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The Atmospheric Photolysis of o-Tolualdehyde Grainne M. Clifford,† Aurelie Hadj-Aïssa,† Robert M. Healy,† Abdelwahid Mellouki,‡ Amalia Mu~noz,§ Klaus Wirtz,§ Montserrat Martín Reviejo,§ Esther Borras,§ and John C. Wenger†,* †
Department of Chemistry and Environmental Research Institute, University College Cork, Cork, Ireland CNRS ICARE, F-45071 Orleans 2, France § Instituto Universitario UMH-CEAM, C/Charles R. Darwin 14, Parque Tecnologico, 46980 Paterna, Valencia, Spain ‡
bS Supporting Information ABSTRACT: The photolysis of o-tolualdehyde by natural sunlight has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photolysis rate coefficient was measured directly under different solar flux levels, with values in the range j(o-tolualdehyde) = (1.622.15) 104 s1 observed, yielding an average value of j(o-tolualdehyde)/j(NO2) = (2.53 ( 0.25) 102. The estimated photolysis lifetime is 12 h, confirming that direct photolysis by sunlight is the major atmospheric degradation pathway for o-tolualdehyde. Published UV absorption cross-section data were used to derive an effective quantum yield (290400 nm) close to unity, within experimental error. Possible reaction pathways for the formation of the major photolysis products, benzocyclobutenol (tentatively identified) and o-phthalaldehyde, are proposed. Appreciable yields (513%) of secondary organic aerosol (SOA) were observed at EUPHORE and also during supplementary experiments performed in an indoor chamber using an artificial light source. Off-line analysis by gas chromatographymass spectrometry allowed identification of o-phthalaldehyde, phthalide, phthalic anhydride, o-toluic acid, and phthalaldehydic acid in the particle phase.
’ INTRODUCTION o-Tolualdehyde is an aromatic aldehyde regularly detected in ambient air.13 It is emitted into the atmosphere as a primary pollutant from automobile exhausts 4,5 and can also be formed in situ from the hydroxyl radical (OH) initiated oxidation of o-xylene.3,6,7 The subsequent degradation of o-tolualdehyde in the atmosphere may contribute to the formation of secondary species such as ozone, nitrates, and organic aerosol, which are major constituents of polluted air in the troposphere.8 A detailed knowledge of the kinetics and mechanisms of these atmospheric degradation processes is therefore required to fully understand the environmental impact of o-tolualdehyde and its parent compound, o-xylene. The potential gas-phase removal processes for o-tolualdehyde are reaction with OH, the nitrate radical (NO3), ozone, chlorine atoms, and direct photolysis by sunlight.8 Laboratory kinetic studies indicate that the reactions with ozone and Cl atoms are of negligible importance and that reaction with NO3 is a minor pathway compared to OH-initiated oxidation.9,10 The gas-phase absorption cross section of o-tolualdehyde is relatively strong in the actinic region, indicating that photolysis may also be an important loss process if the quantum yield is close to unity.11 Some preliminary studies of the gas-phase photolysis of o-tolualdehyde have been performed at the outdoor European r 2011 American Chemical Society
Photoreactor (EUPHORE) in Valencia, Spain;12,13 however, the reported value for the photolysis rate coefficient is somewhat uncertain and no mechanistic information was obtained. In this work a more detailed investigation of the photolysis of otolualdehyde by natural sunlight has been performed at EUPHORE. The photolysis rate coefficient has been determined, reaction products identified and secondary organic aerosol formation observed. Supplementary experiments have also been performed in an indoor simulation chamber using an artificial light source. The results provide new information on the atmospheric degradation of o-tolualdehyde and its potential impact on the environment.
