Tara F. Kahan

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 Earths 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.

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.

Self-Association of Naphthalene at the Air―Ice Interface

We present a molecular dynamics study of the interactions between two molecules of naphthalene present at air-water versus air-ice interfaces. In agreement with the inference from our previous experimental work [Kahan, T. F.; Donaldson, D. J. J. Phys. Chem. A 2007, 111, 1277], the results suggest that self-association of the molecules is more likely to take place on the ice surface than on the water surface. A shorter average distance between the two naphthalene molecules, in conjunction with a stronger interaction energy and free energy of association, point to a stronger tendency to self-associate on ice than on water. The distinct behavior at the two interfaces appears be due to more favorable interactions between naphthalene molecules on liquid water surfaces than on ice surfaces.

Heterogeneous ozonation kinetics of phenanthrene at the air–ice interface

Phenanthrene ozonation kinetics were measured on ice at ?30 and ?10??C, and on a water surface at 22??C using glancing angle laser-induced fluorescence. A Langmuir?Hinshelwood type kinetic mechanism was observed on ice. The maximum ozonation rates were a factor of ten higher on ice than on water. No temperature dependence to the kinetics was observed between ?30 and ?10??C, suggesting that the differences in the rates on ice and water are due to different physical properties at the two surfaces. Fluorescence spectra of phenanthrene show significant self-association on ice that is not observed on water, adding further evidence for the hypothesis that the quasi-liquid layer at the air?ice interface presents a very different environment to liquid water.

A pinch of salt is all it takes: chemistry at the frozen water surface.

Chemical interactions at the air-ice interface are of great importance to local atmospheric chemistry but also to the concentrations of pollutants deposited onto natural snow and ice. However, the study of such processes has been hampered by the lack of general, surface-specific probes. Even seemingly basic chemical properties, such as the local concentration of chemical compounds, or the pH at the interface, have required the application of assumptions about solute distributions in frozen media. The measurements that have been reported have tended for the most part to focus on entire ice or snow samples, rather than strictly the frozen interface with the atmosphere. We have used glancing-angle laser spectroscopy to interrogate the air-ice interface; this has yielded several insights into the chemical interactions there. The linear fluorescence and Raman spectra thus measured have the advantage of easy interpretability; careful experimentation can limit their probe depth to that which is relevant to atmospheric heterogeneous processes. We have used these techniques to show that the environment at the interface between air and freshwater ice surfaces is distinct from that at the interface between air and liquid water. Acids such as HCl that adsorb to ice surfaces from the gas phase result in significantly different pH responses than those at liquid water surfaces. Further, the solvation of aromatic species is suppressed at freshwater ice surfaces compared with that at liquid water surfaces, leading to extensive self-association of aromatics at ice surfaces. Photolysis kinetics of these species are much faster than at liquid water surfaces; this can sometimes (but not always) be explained by red shifts in the absorption spectra of self-associated aromatics increasing the extent to which solar radiation is absorbed. The environment presented by frozen saltwater surfaces, in contrast, appears to be reasonably well-described by liquid water. The extent of hydrogen bonding and the solvation of adsorbed species are similar at liquid water surfaces and at frozen saltwater surfaces. Adsorbed acids and bases evoke similar pH responses at frozen saltwater ice surfaces and liquid water surfaces, and photochemical kinetics of at least some aromatic compounds at frozen saltwater ice surfaces are well-described by kinetics in liquid water. These differences are not observed in experiments that interrogate the entire ice sample (i.e., that do not distinguish between processes occurring in liquid regions within bulk ice and those at the air-ice interface). Our work has shown that in general, the chemistry occurring at salty frozen interfaces is well described as being cold aqueous chemistry, whereas that seen at the pure ice interface is not. These findings have significant implications for heterogeneous atmospheric processes in ice-covered environments.

Cavity-enhanced measurements of hydrogen peroxide absorption cross sections from 353 to 410 nm.

