Dynamics at the Air-Water Interface Determine the Ozonolysis Kinetics of Anions in Single Microdroplets

ALEXANDER PROPHET, Kevin R. Wilson, Lawrence Berkeley National Laboratory

     Abstract Number: 620
     Working Group: Aerosol Chemistry

Abstract
Chemical and physical transformations of aerosol are greatly influenced by heterogeneous reactions occurring both within the particle interior and at the surface. However, the molecular details of reactive gas uptake into aerosol may be obscured by overlapping timescales of chemical reactions at the surface and physical transport processes such as adsorption and solvation. Here we present ozonolysis kinetics for a set of inorganic anions (I^-, S₂O₃^2-, SO₃^2-, SCN^-) contained in levitated microdroplets to explore the effects of ion surface propensity and bimolecular ozonolysis rates on the overall droplet reactivity and uptake of ozone. Measurements on single microdroplets are performed using a quadrupole electrodynamic trap coupled to mass spectrometry to monitor reaction progress during trace ozone exposure. Kinetic and molecular dynamics simulations are used to study the underlying physical steps governing ozone equilibration times at the air-water interface with emphasis on how gas adsorption, solvation, and diffusion are coupled to chemical reactions occurring on the surface and throughout the particle. Analytical expressions describing heterogenous ozonolysis at the droplet surface and in the bulk are used to construct a general characterization of heterogeneous chemistry occurring in liquid aerosol for a range of surface reactivies and droplet compositions.