’ EXPERIMENTAL SECTION EUPHORE Chamber. The sunlight photolysis of o-tolualdehyde was investigated in Chamber B at the EUPHORE facility which consists of a 204.5 m3 hemispherical reactor made of FEP Teflon foil. Technical details concerning the facility and its Received: August 1, 2011 Accepted: October 18, 2011 Revised: October 17, 2011 Published: October 18, 2011 9649
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Environmental Science & Technology application for photolysis experiments have been previously reported in the literature.1418 The chamber was cleaned by flushing with purified air overnight and filled to atmospheric pressure. o-Tolualdehyde was introduced into the chamber and allowed to mix for approximately one hour while its loss to the chamber walls was measured. Photolysis was initiated by opening the protective housing to expose the contents of the chamber to sunlight for 23 h. The temperature inside the chamber increased slightly as the experiments progressed but was always within the range 296308 K. The solar actinic flux over the range 290520 nm was measured using a calibrated spectroradiometer (Bentham DM300). A full spectral scan took 420430 s. Chemical analysis was performed by in situ FTIR spectroscopy and gas chromatography (GC). The FTIR spectrometer (Nicolet Magna 550) was operated at 1 cm1 resolution and spectra over the range 6004000 cm1 were derived from the coaddition of 270 scans collected over 5 min. The GC (Fisons 8160), equipped with flame ionization and photoionization detectors (FID and PID), was operated using a 30 m DB-624 fused silica capillary column (J&W Scientific, 0.32 mm i.d., 1.8 μm film). Air was sampled from the chamber into a 3 cm3 sampling loop and injected onto the column operated at 150 °C. The reactants and products were quantified using calibrated FTIR spectra and GC sensitivity factors obtained by introducing known volumes of pure materials into the chamber. The leak rate was determined by adding about 20 ppbV of SF6 to the chamber and measuring its loss by FTIR spectroscopy. Additional chemical analysis was afforded by gas chromatographymass spectrometry (GCMS) using a Varian GC 3400 interfaced to a Saturn 2000 ion trap mass spectrometer. The GCMS incorporated a cryogenic sample preconcentration trap (SPT) containing glass beads cooled to 160 °C. The SPT was operated for 5 min at a flow of 40 cm3 min1. Desorption was performed at 270 °C onto a 30 m HP Innowax column (0.25 mm id, 0.25 μm film thickness). The column was held at 20 °C for 7 min and increased to 250 at 10 °C min1. The GCMS was operated in electron ionization (EI) mode over the m/z range 46250. Products were identified by comparison with chromatographic retention times and mass spectra of authentic standards. A scanning mobility particle sizer (SMPS), comprising of a condensation particle counter (TSI 3022A) and differential mobility analyzer (TSI 3081), was used to measure particle size distribution, number and volume concentrations with a time resolution of ca. 5 min. Aerosol mass concentrations were measured using a tapered element oscillating microbalance (TEOM, Rupprecht and Patashnick 1400a) fitted with a PM1 inlet and operated at a total flow of 16.7 L min1. For stable operation of the TEOM system, the sampling line and the sensor unit were held at 27 °C. The cabinet temperature of the SMPS was 25 °C. Both the TEOM and SMPS systems were started prior to the introduction of reactants to confirm that no particles were present in the chamber before photolysis was initiated. The instruments were also operated for several hours after the chamber was closed to measure the loss of particles to the chamber walls. Off-line chemical analysis of the particles was performed by GCMS using a method reported previously.19 Particles were collected onto a 47 mm quartz fiber filter at a flow rate of 80 L min1 for 1 h and subsequently extracted under sonication in 5 mL of a CH2Cl2/CH3CN (1:1) mixture. The extract was derivatized with O-(2,3,4,5,6-
