Anion-Catalyzed Dissolution of NO_2 on Aqueous Microdroplets

Abstract

Fifty-seven years after NOx (NO + NO_2) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict •OH/HO_2• concentrations under NO_x-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO_2(g) (2NO_2 + H_2O = NO_3^− + H^+ + HONO) in fog and aerosol droplets. Thus, state-of-the-art models parametrize the uptake of NO_2 by atmospheric aerosol from data obtained on "deactivated tunnel wall residue". Here, we report experiments in which NO_3^− production on the surface of microdroplets exposed to NO_2(g) for 1 ms is monitored by online thermospray mass spectrometry. NO_2 does not dissolve in deionized water (NO_3^− signals below the detection limit) but readily produces NO_3^− on aqueous NaX (X = Cl, Br, I) microdroplets with NO_2 uptake coefficients γ that vary nonmonotonically with electrolyte concentration and peak at γ_(max) ~ 10^(−4) for [NaX] ~ 1 mM, which is >10^3 larger than that in neat water. Since I^− is partially oxidized to I_2•^− in this process, anions seem to capture NO2(g) into X−NO_2•^− radical anions for further reaction at the air/water interface. By showing that γ 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 NO_2(g) to (NO_3^− + HONO) on the surface of aqueous media.

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