Production of Molecular Iodine and Tri-iodide in the Frozen Solution of Iodide: Implication for Polar Atmosphere.

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 (I3(-)) 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 I2 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 I2 in ice appear to be related with the freeze concentration of iodide and dissolved O2 trapped in the ice crystal grain boundaries. We propose that an accelerated abiotic transformation of iodide to gaseous I2 in ice media provides a previously unrecognized formation pathway of active iodine species in the polar atmosphere.

Tropospheric aerosol as a reactive intermediate.

In tropospheric chemistry, secondary organic aerosol (SOA) is deemed an end product. Here, on the basis of new evidence, we make the case that SOA is a key reactive intermediate. We present laboratory results on the catalysis by carboxylate anions of the disproportionation of NO2 'on water': 2NO2 + H2O = HONO + NO(9-) + H+ (R1), and supporting quantum chemical calculations, which we apply to reinterpret recent reports on (i) HONO daytime source strengths vis-à-vis SOA anion loadings and (ii) the weak seasonal and latitudinal dependences of NO(x) decay kinetics over several megacities. HONO daytime generation via R1 should track sunlight because it is generally catalyzed by the anions produced during the photochemical oxidation of pervasive gaseous pollutants. Furthermore, by proceeding on the everpresent substrate of aquated airborne particulates, R1 can eventually overtake the photolysis of NO2: NO2 + hv = NO + O(3P) (R2), at large zenith angles. Thus, since R1 leads directly to *OH-radical generation via HONO photolysis: HONO + hv = NO + *OH, whereas the path initiated by R2 is more circuitous and actually controlled by the slower photolysis of O3: O3 + hv (+H2O) = O2 + 2*OH, the competition between R1 and R2 provides a mechanistic switch that buffers *OH concentrations and NO2 decay (via R1 and/or NO2 + *OH = HNO3) from actinic flux variations.

Accounting for changes in particle charge, dry mass and composition occurring during studies of single levitated particles.

The most used instrument in single particle hygroscopic analysis over the past thirty years has been the electrodynamic balance (EDB). Two general assumptions are made in hygroscopic studies involving the EDB. First, it is assumed that the net charge on the droplet is invariant over the time scale required to record a hygroscopic growth cycle. Second, it is assumed that the composition of the droplet is constant (aside from the addition and removal of water). In this study, we demonstrate that these assumptions cannot always be made and may indeed prove incorrect. The presence of net charge in the humidified vapor phase reduces the total net charge retained by the droplet over prolonged levitation periods. The gradual reduction in charge limits the reproducibility of hygroscopicity measurements made on repeated RH cycles with a single particle, or prolonged experiments in which the particle is held at a high relative humidity. Further, two contrasting examples of the influence of changes in chemical composition changes are reported. In the first, simple acid-base chemistry in the droplet leads to the irreversible removal of gaseous ammonia from a droplet containing an ammonium salt on a time scale that is shorter than the hygroscopicity measurement. In the second example, the net charge on the droplet (<100 fC) is high enough to drive redox chemistry within the droplet. This is demonstrated by the reduction of iodic acid in a droplet made solely of iodic acid and water to form iodine and an iodate salt.

Iodine emission in the presence of humic substances at the water's surface.

Humic substances that preferentially adsorb at the air/water interfaces of water or aerosols consist of both fulvic and humic acid. To investigate the chemical reactivity for the heterogeneous reaction of gaseous ozone, O(3)(g), with aqueous iodide, I(-)(aq), in the presence of standard fulvic acid, humic acid, or alcohol, cavity ring-down spectroscopy was used to detect gaseous products, iodine, I(2)(g) and an iodine monoxide radical, IO(g). Fulvic acid enhanced the I(2)(g) production yield, but not the IO(g) yield. Humic acid, n-hexanol, n-heptanol, and n-octanol did not affect the yields of I(2)(g) or IO(g). We can infer that the carboxylic group contained in fulvic acid promotes the I(2)(g) emission by supplying the requisite interfacial protons more efficiently than water on its surface.