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pentafluorobenzyl)-hydroxylamine (PFBHA) and N-methylN-trimethylsilyltrifluoro-acetamide (MSTFA) to react with carbonyls and hydroxyl-containing compounds respectively. One μL was injected into the GCMS (TRACE-DSQ II, Thermo Fisher Scientific Co., Waltham, MA) fitted with an RTX-5MS column (30 m 0.25 mm i.d. 0.25 μm film thickness, Thermo Fisher Scientific Co). Indoor Chamber. A number of experiments were also conducted in the 3.91 m3 FEP Teflon indoor chamber at University College Cork, described in detail elsewhere.20 The chamber was surrounded by 18 lamps (Philips TL05, 40 W; 320 nm < λ > 480 nm; λmax = 360 nm) which deliver a light intensity equivalent to j(NO2) = 1.56 103 s1. Photolysis of o-tolualdehyde was initiated by turning on the lamps for 23 h. Quantitative analysis of gas-phase species was performed by in situ FTIR spectroscopy using the procedures outlined above. A SMPS, consisting of a condensation particle counter (TSI 3010) and a differential mobility analyzer (TSI 3080), was used to measure particle properties at ca. 5 min intervals. Additional chemical analysis was provided by GCMS. The contents of the chamber were sampled using a 1 cm3 gastight syringe (Hamilton) and injected directly into the GCMS instrument (Varian GC 3800 interfaced to a MS Saturn 2000). Typically five samples were taken per experiment. The GCMS was operated in electron ionization (EI) mode over the m/z range 46250. Chromatographic separation was achieved using a CP-Sil8 fused silica capillary column (Varian, 30 m, 0.25 mm i.d, 0.25 μm film thickness) operated at 60 °C for 1 min and then increased to 290 °C at a rate of 10 °C per minute. A flow rate of 5 cm3 min1 was used and the injector temperature was 250 °C. Products were identified and quantified by comparison with chromatographic retention times and mass spectra of authentic standards. It should be noted however, that reliable quantitative information could not be obtained for all products due to sampling losses associated with use of the syringe. Further off-line detection of carbonyl reaction products in the gas and particle phase was performed using a denuder-filter sampling method coupled with PFBHA derivatization and GCMS analysis.21,22 Materials. All organic compounds were obtained from Aldrich Chemical Co. at the highest purity available (stated purities >97%) and used without further purification. Sulfur hexafluoride (99.9%) was obtained from Messer Griesheim, Germany.
’ RESULTS AND DISCUSSION Photolysis Rate Coefficient. Photolysis of o-tolualdehyde follows first order kinetics: j
o-tolualdehyde þ hν f products
ðIÞ
where j is the photolysis rate coefficient. Assuming photolysis is the only loss process, j can be determined from a simple first order kinetic plot: ln½o-tolualdehydet =½o-tolualdehyde0 ¼ jt
ðIIÞ
where the subscripts 0 and t refer to the concentrations at initial time 0 and elapsed time t, respectively. Test experiments carried out in the presence and absence of excess amounts (1025 ppmV) of OH radical scavenger (isopropanol and cyclohexane in the indoor and outdoor chambers, respectively) produced very similar decay rates, indicating that loss of o-tolualdehyde due to reaction with OH in the chambers was negligible. However, 9650
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Figure 1. Concentrationtime profile and j(NO2) during the photolysis of o-tolualdehyde at EUPHORE on 3 July 2003. The vertical dotted lines indicate the time the chamber was opened (09:44) and closed (12:27).
