Nitrogen dioxide (NO_2), a sparingly water-soluble π-radical gas, is a criteria air pollutant that induces adverse health effects. How is inhaled NO_2(g) incorporated into the fluid microfilms lining respiratory airways remains an open issue because its exceedingly small uptake coefficient (γ 10^(−7)−10^(−8)) limits physical dissolution on neat water. Here, we investigate whether the biological antioxidants present in these fluids enhance NO_2(g) dissolution by monitoring the surface of aqueous ascorbate, urate, and glutathione microdroplets exposed to NO_2(g) for 1 ms via online thermospray ionization mass spectrometry. We found that antioxidants catalyze the hydrolytic disproportionation of NO_2(g), 2NO_2(g) + H_2O(l) = NO_3^−(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 NO_2(g) delivers H^+, HONO, and NO_3^− as primary transducers of toxic action without antioxidant participation