Ab initio theoretical calculations of the electronic excitation energies of small water clusters.

A direct ab initio molecular dynamics method has been applied to a water monomer and water clusters (H(2)O)(n) (n = 1-3) to elucidate the effects of zero-point energy (ZPE) vibration on the absorption spectra of water clusters. Static ab initio calculations without ZPE showed that the first electronic transitions of (H(2)O)(n), (1)B(1)←(1)A(1), are blue-shifted as a function of cluster size (n): 7.38 eV (n = 1), 7.58 eV (n = 2) and 8.01 eV (n = 3). The inclusion of the ZPE vibration strongly affects the excitation energies of a water dimer, and a long red-tail appears in the range of 6.42-6.90 eV due to the structural flexibility of a water dimer. The ultraviolet photodissociation of water clusters and water ice surfaces is relevant to these results.

A theoretical and experimental study on translational and internal energies of H2O and OH from the 157 nm irradiation of amorphous solid water at 90 K.

The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.

Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices.

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.

Weak acids enhance halogen activation on atmospheric water's surfaces.

We report that rates of I(2)(g) emissions, measured via cavity ring-down spectroscopy, during the heterogeneous ozonation of interfacial iodide: I(-)(surface, s) + O(3)(g) + H(+)(s) →→ I(2)(g), are enhanced several-fold, whereas those of IO·(g) are unaffected, by the presence of undissociated alkanoic acids on water. The amphiphilic weak carboxylic acids appear to promote I(2)(g) emissions by supplying the requisite interfacial protons H(+)(s) more efficiently than water itself, at pH values representative of submicrometer marine aerosol particles. We infer that the organic acids coating aerosol particles ejected from ocean's topmost films should enhance I(2)(g) production in marine boundary layers.

Conversion of gaseous nitrogen dioxide to nitrate and nitrite on aqueous surfactants.

The hydrolytic disproportionation of gaseous NO(2) on water's surface (2 NO(2) + H(2)O → HONO + NO(3)(-) + 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.

Role of OH radicals in the formation of oxygen molecules following vacuum ultraviolet photodissociation of amorphous solid water.

Photodesorption of O(2)(X (3)Σ(g) (-)) and O(2)(a (1)Δ(g)) from amorphous solid water at 90 K has been studied following photoexcitation within the first absorption band at 157 nm. Time-of-flight and rotational spectra of O(2) reveal the translational and internal energy distributions, from which production mechanisms are deduced. Exothermic and endothermic reactions of OH+O((3)P) are proposed as plausible formation mechanisms for O(2)(X (3)Σ(g) (-) and a (1)Δ(g)). To examine the contribution of the O((3)P)+O((3)P) recombination reaction to the O(2) formation following 157 nm photolysis of amorphous solid water, O(2) products following 193 nm photodissociation of SO(2) adsorbed on amorphous solid water were also investigated.

A desorption mechanism of water following vacuum-ultraviolet irradiation on amorphous solid water at 90 K.

Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H(2)O and D(2)O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D(2)O following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism.

Heterogeneous reaction of gaseous ozone with aqueous iodide in the presence of aqueous organic species.

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.

Translational and internal energy distributions of methyl and hydroxyl radicals produced by 157 nm photodissociation of amorphous solid methanol.

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH(3)(v=0) and OH(v=0,1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH(3) and OH radicals from solid methanol are discussed. The formation of O((1)D and (3)P) atoms and H(2)O was investigated, but the yield of these species was found to be negligible. CH(3) products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice.

Formation mechanisms of oxygen atoms in the O((1)D(2)) state from the 157 nm photoirradiation of amorphous water ice at 90 K.

Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O((1)D) and OH(v=0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O((1)D) atoms from the ice surface. One is unimolecular dissociation of H(2)O to H(2)+O((1)D) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H(2)O(2) is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice.