Table 1. Experimental Details for the Photolysis of o-Tolualdehyde in the Outdoor (EUPHORE) and Indoor Simulation Chambersa EUPHORE 19 September 2002
indoor chamber
3 July 2003
4 July 2003
four experiments
initial concentration (ppbV)
525
508
256
364, 518, 712, 1354
irradiation period j(NO2)average (s1)
11.59 14:24 (6.41 ( 0.64) 103
09:4412:27 (8.13 ( 0.81) 103
11:3613:53 (8.44 ( 0.84) 103
13 h 1.56 103
kwall (s1)
(7.80 ( 0.47) 106
(1.03 ( 0.47) 105
(1.68 ( 0.50) 105
(7.03 ( 0.90) 106
6
(5.48 ( 0.90) 10
6
4
1
kSF6 (s )
(6.57 ( 0.72) 10
j(o-tolualdehyde)FTIR (s1)
(1.97 ( 0.05) 104
1
4
4
(4.16 ( 0.23) 105
j(o-tolualdehyde)GC‑PID (s )
(1.62 ( 0.05) 10
(2.12 ( 0.05) 10
(2.15 ( 0.06) 10
j(o-tolualdehyde)average (s1)
(1.62 ( 0.05) 104
(2.05 ( 0.05) 104
(2.15 ( 0.06) 104
(4.16 ( 0.23) 105
(2.53 ( 0.25) 10
2
2
(2.55 ( 0.26) 10
2
(2.66 ( 0.23) 102
(1.33 ( 0.13) 10 1.22 ( 0.03
4
(1.90 ( 0.19) 10 1.13 ( 0.05
4
j(o-tolualdehyde)/j(NO2) 1
maximum theoretical loss rate (s ) effective quantum yield
(2.52 ( 0.25) 10
4
(1.85 ( 0.19) 10 1.11 ( 0.03
molar yield of benzocyclobutenol
0.77 ( 0.04
0.71 ( 0.06
molar yield of o-phthalaldehyde
0.21 ( 0.02
0.22 ( 0.02
yield of aerosol
0.104
0.049, 0.059, 0.080, 0.133
Except for j(NO2), quoted errors are twice the standard deviation arising from the least squares fit of the data and include the uncertainty in calibration and response factors. For j(NO2) and the maximum theoretical loss rate, the estimated error is 10%. The molar yield of benzocyclobutenol is based on the use of 1-indanol as a surrogate compound. a
o-tolualdehyde was found to undergo a small amount of deposition to the walls of the reactor. The rate coefficient for this process (kwall) was determined by measuring the first order decay of the compound in the dark for about one hour prior to photolysis. This value was incorporated into the overall decay as follows: ln½o-tolualdehydet =½o-tolualdehyde0 kwall t ¼ jt
ðIIIÞ
Although dilution was also observed during the EUPHORE experiments, the rate determined by measuring the loss of SF6 from the chamber, kSF6, was lower than the wall loss and is therefore already incorporated into kwall. Thus a plot in the form of eq III should yield a straight line with gradient j.
Concentrationtime profiles and kinetic plots in the form of eq III were generated for all experiments. The concentration time profile for the EUPHORE experiment conducted on third July is presented in Figure 1 and clearly shows the rapid decay of o-tolualdehyde following exposure of the chamber to sunlight. The light intensity is represented by the photolysis rate coefficient for NO2, j(NO2), which was calculated from the solar flux measurements of the spectroradiometer and recommended values for the absorption cross-section and quantum yield.23 The corresponding kinetic plot used to calculate j from these FTIR spectroscopic measurements is shown in Figure S1 (Supporting Information) and exhibits good linearity and a near-zero intercept. The value for j(o-tolualdehyde) is listed in Table 1 along 9651
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Figure 2. Yield plot of the products detected by FTIR spectroscopy during the photolysis of o-tolualdehyde at EUPHORE on 3 July 2003.
