Fifty-seven years after NOx (NO + NO2) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict center dot OH/HO2 center dot concentrations under NOx-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO2(g) (2NO(2) + 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 NO,(g) for similar to 1 ms 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 gamma that vary nonmonotonically with electrolyte concentration and peak at gamma(max) similar to 10(-4) for [NAX] similar to 1 mM, which is > 10(3) larger than that in neat water. Since I- is partially oxidized to I-2(center dot-) in this process, anions seem to capture NO2(g) into X-NO2 center dot- radical anions for further reaction at the air/water interface. By showing that gamma 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.