We report near-ultraviolet and visible absorption cross sections of hydrogen peroxide (H(2)O(2)) using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS), a recently developed, high-sensitivity technique. The measurements reported here span the range of 353-410 nm and extend published electronic absorption cross sections by 60 nm to absorption cross sections below 1 × 10(-23) cm(2) molecule(-1). We have calculated photolysis rate constants for H(2)O(2) in the lower troposphere at a range of solar zenith angles by combining the new measurements with previously reported data at wavelengths shorter than 350 nm. We predict that photolysis at wavelengths longer than those included in the current JPL recommendation may account for up to 28% of the total hydroxyl radical (OH) production from H(2)O(2) photolysis under some conditions. Loss of H(2)O(2) via photolysis may be of the same order of magnitude as reaction with OH and dry deposition in the lower atmosphere; these processes have very different impacts on HO(x) loss and regeneration.

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 SO(2) 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 SO(2).H(2)O, although possessing a barrier, is still more energetically favorable than its ozonation. The ozonation of HOSO(2)(-) and SO(3)(2)(-) 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 (HSO(3)(-)), it is the dominant form of S(IV) in solutions of neutral pH and is the species through which ozonation occurs.

Photolysis Kinetics of Toluene, Ethylbenzene, and Xylenes at Ice Surfaces

Benzene, toluene, ethylbenzene, and xylenes (BTEX) are important organic pollutants. These compounds do not undergo direct photolysis in natural waters because their absorbance spectra do not overlap with solar radiation at the Earths surface. Recent research has suggested that benzene is able to undergo direct photolysis when present at ice surfaces. However, the photolysis of toluene, ethylbenzene, and xylenes (TEX) at ice surfaces has not been investigated. Using fluorescence spectroscopy, photolysis rate constants were measured for TEX in water, in ice cubes, and in ice granules which reflect reactivity at ice surfaces. No photolysis was observed in water or ice cubes. Photolysis was observed in ice granules; rate constants were (4.5 ± 0.5) × 10(-4) s(-1) (toluene), (5.4 ± 0.3) × 10(-4) s(-1) (ethylbenzene), and (3.8 ± 1.2) × 10(-4) s(-1) (xylenes). Photolysis of TEX molecules appears to be enabled by a red shift in the absorbance spectra at ice surfaces, although photosensitization may also occur. The results suggest that direct photolysis could be an important removal pathway for TEX in snow-covered environments.

Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home

Indoor oxidizing capacity in occupied residences is poorly understood. We made simultaneous continuous time-resolved measurements of ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), and nitrous acid (HONO) for two months in an occupied detached home with gas appliances in Syracuse, NY. Indoor NO and HONO mixing ratios were higher than those outdoors, whereas O3 was much lower (sub-ppbv) indoors. Cooking led to peak NO, NO2, and HONO levels 20-100 times greater than background levels; HONO mixing ratios of up to 50 ppbv were measured. Our results suggest that many reported NO2 levels may have a large positive bias due to HONO interference. Nitrous acid, NO2, and NO were removed from indoor air more rapidly than CO2, indicative of reactive removal processes or surface uptake. We measured spectral irradiance from sunlight entering the residence through glass doors; hydroxyl radical (OH) production rates of (0.8-10) × 107 molecules cm-3 s-1 were calculated in sunlit areas due to HONO photolysis, in some cases exceeding rates expected from ozone-alkene reactions. Steady-state nitrate radical (NO3) mixing ratios indoors were predicted to be lower than 1.65 × 104 molecules cm-3. This work will help constrain the temporal nature of oxidant concentrations in occupied residences and will improve indoor chemistry models.

Response to Comment on “Wavelength-Resolved Photon Fluxes of Indoor Light Sources: Implications for HOx Production”

Indoor Light Sources: Implications for HOx Production” D Kleffmann raises two concerns in his response to our work. The first is that photon fluxes rather than actinic fluxes should be used to calculate photolysis rate constants. We agree with this, and in fact we did use photon fluxes to calculate photolysis rate constants. This is explained in detail in the Supporting Information of Kowal et al. Dr. Kleffmann’s major criticism is that sunlight is brighter than fluorescent lamps, and that photolysis rates should always be greater for molecules irradiated by sunlight than by fluorescent bulbs. We agree that sunlight is generally more intense than fluorescent bulbs. However, this does not hold true at all wavelengths when sunlight is attenuated by windows. The windows studied in Kowal et al. completely attenuated sunlight at wavelengths shorter than 330 nm, whereas emission from fluorescent tubes was observed at wavelengths as short as 300 nm, as shown in Figure 1b of Kowal et al., and reproduced below. The lack of sunlight at short wavelengths means that