Red-light initiated atmospheric reactions of vibrationally excited molecules.

We present a brief review of long wavelength, red-light initiated chemistry from excited vibrational levels of the ground electronic state of atmospheric trace species. When sunlight driven electronic state reactions are not effective, photochemical processes occurring by vibrational overtone excitation have been found to be important in reactions of oxidized atmospheric compounds (acids, alcohols and peroxides) prevalent in the Earth's atmosphere. This review focuses on the fundamental energetic, mechanistic and dynamical aspects of unimolecular reactions of vibrationally excited atmospheric species. We will discuss the relevance of these red light initiated reactions to address the discrepancies between atmospheric measurements and results of standard atmospheric models.

Emerging areas in atmospheric photochemistry.

Sunlight is a major driving force of atmospheric processes. A detailed knowledge of atmospheric photochemistry is therefore required in order to understand atmospheric chemistry and climate. Considerable progress has been made in this field in recent decades. This contribution will highlight a set of new and emerging ideas (and will therefore not provide a complete review of the field) mainly dealing with long wavelength photochemistry both in the gas phase and on a wide range of environmental surfaces. Besides this, some interesting bulk photochemistry processes are discussed. Altogether these processes have the potential to introduce new chemical pathways into tropospheric chemistry and may impact atmospheric radical formation.

Heterogeneous photooxidation of fluorotelomer alcohols: a new source of aerosol-phase perfluorinated carboxylic acids.

Little is known of the atmospheric fate(s) of fluorotelomer alcohols (FTOHs), a class of high-production-volume chemicals used in the production of water- and oil-repelling surface coatings and which have been detected in a wide variety of urban and remote environmental matrices. In the present study, we investigated the uptake and photochemistry of FTOHs at the surface of TiO2, Fe2O3, Mauritanian sand, and Icelandic volcanic ash. Gas-phase 3,3,3-trifluoropropanol, 4:2 FTOH, and 6:2 FTOH exhibited significant uptake to each of the surfaces under study. The sand- and ash-catalyzed heterogeneous photooxidation of 6:2 FTOH resulted in the rapid production and subsequent slow degradation of surface-sorbed perfluorinated carboxylic acids (PFCAs). We suggest that this transformation, which proceeds via saturated and unsaturated fluorotelomer carboxylic acid intermediates (6:2 FTCA/FTUCA), is catalyzed by Fe and Ti contained within the samples. These results provide the first evidence that the heterogeneous oxidation of FTOHs at metal-rich atmospheric surfaces may provide a significant loss mechanism for these chemicals and also act as a source of aerosol-phase PFCAs close to source regions. Subsequent long-range transport of these aerosol-sorbed PFCAs has the potential to join oceanic transport and local gas-phase FTOH oxidation as a source of PFCAs to Arctic regions.

Can we model snow photochemistry? Problems with the current approaches.

Snow is a very active photochemical reactor that considerably affects the composition and chemistry of the lower troposphere in polar regions. Snow photochemistry models have therefore been recently developed to describe these processes. In all those models, the chemically active medium is a brine formed at the surface of snow crystals by impurities whose presence cause surface melting. Reaction and photolysis rate coefficients are those measured in dilute liquid solutions. Here, we critically examine the basis for these models by considering the structure of ice crystal surfaces, the processes involved in the interactions between impurities and ice crystals, the location of impurities in snow, and the reactivity of impurities in the various media present in snow. We conclude that the brine formed by impurities can only be present in grooves at grain boundaries and cannot cover ice crystal surfaces because of insufficient ice wettability. It is then very likely that most reactions in snow do not take place in liquids, but rather either on an actual ice surface highly different from a liquid or in particulate matter contained in snow, such as organic particles that are thought to contain most snow chromophores. We discuss why some snow models appear to adequately reproduce some observations, concluding that they are insufficiently constrained and that the use of adjustable parameters allows acceptable fits. We discuss the complexity of developing a snow model without adjustable parameters and with a predictive value. We conclude that reaching this goal in the near future is a tremendous challenge. Modeling attempts focused on snow where the impact of organic particles is minimal, such as on the east Antarctic plateau, represents the best chance of midterm success.

