Anion-catalyzed dissolution of NO2 on aqueous microdroplets

Fifty-seven years after NOx (NO + NO2) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict ·OH/HO2« concentrations under NOx-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO2(g) (2NO2 + H2O = NO3- + H+ + HONO) in fog and aerosol droplets. Thus, state-of-the-art models parametrize the uptake of NO2 by atmospheric aerosol from data obtained on "deactivated tunnel wall residue". Here, we report experiments in which NO3- production on the surface of microdroplets exposed to NO2(g) for ∼1ms is monitored by online thermospray mass spectrometry. NO2 does not dissolve in deionized water (NO3- signals below the detection limit) but readily produces NO3- on aqueous NaX (X = Cl, Br, I) microdroplets with NO2 uptake coefficients y that vary nonmonotonically with electrolyte concentration and peak at ymax ~ 10-4 for [NaX] ~ 1 mM, which is > 103 larger than that in neat water. Since I- is partially oxidized to I/in this process, anions seem to capture NO2(g) into X-NO/- radical anions for further reaction at the air/ water interface. By showing that y is strongly enhanced by electrolytes, these results resolve outstanding discrepancies between previous measurements in neat water versus NaCl-seeded clouds. They also provide a general mechanism for the heterogeneous conversion of NO2(g) to (NO3- + HONO) on the surface of aqueous media.