Atmospheric Photosensitized Heterogeneous and Multiphase

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Atmospheric Photosensitized Heterogeneous and Multiphase Reactions: From Outdoors to Indoors Elena Gómez Alvarez,†,‡ Henri Wortham,†,‡ Rafal Strekowski,†,‡ Cornelius Zetzsch,§ and Sasho Gligorovski*,†,‡ †

Aix-Marseille Université, Laboratoire Chimie Environnement, FRE 3416, Equipe Instrumentation et Réactivité Atmosphérique, Case courrier 29, 3 Place Victor Hugo, 13331, Marseille Cedex 03, France ‡ CNRS, Laboratoire Chimie Environnement, FRE 3416, Equipe Instrumentation et Réactivité Atmosphérique, Case courrier 29, 3 Place Victor Hugo, 13331, Marseille Cedex 03, France § Forschungsstelle für Atmosphärische Chemie, Universität Bayreuth, Dr.-Hans-Frisch-Strasse 1-3, D-95448 Bayreuth, Germany

Figure 1. Impact of photosensitized processes in atmospheric chemistry. •

OH radical formation and to better differentiate significant pollution risks.

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he concentration of atmospheric organic compounds is strongly influenced by their emissions followed by their atmospheric degradation which is mainly controlled by photochemistry, including photoinduced multiphase processes and heterogeneous reactivity. In addition to atmospheric loss of organic compounds through direct photolysis, activation processes of certain organic compounds by solar radiation can also occur by indirect photoinduced or photosensitized processes.1 In a photosensitized process, a photosensitizer induces different physicochemical processes on the neighboring molecules that, in turn, can lead to a number of processes, oligomerization among them, with possible climate change impacts on regional and global scales. The atmospheric chemistry impacts of photosensitized processes are shown in Figure 1. Photosensitized processes for instance could influence the reactivity of semivolatile compounds (pesticides and PAHs among others) at the interface ocean-atmosphere and also the heterogeneous formation of HONO (Figure 1). In this article, the impact of photosensitized phenomena on the atmospheric reactivity is explored and the importance of atmospheric photosensitization processes is evaluated. Light-induced multiphase processes and light-induced heterogeneous reactivity are explored which lead to outdoor and indoor • OH radical formation. Given that a considerable fraction of solar radiation penetrates indoor environments through the windows, more research should focus on indoor environments to assess © 2011 American Chemical Society

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PHOTOSENSITIZED PROCESSES: A BRIEF OVERVIEW Photosensitization refers to a light-activated process that requires the presence of a light-absorbing molecule, that is, a photosensitizer, that induces physicochemical changes in nonabsorbing substrate molecules. In other words, a photosensitizer absorbs light and transfers the resulting excess energy to another molecule. Atmospheric photosensitization loss processes may be important for molecules that are themselves not readily photolyzed. As shown in Scheme 1, the atmospheric destruction of phenols results from a reaction with an excited photosensitizer, such as a carbonyl compound. The absorption of radiation by a light absorbing compound, that is, a chromophore, is the initial step in all photophysical and photochemical reactions. The energy of the absorbed light promotes molecules from their ground state to an excited singlet state, followed by a spin conversion by intersystem crossing from singlet to triplet with a certain quantum yield, ΦISC. The resulting change in spin multiplicity modifies the properties of the excited state. As a result the triplet excited state has much longer lifetime than the singlet state. The observed increase in the excited triplet lifetime enhances the probability of interaction with another Published: December 7, 2011 1955

dx.doi.org/10.1021/es2019675 | Environ. Sci. Technol. 2012, 46, 1955−1963

Environmental Science & Technology

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Scheme 1

Scheme 2

and leads to the formation of singlet state oxygen O2(a1Δg).

molecule, therefore, explaining the prevailing role of the triplet state in the photosensitization process. A schematic representation of a sensitization process (Type I) is shown in Scheme 1. As shown, the triplet state of a photosensitizer reacts first with a substrate other than molecular oxygen. The type I process is generally considered to involve a reactive radical species which is derived from a photoinitiated electron transfer or H-atom abstraction reaction between the light-absorbing compound (photosensitizer) and another organic compound that is ultimately oxidized. The photosensitization process shown in Scheme 1 involves a carbonyl compound as a photosensitizer. Solid scientific evidence for this mechanism in atmospheric chemistry exists.2−5 The process takes place through the electronic state configuration (ππ* or nπ*) of the lowest triplet excited state. In this scheme, a phenolic substituent is presented as a nonabsorbing substrate that interacts with the photosensitizer via a H-atom abstraction mechanism. In the alternative Type II reaction pathway, the triplet state of the photosensitizer reacts first with molecular oxygen. This reaction usually proceeds via energy transfer from the photosensitizer triplet excited state to the molecular oxygen. This process R-1 regenerates the ground state photosensitizer

3

photosensitizer* + O2 → photosensitizer + O2 (a1Δg )

(R-1)

In the Type II reaction mechanism the energy transfer to produce the singlet oxygen R-1 sometimes competes with the electron transfer channel, that is, reduction of O2(X3Σ−g) to yield the superoxide anion, O2−. For this reason the definitions should reflect whether the excited state of the sensitizer is quenched by oxygen, Type II, or an organic molecule, Type I. In this article we hold the nomenclature that reflects a photosensitized phenomenon which is easily determined experimentally not a process that first involves resolving a reaction mechanism. Type I and Type II reactions of the photosensitization process are compared in Scheme 2.

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IMPACT OF PHOTOSENSITIZED PROCESSES ON THE •OH RADICAL PRODUCTION Among the others excited singlet states of molecular oxygen three are the most important. Two of them are low-lying singlet excited states, that is, O2(a1Δg) and O2(b1Σ+g). Electronic configurations of those two states differ only by the structure of the 1956



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formation rate of 34 μM h−1 was observed for the methoxysubstituted carbonyls.9 Other important species present in deliquescent aerosol particles are ionic iron-organic complexes such as iron-oxalate. Iron-organic complexes have been studied by atmospheric scientists for more than 20 years.10−12 These complexes are excellent absorbers in the UV-vis actinic region (for certain complexes the absorption band extends well into the visible region) and as a consequence can potentially induce further photochemistry. Such photochemistry has been shown to initiate a radical chain production in the condensed phase and it may prove to be a considerable in situ •OH radical source in aerosols (Figure 2) which in