Enhanced Surface Partitioning of Nitrate Anion in Aqueous Bromide Solutions.

The proximity of nitrate anions to the air-water interface is thought to strongly influence their photodissociation quantum yield, due to a reduced solvent cage effect at the water surface. Although nitrate in aqueous solution exhibits little or no surface affinity, the release of gas phase NO2 (nitrate's primary photodissociation product) has been reported to be enhanced when halides, in particular bromide, are also present in solution. Here, we use glancing-angle Raman spectroscopy to investigate whether solutions containing both nitrate and halides show different propensities for nitrate at the air-water interface. We find that bromide enhances, and chloride has little effect on (or perhaps suppresses) the surface partitioning of nitrate anions.

Photochemical renoxification of nitric acid on real urban grime.

The fate of NO(x) (=NO + NO(2)) is important to understand because NO(x) is a significant player in air quality determination through its role in O(3) formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO(3) deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10(-3) s(-1) for nitrate on urban grime and 1.0 × 10(-7) s(-1) for aqueous nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aqueous nitrate photolysis, involving the formation of NO(2), OH, and possibly HONO. Thus NO(x) may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off.

Heterogeneous photochemistry of oxalic acid on Mauritanian sand and Icelandic volcanic ash.

Teragram quantities of crustal and volcanic aerosol are released into the atmosphere on an annual basis. Although these substrates contain photoactive metal oxides, little is known about the role that they may play in catalyzing the heterogeneous phototransformation of semivolatile organic species. In the present study, we have investigated oxalic acid photochemistry at the surface of Fe(2)O(3), TiO(2), Mauritanian sand, and Icelandic volcanic ash in the presence and absence of oxygen using a photochemical Knudsen cell reactor. Illumination of all sample types resulted in the production of gas-phase CO(2). In the case of Mauritanian sand, the production of gas-phase CO(2) scaled with the loss of surface oxalic acid. In the absence of oxygen, the production of CO(2) by the sand and ash films scaled with the absorption spectrum of iron oxalate, which suggests that the reaction is at least in part iron-mediated. The presence of oxygen suppressed CO(2) production at the Fe(2)O(3) surface, enhanced CO(2) production at the Mauritanian sand surface, and did not have a net effect upon CO(2) production at the Icelandic ash surface. These different oxygen dependencies imply that oxalic acid photochemistry at the authentic surfaces under study was not solely iron-mediated. Experiments at the TiO(2) surface, which showed enhanced CO(2) production from oxalic acid in the presence of oxygen, suggest that Ti-mediated photochemistry played an important role. In summary, these results provide evidence that solid-phase aerosol photochemistry may influence the atmospheric lifetime of oxalic acid in arid regions, where its removal via wet deposition is insignificant.

Sea-surface chemistry and its impact on the marine boundary layer.

The abundance of organic compounds at the surface of oceans provides a link between ocean biogeochemistry and the atmospheric chemistry of the marine boundary layer through physicochemical processes at and near the air-water interface. These processes, in turn, affect the formation and growth of marine boundary layer aerosols, being involved in primary and secondary aerosol formation and evolution in the atmosphere. The photochemistry and photosensitizing properties of the kinds of biogenically derived organic coatings present at the ocean surface have until now only been suggested but never fully addressed. We present the current state of understanding, and make some suggestions for where the field may go, for greater understanding of the possible feedbacks of air/sea exchanges on air quality and climate change.

Standard states and thermochemical kinetics in heterogeneous atmospheric chemistry.

The significance of the (often implicit) choice of standard state in the analysis and interpretation of heterogeneous chemical processes is not well acknowledged. This paper attempts to illuminate how the specific choice of standard state influences the numerical values of the parameters obtained from such analysis. Examples are drawn from air-solution and air-surface equilibria.

