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.

SI text Experimental detailsThe cavity ring-down mirrors (Research Electro-Optics, 7.8 mm dia. and 1 m curvature) had a specified maximum reflectivity of 0.9994 and were mounted 1.04 m apart. Light leaking from the end mirror was detected by a photomultiplier tube (Hamamatsu Photonics, R212UH) through a band pass filter. The ring-down signal of the light intensity was recorded in a personal computer. The decay of the light intensity is represented by equation (I) 1 ;where I 0 and I(t) are the light intensities at time 0 and t, respectively. τ 0 is the cavity ring-down time without any absorbed species (about 20 µs at 435 nm), τ the measured cavity ring-down time with absorbed species, c the velocity of light, l and L are the length of the reaction surface where absorbers are considered to be present (l = 70 + 10 cm) and the length between mirrors (L = 104 cm), N and σ are the concentration and absorption cross section of the species of interest, respectively. Each ring-down trace was digitized with a time resolution of 100 MHz. The digitized traces were transferred to a personal computer and averaged over 16 runs to calculate the ring-down rate, τ −1 .

Ethylene ozonolysis was investigated in laboratory experiments using a Teflon bag reactor. A negative ion chemical ionization mass spectrometer (NI-CIMS) using SO2Cl(-) and Cl(-) as reagent ions was used for product analysis. In addition to the expected gas-phase products, such as formic acid and hydroperoxymethyl formate, oligomeric hydroperoxides composed of the Criegee intermediate (CH2OO) as a chain unit were observed. Furthermore, we observed secondary organic aerosol (SOA) formation from the ethylene ozonolysis, and the particle-phase products were also analyzed by NI-CIMS. The CH2OO oligomers were also observed as particle-phase components, suggesting that the oligomeric hydroperoxides formed in the gas phase partition into the particle phase. By adding methanol as a stabilized Criegee intermediate scavenger, both the gas-phase oligomer formation and SOA formation were strongly suppressed. This indicates that CH2OO plays a critical role in the formation of oligomeric hydroperoxides followed by SOA formation in ethylene ozonolysis. A new formation mechanism for the oligomeric hydroperoxides, which includes sequential addition of CH2OO to hydroperoxides, is proposed.

Fifty-seven years after NO(x) (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(2)O = 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 approximately 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 gamma that vary nonmonotonically with electrolyte concentration and peak at gamma(max) approximately 10(-4) for [NaX] approximately 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 NO(2)(g) into X-NO(2)(*-) 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 NO(2)(g) to (NO(3)(-) + HONO) on the surface of aqueous media.