Photosensitized Heterogeneous Chemistry of Ozone on Organic Films

Heterogeneous chemistry of ozone with floor cleaning agent: Implications of secondary VOCs in the indoor environment

Human daily activities such as cooking, and cleaning can affect the indoor air quality by releasing primary emitted volatile organic compounds (VOCs), as well as by the secondary product compounds formed through reactions with ozone (O₃) and hydroxyl radicals (OH). However, our knowledge about the formation processes of the secondary VOCs is still incomplete. We performed real-time measurements of primary VOCs released by commercial floor-cleaning detergent and the secondary product compounds formed by heterogeneous reaction of O₃ with the constituents of the cleaning agent by use of high-resolution mass spectrometry. We measured the uptake coefficients of O₃ on the cleaning detergent at different relative humidities in dark and under different light intensities (320 nm < λ < 400 nm) relevant for the indoor environment. On the basis of the detected compounds we developed tentative reaction mechanisms describing the formation of the secondary VOCs. Intriguingly, under light irradiation the formation of valeraldehyde was observed based on the photosensitized chemistry of acetophenone which is a constituent of the cleaning agent. Finally, we modeled the observed mixing ratios of three aldehydes, glyoxal, methylglyoxal, and 4-oxopentanal with respect to real-life indoor environment. The results suggest that secondary VOCs initiated by ozone chemistry can additionally impact the indoor air pollution.

Emerging investigator series: ozone uptake by urban road dust and first evidence for chlorine activation during ozone uptake by agro-based anti-icer: implications for wintertime air quality in high-latitude urban environments

High-latitude urban regions provide a unique and complex range of environmental surfaces for uptake of trace pollutant gases, including winter road maintenance materials (e.g., gravel, rock salts, and anti-icer, a saline solution applied to roads during winter). In an effort to reduce the negative environmental and economic impacts of road salts, many municipalities have turned to agro-based anti-icing materials that are rich in organic material. To date, the reactivity of both anti-icer and saline road dust with pollutant gases remain unexplored, which limits our ability to assess the potential impacts of these materials on air quality in high-latitude regions. Here, we used a coated-wall flow tube to investigate the uptake of ozone, an important air pollutant, by road dust collected in Edmonton, Canada. At 25% relative humidity (RH) and 50 ppb ozone, γ_BET for ozone uptake by this sample is (8.0 ± 0.7) × 10^-8 under dark conditions and (2.1 ± 0.1) × 10^-7 under illuminated conditions. These values are 2-4× higher than those previously obtained by our group for natural mineral dusts, but are not large enough for suspended road dust to influence local ozone mixing ratios. In a separate set of experiments, we also investigated the uptake of ozone by calcium chloride (i.e., road salt) and commercial anti-icer solution. Although ozone uptake by pure calcium chloride was negligible, ozone uptake by anti-icer was significant, which implies that the reactivity of anti-icer is conferred by its organic content. Importantly, ozone uptake by anti-icer-and, to a lesser extent, road dust doped with anti-icer-leads to the release of inorganic chlorine gas, which we collected using inline reductive trapping and quantified using ion chromatography. To explain these results, we propose a novel pathway for chlorine activation: here, ozone oxidation of the anti-icer organic fraction (in this case, molasses) yields reactive OH radicals that can oxidize chloride. In summary, this study demonstrates the ability of road dust and anti-icer to influence atmospheric oxidant mixing ratios in cold-climate urban areas, and highlights previously unidentified air quality impacts of winter road maintenance decisions.

Inorganic Ions Enhance the Number of Product Compounds through Heterogeneous Processing of Gaseous NO2 on an Aqueous Layer of Acetosyringone

Methoxyphenols represent important pollutants that can participate in the formation of secondary organic aerosols (SOAs) through chemical reactions with atmospheric oxidants. In this study, we determine the influence of ionic strength, pH, and temperature on the heterogeneous reaction of NO₂ with an aqueous film consisting of acetosyringone (ACS), as a proxy for methoxyphenols. The uptake coefficient of NO₂ (50 ppb) on ACS (1 × 10^-5 mol L^-1) is γ = (9.3 ± 0.09) × 10^-8 at pH 5, and increases by one order of magnitude to γ = (8.6 ± 0.5) × 10^-7 at pH 11. The lifetime of ACS due to its reaction with NO₂ is largely affected by the presence of nitrate ions and sulfate ions encountered in aqueous aerosols. The analysis performed by membrane inlet single-photon ionization-time-of-flight mass spectrometry (MI-SPI-TOFMS) reveals an increase in the number of product compounds and a change of their chemical composition upon addition of nitrate ions and sulfate ions to the aqueous thin layer consisting of ACS. These outcomes indicate that inorganic ions can play an important role during the heterogeneous oxidation processes in aqueous aerosol particles.

Ionic Strength Effect Triggers Brown Carbon Formation through Heterogeneous Ozone Processing of Ortho-Vanillin

Methoxyphenols are an important class of compounds emerging from biomass combustion, and their reactions with ozone can generate secondary organic aerosols in the atmosphere. Here, we use a vertical wetted wall flow tube reactor to evaluate the effect of ionic strength on the heterogeneous reaction of gas-phase ozone (O₃) with a liquid film of o-vanillin (o-VL) (2-hydroxy-3-methoxybenzaldehyde), as a proxy for methoxyphenols. Typical for moderately acidic aerosols, at fixed pH = 5.6, the uptake coefficients (γ) of O₃ on o-VL ([o-VL] = 1 × 10^-5 mol L^-1) increase from γ = (1.9 ± 0.1) × 10^-7 in the absence of Na₂SO₄ to γ = (6.8 ± 0.3) × 10^-7 at I = 0.2 mol L^-1, and then, it decreases again. The addition of NO₃^- ions only slightly decreases the uptakes of O₃. Ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) reveals that the formation of multicore aromatic compounds is favored upon heterogeneous O₃ reaction with o-VL, in the presence of SO₄^2- and NO₃^- ions. The addition of NO₃^- ions favors the formation of nitrooxy (-ONO₂) or oxygenated nitrooxy group of organonitrates, which are components of brown carbon that can affect both climate and air quality.

Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property

Heterogeneous photochemical reaction of O₃ with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O₃ analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O₃ on HA. The initial uptake coefficient (γ_i) and the steady-state uptake coefficient (γ_ss) of O₃ under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γ_i and γ_ss on HA varied in the range of 0.76-2.77 × 10^-5 and 1.50-9.55 × 10^-6, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O₃ concentration, O₂ content, temperature, relative humidity (RH) and HA solution pH. Both γ_i and γ_ss showed linear dependence on the total irradiance (0-2.07 × 10¹⁶ photons/(cm²⋅s)) of the light source, and increased with the HA mass (0-3.2 μg/cm²), temperature (278-298 K) and HA solution pH (4.0-9.6). However, they showed negative correlations with the initial O₃ concentration and O₂ content. The γ_i remained constant in the RH range of 7%-60%, while γ_ss exhibited the maximum value at RH = 20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH₂, CH₃, aromatic CC, OH, CO, COOH and COO^-. HA aged by O₃ exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600 nm wavelength (AAE_300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.

Ionic Strength Effect Alters the Heterogeneous Ozone Oxidation of Methoxyphenols in Going from Cloud Droplets to Aerosol Deliquescent Particles

Methoxyphenols are one of the most abundant classes of biomarker tracers for atmospheric wood smoke pollution. The reactions of atmospheric oxidants (ozone, OH) with methoxyphenols can contribute to the formation of secondary organic aerosols (SOA). Here, for the first time, we use the well-established vertical wetted wall flow tube (VWWFT) reactor to assess the effect of ionic strength (I), pH, temperature, and ozone concentration on the reaction kinetics of ozone with acetosyringone (ACS), as a representative methoxyphenol compound. At fixed pH 3, typical for acidic atmospheric deliquescent particles, and at I = 0.9 M adjusted by Na₂SO₄, the uptake coefficient (γ) of O₃ increases by 2 orders of magnitude from γ = (5.0 ± 0.8) × 10^-8 on neat salt solution (Na₂SO₄) to γ = (6.0 ± 0.01) × 10^-6 on a mixture of ACS and Na₂SO₄. The comparison of the uptake coefficients of O₃ at different pH values indicates that the reaction kinetics strongly depends on the acidity of the phenolic group of ACS. The observed different reactivity of gas-phase ozone with ACS has implications for ozone uptake by the dilute aqueous phase of cloud droplets and by aerosol deliquescent particles loaded with inorganic salts, and it can affect the formation of SOA in the atmosphere.

Dry Deposition of Ozone over Land: Processes, Measurement, and Modeling

Dry deposition of ozone is an important sink of ozone in near surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely-used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Heterogeneous reaction of ozone with syringic acid: Uptake of O3 and changes in the composition and optical property of syringic acid

Syringic acid, which is a typical methoxyphenol emitted from wood combustion, can provide heterogeneous reaction sites for gaseous active components, influencing the concentrations of trace gases and the compositions of syringic acid. The heterogeneous uptake of O₃ on syringic acid was investigated using a flow tube reactor under ambient pressure. The initial uptake coefficient (γ_i) and the steady-state uptake coefficient (γ_ss) of O₃ linearly increased with syringic acid mass (0-0.16 μg cm^-2) and temperature (278-328 K), while they decreased with increasing the O₃ concentration and the O₂ content. The γ_i was independent of relative humidity (20%-70%), whereas γ_ss decreased with relative humidity (7%-70%). The compositional changes of syringic acid by the ozonization were analyzed by the Fourier transform infrared spectrometer (FT-IR) and the gas chromatography-mass spectrometry (GC-MS), confirming the generation of 2,6-dimethoxy-1,4-benzoquinone. In addition, compared to that of fresh syringic acid, the mass absorption efficiency of syringic acid aged by O₃ exhibited an increase in the range of 290-320 nm.

Enhanced photochemical conversion of NO2 to HONO on humic acids in the presence of benzophenone

The photochemical conversion of NO₂ to HONO on humic acids (HA) in the presence of benzophenone (BP) was investigated using a flow tube reactor coupled to a NO_x analyzer at ambient pressure. BP significantly enhanced the reduction of NO₂ to HONO on HA under simulated sunlight, as shown by the increase of NO₂ uptake coefficient (γ) and HONO yield with the mass ratio of BP to HA. The γ and HONO yield on the mixtures of HA and BP obviously depended on the environmental conditions. Both γ and HONO yield increased with the increase of irradiation intensity and temperature, whereas they decreased with pH. The γ exhibited a negative dependence on the NO₂ concentration, which had slight influences on the HONO yield. There were maximum values for the γ and HONO yield at relative humidity (RH) of 22%. Finally, atmospheric implications about the photochemical reaction of NO₂ and HA in the presence of photosensitive species were discussed.

A model system to mimic environmentally active surface film roughness and hydrophobicity

This work presents the development and initial assessment of a laboratory platform to allow quantitative studies on model urban films. The platform consists of stearic acid and eicosane mixtures that are solution deposited from hexanes onto smooth, solid substrates. We show that this model has distinctive capabilities to better mimic a naturally occurring film's morphology and hydrophobicity, two important parameters that have not previously been incorporated into model film systems. The physical and chemical properties of the model films are assessed using a variety of analytical instruments. The film thickness and roughness are probed via atomic force microscopy while the film composition, wettability, and water uptake are analyzed by Fourier transform infrared spectroscopy, contact angle goniometry, and quartz crystal microbalance, respectively. Simulated environmental maturation is achieved by exposing the film to regulated amounts of UV/ozone. Ultimately, oxidation of the film is monitored by the analytical techniques mentioned above and proceeds as expected to produce a utile model film system. Including variable roughness and tunable surface coverage results in several key advantages over prior model systems, and will more accurately represent native urban film behavior.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

The photoenhanced aging process of soot by the heterogeneous ozonization reaction

Light can dramatically enhance the heterogeneous aging process of soot by O₃, leading to the formation of various oxygen-containing species.

The Nevada Rural Ozone Initiative (NVROI): Insights to understanding air pollution in complex terrain

The Nevada Rural Ozone Initiative (NVROI) was established to better understand O3 concentrations in the Western United States (US). The major working hypothesis for development of the sampling network was that the sources of O3 to Nevada are regional and global. Within the framework of this overarching hypothesis, we specifically address two conceptual meteorological hypotheses: (1) The high elevation, complex terrain, and deep convective mixing that characterize Nevada, make this state ideally located to intercept polluted parcels of air transported into the US from the free troposphere; and (2) site specific terrain features will influence O3 concentrations observed at surface sites. Here, the impact of complex terrain and site location on observations are discussed. Data collected in Nevada at 6 sites (1385 to 2082 m above sea level (asl)) are compared with that collected at high elevation sites in Yosemite National Park and the White Mountains, California. Average daily maximum 1-hour concentrations of O3 during the first year of the NVROI ranged from 58 to 69 ppbv (spring), 53 to 62 ppbv (summer), 44 to 49 ppbv (fall), and 37 to 45 ppbv (winter). These were similar to those measured at 3 sites in Yosemite National Park (2022 to 3031 m asl), and at 4 sites in the White Mountains (1237 to 4342 m asl) (58 to 67 ppbv (summer) and 47 to 58 ppbv (fall)). Results show, that in complex terrain, collection of data should occur at high and low elevation sites to capture surface impacts, and site location with respect to topography should be considered. Additionally, concentrations measured are above the threshold reported for causing a reduction in growth and visible injury for plants (40 ppbv), and sustained exposure at high elevation locations in the Western USA may be detrimental for ecosystems.

Photosensitised humic-like substances (HULIS) formation processes of atmospheric significance: a review

Photosensitised reactions can produce compounds that closely resemble the humic-like substances (HULIS) occurring in atmospheric aerosols. The relevant processes have been observed in the laboratory, in both gas-solid systems and the aqueous phase. They involve triplet sensitisers (such as benzophenones, anthraquinones and nitroaromatic compounds, which yield reactive triplet states after sunlight absorption) or photogenerated oxidants like (•)OH, in the presence of substrates that undergo oligomerisation reactions upon oxidation. Formation of higher molecular weight compounds, modification of the wettability properties of organic films and photoproduction of substances with humic-like fluorescence properties have been observed as a consequence of the photosensitised reactions. Ozone plays an important but still not completely clear role in gas-solid systems.

Measurement of humic-like substances in aerosols: a review

Aerosol-phase humic-like substances (HULIS) have received increasingly attention due to their universal ambient presence, active participation in atmospheric chemistry and important environmental and health effects. In last decade, intensive field works have promoted development of quantification and analysis method, unearthed spatio-temporal variation, and proved evidence for source identification of HULIS. These important developments were summarized in this review to provide a global perspective of HULIS. The diverse operational HULIS definitions were gradually focused onto several versions. Although found globally in Europe, Asia, Australasia and North America, HULIS are far more typical in continental and near-ground aerosols. HULIS concentrations varied from <1 μg/m(3) to >13 μg/m(3), with their carbon fraction making up 9%-72% of water soluble organic carbon. Dominant HULIS source was suggested as secondary processes and biomass burning, with the detailed formation pathways suggested and verified in laboratory works.

Key role of organic carbon in the sunlight-enhanced atmospheric aging of soot by O2

Soot particles are ubiquitous in the atmosphere and have important climatic and health effects. The aging processes of soot during long-range transport result in variability in its morphology, microstructure, and hygroscopic and optical properties, subsequently leading to the modification of soot's climatic and health effects. In the present study the aging process of soot by molecular O(2) under simulated sunlight irradiation is investigated. Organic carbon components on the surface of soot are found to play a key role in soot aging and are transformed into oxygen-containing organic species including quinones, ketones, aldehydes, lactones, and anhydrides. These oxygen-containing species may become adsorption centers of water and thus enhance the cloud condensation nuclei and ice nuclei activities of soot. Under irradiation of 25 mW·cm(-2), the apparent rate constants (k(1,obs)) for loss or formation of species on soot aged by 20% O(2) were larger by factors of 1.5-3.5 than those on soot aged by 100 ppb O(3). Considering the abundance of O(2) in the troposphere and its higher photoreactivity rate, the photochemical oxidation by O(2) under sunlight irradiation should be a very important aging process for soot.

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.

Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors

This proposal involves direct photolysis processes occurring in the troposphere incorporating photochemical excitation and intermolecular energy transfer. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atmosphere and in consequence, of a more accurate prediction of the oxidative capacity of the atmosphere. Compounds that readily absorb in the tropospheric actinic window (ionic organic complexes, PAHs, aromatic carbonyl compounds) acting as potential photosensitizers of atmospheric relevant processes are explored. The impact of hotosensitation on relevant systems which could act as powerful atmospheric reactors,that is, interface ocean-atmosphere, urban and forest surfaces and indoor air environments is also discussed.

Oxidation of atmospheric humic like substances by ozone: a kinetic and structural analysis approach

This work explores the heterogeneous reaction between HUmic-LIke Substances (so-called HULIS) and ozone. Genuine atmospheric HULIS were extracted from aerosol samples collected in Chamonix (France) in winter and used in coated flow tube experiments to evaluate heterogeneous uptake of O₃ on such mixtures. The uptake coefficient (γ) was investigated as a function of pH (from 2.5 to 10), O₃ concentration (from 8 to 33 × 10¹¹ molecules cm⁻³), relative humidity (20 to 65%) and photon flux (from 0 to 1.66 × 10¹⁵ photons cm⁻² s⁻¹). Reactive uptake was found to increase in the irradiated experiment with pH, humidity and photon flux. The extract was characterized before and after exposure to O₃ and/or UV light in the attempt to elucidate the effect of the photochemical aging. Carbon content measurements, UV-vis spectroscopy and functional groups analysis revealed a decrease of the UV absorbance as well as of the carbon mass content, while the functionalization rate (COOH and C═O) and therefore the polarity increased during the simulated photochemical exposure.

Photoenhanced NO2 loss on simulated urban grime

The present study focuses on the heterogeneous reaction between gaseous NO(2) and solid pyrene/KNO(3) films, used as a simplified proxy of urban grime. This reaction is investigated under simulated atmospheric conditions with respect to relative humidity, NO(2) concentration and irradiation in a coated-wall flow-tube reactor. The geometric steady-state uptake coefficients γ(geo) for pyrene/KNO(3) films exposed to 50 ppbv of NO(2) ranged from 1.12×10(-7) in the dark to 2.67×10(-6) under near-UV irradiation (300-420 nm) and decreased with increasing NO(2) concentration in the range 30-120 ppbv. NO(2) removal is linearly dependent on light intensity, with release of gas-phase NO and HONO. Analysis of the solid film by ion chromatography and GC-MS showed the formation of nitrite ions and traces of 1-nitropyrene. A light-induced reaction mechanism is proposed. The results discussed herein suggest that PAH-containing urban grime on windows and buildings may be a key player in urban air pollution.

Photochemistry of atmospheric dust: ozone decomposition on illuminated titanium dioxide

The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 x 10(-6) to 3 x 10(-5). The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.

DRIFTS studies on the photodegradation of tannic acid as a model for HULIS in atmospheric aerosols

Humic like substances (HULIS) are important components of atmospheric aerosols, yet little is known about their photochemical transformation and the role of adsorbed water in this photochemistry. We report herein in situ and surface-sensitive spectroscopic studies on (1) the photodegradation of solid tannic acid, (2) structure of adsorbed water before and after photodegradation, and (3) the change in the hydrophilicity of tannic acid as a result of this photochemistry. Tannic acid (TA) was chosen as a synthetic proxy for HULIS because it has a defined molecular structure. Photochemical studies were conducted using diffuse reflectance infrared spectroscopy (DRIFTS) as a function of time (3 h), relative humidity (5-30%) and total irradiance (7, 20, 290 W m(-2) at 555 nm). Water adsorption isotherm measurements were recorded before and after photodegradation, which provided information on the structure of interfacial water and the thermodynamics of adsorption. The structure of water adsorbed on TA resembles that of water at the interface with polar organic solvents. Difference spectral data collected during irradiation shows loss features in the 1700-1000 cm(-1) range and growth in carbonyl features that are blue shifted relative to the starting material, suggesting oxidative photodegradation of TA and formation of aryl aldehydes. Under our experimental conditions, we observed no enhancement in water uptake after photodegradation relative to that on unirradiated samples. The implications of our results to the understanding of heterogeneous photochemistry of HULIS and the role of adsorbed water in these reactions are discussed.

Photoenhanced uptake of NO2 by pyrene solid films

We report uptake kinetics measurements of the heterogeneous reaction of gas phase NO2 with solid films of pyrene. By using a coated flow tube equipped with several near-ultraviolet (UV) emitting lamps (range 300-420 nm), we examined the effect of actinic radiation on the heterogeneous loss kinetics of nitrogen dioxide. With atmospherically relevant concentrations of NO2, (20-119 ppbv), the uptake ranged from below 10(-7) in the dark to 3.5 x 10(-6) under near-UV irradiation. Under illuminated conditions, the uptake coefficient decreased markedly with increasing gas-phase concentration, suggestive of a Langmuir-Hinshelwood-type surface reaction mechanism. The NO2 reactivity was not a function of deposited Pyrene mass or of the relative humidity (in the range 10-89%) and depended linearly on the intensity of illumination. Gas-phase product analysis indicated that approximately 50% of the NO2 loss could be accounted for by HONO and NO release. These experimental results are discussed along with a possible nitration mechanism.