π-antibonding orbitals. Since the transition from the O2(a1Δg) state to the O2(X3Σ−g) state is spin forbidden, the O2(a1Δg) has a relatively long lifetime. On the other hand, the second excited state of oxygen is short-lived due to a spin-allowed transition to the O2(a1Δg) state. A transition Δ → Σ is not allowed either (change of quantum number by two). This difference in stability is best explained in terms of the corresponding radiative lifetimes of O2(a1Δg) and O2(b1Σ+g) which are 67 min and 7−12 s, respectively, in the gas phase and 10−6 s, and 10−1 s, respectively, in aqueous solution.6 Of the two low-lying singlet states, O2(a1Δg) is the one with the lowest excitation energy and a longer radiative lifetime. For example, the most intense feature of the oxygen dayglow, that is to say, the infrared atmospheric band, comes from O2(a1Δg), produced by photolysis of ozone. However, the photolysis of ozone is not the sole process responsible for the formation of atmospheric O2(a1Δg). It can be generated in a photosensitized process wherein a sensitizer absorbs light and subsequently the excited triplet of the sensitizer transfers a fraction of its excitation energy to ground state oxygen, O2(X3Σ−g), to form singlet oxygen. Furthermore, in the lower troposphere polycyclic aromatic hydrocarbons (PAHs) may be involved in the formation of atmospheric O2(a1Δg).1 PAHs absorb sunlight in the UV-A region (320−400 nm). It is shown that the O2(a1Δg) production by photosensitization is favored by the high triplet yields and long lifetimes of the triplet states.7 The photochemical reaction efficiency and relaxation behavior of carbonyl compounds can be correlated with the electronic state configuration (ππ* or nπ*) of the lowest triplet excited state. In addition to the electronic nature of the triplet excited state, the structure/orientation also plays an important role which can be correlated with their reactivity.8 Because we live in a “world of light” and because excited electronic states are readily produced by atmospheric processes that result from absorption of ultraviolet (UV) or visible radiation that involve molecular oxygen and certain organic compounds that are efficient photosensitizers, photosensitizer mechanisms that form atmospheric singlet oxygen are potentially important in the atmosphere. It has been shown that each photosensitizer molecule will typically produce 103 to 105 molecules of singlet oxygen, 1O2, before being consumed by oxidation under given atmospheric conditions.7 Most commonly the O2(a1Δg) first reacts with organic molecules through direct insertion to form peroxides. Given the peroxide O−O bond low dissociation energy (BDE = 145 kJ mol−1), it cleaves readily to give oxygen-based radicals (e.g., the •OH radicals from hydrogen peroxide). Hydrogen peroxide can be formed and destroyed via redox cycles between HOO•/•O2− and transition metals, which in atmospheric condensed phase (aerosols) may be complexed by organic compounds. The photosensitized formation of atmospheric H2O2 was suggested by Anastasio et al. in 1997.9 These authors proposed a mechanism that involves light induced formation of the triplet excited state of the carbonyl compound (and its protonated form), oxidation of phenol by the protonated and nonprotonated triplets to form a ketyl radical (and a phenoxyl radical), and subsequent reduction of O2 by the ketyl radical to form HOO•/•O2− and regenerate the parent carbonyl. Hence, photosensitized degradation of organic compounds induced by aromatic carbonyls and PAHs as photosensitizers is a potentially important source of hydrogen peroxide and thus •OH radicals in atmospheric aerosols. The photoformation rates for hydrogen peroxide have been shown to vary between 1 and 10 μM h−1 depending on the chromophores and the pH of the solution.9 The highest photo-

Figure 2. Photodissociation of Fe-oxalate in aqueous phase leading to the formation of •OH radicals through a chain of reactions.

turn may influence the oxidizing capacity of the atmosphere.13 In Figure 2 this is illustrated taking iron-oxalate as an example. Under remote conditions, FeOH+ and FeOH2+ are the most important in situ sources of •OH radicals in the atmospheric condensed phase.

H2O2 + Fe2 + → Fe3 + + •OH + OH−

(R-2)

It has been shown that reaction R-2 contributes as much as 33% to the total •OH radical concentration present in the atmospheric condensed phase.14 Photolysis of iron(III) complexes and the Fenton reaction both lead to •OH radical formation in the condensed phase. Iron, in both, oxidized and reduced states, acts as a “catalyst” for • OH radical formation in the aqueous phase because it is simply regenerated.15 The organic acidic−Fe(III) complexes have been shown to catalyze the photochemical decomposition of oxalic, glyoxalic and pyruvic acids.16 According to the last version of CAPRAM model14 the Fe2+-oxalate photochemical decay contributes about 8% of the Fe2+ source which implies that it is an important indirect in situ •OH radical source. Also, the formation of organic-iron complexes is likely the reason for the formation of light absorbing species (HULIS) observed by Gelenscér et al.17 and Hoffer et al.,18 during the • OH-oxidation of 3,5-dihydroxy-benzoic acid. Recently, Wentworth and Al-Abadleh19 have demonstrated that ferric chloride (FeCl3) may act as a photosensitizer inducing a photochemical transformation of gallic acid which is used as a proxy for humic like substances (HULIS). These 1957



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reactor and found that reduction of NO2 on humic acid in the presence of light represents an important source of HONO. This study,30 clearly indicates that HONO becomes a very important source of •OH not only at dawn but during the day, as well. The daytime HONO formation discussed in an overview paper by Kleffmann31 was found to be about 60 times faster than the heterogeneous night-time formation, which strongly suggests missing sources of HONO during the day. It is extremely important to study the daylight formation of HONO since •OH production from HONO photolysis was found to be larger (by 20%) than the traditionally estimated to be the highest contribution of •OH coming from the photolysis of ozone integrated over the day29 (Figure 3).

authors observed that the degradation rate of gallic acid in the presence of a photosensitizer was much faster compared to UV photo-Fenton reactions of gallic acid in the aqueous phase containing H2O2, Fe2+, or Fe3+ ions. Considering the number of organic compounds which readily absorb in the tropospheric actinic window (PAHs and aromatic carbonyl compounds among others) and thus, potentially act as powerful photosensitizers, photosensitized processes could potentially be very important with respect •OH radical formation in the atmospheric condensed phase. For example, Styler et al.20 have shown that the lowest triplet excited state of pyrene adsorbed on liquid films is reactive toward gas-phase ozone and leads to formation of O3− which further reacts with water molecules to yield •OH radicals.

