microdroplets is still under development. Here, we use a stochastic kinetic model of the oxidation of thiosulfate by ozone to examine the multiphase control of a complex reaction mechanism in the microdroplet environment. Previous work has shown that a detailed description of the multiphase dynamics of ozone is crucial for accurate kinetic modeling. We demonstrate that this can also be true for reaction intermediates, implying that complex reaction mechanisms may be impacted by the multiphase nature of microdroplets at several stages. Namely, we find that the thiosulfate oxidation by ozone is sensitive to SO2 evaporation and its equilibria with HSO3- and SO32-. To better understand the role of surface activity on reaction kinetics, we employ Deep-UV Second Harmonic Generation (DUV-SHG) spectroscopy to directly probe thiosulfate and sulfite at the air-water interface and quantify their surface affinity. These results show thiosulfate exhibits a strong propensity for the interface with a Gibbs free energy of adsorption of -7.2 kJ/mol while sulfite exhibits negligible surface adsorption.
Finally, we take a closer look at model results, focusing on how each phase (gas, interface, bulk) affects the overall mechanism and product distributions. We find that the primary reaction between thiosulfate and ozone occurs predominantly at the interface while subsequent reaction steps occur predominantly in the bulk. This division is likely true for other atmospherically relevant reaction mechanisms, and we attempt to provide reaction rate/surface activity envelopes which atmospheric scientists can use to screen for reactions with ozone that are likely to occur at the interface, in the bulk, or across phases.