with a summary of the reaction conditions and results obtained for all of the photolysis experiments. As shown in Table 1, there is very good agreement between the j values determined using FTIR spectroscopy and GC-PID in the experiment performed on third July 2003. In the experiment performed on fourth July 2003, o-tolualdehyde was among several aromatic aldehydes subjected to photolysis in the chamber and FTIR spectroscopy could not be used for analysis due to overlapping absorption bands. The value for j(o-tolualdehyde) obtained by GC-PID in this experiment is very similar to that obtained on third July 2003, indicating that the presence of the other compounds (2,3-dimethylbenzaldehyde and 2,6-dimethylbenzaldehyde) did not affect the photolysis rate. The average values for j(NO2) and hence j(o-tolualdehyde)/j(NO2) in these two EUPHORE experiments were also very similar, reflecting the fact that they were performed under almost identical, cloud-free conditions. As expected, the sunlight intensity was somewhat lower for the experiment performed on 19th September 2002 and although a slightly lower value was observed for j(o-tolualdehyde), the j(o-tolualdehyde)/j(NO2) ratio was virtually the same. This indicates that the average value of j(o-tolualdehyde)/j(NO2) = (2.53 ( 0.25) 102 is a useful parameter for calculating the photolysis rate of o-tolualdehyde under different light conditions in chambers or in the real atmosphere. The effective quantum yield for photolysis of o-tolualdehyde, jeff was determined using the following expression; jeff ¼ jexp =jmax
ðIVÞ
where jexp and jmax are the experimentally observed and maximum theoretical values of the photolysis rate coefficient respectively. The latter term was calculated using the solar flux intensity measurements of the spectroradiometer, the absorption cross section data reported by Thiault et al.,11 and assuming a quantum yield of unity over the atmospheric absorption range of the compound. The values obtained for jeff during the EUPHORE experiments are in reasonable agreement, yielding an average of jeff = (1.15 ( 0.05). However, the calculated value of jmax does not include the uncertainty in the reported absorption cross
sections, which is estimated to be 15% below 340 nm and 20% in the range 340363 nm.11 The results therefore suggest that, within experimental error, the effective quantum yield for photolysis of o-tolualdehyde by natural sunlight is unity. The results of this work can be compared to those obtained in the preliminary studies also performed at EUPHORE. A value of j(o-tolualdehyde) = (2.00 ( 0.10) 104 s1 was obtained by Volkamer et al.12 in one experiment during February, where the solar zenith angle was 50° and the UV flux reduced by around a factor of 2 compared to midsummer. In contrast, Thiault et al.13 obtained a value of j(o-tolualdehyde) = (1.10 ( 0.20) 104 s1 during an experiment performed in April. Values of j(o-tolualdehyde)/j(NO2) = 0.032 and jeff = 0.6 were also reported, but no information was provided on whether the wall loss of o-tolualdehyde was taken into account during data analysis. It is interesting note that in these preliminary studies,12,13 the photolysis of benzaldehyde, m- and p-tolualdehyde was found to be negligible, suggesting that the presence of the methyl group in the ortho position is a key factor in determining the photolysis efficiency of o-tolualdehyde. The photolysis rate of o-tolualdehyde in the indoor chamber was around a factor of 5 slower than in the outdoor chamber. This result was expected since the intensity and wavelength distribution of UV light produced from the TL05 lamps is quite different from natural sunlight. However, a useful comparison between the two chambers can be made by examining the values determined for j(o-tolualdehyde)/j(NO2), shown in Table 1. The values are in very close agreement, indicating that the TL05 lamps used in the indoor chamber studies provide reasonably realistic light conditions for studies of the atmospheric photolysis of the aromatic aldehydes. Photolysis Products. Gas-phase products arising from the photolysis of o-tolualdehyde were determined by FTIR spectroscopy and GCMS. The FTIR spectra obtained in both chambers enabled identification and quantification of o-phthalaldehyde and carbon monoxide as reaction products. Phthalide and formic acid were also detected during the later stages of the experiments, but below the limits of quantification. The FTIR product spectra also contain significant absorption features around 1050 cm1 9652