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THE ROLE OF PHOTOSENSITIZED PROCESSES AT ATMOSPHERIC SURFACES Direct and indirect (photosensitized) processes are more efficient on atmospheric surfaces.21 Atmospheric aerosols include a significant amounts of phenols, substituted phenols and photosensitizers, such as quinones and aromatic carbonyls (e.g., 4-carboxybenzophenone), which may be also products of PAH photolysis.1,9,22−24 It was shown that, in solution, the triplet excited states of aromatic carbonyls such as 4carboxybenzophenone can be efficiently quenched by phenols and substituted phenols.25 The triplet states of these molecules can act as electron oxidants and react rapidly with electron donor substituted phenols at almost diffusion controlled rates. The resulting products are phenoxy radicals and reduced benzophenones (diphenyl ketyl radicals). George et al.2 have suggested a reaction mechanism describing the formation of nitrous acid following the heterogeneous reaction of NO2 with phenols and substituted phenols on bulk surfaces in presence of 4-carboxybenzophenone as a photosensitizer. Diphenylketyl radical was identified as key species in the reduction of NO2. Hence, the photosensitized reaction could be a source of radical intermediates which can be reactive toward atmospheric oxidants such as O3 and NO2. It is assumed that the same mechanism might occur on the surface or in the bulk of atmospheric aerosols. In fact the fastest PAH degradation takes place on solid particles that are white in color such as pure silica and alumina, which afford the least absorption of solar radiation.1 The photosensitized heterogeneous reactions of gaseous ozone with adsorbed phenolic substances in presence of 4carboxybenzophenone as a photosensitizer were largely studied by Gligorovski and his co-workers.3,4,26,27 Experimental evidence of enhanced reactivity of adsorbed phenols toward ozone in presence of 4-carboxybenzophenone and formation of oligomers during such processes was obtained in those studies. Although a reasonable starting point, the role of photosensitized processes at the air−liquid and air−solid surfaces of atmospheric relevance (aerosols, ocean and urban surfaces among others) remains largely unexplored.

Figure 3. Relative contribution of ozone vs HONO photolysis in the production of •OH radicals in the Fichtelgebirge mountainous area (Source: Sörgel et al., 2011.29).

The heterogeneous reactions of NO2 with ground surfaces containing humic acid could be the main reason among the other important processes that could play a role such as turbulent exchange of air masses.29 Zhou et al.32 have suggested that high nitrous acid mixing ratios, observed in the forest canopy, could be correlated with the product of leaf surface nitrate loading and the rate constant of nitrate photolysis. High PAH concentrations can be deposited onto pine needles and leaves of deciduous trees.33 On the other hand, PAH are excellent candidates that can initiate photosensitized processes and thus enhance the formation of HONO through reduction of NO2 on forest surfaces. The PAH can undergo electron transfer followed by hydrolysis and/or formation of nitro compounds. Both reaction mechanisms lead to formation of HONO. The readers are referred to Monge et al.34 where a reaction mechanism has been suggested for the formation of HONO via heterogeneous NO2 reactions with the triplet excited state of pyrene as a representative PAH. There is also a missing daytime HONO source in the urban environments since up to 2 ppbv h−1 of HONO was measured.35 Buildings can strongly influence HONO formation in the urban environments because their surfaces chemically process gases, such as NO2. Buildings can occupy 35% of volume at ground level, decreasing with altitude. Building surface area to air volume can be 0.25/meter, also decreasing with altitude.36 In fact, the grime, settling on all surfaces outdoors, may provide the required hydrophilic and slightly acidic reaction medium for the uptake of NO2 and release of HONO. George and coworkers have demonstrated that indeed the heterogeneous loss

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HETEROGENEOUS PHOTOCHEMISTRY LEADING TO HONO FORMATION Surface/volume ratios in forests have recently attracted significant interest triggered by the unexpectedly high levels of HONO during daytime.28−30 This could be partially due to the presence of surfaces on which reactions leading to HONO generation could potentially take place, that is, stems, branches and leaves/needles, humic layer and soil. Stemmler et al., 200630 exposed humic acid films to gaseous NO2 in a gas flow 1958



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in biological and photochemical processes at the sea surface.46 The penetration of solar radiation into the upper water columns depends strongly on the wavelength and degree of water color. However, there is no attenuation of the solar light passing through the sea surface microlayer.46 Major photochemical intermediates at the sea surface microlayer include singlet oxygen, IO2, superoxide/ hydroperoxide, O2−/HO2, hydrogen peroxide, H2O2, and peroxy radicals, RO2.46 Surface seawater contains a variety of substances which act as photosensitizers. They include components of dissolved organic matter known as marine humic material,47 and anthropogenic organic compounds such as polycyclic aromatic ketones. 48,49 Carlson and Mayer, and Carlson 50−52 have demonstrated that the surface microlayer is enriched in organic material absorbing at wavelengths >300 nm relative to the bulk waters, which is predominant in light-absorbing species. There is a growing body of evidence that the heterogeneous reactive loss of gas phase NO2 and ozone at surfaces containing photoactive compounds (including chlorophyll) may be significantly enhanced under illumination.2,3,5,26,30,53,54 In the case of NO2, a good portion of the enhancement is due to heterogeneous reduction of the gaseous compound to HONO, following photoexcitation of the substrate.30 The substrates which demonstrate this effect to the greatest extent are those which act as photosensitizing or photoreducing agents such as humic acid for instance.2 As a matter of fact, reactive uptake of ozone by chlorophyll (and, by extension, other such biogenics) at the air−water interface is sufficiently fast that it could provide a major dry deposition route for ozone in the marine boundary layer.55 Recently, Ganzeveld et al.56 presented an evaluation of a globalscale analysis with a new mechanistic representation of atmosphereocean ozone exchange. They highlighted the important role of organic chemistry in oceanic O3 dry deposition considering the chlorophyll-ozone chemical interaction as proxy for the role of DOM-O3 reactivity in oceanic ozone uptake. They also pointed out that more studies are required to substantiate their findings. A photosensitizer that deserves special attention is pyruvic acid (PA) which is deposited on atmospheric condensed particles57 and can be present in significant amounts at the seasurface micro layer.58 PA is a keto acid with a π* ← n transition of the keto group around 325 nm. The π* ← n excitation of PA induces a long-range electron transfer from the carbonyl chromophore to neighboring carbonyl acceptors, rather than homolysis into contact radical pairs 3[CH3C(O• •COOH] or concerted decarboxylation into the carbene 1-hydroxyethylidene (3CH3C(•OH):).59,60 Long-range electron transfer involves redox centers separated by long distances. The electronic interaction between redox sites is relatively weak and the transition state for the electron transfer must be formed many times before there is a successful conversion from reactants to products.

of NO2 on simulated urban grime (KNO3/pyrene) under solar light irradiation lead to HONO formation.37 A recent study performed by the group of Ammann has shown that photoenhanced NO2 → HONO conversion might occur even under visible light irradiation of adsorbed phenolic moieties in presence of methylene blue as a photosensitizer.38 Despite the above-mentioned facts; the modeled daytime HONO concentrations are still underestimated which implies that there is room for further development and this topic should be pursued in future studies. The aforementioned suggests that atmospheric surfaces can represent at the same time a sink and a source of reactive species.

