Significance: The Fenton reaction, Fe ²⁺ + H ₂ O ₂ , plays fundamental roles in vivo and in advanced oxidation processes. Its mechanism and the identity of the intermediates involved, however, remain controversial. Here we present direct, mass-specific evidence of the prompt formation of mono- and poly-iron Fe ^IV =O (ferryl) species on the surface of aqueous FeCl ₂ microjets exposed to gaseous H ₂ O ₂ or O ₃ beams. Remarkably, Fe ²⁺ ions at the aqueous surface react with H ₂ O ₂ and O ₃ >10 ³ times faster than Fe(H ₂ O) ₆ ²⁺ in bulk water. Our results suggest that interfacial Fenton and Fenton-like chemistries could play a more significant role than hitherto envisioned.

Fifty-seven years after NO_x (NO + NO₂) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict •OH/HO₂• concentrations under NO_x-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 ∼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 γ that vary nonmonotonically with electrolyte concentration and peak at γ_max ∼ 10⁻⁴ for [NaX] ∼ 1 mM, which is >10³ larger than that in neat water. Since I⁻ is partially oxidized to I₂^•− in this process, anions seem to capture NO₂(g) into X−NO₂^•− 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 NO₂(g) to (NO₃⁻ + HONO) on the surface of aqueous media.