Influence of organic coatings on pyrene ozonolysis at the air-aqueous interface.

Glancing angle laser-induced fluorescence was used to investigate the effects of organic monolayer coatings on the ozonation kinetics of pyrene at the air-aqueous interface. Fluorescence spectra show that both 1-octanol and octanoic acid coatings give rise to similar decreased polarity at the interface relative to the uncoated surface and show a similar propensity of pyrene to partition to the interface. Ozonation kinetics follow a Langmuir-Hinshelwood mechanism, indicating a surface reaction. At high ozone concentrations, a monolayer coating of 1-octanol enhances the rate relative to the uncoated surface and a coating of octanoic acid decreases the rate. Pyrene fluorescence is most efficiently quenched by ozone in the presence of a 1-octanol coating, followed by the uncoated surface, and least efficiently quenched by ozone in the presence of octanoic acid. In agreement with earlier work, a significant photoenhancement of the ozonation is observed at the uncoated surface; however, no enhancement is observed with monolayer coatings of either organic. Quantum chemical calculations indicate a reasonable binding of ozone by the carboxylic acid group (in both its dissociated and undissociated forms). We suggest that the inhibition of the water surface reaction by a monolayer of octanoic acid is due to the sequestration of ozone by the carboxylic acid group.

Photooxidation of atmospheric alcohols on laboratory proxies for mineral dust.

We have used a novel photochemical Knudsen cell reactor to investigate the uptake and phototransformation of some atmospherically important trace organics on TiO(2) and TiO(2)-SiO(2) mixed films. Illumination of TiO(2) films led to an enhanced uptake of isopropanol and n-propanol and the concurrent production of gas-phase acetone and propionaldehyde, respectively, with high efficiency. Acetone production from isopropanol on illuminated TiO(2) films displayed a significant enhancement in the presence of cosorbed AgNO(3) or KNO(3). Uptake of cyclohexene by TiO(2) films required the presence of both nitrate anion and light. The wavelength and substrate (TiO(2) vs SiO(2)) dependence of the nitrate-induced enhancement in uptake indicates that it was not caused by direct photolysis of nitrate anion. We propose a 2-fold role for nitrate anion in the present experiments: first, as an electron trapping agent that activates the TiO(2) surface toward photooxidation; second, as suggested by our results for cyclohexene, as a source of reactive nitrate radical at the TiO(2) surface. These observations suggest that mineral dust containing photoactive components may catalyze the transformation of photochemically inactive organic compounds into species that absorb in the actinic region.

Benzene photolysis on ice: implications for the fate of organic contaminants in the winter.

The members of the important class of organic pollutants known as BTEX (benzene, toluene, ethylbenzene, and xylenes) do not undergo direct photolysis in natural waters, because their absorption spectra do not overlap that of the solar radiation which reaches the Earth's surface. Recent work has shown that aromatic compounds undergo significant red-shifts in their absorption spectra when they are present at air-ice interfaces, suggesting that BTEX components could undergo direct photolysis at ice surfaces. Using glancing-angle laser-induced fluorescence (LIF) as a probe, we measured benzene photodegradation at lambda > 295 nm having a rate constant of (3 +/- 1 x 10(-4) s(-1)) under our experimental conditions. We predict that the photolysis rate at environmental ice surfaces will be similar, based on the photon flux dependence we measured. This study presents the first report of direct benzene photolysis under environmentally relevant conditions. The results suggest that direct photolysis could be an important removal pathway for organic pollutants such as BTEX in snow-covered regions, for example, in polar or urban areas contaminated by oil spills or leaks.

Glancing-angle Raman spectroscopic probe for reaction kinetics at water surfaces.