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REACTIVITY AT THE OCEAN−ATMOSPHERE INTERFACE Ozone-induced heterogeneous reactions of long-lived reactive oxygen intermediates can be formed with lifetimes of 100 s which is far longer than the residence time of adsorbed ozone that is in the order of a few ns.39 These reactive oxygen species emitted from the aerosol surface can further participate in the formation of secondary organic aerosols. Production of free radicals could be enhanced on aerosol surfaces and in thin liquid films generating more active photo-oxidation than is presently used in atmospheric models.21 For example, the concentrations of O3 and H2O2, both sources of •OH in the atmosphere, are predicted to be enhanced at the interface by factors of 10 and 2, respectively, compared to the bulk.40 The latter presumably implies an enhanced radical production at the air−ocean interface. Considering that oceans cover 75% of our planet the ocean surface microlayer can contribute to an countless amount of radicals (e.g., •OH) and reactive species in the free atmosphere. However, about ten years ago the decline of atmospheric concentrations of •OH were reported,41 implicating anthropogenic global warming as a likely cause. As the •OH radical is particularly active as cleansing reagent toward atmospheric pollutants (like low level of ozone), the implication was that anthropogenic global warming was inhibiting the atmosphere’s self-oxidizing properties. On the other hand, variations in •OH concentrations of up to 20% have been estimated from 14CO, in the period of a few months.42 In a very recent study, global mean •OH concentrations were estimated from atmospheric observations in the period 1998−2007, of a trace gas (methyl chloroform) whose predominant sink is reaction with •OH and they found negligible variations of 2−3% of [•OH] on a global level.43 In the future, it is likely that human-induced pollution coupled with a climate change trend will possibly lead to important changes in the •OH global budget. Therefore, further studies ought to be performed in order to come to a conclusion about this complex issue. In particular, high resolution models are required which include interactive coupling with biosphere processes and climate change. Certain studies44,45 report high levels of organic contamination of the surface microlayer, in marine waters.46 Among the organics of particular interest are light absorbing organic compounds which can effectively contribute triplet excited states, thereby enhancing the reactivity toward atmospheric oxidants such as NO2 or ozone and by this impacting the formation of gas phase compounds and the budget of reactive species in the marine boundary layer. Increased UV-B influences the production of important trace gases in the sea through changes

CH3C(O)C(O)OH + hν → 3[CH3C(O)C(O)OH]* → CO2 + products

(R-3) 59

The results by Guzman et al. have demonstrated that aqueous PA undergoes efficient photodecarboxylation R-3 from a triplet excited state. 3

[CH3C(O)C(O)OH]* → 3CH3C(OH): + CO2

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(R-4)



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Vaida et al.61 reported that decarboxylation of pyruvic acid is initiated by hydrogen atom chattering, which is a very fast process on the order of 150 fs. Decarboxylation of organic acids is likely to be competitive with other atmospheric processing mechanisms, primarily excited electronic state photoreaction and oxidation processes. Recently, Grgic et al.62 suggested that the triplet excited state of PA plays the role of a photosensitizer in the multiphase reactions between gas-phase ozone and aqueous phase oxalic acid. They demonstrated that the presence of pyruvic acid can potentially influence the reaction mechanism of the ozone multiphase chemistry of aqueous oxalic acid. In addition, it was suggested that there is also a possibility of a H-atom transfer between the excited triplet of PA and oxalic acid. Such interaction between the triplet excited state of PA and dicarboxylic acids in the presence of atmospheric oxidants such as ozone can lead to the oligomer formation in aerosols.

At homes, offices, hospitals, schools etc., the surfaces are diverse (aerosol particles, dust, clothes, ceilings, carpets, walls, glass, and paint) and complex with a high content of organic compounds.66 In the past decade it has become clear that surface reactions often have a larger impact on indoor settings than gas phase processes. The light induced heterogeneous chemistry on indoor surfaces may have significant influence on human exposure to primary reactant species and to secondary reaction products as well. Several studies (see Carslaw65 and references therein) have predicted through model simulations or directly from measurements that significant concentration of radicals will be formed indoors. Predicted/inferred •OH concentrations are between 1 and 7 × 105 molecules cm−3.60 Clearly, there is a gap to fill for a better understanding of the radical formation, particularly the formation of •OH radical in the indoor atmosphere. In this manner, online and continuous measurements of •OH radicals indoors by, for example, laser induced fluorescence (LIF) are highly desirable. Benzophenone has been extensively studied as a photosensitizer and therefore serves as a prototype aromatic ketone. It is known to form triplet excited states in high yields when irradiated in the UV-A spectral region in benzene and in water containing solutions.25 Aromatic carbonyl compounds such as benzophenone (a well-known photosensitizer) adsorbed on surfaces may play an important role in the indoor atmosphere just as they do in the troposphere. Based on the findings by George et al.2 a possible reaction mechanism is depicted below.

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PHOTOSENSITIZED PROCESSES: FROM OUTDOORS TO INDOORS Considering that human beings typically spend 90% of the time indoors, the understanding of the formation processes of oxidizing species such as •OH radicals and their behavior in confined environments becomes of crucial importance. Indoor chemistry is extremely important from the health point of view. We can learn a lot from outdoor processes, since similar processes occur indoors, where ozone and •OH radicals initiate and undergo the same photochemical oxidation cycles only under larger organic trace gas concentrations from various sources such as outgassing from furniture, paint and carpets, among others and a considerably higher impact of surface reactions. Indoors, oxidation processes occur under lower ventilation conditions and in consequence in increased fuel-rich conditions, where the initiators of oxidation (ozone and NOx) are thought to come less and less from outdoors. In homes with fireplaces, stoves, gas heating and cooking, indoor air can be surprisingly rich in NOx oxides. The emphasis should be placed on exploring indoor sources of •OH since •OH has been identified in early simulation chamber experiments of outdoor chemistry to originate from wall reactions. It is known63 that the formation of HONO takes place from the heterogeneous reaction of NO2 and water even in darkness.64 Whether it is adsorption or there are other mechanisms involved which are crucial for HONO formation, HONO photolyzes rapidly to form •OH. Indoor photolysis coefficients are approximately 10 times smaller in comparison to the outdoors ones.65 However, this ratio of 1/10 is based on only one single study and therefore further experimental evidence is necessary in order to establish the indoor photolysis rate coefficients in various indoor settings. Depending on the building and window design, there can be a huge difference in the fraction of outdoor light that penetrates indoors. Indoors, light is typically of longer wavelengths, and it comes from exterior light that enters through the windows, together with electric lighting providing sufficient light to cause this photolysis. In comparison with the urban atmosphere, indoor environments feature large surface to volume ratios. Namely, typical outdoor surface area is estimated to be roughly 10−5 cm2/cm3 (urban) while typical indoor surface area, ∼ 0.02 cm2/cm3 (unfurnished).65

benzophenone* + red → red+ + benzophenone−*

(R-5)

benzophenone−* + NO2(ads) → NO− 2 + benzophenone

(R-6)

