Water Surfaces and Multiphase Chemistry in/on Droplets: Towards a Molecular View of Atmospheric Chemistry at the Air-Aqueous Solution Interface
ALEXANDRA DEAL, Franky Bernal, Andreas Siebert, Alexander Prophet, Mauricio Lopez Luna, Monika Blum, Richard Saykally, Kevin R. Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 421
Working Group: Aerosol Chemistry
Abstract
Recent evidence for novel chemistry and enhanced reaction rates on water surfaces has implications for environmental and atmospheric chemistry. However, the fundamental causes for these observations are still under investigation, and direct assessment of potential drivers, including increased concentrations, pH effects, and competitive adsorption, is challenging. Levitated microdroplets are used to study the pH-dependent ozonation of thiosulfate, a surprisingly surface-active doubly charged anion, and the associated MS analysis provides a high level of kinetic detail. Kinetic modeling of the microdroplet reaction kinetics demonstrates that the droplet interface significantly mediates sulfate production, which could impact droplet growth, cloud condensation, and radiative forcing. Sulfate can be produced by two different pathways at pH 5, one with a contribution from the interface of >70% and the other occurring predominantly in the bulk (>98%). Additionally, the overall reaction rate decreases with increasing pH, suggesting competitive adsorption between OH- and the anion of interest, which has significant implications for understanding the overall ‘acidity’ and ‘basicity’ of the water surface. This is further investigated using surface-specific techniques including deep UV second harmonic generation (DUV-SHG) and ambient pressure X-ray photoelectron spectroscopy (APXPS) The findings from these works demonstrate the importance of using multiple techniques to obtain a molecular view of air–water interfaces and the utility of fundamental studies in piecing together a thorough understanding of atmospheric chemistry involving water surfaces.