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 .

The chemistry of reactive halogens in the polar atmosphere plays important roles in ozone and mercury depletion events, oxidizing capacity, and dimethylsulfide oxidation to form cloud-condensation nuclei. Among halogen species, the sources and emission mechanisms of inorganic iodine compounds in the polar boundary layer remain unknown. Here, we demonstrate that the production of tri-iodide (I 3 − ) via iodide oxidation, which is negligible in aqueous solution, is significantly accelerated in frozen solution, both in the presence and the absence of solar irradiation. Field experiments carried out in the Antarctic region (King George Island, 62°13′S, 58°47′W) also showed that the generation of tri-iodide via solar photo-oxidation was enhanced when iodide was added to various ice media. The emission of gaseous I 2 from the irradiated frozen solution of iodide to the gas phase was detected by using cavity ring-down spectroscopy, which was observed both in the frozen state at 253 K and after thawing the ice at 298 K. The accelerated (photo-)oxidation of iodide and the subsequent formation of tri-iodide and I 2 in ice appear to be related with the freeze concentration of iodide and dissolved O 2 trapped in the ice crystal grain boundaries. We propose that an accelerated abiotic transformation of iodide to gaseous I 2 in ice media provides a previously unrecognized formation pathway of active iodine species in the polar atmosphere.

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.

The fast reaction of gaseous ozone, O(3)(g), with aqueous iodide, I(-)(aq), was found to be affected by environmentally relevant cosolutes in experiments using cavity ring-down spectroscopy (CRDS) and electrospray ionization mass spectrometry (ESIMS) for the detection of gaseous and interfacial products, respectively. Iodine, I(2)(g), and iodine monoxide radical, IO(g), product yields were suppressed in the presence of a few millimolar phenol (pK(a) = 10.0), p-methoxyphenol (10.2), or p-cresol (10.3) at pH > or = 3 but unaffected by salicylic acid (pK(a(2)) = 13.6), tert-butanol, n-butanol, or malonic acid. We infer that reactive anionic phenolates inhibit I(2)(g) and IO(g) emissions by competing with I(-)(aq) for O(3)(g) at the air/water interface. ESIMS product analysis supports this mechanism. Atmospheric implications are discussed.

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The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.

The photodissociation dynamics of chlorine molecules adsorbed on amorphous and crystalline water ice films was investigated at 351 nm. The ice films were prepared on a gold polycrystalline substrate at 80−140 K. Time-of-flight spectra of the photofragment chlorine atoms, measured with the resonance-enhanced multiphoton ionization technique, were simulated with a composite of two translational energy distributions:  a Gaussian distribution and a flux-weighted Maxwell−Boltzmann distribution. For both amorphous and crystalline ice films, the Gaussian distribution is characterized by the average energy 〈E t〉 = 0.38 ± 0.02 eV, while the Maxwell−Boltzmann one by 〈E t〉 = 0.12 ± 0.01 eV. The Gaussian distribution is attributable to the chlorine atoms produced from the direct photodissociation of Cl2, while the Maxwell−Boltzmann characterizes those having undergone strong relaxation processes. The observed translational energy distributions for amorphous and crystalline ice films were similar to each other, but the relative contribution of the two energy distributions as well as the photodissociation yield of Cl atoms depend on the states of the ice films. Free OH groups and surface morphology of an ice film surface have a strong influence on the photodissociation quantum yield. The adsorbate−water interaction for Cl2 and an ice surface is discussed on the basis of the measurements of time-of-flight, infrared absorption, and temperature-programmed desorption spectra.

The hydrolytic disproportionation of gaseous NO 2 on water's surface (2 NO 2 + H 2 O -HONO + NO 3 À + H + ) (R1) has long been deemed to play a key, albeit unquantifiable role in tropospheric chemistry. We recently found that (R1) is dramatically accelerated by anions in experiments performed on aqueous microjets monitored by online electrospray mass spectrometry. This finding let us rationalize unresolved discrepancies among previous laboratory results and suggested that under realistic environmental conditions (R1) should be affected by everpresent surfactants. Herein, we report that NO 2 (g) uptake is significantly enhanced by cationic surfactants, weakly inhibited by fulvic acid (FA, a natural polycarboxylic acid) and anionic surfactants, and unaffected by 1-octanol. Surfactants appear to modulate interfacial anion coverage via electrostatic interactions with charged headgroups. We show that (R1) should be the dominant mechanism for the heterogeneous conversion of NO 2 (g) to HONO under typical atmospheric conditions throughout the day. The photoinduced reduction of NO 2 into HONO on airborne soot might play a limited role during daytime.