NO2− + H3O+ ⇄ HONO + H2O

(R-7)

In this reaction mechanism the triplet excited state of benzophenone will initiate a series of potential reaction pathways, possibly involving a reducing agent (red) to form oxidized (ionized) agent benzophenone radical species, R-5. This mechanism is different from the ordinary charge transfer mechanism in the reaction between phenolate and NO2 leading to phenoxy and nitrite (a well known pathway that competes with R-6 at high pH). The reduced benzophenone radical species formed in this way may react with the adsorbed NO2, R-6, to form nitrite ion (NO2−). Protonation of nitrite leads to production of HONO which in turn is in equilibrium with nitrite strongly depending on pH R-7. There is solid evidence that electron rich phenolic and polyaromatic light absorbing compounds such as benzophenone, can be present indoors. The emission factors, for instance, of some polyaromatic compounds, generated during various activities, which are known as light absorbing chromophores (phenanthrene, pyrene, anhracene, benzoperylene) were measured indoors.67,68 In many countries, legal requirements regarding the consumption and release of volatile solvents have encouraged manufacturers of coating systems to increasingly introduce UV-curable formulations.69 For these reasons, photoinitiators are generally added to furniture lacquers and flooring materials in nonstoichiometric quantity which causes uncontrolled reactions during their use. Apart from release of secondary emissions, the presence of such photoinitiators in surface coatings may induce adverse health effects such as skin irritation and dermal burns. Another undesired effect is a rapid yellowing of the parquet surface 1960



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which is most probably caused by the photoinduced oligomerization of photoinitiators such as benzophenone,5,26 a compound commonly used in UV-lacquers.69 HONO could represent an additional source of •OH radicals indoors under long wavelengths (320 nm < λ < 400 nm). The recognition of such new sources of HONO and thus •OH radicals will certainly shed a new light on the role of surfaces in affecting the indoor air chemistry.

aromatic compounds which was carried out in the photo-smog chambers EUPHORE located in Valencia (Spain). She also holds a MSc in Instrumental Analysis awarded by Dublin City University (Dublin, Ireland). Throughout her career she has specialized in the development of analytical methodologies to address challenges in the determination of atmospheric pollutants. She was contracted as a research scientist in the EUPHORE chambers for over eight years, during which she was involved in a number of national and international projects. In the last year, she holds a research position at the Aix-Marseille Université (Marseille, France) during which her work has focused on the quality of indoor air and more in particular indoor sources of nitrous acid and the role of photo-sensitization in atmospheric chemical transformations both indoors and outdoors. She is author of 12 articles in international journals and has participated in several international conferences.

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OUTLOOK Near-UV/vis light-absorbing species, present on surfaces of atmospheric relevance and in the atmospheric condensed phase, interacting with trace gases (O3, NO2) can initiate a new and potentially important light-induced heterogeneous and multiphase chemistry (see for example references 2, 14, 15, 30). Pyruvic acid, for instance will always be accompanied by other carboxylic acids (and also other carbonyl compounds) which are constituents of either aerosol particles or enriched in the sea surface microlayer. The effects of a possible photochemistry triggered by pyruvic acid should be experimentally studied in depth and under natural conditions as close as possible to reality. Further experimental work concerning the iron-organic photochemical interactions is highly recommended to clarify open scientific issues and mechanistic limitations. One of the questions which arise is: May such iron-organic charged complexes play a role as photosensitizers in deliquescent aerosol particles? This newly identified near UV/visible sunlight−driven photochemistry may influence the oxidative capacity of the atmosphere,70 both outdoors and indoors and, thus, it must be seriously considered by the international scientific community. Also, such photochemical processes may alter aerosol wettability and optical properties13 which in turn can affect the direct and indirect aerosol radiative forcing of climate. The elucidation of the reaction mechanism for the lightinduced heterogeneous oxidation is of crucial importance to comprehensively understand the impact of this chemistry on the atmosphere. However, these mechanisms are not necessarily complete and in the future important efforts are required to shed some light on this vital issue. The formation of •OH radical which represents the most important oxidant in both outdoor and indoor atmosphere through photosensitized processes appears to be of tremendous importance and it should be studied in greater detail. Although the concentration of •OH radicals is less than one part per trillion, the large amounts of greenhouse gases would be many orders of magnitude higher in the absence of •OH radicals.71 In the indoor environments the •OH concentrations are not known at all and their direct measurements in future studies is highly recommended. The ideas given here emerge from our own research plan. During our discussion, we have also commented on what might be important and interesting topic for future research.

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Henri Wortham holds his doctoral degree in chemistry since 1991 from University of Paris 7, Paris, France. He was Associate Professor at the University Louis Pasteur of Strasbourg, Strasbourg, France, and since 1998 he is a Full Professor at the Aix-Marseille University, Marseille, France. Currently he is a head of both the laboratory of Environmental Chemistry and the Atmospheric chemistry research team at the Aix-Marseille University. His research interests focus on atmospheric chemistry and include specifically developments of methodologies in analytical chemistry and photochemical heterogeneous processes in the troposphere and indoors. He is author of about 70 articles in international journals and has contributed to more than 150 conferences. Rafal Strekowski holds a doctoral degree in Atmospheric Chemistry from ̂ de Georgia Institute of Technology, Atlanta, Georgia. He is now a Maitre Conférences at the Aix-Marseille Univ., Marseille, France. Since 2004, he has conducted research in the field of atmospheric chemistry, written and lectured internationally in areas related to environmental chemistry and pollution. Current research interests include gas phase reaction kinetic processes that are particularly important in the Earth’s troposphere and stratosphere. Another focus of research effort is on aerosol speciation and formation processes that are particularly important in marine environments and of nuclear safety interest. Cornelius Zetzsch holds a doctoral degree in chemistry from the University of Göttingen, Germany, with a PhD work in Physical Chemistry on elementary reactions of F atoms, and he is Professor of Environmental Chemistry and director of the Laboratory for Atmospheric Chemistry Research at the University of Bayreuth since 2003. He has pioneered development of aerosol smog chamber techniques for studies on photocatalysis by TiO2 aerosol, on halogen activation by sea spray and on photochemical degradation of semivolatile compounds, including pesticides, in the Fraunhofer-Institute at Hannover, Germany, from 1983. Current research includes elementary reactions of •OH radicals with aromatics, simulation chamber studies of halogen activation and its impact on secondary aerosol and its chemical and optical properties, and field observations of nitrous acid, formed by heterogeneous chemistry in the atmosphere. He has authored about 150 scientific articles in international journals and has contributed to more than 180 conferences.