We report glancing-angle Raman spectra acquired at the surface of aqueous dimethyl sulfoxide solutions and demonstrate that this technique can be used to measure the surface concentration of solutes. The presence of some solute molecules at the surface suppresses the intensity of the OH-stretching band of water there. We used this phenomenon to study the interfacial reaction of gas-phase ozone with aqueous NaX solutions (X = Br, I) by monitoring the decrease in intensity of the OH-stretching band of water over time. UV-VIS analysis of the product solutions indicates that X(3)(-), formed from X(2) evolved in the ozonation reaction, is the species most likely responsible for the decrease in OH-stretching intensity at the surface. The dependence of the rate of OH-Raman signal loss at the water surface on the bulk halide concentration is well described by a Langmuir-Hinshelwood kinetic model. The Langmuir-Hinshelwood parameters indicated that iodide has a approximately 50 times greater propensity for the surface compared to bromide.

Adsorption and reaction of trace gas-phase organic compounds on atmospheric water film surfaces: a critical review.

The air-water interface in atmospheric water films of aerosols and hydrometeors (fog, mist, ice, rain, and snow) presents an important surface for the adsorption and reaction of many organic trace gases and gaseous reactive oxidants (hydroxyl radical (OH(.)), ozone (O(3)), singlet oxygen (O(2)((1)Delta(g))), nitrate radicals (NO(3)(.)), and peroxy radicals (RO(2)(.)). Knowledge of the air-water interface partition constant of hydrophobic organic species is necessary for elucidating the significance of the interface in atmospheric fate and transport. Various methods of assessing both experimental and theoretical values of the thermodynamic partition constant and adsorption isotherm are described in this review. Further, the reactivity of trace gases with gas-phase oxidants (ozone and singlet oxygen) at the interface is summarized. Oxidation products are likely to be more water-soluble and precursors for secondary organic aerosols in hydrometeors. Estimation of characteristic times shows that heterogeneous photooxidation in water films can compete effectively with homogeneous gas-phase reactions for molecules in the atmosphere. This provides further support to the existing thesis that reactions of organic compounds at the air-water interface should be considered in gas-phase tropospheric chemistry.

Anthracene photolysis in aqueous solution and ice: photon flux dependence and comparison of kinetics in bulk ice and at the air-ice interface.

We report an investigation of the photolysis kinetics of polycyclic aromatic hydrocarbons (PAHs) in aqueous solution, frozen in ice, and at air-ice interfaces. Measurements of PAH photolysis rates in aqueous solution and at air-ice interfaces as a function of lamp power show that the kinetics depend nonlinearly on photon flux. In both media, the rates do not increase when lamp powers are above 0.1 W, which corresponds to total photon fluxes lower than 10(13) photon cm(-2) s(-1) in the actinic region. This suggests that extrapolating laboratory-measured rates to expected atmospheric photon fluxes may not yield accurate lifetimes for some species. In the plateau region of the photon flux dependence, anthracene located within the ice matrix (or in liquid pockets or veins in the ice) undergoes photolysis at a similar rate to that in room temperature aqueous solution, but the rate of anthracene photolysis at air-ice interfaces is five times greater. This indicates that in order to accurately predict the lifetimes of aromatic pollutants in snow and ice, the quasi-liquid layer (QLL) must be treated separately from bulk ice.

Light changes the atmospheric reactivity of soot.

Soot particles produced by incomplete combustion processes are one of the major components of urban air pollution. Chemistry at their surfaces lead to the heterogeneous conversion of several key trace gases; for example NO(2) interacts with soot and is converted into HONO, which rapidly photodissociates to form OH in the troposphere. In the dark, soot surfaces are rapidly deactivated under atmospheric conditions, leading to the current understanding that soot chemistry affects tropospheric chemical composition only in a minor way. We demonstrate here that the conversion of NO(2) to HONO on soot particles is drastically enhanced in the presence of artificial solar radiation, and leads to persistent reactivity over long periods. Soot photochemistry may therefore be a key player in urban air pollution.