Overtone-Induced Degradation of Perfluorinated Alcohols in the Atmosphere

Perfluorinated alcohols (PFOHs) are thermally unstable and degrade via loss of HF, ultimately forming perfluorocarboxylic acids. Experiments and calculations of the high activation barrier for the decomposition of CF3OH suggest that the reaction occurs exclusively heterogeneously, perhaps with the involvement of water. Here, we use density functional theory calculations to show that PFOHs may readily complex with water and are expected to be present as a few percent of the total PFOH concentration under ambient atmospheric conditions. The presence of water lowers the HF elimination barrier sufficiently that excitation to low-lying O−H vibrational overtone levels (vOH ≥ 3) may cause reaction. Photolysis rate constants for CF3OH·H2O and CF3CF2OH·H2O were estimated to be 6.1 × 10-8 and 5.6 × 10-8 s-1, respectively. PFOH−water complexes should undergo degradation much faster than the corresponding gas-phase unimolecular decomposition of PFOHs, which requires excitation into the vOH = 5 or 6 vibrational level. Overtone-driven gas-phase reactions of PFOH−water complexes could be moderately competitive with heterogeneous reactions with water in dry locations. Overtone-induced degradation of PFOHs is likely a modest atmospheric source of PFCAs to the environment

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 Fe₂O₃, TiO₂, 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 CO₂. In the case of Mauritanian sand, the production of gas-phase CO₂ scaled with the loss of surface oxalic acid. In the absence of oxygen, the production of CO₂ 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 CO₂ production at the Fe₂O₃ surface, enhanced CO₂ production at the Mauritanian sand surface, and did not have a net effect upon CO₂ 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 TiO₂ surface, which showed enhanced CO₂ 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

Overtone-Induced Chemistry of Trifluoroacetic Acid: An Experimental and Theoretical Study

The effect of overtone-induced chemistry on the atmospheric fate of trifluoroacetic acid has been investigated. We report the absolute absorption intensities for the vOH = 3, 4, and 5 overtone transitions as well as an ab initio calculation of the energetics of the dissociation pathways. Calculations at the MP2 level give the lowest dissociation barrier as 50.3 kcal mol-1 for the elimination of HF. Integrated cross sections for vOH = 3, 4, and 5 are 2.70 × 10-20, 1.68 × 10-21, and 1.5 × 10-22 cm2 molecule-1 cm-1, respectively. Dissociation may proceed after absorption into v = 5 or 6, giving an upper limit to the photodissociation rate constants of 3.7 × 10-9 s-1 or 2.5 × 10-10 s-1, respectively. These correspond to a tropospheric lifetime of between 8 and 127 years. The overtone-driven photodissociation is more important than the ultraviolet photodissociation in the troposphere but is insignificant in comparison to wet deposition and the reaction with OH radicals

Photochemical Renoxification of Nitric Acid on Real Urban Grime

The fate of NO_<i>x</i> (=NO + NO₂) is important to understand because NO_<i>x</i> is a significant player in air quality determination through its role in O₃ formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO₃ deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with <i>J</i> 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 NO₂, OH, and possibly HONO. Thus NO_<i>x</i> may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off

Laboratory Study of pH at the Air–Ice Interface

A good understanding of pH at the air–ice interface is required in order to better understand phenomena occurring on frozen media. In this study, we used glancing-angle laser-induced fluorescence in conjunction with the surface- and pH-sensitive fluorescent dyes harmine and acridine to investigate pH at the air–ice interface. We followed changes in the surface region pH due to the freezing of water samples containing HCl, HNO₃, or NaOH. Freezing leaves the surface pH largely unchanged with respect to the prefreezing pH, indicating that protons are not strongly excluded during freezing. Exclusion of chloride to the ice surface on freezing was inferred via the enhanced quenching of the acridine fluorescence lifetime upon freezing samples containing HCl. Changes in surface pH due to the deposition of HCl­(g) to frozen water surfaces were smaller than those seen on liquid surfaces, for the same acid loading

Exclusion of Nitrate to the Air–Ice Interface During Freezing

During freezing, the majority of solutes are rejected from the growing ice lattice and are concentrated at grain boundaries or nodes within the ice crystal or at the ice crystal surface itself. The degree of solute enrichment as well as the location of the rejected solutes has important consequences for reactions occurring in or on frozen media. We have used glancing-angle Raman spectroscopy to probe the exclusion of nitrate to the air–ice interface during freezing. This work represents the first use of this technique to measure solutes at the ice surface. Our results show that nitrate is excluded to the ice surface but not to the extent predicted by equilibrium thermodynamics. These findings have important implications for understanding the mechanism of snowpack nitrate photolysis

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 TiO₂ and TiO₂–SiO₂ mixed films. Illumination of TiO₂ films led to an enhanced uptake of isopropanol and <i>n</i>-propanol and the concurrent production of gas-phase acetone and propionaldehyde, respectively, with high efficiency. Acetone production from isopropanol on illuminated TiO₂ films displayed a significant enhancement in the presence of cosorbed AgNO₃ or KNO₃. Uptake of cyclohexene by TiO₂ films required the presence of both nitrate anion and light. The wavelength and substrate (TiO₂ vs SiO₂) 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 TiO₂ surface toward photooxidation; second, as suggested by our results for cyclohexene, as a source of reactive nitrate radical at the TiO₂ 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

Mechanism of Aqueous-Phase Ozonation of S(IV)

The ozonation of dissolved sulfur dioxide is an important route for sulfate formation, especially in fog and cloud droplets of high pH. However, little is known about the detailed chemical mechanism of this process. We have mapped out the fate of aqueous SO2 in the presence of ozone by use of density functional theory (DFT) calculations in solution (via the polarized continuum model, PCM), including up to two explicit water molecules. The calculations predict that the hydrolysis of SO2·H2O, although possessing a barrier, is still more energetically favorable than its ozonation. The ozonation of HOSO2− and SO32− proceeds without barriers and gives S(VI) products that are more stable than the reagents by 77.1 and 88.6 kcal/mol, respectively. By comparing our calculated pH dependence of the ozonation kinetics to those determined experimentally, we conclude that, despite a high calculated energy barrier to the ozonation of sulfonate (HSO3−), it is the dominant form of S(IV) in solutions of neutral pH and is the species through which ozonation occurs

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

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