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 S2O62-, 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 S2O32- in bulk solution, only S2O32- is significantly depleted in the interfacial layers of [I-]/[S2O32-] = 10 microdroplets below [O3(g)] ∼ 50 ppm.