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Environmental Science & Technology and just below 1400 cm1 and 750 cm1 (Figure S2, Supporting Information). The peak at 1050 cm1 is characteristic of the OH bending vibration in an alcohol, and inspection of the literature reveals that UV photolysis of o-alkyl aromatic carbonyl compounds in solution and in the condensed phase leads to the efficient formation of the corresponding dienols and benzocyclobutenols.24,25 The dienol and benzocyclobutenol expected from photolysis of o-tolualdehyde are not commercially available, however, their IR absorption spectra (not quantified) in a low temperature matrix have been reported previously.26 The main absorption bands of benzocyclobutenol in the fingerprint region, located at 1055 cm1 (OH bend), 1212 cm1 and 1401 cm1, are all apparent in the product spectra obtained here, while there is no evidence for the presence of the dienol (at 1256 cm1 (OH bend) and 1102 cm1 (CO stretch)). It is therefore proposed that benzocyclobutenol is a product of the gas-phase photolysis of o-tolualdehyde. Since benzocyclobutenol is not commercially available, 1-indanol, the similarly structured aromatic cyclic alcohol, was used as a surrogate compound:
The main absorption features of 1-indanol are virtually identical to those observed in the product spectra (Supporting Information Figure S2) thus providing further evidence to support the formation of benzocyclobutenol as a reaction product from photolysis of o-tolualdehyde. Calibrated infrared spectra of 1-indanol were subsequently used for quantification of benzocyclobutenol and the concentrationtime profile in Figure 1 shows that the aromatic cyclic alcohol is in fact the major reaction product. The corresponding product yield plots for benzocyclobutenol and o-phthalaldehyde displayed in Figure 2 are linear, indicating that these compounds are primary products of the reaction and are not removed to any significant extent during the time scale of the experiments. Only trace amounts of carbon monoxide were observed and the yield plot in Figure 2 is curved, indicating that it is most likely a secondary product. Similar results were also obtained from analysis of the FTIR spectra obtained in the indoor chamber experiments, Table 1. A number of gas-phase photolysis products were also detected using GCMS. During the indoor chamber experiments, ophthalaldehyde, phthalide, phthalic anhydride, o-toluic acid, and o-cresol were identified by comparison of the retention times and mass spectra with those of standards (Figure S3 and Table S1, Supporting Information). The molar yields of o-phthalaldehyde and phthalide were (0.25 ( 0.04) and (0.02 ( 0.01), in good agreement with the results obtained by FTIR spectroscopy. Analysis of the denuder extracts showed the formation of ophthalaldehyde and very small amounts of glyoxal. Quantification of the other products identified by direct injection GCMS proved difficult due to losses during the sampling procedure. However, there was no evidence for the presence of benzocyclobutenol in the mass spectra of the products. Similar results were obtained at EUPHORE, where o-phthalaldehyde and phthalide were also detected as the major products, along with o-cresol and 2-hydroxymethylbenzaldehyde in trace amounts. A small peak, possibly due to benzocyclobutenol was also observed in the GCMS product spectra, but could not be confirmed due to the lack of a standard. The benzocyclobutenols are known to undergo thermal decomposition above 80 °C24 and it therefore
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Scheme 1
seems very likely that benzocyclobutenol decomposed during the GCMS analysis, either in the heated injector or on the column itself. In fact, degradation of the aromatic cyclic alcohol could be responsible for generating some of the aromatic products (phthalide, phthalic anhydride, o-cresol) which were detected by GCMS but not by in situ FTIR spectroscopy. The information obtained from these product studies can be used to propose a mechanism for the atmospheric photolysis of o-tolualdehyde, shown in Scheme 1. The initial step in the mechanism is photoexcitation of the aldehyde through the nfπ* electronic transition followed by intramolecular hydrogen abstraction (Norrish Type II process) to produce a 1,4-biradical species.24,25 Based on the distribution of identified reaction products, two possible reactions for the 1,4-biradical are proposed, cyclization to form benzocyclobutenol and reaction with O2 to produce o-phthalaldehyde. The cyclization reaction was initially proposed by Yang27 to explain the formation of cyclic alcohols from photolysis of aliphatic ketones in solution. However, more recent