Formation mechanisms of oxygen atoms in the O((3)P(J)) state from the 157 nm photoirradiation of amorphous water ice at 90 K.

Desorption of ground state O((3)P(J=2,1,0)) atoms following the vacuum ultraviolet photolysis of water ice in the first absorption band was directly measured with resonance-enhanced multiphoton ionization (REMPI) method. Based on their translational energy distributions and evolution behavior, two different formation mechanisms are proposed: One is exothermic recombination reaction of OH radicals, OH+OH-->H(2)O+O((3)P(J)) and the other is the photodissociation of OH radicals on the surface of amorphous solid water. The translational and internal energy distributions of OH radicals as well as the evolution behavior were also measured by REMPI to elucidate the roles of H(2)O(2) and OH in the O((3)P(J)) formation mechanisms.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K.

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

Direct emission of I2 molecule and IO radical from the heterogeneous reactions of gaseous ozone with aqueous potassium iodide solution.

Recent studies indicated that gaseous halogens mediate key tropospheric chemical processes. The inclusion of halogen-ozone chemistry in atmospheric box models actually closes the approximately 50% gap between estimated and measured ozone losses in the marine boundary layer. The additional source of gaseous halogens is deemed to involve previously unaccounted for reactions of O(3)(g) with sea surface water and marine aerosols. Here, we report that molecular iodine, I(2)(g), and iodine monoxide radical, IO(g), are released ([I(2)(g)] > 100[IO(g)]) during the heterogeneous reaction of gaseous ozone, O(3)(g), with aqueous potassium iodide, KI(aq). It was found that (1) the amounts of I(2)(g) and IO(g) produced are directly proportional to [KI(aq)] up to 5 mM and (2) IO(g) yields are independent of bulk pH between 2 and 11, whereas I(2)(g) production is markedly enhanced at pH < 4. We propose that O(3)(g) reacts with I(-) at the air/water interface to produce I(2)(g) and IO(g) via HOI and IOOO(-) intermediates, respectively.

Translational and internal states of hydrogen molecules produced from the ultraviolet photodissociation of amorphous solid methanol.

Translationally and internally hot H(2) molecules are produced from the 157 nm photodissociation of amorphous solid methanol at 90 K by two distinct mechanisms: exothermic recombination of two H-atom photoproducts bound to the surface and unimolecular molecular elimination of H(2) from the photoexcited methanol. The vibrationally hot H(2)(v=2-5) products are characterized by high translational and rotational temperatures. A third mechanism, the almost thermoneutral abstraction of a hydrogen atom from methanol parent molecule by the photolytically produced hydrogen atom, yields translationally and rotationally cold H(2)(v=0 and 1) products. Comparison with the results of the vacuum ultraviolet photolysis of water ice is discussed. Production of translationally hot and cold hydrogen atoms is also confirmed.

Direct observation of OH radicals ejected from water ice surface in the photoirradiation of nitrate adsorbed on ice at 100 K.

Production of gaseous OH radicals in the 248-350 nm photoirradiation of NO3(-) doped on amorphous ice at 100 K was monitored directly by using resonance-enhanced multiphoton ionization. The translational energy distribution of the OH product was represented by a Maxwell-Boltzmann energy distribution with the translational temperature of 3250 +/- 250 K. The rotational temperature was estimated to be 175 +/- 25 K. We have confirmed that the OH production should be attributed to the secondary photolysis of H2O2 produced on ice surface on the basis of the results of controlled photolysis experiments for H2O2 doped on ice surface.

Release of hydrogen molecules from the photodissociation of amorphous solid water and polycrystalline ice at 157 and 193 nm.

The production of H(2) in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H(2)(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H(2)(v=0 and 1) populations in this case were below the detection limit. The H(2) products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H(2)O by a photolytically produced H atom yields vibrationally cold H(2) products, whereas exothermic recombination of two H-atom photoproducts yields H(2) molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations.