Aqueous Phase Photooxidation Research Articles

Aqueous-Phase Photooxidation of Vanillin in the Presence of Nitrite: Characteristics, Products, and Mechanism

Aqueous-Phase Photooxidation of Vanillin in the Presence of Nitrite: Characteristics, Products, and Mechanism

Glycerol photocatalytic oxidation to higher value-added compounds via bismuth oxyhalide photocatalysts

Bismuth oxyhalides (BiOX) including BiOCl, BiOBr, and BiOI, were well synthesized using solvothermal technique and then used in the aqueous phase photooxidation of glycerol as a catalyst. The as-synthesized BiOBr could achieve the highest glycerol transformation of around 85.6% in 8 h under ultraviolet light (UV) irradiation among as-synthesized BiOXs. Moreover, the BiOBr/TiO2 heterojunction was also prepared through an ethylene glycol-assisted solvothermal process. This new BiOBr/TiO2 heterostructure exhibited excellent photocatalytic activity (97.4%) for the oxidation of glycerol compared with pure BiOBr (74%) under ultraviolet light irradiation at 6 h. This obtained behavior was confirmed by more produced OH• radicals of BiOBr/TiO2.

Aqueous-phase photo-oxidation of selected green leaf volatiles initiated by [rad]OH radicals: Products and atmospheric implications

C5- and C6- unsaturated oxygenated organic compounds emitted by plants under stress like cutting, freezing or drying, known as Green Leaf Volatiles (GLVs), may clear some of the existing uncertainties in secondary organic aerosol (SOA) budget. The transformations of GLVs are a potential source of SOA components through photo-oxidation processes occurring in the atmospheric aqueous phase. Here, we investigated the aqueous photo-oxidation products from three abundant GLVs (1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al) induced by OH radicals, carried out in a photo-reactor under simulated solar conditions. The aqueous reaction samples were analyzed using advanced hyphenated mass spectrometry techniques: capillary gas chromatography mass spectrometry (c-GC–MS); and reversed-phase liquid chromatography high resolution mass spectrometry (LC-HRMS). Using carbonyl-targeted c-GC–MS analysis, we confirmed the presence of propionaldehyde, butyraldehyde, 1-penten-3-one, and 2-hexen-1-al in the reaction samples. The LC-HRMS analysis confirmed the presence of a new carbonyl product with the molecular formula C6H10O2, which probably bears the hydroxyhexenal or hydroxyhexenone structure. Density functional theory (DFT)-based quantum calculations were used to evaluate the experimental data and obtain insight into the formation mechanism and structures of the identified oxidation products via the addition and hydrogen-abstraction pathways. DFT calculations highlighted the importance of the hydrogen abstraction pathway leading to the new product C6H10O2. Atmospheric relevance of the identified products was evaluated using a set of physical property data like Henry's law constant (HLC) and vapor pressure (VP). The unknown product of molecular formula C6H10O2 has higher HLC and lower VP than the parent GLV and thus has potential to remain in the aqueous phase leading to possible aqueous SOA formation. Other observed carbonyl products are likely first stage oxidation products and precursors of aged SOA.

PH affects the aqueous-phase nitrate-mediated photooxidation of phenolic compounds: implications for brown carbon formation and evolution.

Brown carbon (BrC) is known to have important impacts on atmospheric chemistry and climate. Phenolic compounds are a prominent class of BrC precursors that are emitted in large quantities from biomass burning and fossil fuel combustion. Inorganic nitrate is a ubiquitous component of atmospheric aqueous phases such as cloudwater, fog, and aqueous aerosols. The photolysis of inorganic nitrate can lead to BrC formation via the photonitration of phenolic compounds in the aqueous phase. However, the acidity of the atmospheric aqueous phase adds complexity to these photonitration processes and needs to be considered when investigating BrC formation from the nitrate-mediated photooxidation of phenolic compounds. In this study, we investigated the influence of pH on the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol, catechol, 5-nitroguaiacol, and 4-nitrocatechol initiated by inorganic nitrate photolysis. The reaction rates, BrC composition and quantities were found to depend on the aqueous phase pH. Guaiacol, catechol, and 5-nitroguaiacol reacted substantially faster at lower pH. In contrast, 4-nitrocatechol reacted at slower rates at lower pH. For all four phenolic compounds, the initial stages of photooxidation resulted in an increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range due to the formation of light absorbing products formed via the addition of nitro and/or hydroxyl groups to the phenolic compound. Greater photo-enhancement was observed at lower pH during the nitrate-mediated photooxidation of guaiacol and catechol. In contrast, greater photo-enhancement was observed at higher pH during the nitrate-mediated photooxidation of 5-nitroguaiacol and 4-nitrocatechol. This indicated that the effect that the aqueous phase pH has on the composition and yields of BrC formed is not universal, and will depend on the initial phenolic compound. These results provide new insights into how the atmospheric aqueous phase acidity influences the reactivities of different phenolic compounds and BrC formation/evolution during photooxidation initiated by inorganic nitrate photolysis, which will have significant implications for how the atmospheric fates of phenolic compounds and BrC formation/evolution are modeled for areas with high levels of inorganic nitrate.

Formation and evolution of brown carbon during aqueous-phase nitrate-mediated photooxidation of guaiacol and 5-nitroguaiacol

Light absorbing organic aerosols, which are commonly referred to as brown carbon (BrC), have important climate impacts. Phenolic compounds are a class of aromatic compounds that are known BrC precursors. Inorganic nitrate is a ubiquitous component of atmospheric aerosols, clouds, and fog. The photolysis of inorganic nitrate in atmospheric aqueous phases generates a variety of reactive oxygen species and reactive nitrogen species that can facilitate the photooxidation of organic compounds to form BrC. In this study, we investigated the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol and 5-nitroguaiacol initiated by inorganic nitrate photolysis. Guaiacol and 5-nitroguaiacol are two water-soluble phenolic compounds that are commonly found in fossil fuel and biomass burning emissions. Upon illumination in the presence of sodium nitrate, guaiacol and 5-nitroguaiacol reacted rapidly to form BrC. The reaction rates and quantities of BrC formed depended on the initial sodium nitrate concentration. The reaction rates of guaiacol were substantially faster than those of 5-nitroguaiacol. The electron-withdrawing nitro functional group on 5-nitroguaiacol's aromatic ring likely had a deactivating effect on the ring's reactivity, which contributed to 5-nitroguaiacol's lower reactivity compared to guaiacol. The major products produced during the initial stages of photooxidation were formed by the addition of nitro and/or nitroso groups to guaiacol and 5-nitroguaiacol. These products likely strongly absorbed near-UV and/or visible light, which led to an observed increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range. Greater photo-enhancement was observed during the nitrate-mediated photooxidation of guaiacol. This indicated that the presence of nitro functional group(s) on the aromatic ring of phenolic compounds will impact the extent of photo-enhancement during BrC formation. Further photooxidation caused the initial products to fragment into smaller molecules that do not absorb near-UV and visible light. This resulted in an observed decrease in light absorption (i.e., photo-bleaching). These results highlight how aqueous-phase inorganic nitrate photolysis can drive the formation of BrC by facilitating the photooxidation of organic compounds.

Aqueous Phase Photo-oxidation of Brown Carbon Nitrophenols: Reaction Kinetics, Mechanism, and Evolution of Light Absorption

Light absorbing organic aerosol particles, referred to as brown carbon, are geographically widespread and can have an important climate impact through the absorption of solar radiation. Recent studies, both in the laboratory and the field, have shown that brown carbon aerosols can be bleached of their color by direct photolysis and photo-oxidation reactions on the time scale of hours to days. However, the photo-oxidation of nitrophenol molecules, which are colored compounds often associated with biomass burning organic aerosol, show an enhancement in light absorption before the color is lost. This study investigates the mechanism of color enhancement and the fate of three nitrophenol compounds, specifically nitrocatechol, nitroguaiacol, and dinitrophenol, in the aqueous phase using online aerosol chemical ionization mass spectrometry (aerosol-CIMS). The second-order rate constants for the three nitrophenols with OH radicals in the aqueous phase at pH 7 (298 K), were determined to be 5 × 109 M–1 s–1, 5.2 ×...

Long-term (2001\u20132013) observations of water-soluble dicarboxylic acids and related compounds over the western North Pacific: trends, seasonality and source apportionment

To better understand the impact of East Asian pollutants on the molecular composition of marine organic aerosols, we conducted long-term (2001–2013) observations of water-soluble dicarboxylic acids and related compounds in total suspended particulate samples collected at Chichijima Island in the western North Pacific (WNP). Seasonal variations of all the diacids and related compounds showed maxima in winter and spring and minima in summer, except for azelaic acid (C9), which maximized in summer to autumn. The overall annual concentrations of the total diacids, ω-oxoacids and α-dicarbonyls showed an increase during 2001–2013. We found a significant (p < 0.05) decadal increase in the inter-annual trends of pyruvic and glyoxylic (p > 0.05) acids, and methylglyoxal (MeGly). In contrast, phthalic acid (p < 0.05) and glyoxal (Gly) showed a decrease in their trends. We also found a significant decrease in the trend of the Gly/MeGly mass ratios. These results demonstrate that the enhanced concentrations of diacids over the WNP are majorly attributed to the aqueous-phase photooxidation of biogenic volatile organic compounds from East Asia followed by long-range atmospheric transport. Further, positive matrix factorization analysis showed a biogenic photochemical contribution (42%) was the dominant source of oxalic acid in the WNP.

Rapid Aqueous-Phase Photooxidation of Dimers in the α-Pinene Secondary Organic Aerosol

Chemical complexity significantly hinders our understanding of the formation and evolution of secondary organic aerosol (SOA), which is known to have impacts on air quality and global climate. Dimeric substances present in SOA comprise a major fraction of extremely low-volatile organic compounds that are especially poorly characterized. Using online mass spectrometry, we have investigated the aqueous-phase OH oxidation of dimers present in the water-soluble fraction of SOA arising from ozonolysis of α-pinene. This study highlights very rapid OH oxidation of dimeric compounds. In particular, using pinonic acid as a reference compound, we obtained second-order rate constants for the loss of 12 dimers, with an average value of (1.3 ± 0.5) × 109 M–1 s–1 at room temperature. For the first time, this study demonstrates that rapid loss of dimeric compounds will occur in cloudwater and potentially also in aqueous aerosols.

Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α-Pinene Oxidation Products.

Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueous-phase OH oxidation chemistry of oxidation products of α-pinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, cis-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToF-CIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (± 0.5) × 10(9) and 3.1 (± 0.2) × 10(8) M(-1) s(-1), respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation.

Aqueous-phase photooxidation of levoglucosan – a mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS)

Abstract. Levoglucosan (LG) is a widely employed tracer for biomass burning (BB). Recent studies have shown that LG can react rapidly with hydroxyl (OH) radicals in the aqueous phase despite many mass balance receptor models assuming it to be inert during atmospheric transport. In the current study, aqueous-phase photooxidation of LG by OH radicals was performed in the laboratory. The reaction kinetics and products were monitored by aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS). Approximately 50 reaction products were detected by the Aerosol ToF-CIMS during the photooxidation experiments, representing one of the most detailed product studies yet performed. By following the evolution of mass defects of product peaks, unique trends of adding oxygen (+O) and removing hydrogen (−2H) were observed among the products detected, providing useful information for determining potential reaction mechanisms and sequences. Additionally, bond-scission reactions take place, leading to reaction intermediates with lower carbon numbers. We introduce a data analysis framework where the average oxidation state (OSc) is plotted against a novel molecular property: double-bond-equivalence-to-carbon ratio (DBE/#C). The trajectory of LG photooxidation on this plot suggests formation of polycarbonyl intermediates and their subsequent conversion to carboxylic acids as a general reaction trend. We also determined the rate constant of LG with OH radicals at room temperature to be 1.08 ± 0.16 × 109 M−1 s−1. By coupling an aerosol mass spectrometer (AMS) to the system, we observed a rapid decay of the mass fraction of organic signals at mass-to-charge ratio 60 (f60), corresponding closely to the LG decay monitored by the Aerosol ToF-CIMS. The trajectory of LG photooxidation on a f44–f60 correlation plot matched closely to literature field measurement data. This implies that aqueous-phase photooxidation might be partially contributing to aging of BB particles in the ambient atmosphere.

Investigation of Aqueous-Phase Photooxidation of Glyoxal and Methylglyoxal by Aerosol Chemical Ionization Mass Spectrometry: Observation of Hydroxyhydroperoxide Formation

Aqueous-phase processing of glyoxal (GLY) and methylglyoxal (MG) produces highly oxygenated, less volatile organic acids that can contribute to SOA formation and aging. In this study, aerosol chemical ionization mass spectrometry (aerosol CIMS) is employed to monitor aqueous-phase photooxidation of GLY and MG. Using iodide (I(-)) as the reagent ion, aerosol CIMS can simultaneously detect important species involved in the reactions: organic acids, peroxides, and aldehydes, so that the reconstructed total organic carbon (TOC) concentrations from aerosol CIMS data agree well with offline TOC analysis. This study also reports the first direct detection of hydroxyhydroperoxide (HHP) formation from the reaction of H(2)O(2) with GLY or MG. The formation of HHPs is observed to be reversible and an estimate of their equilibrium constants is made to be between 40 and 200 M(-1). Results of this study suggest that HHPs can form additional formic acid and acetic acid via photooxidation and regenerate GLY or MG during photooxidation, compensating their loss. HHP formation needs to be further studied for inclusion in aqueous-phase chemical models given that it may affect the aqueous partitioning of carbonyls in the atmosphere.

Oligomer and SOA formation through aqueous phase photooxidation of methacrolein and methyl vinyl ketone

Abstract This work investigates the ability of methacrolein (MACR) and methyl vinyl ketone (MVK) (the two main gas phase atmospheric oxidation products of isoprene) to form oligomers and secondary organic aerosol (SOA) upon aqueous phase OH-oxidation and subsequent water evaporation. For the two precursors, electrospray mass spectrometry (in infusion and coupled to liquid chromatography) analysis of the reacting solutions brought clear evidence for the formation of oligomer systems having a mass range of up to 1400 Da. More than 11 series of oligomers were found. For MVK, the intensity and masses of oligomers became increasingly important as MVK initial concentrations increased from 0.2 to 20 mM. For both precursors, the oligomers were responsible for the SOA formation during nebulization experiments. The evaluated SOA mass yield ranged from 3.9 to 9.9% for MVK. These yields were time dependent and were in good agreement with the range (1.6–11.7%) obtained for MACR under the same conditions by El Haddad et al. (2009) .

In-cloud multiphase behaviour of acetone in the troposphere: Gas uptake, Henry’s law equilibrium and aqueous phase photooxidation

Acetone is ubiquitous in the troposphere. Several papers have focused in the past on its gas phase reactivity and its impact on tropospheric chemistry. However, acetone is also present in atmospheric water droplets where its behaviour is still relatively unknown. In this work, we present its gas/aqueous phase transfer and its aqueous phase photooxidation. The uptake coefficient of acetone on water droplets was measured between 268 and 281 K ( γ = 0.7 × 10 −2–1.4 × 10 −2), using the droplet train technique coupled to a mass spectrometer. The mass accommodation coefficient α (derived from γ) was found in the range (1.0–3.0 ± 0.25) × 10 −2. Henry’s law constant of acetone was directly measured between 283 and 298 K using a dynamic equilibrium system ( H (298K) = (29 ± 5) M atm −1), with the Van’t Hoff expression ln H ( T ) = (5100 ± 1100)/ T − (13.4 ± 3.9). A recommended value of H was suggested according to comparison with literature. The OH-oxidation of acetone in the aqueous phase was carried out at 298 K, under two different pH conditions: at pH = 2, and under unbuffered conditions. In both cases, the formation of methylglyoxal, formaldehyde, hydroxyacetone, acetic acid/acetate and formic acid/formate was observed. The formation of small amounts of four hydroperoxides was also detected, and one of them was identified as peroxyacetic acid. A drastic effect of pH was observed on the yields of formaldehyde, one hydroperoxide, and, (to a lesser extent) acetic acid/acetate. Based on the experimental observations, a chemical mechanism of OH-oxidation of acetone in the aqueous phase was proposed and discussed. Atmospheric implications of these findings were finally discussed.

In-cloud processes of methacrolein under simulated conditions – Part 1: Aqueous phase photooxidation

Abstract. The photooxidation of methacrolein was studied in the aqueous phase under simulated cloud droplet conditions. The obtained rate constant of OH-oxidation of methacrolein at 6°C in unbuffered solutions was 5.8(±0.9)×109 M−1 s−1. The measured rate coefficient is consistent with OH-addition on the C=C bond. This was confirmed by the mechanism established on the study of the reaction products (at 25°C in unbuffered solutions) where methylglyoxal, formaldehyde, hydroxyacetone and acetic acid/acetate were the main reaction products. An upper limit for the total carbon yield was estimated to range from 53 to 85%, indicating that some reaction products remain unidentified. A possible source of this mismatch is the formation of higher molecular weight compounds as primary reaction products which are presented in El Haddad et al. (2009) and Michaud et al. (2009).

In-cloud processes of methacrolein under simulated conditions – Part 3: Hygroscopic and volatility properties of the formed secondary organic aerosol

Abstract. The hygroscopic and volatility properties of secondary organic aerosol (SOA) produced from the nebulization of solutions after aqueous phase photooxidation of methacrolein was experimentally studied in a laboratory, using a Volatility-Hygroscopicity Tandem DMA (VHTDMA). The obtained SOA were 80% 100°C-volatile after 5 h of reaction and only 20% 100°C-volatile after 22 h of reaction. The Hygroscopic Growth Factor (HGF) of the SOA produced from the nebulization of solutions after aqueous-phase photooxidation of methacrolein is 1.34–1.43, which is significantly higher than the HGF of SOA formed by gas-phase photooxidation of terpenes, usually found almost hydrophobic. These hygroscopic properties were confirmed for SOA formed by the nebulization of the same solutions where NaCl was added. The hygroscopic properties of the cloud droplet residuals decrease with the reaction time, in parallel with the formation of more refractory compounds. This decrease was mainly attributed to the 250°C-refractive fraction (presumably representative of the highest molecular weight compounds), which evolved from moderately hygroscopic (HGF of 1.52) to less hygroscopic (HGF of 1.36). Oligomerization is suggested as a process responsible for the decrease of both volatility and hygroscopicity with time. The NaCl seeded experiments enabled us to show that 19±4 mg L−1 of SOA was produced after 9.5 h of reaction and 41±9 mg L−1 after 22 h of in-cloud reaction. Because more and more SOA is formed as the reaction time increases, our results show that the reaction products formed during the aqueous-phase OH-oxidation of methacrolein may play a major role in the properties of residual particles upon the droplet's evaporation. Therefore, the specific physical properties of SOA produced during cloud processes should be taken into account for a global estimation of SOA and their atmospheric impacts.

Oligomers formed through in-cloud methylglyoxal reactions: Chemical composition, properties, and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry

Secondary organic aerosol (SOA) is a substantial component of total atmospheric organic particulate matter, but little is known about the composition of SOA formed through cloud processing. We conducted aqueous phase photo-oxidation experiments of methylglyoxal and hydroxyl radical to simulate cloud processing. In addition to predicted organic acid monomers, oligomer formation from methylglyoxal–hydroxyl radical reactions was detected by electrospray ionization mass spectrometry (ESI-MS). The chemical composition of the oligomers and the mechanism of their formation were investigated by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and LCQ DUO ion trap mass spectrometry (ESI-MS-MS). Reaction products included 415 compounds detected in the mass range 245–800 Da and the elemental composition of all 415 compounds were determined by ultra-high resolution FT-ICR MS. The ratio of total organic molecular weight per organic carbon weight (OM:OC) of the oligomers (1.0–2.5) was lower than the OM:OC of the organic acid monomers (2.3–3.8) formed, suggesting that the oligomers are less hygroscopic than the organic acid monomers formed from methylglyoxal–hydroxyl radical reaction. The OM:OC of the oligomers (average=2.0) is consistent with that of aged atmospheric aerosols and atmospheric humic-like substances (HULIS). A mechanism is proposed in which the organic acid monomers formed through hydroxyl radical reactions oligomerize through esterification. The mechanism is supported by the existence of series of oligomers identified by elemental composition from FT-ICR MS and ion fragmentation patterns from ESI-MS-MS. Each oligomer series starts with an organic acid monomer formed from hydroxyl radical oxidation, and increases in molecular weight and total oxygen content through esterification with a hydroxy acid (C 3H 6O 3) resulting in multiple additions of 72.02113 Da (C 3H 4O 2) to the parent organic acid monomer. Methylglyoxal is a water-soluble product of both gas phase biogenic (i.e., isoprene) and anthropogenic (i.e., aromatics, alkenes) hydrocarbon oxidation. The varied and multiple sources of methylglyoxal increase the potential for these low volatility cloud processing products (e.g., oxalic acid and oligomers) to significantly contribute to SOA. Aqueous phase oligomer formation investigated here and aerosol phase oligomer formation appear to be more similar than previously realized, which may simplify the incorporation of oligomers into atmospheric SOA models.

Atmospheric oxalic acid and SOA production from glyoxal: Results of aqueous photooxidation experiments

Aqueous-phase photooxidation of glyoxal, a ubiquitous water-soluble gas-phase oxidation product of many compounds, is a potentially important global and regional source of oxalic acid and secondary organic aerosol (SOA). Reaction kinetics and product analysis are needed to validate and refine current aqueous-phase mechanisms to facilitate prediction of in-cloud oxalic acid and SOA formation from glyoxal. In this work, aqueous-phase photochemical reactions of glyoxal and hydrogen peroxide were conducted at pH values typical of clouds and fogs (i.e., pH=4–5). Experimental time series concentrations were compared to values obtained using a published kinetic model and reaction rate constants from the literature. Experimental results demonstrate the formation of oxalic acid, as predicted by the published aqueous phase mechanism. However, the published mechanism did not reproduce the glyoxylic and oxalic acid concentration dynamics. Formic acid and larger multifunctional compounds, which were not previously predicted, were also formed. An expanded aqueous-phase oxidation mechanism for glyoxal is proposed that reasonably explains the concentration dynamics of formic and oxalic acids and includes larger multifunctional compounds. The coefficient of determination for oxalic acid prediction was improved from 0.001 to >0.8 using the expanded mechanism. The model predicts that less than 1% of oxalic acid is formed through the glyoxylic acid pathway. This work supports the hypothesis that SOA forms through cloud processing of glyoxal and other water-soluble products of alkenes and aromatics of anthropogenic, biogenic and marine origin and provides reaction kinetics needed for oxalic acid prediction.

Uptake and UV-Photooxidation of Gas-Phase PAHs on the Surface of Atmospheric Water Films. 1. Naphthalene

The adsorption and photochemical reaction of naphthalene vapor at the air-water interface of water films (22 microm and 450 microm) were studied in a horizontal flow reactor. Experiments were conducted in the regime where gas-phase mass transfer resistance did not limit the uptake. The equilibrium uptake was dependent on water film thickness only below 1 microm. Bulk water-air and air-to-interface partition constants were estimated from the experiments. The equilibrium partition constant between the water film and air decreased with increasing temperature. Photochemical reaction products were isolated in the water film after exposure to UV light. Four main oxygenated products were identified (1,3-indandione, 1(3H)-isobenzofuranone (phthalide), 2H-1-benzopyran-2-one (coumarin), and 1-naphthol). The initial rates of product formation were 46 to 154% larger for the thin film (22 microm) compared to both a thick film (450 microm) and bulk aqueous phase photooxidation. The atmospheric implications of reactions in water films are discussed.