Direct experimental evidence for a heterogeneous reaction of ozone with bromide at the air-aqueous interface

Recent experimental and theoretical evidence has indicated an enhancement of the heavier halide ions at the air-aqueous interface, relative to their bulk concentrations. This, along with an order of magnitude discrepancy between measured and predicted Br2 production in the reaction of ozone with deliquesced NaBr aerosol, has led to the suggestion that an interface reaction occurs between ozone and bromide. We have used harmine, a beta-carboline alkaloid, as an interface-sensitive fluorescent pH probe in order to measure pH changes associated with the interfacial reaction of ozone and bromide. The rate of pH change depends upon the bulk bromide concentration in a way which is well described by a Langmuir-Hinshelwood kinetic model. In the presence of octanol at the interface, the rate of pH change tracks the octanol adsorption isotherm, as expected if octanol enhances the concentration of ozone at the surface.