AUTHOR INFORMATION

Sasho Gligorovski holds his doctoral degree in Physics since 2005 from University of Leipzig, Germany. In 2006 he was appointed as Associate Professor at the Aix-Marseille Université, Marseille, France, to establish and promote a new area of study dealing with the physico−chemical processes relevant to understanding and improving the quality of indoor and outdoor air. Current research interests focus on aqueous atmospheric photo−chemistry and atmospheric photo-sensitized heterogeneous processes in both

Corresponding Author

*Phone: (+33) (0) 4 13 55 10 52; e-mail: saso.gligorovski@ univ-amu.fr. Biographies Elena Gómez Alvarez obtained her doctoral degree in 2008 at the University of Cordoba, Spain, on the atmospheric photo-oxidation of 1961



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lignin-type precursor in model cloud water. Geophys. Res. Lett. 2004, 31, L06115 DOI: 10.1029/2003GL018962. (19) Wentworth, G. R.; Al-Abadleh, H. A. DRIFTS studies on the photo-sensitized transformation of gallic acid by iron(III) chloride as a model for HULIS in atmospheric aerosols. Phys. Chem. Chem. Phys. 2011, 13, 6507−6516. (20) Styler, S. A.; Loiseaux, M.-E.; Donaldson, D. J. Substrate effects in the photoenhanced ozonation of pyrene. Atmos. Chem. Phys. 2010, 11, 1243−1253. (21) Nissenson, P.; Knox, C. J. H.; Finlayson-Pitts, B. J.; Phillips, L. F.; Dabdub, D. Enhanced photolysis in aerosols: Evidence for important surface effects. Phys. Chem. Chem. Phys. 2006, 8, 4700−4710. (22) Simoneit, B. R. T.; Rogge, W. F.; Mazurek, M. A.; Standley, L. J.; Hildemann, L. M.; Cass, G. R. Lignin pyrolysis products, lignans and resin acids as specific tracers of plant classes in emissions from biomass combustion. Environ. Sci. Technol. 1993, 27, 2533−2541. (23) Jang, M.; McDow, S. R. Benz[a]anthracene photodegradation in the presence of known organic constituents of atmospheric aerosols. Environ. Sci. Technol. 1995, 29, 2654−2660. (24) Jang, M.; McDow, S. R. Product of benz[a]anthracene photodegradation in the presence of known organic constituents of atmospheric aerosols. Environ. Sci. Technol. 1997, 31, 1046−1053. (25) Canonica, S.; Hellrung, B.; Wirz, J. Oxidation of phenols by triplet aromatic ketones in aqueous solution. J. Phys. Chem. A 2000, 104, 1226−1232. (26) Nieto-Gligorovski, L.; Net, S.; Gligorovski, S.; Zetzsch, C.; Jammoul, A.; D’Anna, B.; George, C. Interactions of ozone with organic surface films in the presence of simulated sunlight: Impact on wettability of aerosols. Phys. Chem. Chem. Phys. 2008, 10, 2964−2971. (27) Nieto-Gligorovski, L. I.; Net, S.; Gligorovski, S.; Wortham, H.; Grothe, C.; Zetzsch, C. Spectroscopic study of organic coatings on fine particles, exposed to ozone and simulated sunlight. Atmos. Environ. 2010, 44 (40), 5451−5459. (28) Kleffmann, J.; Gavriloaiei, T.; Hofzumahaus, A.; Holland, F.; Koppmann, R.; Rupp, L.; Schlosser, E.; Siese, M.; Wahner, A. Daytime formation of nitrous acid: A major source of ·OH radicals in a forest. Geophys. Res. Lett. 2005, 32, L05818 DOI: 10.1029/2005GL022524. (29) Sörgel, M.; Regelin, E.; Bozem, H.; Diesch, J.-M.; Drewnick, F.; Fischer, H.; Harder, H.; Held, A.; Hosaynali-Beygi, Z.; Martinez, M.; Zetzsch, C. Quantification of the unknown HONO daytime source and its relation to NO2. Atmos. Chem. Phys. 2011, 11, 10433−10447. (30) Stemmler, K.; Ammann, M.; Donders, C.; Kleffmann, J.; George, C. Photo-sensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid. Nature 2006, 440 (7081), 195−198. (31) Kleffmann, J. Daytime sources of nitrous Acid (HONO) in the atmospheric boundary layer. Chem. Phys. Chem. 2007, 8, 1137−1144. (32) Zhou, X.; Zhang, N.; TerAvest, M.; Tang, D.; Hou, J.; Bertman, S.; Alaghmand, M.; Shepson, B. P.; Caroll, A. M.; Griffith, S.; Dusanter, S.; Stevens, S. P. Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid. Nat. Geosci. 2011, 4, 440−443. (33) Tian, X.; Liu, J.; Zhou, G.; Peng, P.; Wang, X.; Wang, C. Estimation of the annual scavenged amount of polycyclic aromatic hydrocarbons by forests in the Pearl River Delta of Southern China. Environ. Pollut. 2008, 156, 306−315. (34) Monge, M. E.; D’Anna, B.; Mazria, L.; Giroir-Fendlera, A.; Ammann, M.; Donaldson, D. J.; George, C. Light changes the atmospheric reactivity of soot. Proc. Natl. Acad. Sci. 2010, 107, 6605− 6610. (35) Acker, K.; Febo, A.; Trick, S.; Perrino, C.; Bruno, P.; Wiesen, P.; Möller, D.; Wieprecht, W.; Auel, R.; Giusto, M.; Geyer, A.; Platt, U.; Allegrini, I. Nitrous acid in the urban area of Rome. Atmos. Environ. 2006, 40, 3123−3133. (36) Stutz, J.; Alicke, B.; Ackermann, R.; Geyer, A.; Wang, S.; White, A.; Williams, E. J.; Spicer, C. W.; Fast, J. D. Relative humidity dependence of HONO chemistry in urban areas. J. Geophys. Res. 2004, 109, D03307 DOI: 10.1029/2003JD004135.

outdoor and indoor environments. He has authored 23 scientific articles in international journals and contributed to more than 60 conferences.

