Nitrogen dioxide (NO2) a sparingly water-soluble pi-radical gas, is a criteria air pollutant that induces adverse health effects. How is inhaled NO2(g) incorporated into the fluid microfilms lining respiratory airways remains an open issue because its exceedingly small uptake coefficient (gamma similar to 10(-7)-10(-8)) limits physical dissolution on neat water. Here, we investigate whether the biological antioxidants present in these fluids enhance NO2(g) dissolution by monitoring the surface of aqueous ascorbate; urate, and glutathione microdroplets exposed to NO2(g) for similar to 1 ms via online thermospray ionization mass spectrometry. We found that antioxidants catalyze the hydrolytic disproportionation of NO2(g), 2NO(2)(g) + H2O(1) = NO3-(aq) + H+(aq) + HONO, but are not consumed in the process. Because this function will be largely performed by chloride, the major anion in airway lining fluids, we infer that inhaled NO2(g) delivers H+, HONO, and NO3- as primary transducers of toxic action without antioxidant participation.