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.

Differences in the extent of protonation of functional groups lying on either side of water–hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol–gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H ₂ O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air–water interface predict that such event is hindered by a significant kinetic barrier unless OH ⁻ ions are present therein. Thus, by detecting RCOO ⁻ we demonstrate the presence of OH ⁻ on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me) ₃ N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH ⁻ densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH ⁻ gradients in the interfacial region. This fact could account for the weak OH ⁻ signals detected by surface-specific spectroscopies.

The pulmonary epithelium, like most aerial biosurfaces, is naturally protected against atmospheric ozone (O ₃ ) by fluid films that contain ascorbic acid (AH ₂ ) and related scavengers. This mechanism of protection will fail, however, if specific copollutants redirect AH ₂ and O ₃ (g) to produce species that can transduce oxidative damage to underlying tissues. Here, the possibility that the synergistic adverse health effects of atmospheric O ₃ (g) and acidic particulate matter revealed by epidemiological studies could be mediated by hitherto unidentified species is investigated by electrospray mass spectrometry of aqueous AH ₂ droplets exposed to O ₃ (g). The products of AH ₂ ozonolysis at the relevant air–water interface shift from the innocuous dehydroascorbic acid at biological pH to a C ₄ -hydroxy acid plus a previously unreported ascorbate ozonide ( m / z = 223) below pH ≈5. The structure of this ozonide is confirmed by tandem mass spectrometry and its mechanism of formation delineated by kinetic studies. Present results imply enhanced production of a persistent ozonide in airway-lining fluids acidified by preexisting pathologies or inhaled particulate matter. Ozonides are known to generate cytotoxic free radicals in vivo and can, therefore, transduce oxidative damage.