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REFERENCES

(1) Vione, D.; Maurino, V.; Minero, C.; Pelizzetti, E.; Harrison, M. A.J.; Olariu, R. I.; Arsene, C. Photochemical reactions in the tropospheric aqueous phase and on particulate matter. Chem. Soc. Rev. 2006, 35, 441−453. (2) George, C.; Strekowski, R. S.; Kleffmann, J.; Stemmler, K.; Ammann, M. Photoenhanced uptake of gaseous NO2 on solid-organic compounds: A photochemical source of HONO? Faraday Discuss. 2005, 130, 195−210. (3) Net, S.; Nieto-Gligorovski, L.; Gligorovski, S.; Temime-Rousell, B.; Barbati, S.; Lazarou, G. Y.; Wortham, H. Heterogeneous lightinduced ozone processing on the organic coatings in the atmosphere. Atmos. Environ. 2009, 43, 1683−1692. (4) Net, S.; Nieto-Gligorovski, L.; Gligorovski, S.; Wortham, H. Heterogeneous ozonation kinetics of 4-phenoxyphenol in the presence of photo-sensitizer. Atmos. Chem. Phys. 2010, 10, 1545−1554. (5) Jammoul, A.; Gligorovski, S.; George, C.; D’Anna, B. Photosensitized heterogeneous chemistry of ozone on organic films. J. Phys. Chem. A 2008, 112 (6), 1268−1276. (6) Finlayson-Pitts, B. J.; Pitts, J. N. Chemistry of the Upper and Lower Atmosphere: Theory Experiments, and Applications; Academic Press, San Diego, CA, 1999. (7) Ogilby, R. P. Singlet oxygen: There is indeed something new under the sun. Chem. Soc. Rev. 2010, 39, 3181−3209. (8) Pandey, R.; Umapathy, S. Time-resolved resonance raman spectroscopic studies on the triplet excited state of thioxanthone. J. Phys. Chem. A 2011, 115, 7566−7573. (9) Anastasio, C.; Faust, B. C.; Janakiram Rao, C. Aromatic carbonyl compounds as aqueous-phase photochemical sources of hydrogen peroxide in acidic sulphate aerosols, fogs, and clouds. I. Non-phenolic methoxybenzaldehydes and methoxyacetophenones with reductants (phenols). Environ. Sci. Technol. 1997, 31, 218−232. (10) Pehkonen, S. O.; Siefert, R.; Erel, Y.; Webb, S.; Hoffmann, M. R. Photoreduction of iron oxyhydroxides in the presence of important atmospheric organic compounds. Environ. Sci. Technol. 1993, 27, 2056−2062. (11) Faust, B.,C; Hoigné, J. Photolysis of Fe (III)-hydroxy complexes as sources of ·OH radicals in clouds, fog and rain. Atmos. Environ. 1990, 24A, 79−89. (12) Zuo, Y.; Hoigné, J. Formation of hydrogen peroxide and depletion of oxalic acid in atmospheric water by photolysis of iron(III)-oxalato complexes. Environ. Sci. Technol. 1992, 26, 1014− 1022. (13) Ervens, B.; George, C.; Willams, J. E.; Buxton, G. V.; Salmon, G .A.; Bydder, M.; Wilkinson, F.; Dentener, F.; Mirabel, P.; Herrmann, H. CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application. J. Geophys. Res. 2003, 108 (D14), 4426. (14) Herrmann, H.; Tilgner, A.; Barzaghi, P.; Majdik, Z.; Gligorovski, S.; Poulain, L.; Monod, A. Towards a more detailed description of tropospheric aqueous phase organic chemistry: CAPRAM 3.0. Atmos. Environ. 2005, 39, 4351−4363. (15) Deguillaume, L.; Leriche, M.; Desboeufs, K.; Mailhot, G.; George, C.; Chaumerliac, N. Transition metals in atmospheric liquid phases: Sources, reactivity, and sensitive parameters. Chem. Rev. 2005, 105 (9), 3388−3431. (16) Zuo, Y.; Hoigné, J. Photochemical decomposition of oxalic, glyoxalic and pyruvic acid catalysed by iron in atmospheric waters. Atmos. Environ. 1994, 28, 1231−1239. (17) Gelencsér, A; Hoffer, A.; Kiss, G.; Tombácz, E.; Kurdi, R.; Bencze, L. In-situ Formation of Light-Absorbing Organic Matter in Cloud Water. J. Atmos. Chem. 2003, 45, 25−33. (18) Hoffer, A.; Kiss, G.; Blazsó, M.; Gelencsér, A. Chemical characterization of humic-like substances (HULIS) formed from a 1962



