Electrospray Mass Spectrometric Detection of Products and Short-Lived Intermediates in Aqueous Aerosol Microdroplets Exposed to a Reactive Gas

The intermediates ISO₃^- (m/z = 207) and IS₂O₃^- (m/z = 239) generated in aqueous (NaI/Na₂S₂O₃) microdroplets traversing dilute O₃ gas plumes are detected via online electrospray mass spectrometry within ∼1 ms, and their stabilities gauged by collisionally induced dissociation. The simultaneous detection of anionic reactants and the S₂O₆^2-, HSO₄^-, IO₃^-, and I₃^- products as a function of experimental conditions provides evidence of genuinely interfacial reaction kinetics. Although O₃(aq) reacts about 3 times faster with I^- than with S₂O₃^2- in bulk solution, only S₂O₃^2- is significantly depleted in the interfacial layers of [I^-]/[S₂O₃^2-] = 10 microdroplets below [O₃(g)] ∼ 50 ppm.