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(37) Ammar, R.; Monge, M. E.; George, C.; D’Anna, B. Photoenhanced NO2 loss on simulated urban grime. Chem.Phys.Chem. 2010, 11 (18), 3956−3961. (38) Sosedova, Y.; Rouvière, A.; Bartels-Rausch, T.; Ammann, M. UVA/Vis-induced nitrous acid formation on polyphenolic films exposed to gaseous NO 2 . Photochem. Photobiol. Sci. 2011, DOI: 10.1039/c1pp05113j. (39) Shiraiwa, M.; Sosedova, Y.; Rouvière, A.; Yang, H.; Zhang, Y.; Abbatt, P. D. J.; Ammann, M.; Poeschl, U. The role of long-lived reactive oxygen intermediates in the reaction of ozone with aerosol particles. Nat. Chem. 2011, 3, 291−295. (40) Vacha, R.; Slavicek, P.; Mucha, M.; Finlayson-Pitts, B. J.; Jungwirth, P. Adsorption of Atmospherically relevant gases at the air/ water interface: Free energy profiles of aqueous solvation of N2, O2, O3, OH, H2O, HO2, and H2O2. J. Phys. Chem. A 2004, 108, 11573− 11579. (41) Prinn, R. G.; Huang, J.; Weiss, R. F.; Cunnold, D. M.; Fraser, P. J.; Simmonds, P. G.; McCulloch, A.; Harth, C.; Salameh, P.; O’Doherty, S.; Wang, R. H. J.; Porter, L.; Miller, B. R. Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science 2001, 292, 1882−1888. (42) Manning, M. R.; Lowe, D. C.; Moss, R. C.; Bodeker, G. E.; Allan, W. Nature 2005, 436, 1001−1004. (43) Montzka, S. A.; Krol, M.; Dlugokencky, E.; Hall, B.; Jöckel, P.; Lelieveld, J. Small interannual variability of global atmospheric hydroxyl. Science 2011, 331, 67−69. (44) Gever, J. R.; Mabury, S. A.; Crosby, D. G. Rice field surface microlayers: Collection, composition and pesticide enrichment. Environ. Toxicol. Chem. 1996, 15, 1676. (45) Wurl, O.; Obbard, J. P. Chlorinated pesticides and PCBs in the sea-surface microlayer and seawater samples of Singapore. Mar. Pollut. Bull. 2005, 50 (11), 1233−1243. (46) Liss, P. S.; Duce, R. A. The Sea Surface and Global Change; Cambridge University Press: Cambridge, 1997. (47) Hoigné, J.; Faust, B. C.; Haag, W. R.; Scully, F. E. Jr; Zepp, R. G. Aquatic humic substances as sources and sinks of photochemicaly produced transient reactants. ACS Adv. Chem. Ser. 1989, 219, 363− 381. (48) Ehrhardt, M. G.; Bouchertall, F.; Hopf, H.-P. Aromatic ketones concentrated from Baltic Sea water. Mar. Chem. 1982, 11, 449−461. (49) Ehrhardt, M. G.; Petrick, G. On the composition of dissolved and particle associated fossil fuel residues in Mediterranean surface water. Mar. Chem. 1993, 42, 57−70. (50) Carlson, D. J. Surface microlayer phenolic enrichments indicate sea surface slicks. Nature 1982, 296, 426−429. (51) Carlson, D. J. Dissolved organic materials in surface microlayers: Temporal and spatial variability and relation to sea state. Limnol. Oceanogr. 1983, 28, 415−431. (52) Carlson, D. J.; Mayer, L. M. Enrichment of dissolved phenolic material in the surface microlayer of coastal waters. Nature 1980, 286 (5772), 482−483. (53) Net, S.; Gligorovski, S.; Pietri, S.; Wortham, H. Photoenhanced degradation of veratraldehyde upon the heterogeneous ozone reactions. Phys. Chem. Chem. Phys. 2010, 12, 7603−7611. (54) Net, S.; Gligorovski, S.; Wortham, H. Light-induced heterogeneous ozonation kinetics of 3,4,5-trimethoxybenzaldehyde: Kinetics and condensed phase products. Atmos. Environ. 2010, 44 (27), 3286−3294. (55) Clifford, D.; Donaldson, D. J.; Brigante, M.; D’Anna, B.; George, C. Reactive uptake of ozone by chlorophyll at aqueous surfaces. Environ. Sci. Technol. 2008, 42 (4), 1138−1143. (56) Ganzeveld, L.; Helmig, D.; Fairall, C. W.; Hare, J.; Pozzer, A. Atmosphere-ocean ozone exchange: A global modeling study of biogeochemical, atmospheric, and waterside turbulence dependencies. Global Biogeochem. Cycles 2009, 23, GB4021 DOI: 10.1029/ 2008GB003301. (57) Kawamura, K.; Yasui, O. Diurnal changes in the distribution of dicarboxylic acids,ketocarboxylic acids and dicarbonyls in the urban Tokyo atmosphere. Atmos. Environ. 2005, 39, 1945−1960.

(58) Steinberg, S. M.; Bada, J. L. Oxalic, glyoxalic and pyruvic acids in eastern Pacific Ocean waters. J. Mar. Res. 1984, 42, 697−708. (59) Guzmán, M. I.; Colussi, A. J.; Hoffmann, M. R. Photoinduced oligomerization of aqueous pyruvic acid. J. Phys. Chem. A 2006, 110, 3619−3626a. (60) Guzmán, M. I.; Colussi, A. J.; Hoffmann, M. R. Photogeneration of distant radical pairs in aqueous pyruvic acid glasses. J. Phys. Chem. A 2006, 110, 931−935b. (61) Vaida, V. Spectroscopy of photoreactive systems: Implications for atmospheric chemistry. J. Phys. Chem. A, 2009, 113 (1), 5−18. (62) Grgic, I.; Nieto-Gligorovski, L; Net, S.; Temime-Roussel, B.; Gligorovski, S; Wortham, H. Light induced multiphase chemistry of gas-phase ozone on aqueous pyruvic and oxalic acids. Phys. Chem. Chem. Phys. 2010, 12, 698−707. (63) Ramazan, K. A.; Syomin, D.; Finlayson-Pitts, B. J. The photochemical production of HONO during the heterogeneous hydrolysis of NO2. Phys. Chem. Chem. Phys. 2004, 6, 3836−3843. (64) Sakamaki, F.; Hatakeyama, S.; Akimoto, H. Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapor in a smog chamber. Int. J. Chem. Kinet. 1983, 15, 1013−1029. (65) Carslaw, N. A new detailed chemical model for indoor air pollution. Atmos. Environ. 2007, 41, 1164−1179. (66) Lam, B.; Diamond, M. L.; Simpson, A. J.; Makar, P. A.; Truong, J.; Hernandez-Martinez, N. A. Chemical composition of surface films on glass windows and implications for atmospheric chemistry. Atmos. Environ. 2005, 39, 6578−6586. (67) Van Winkle, M. R.; Scheff, P. A. Volatile organic compounds, polycyclic aromatic hydrocarbons and elements in the air of ten urban homes. Indoor Air 2001, 11, 49−64. (68) Ohura, T.; Amagai, T.; Fusaya, M.; Matsushita, H. Polycyclic aromatic hydrocarbons in indoor and outdoor environments and factors affecting their concentrations. Environ. Sci. Technol. 2004, 38, 77−83. (69) Uhde, E.; Salthammer, T. Impact of reaction products from building materials and furnishings on indoor air quality-A review of recent advances in indoor chemistry. Atmos. Environ. 2007, 41, 3111− 3128. (70) Donaldson, D. J.; George, C.; Vaida, V. Red sky at night: Longwavelength photochemistry in the atmosphere. Environ. Sci. Technol. 2010, 44, 5321−5326. (71) Wennberg, P. O. Radicals follow the sun. Nature 2006, 442, 145−146.

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Atmospheric Photosensitized Heterogeneous and Multiphase

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