studies on o-alkyl aromatic carbonyl compounds, in solution and in the solid phase, indicate that direct cyclization of the 1,4-biradical is unlikely, and that formation of the dienol, in both (Z)- and (E)-configurations is preferred.24,25 The (Z)-dienol is very short-lived and reverts to the starting aldehyde via a rapid 1,5-H atom shift, whereas the (E)-dienol undergoes thermal conrotatory ring closure to form benzocyclobutenol. As indicated above, there is no evidence for the formation of the dienol in these experiments, although its lifetime in the gas-phase may be too short to be detected. Nevertheless, the possibility that formation of benzocyclobutenol proceeds via the (E)-dienol cannot be ruled out. The formation of o-phthalaldehyde is postulated to occur via reaction of the 1,4-biradical with O2. It is possible that this pathway may involve a number of concurrent or subsequent steps; H-atom abstraction from the OH group and addition of O2 to the methylene unit to form a peroxy radical. In the absence of NO, the peroxy radicals react together, with the major reaction pathway resulting in oxy radicals which also react with O2 to form o-phthalaldehyde. The minor reaction pathway involves combination of the peroxy radicals to produce two molecular products in one step; o-phthalaldehyde and 2-hydroxymethylbenzaldehyde. A very small amount of the latter species was detected by GCMS in the EUPHORE experiment. However, for the sake of simplicity, this mechanistic detail is omitted from Scheme 1. 9653
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Figure 3. Number and size distribution of particles formed during the photolysis of o-tolualdehyde at EUPHORE on 3 July 2003. The numbers in the legend refer to time during the experiment.
Additional reaction pathways are required to explain the formation of the minor products detected by GCMS and FTIR spectroscopy. Phthalide and phthalic anhydride are known to be generated from the UV photolysis of the primary product, ophthalaldehyde.28 The formation of the other aromatic compounds, o-cresol and o-toluic acid, is more difficult to explain and since these products were only detected in very small amounts, speculative mechanisms for their formation will not be considered here. As indicated above, carbon monoxide appears to be formed as a secondary product. Benzocylcobutenol is not expected to undergo rapid photolysis under the conditions employed in the chambers, suggesting that photolysis of ophthalaldehyde is the most likely source of carbon monoxide. However, preliminary experiments on the photolysis of o-phthalaldehyde in both chambers did not generate detectable levels of carbon monoxide. The origin of the secondary carbon monoxide therefore remains unknown. Secondary Organic Aerosol Formation. Secondary Organic Aerosol (SOA) formation was observed in all experiments performed in both chambers. The evolution of the aerosol produced from photolysis of o-tolualdehyde in the EUPHORE chamber is shown in Figure 3. A large number of particles with a mean diameter of around 20 nm were observed within 5 min of the start of photolysis. This initial “burst” of particles also corresponded to the greatest number present during the reaction. As photolysis continued, there was a gradual reduction in particle number and a corresponding increase in average particle diameter due to coagulation. Particles with a mean diameter of around 70 nm were present at the end of the experiment. Very similar results were obtained in the indoor chamber, although the greatest number of particles was typically observed around 10 min later when the mean diameter was 2535 nm. This slight difference in the particle-time profile is probably due to the fact that the photolysis process, and hence particle formation, was slower in the indoor chamber.
The mass and volume concentration of SOA generated in the EUPHORE experiment on third July 2003 is shown in Figure 4. The density of SOA formed in the experiment was determined to be (1.09 ( 0.01) g cm3 by plotting the measured mass (TEOM) versus the volume (SMPS), Figure 4 (inset). The calculated density is lower than the value of 1.35 g cm3 determined for SOA generated from photooxidation of benzene in the EUPHORE chamber using the same method,15 and also at the lower end of the range of effective densities (1.06 1.45 g cm3) for laboratory-generated SOA from anthropogenic precursors.29 The maximum aerosol concentration was observed at the end of the experiment when all of the o-tolualdehyde had reacted. After closing the chamber, the aerosol was found to undergo a first order decay, k = (1.46 ( 0.10) 105 s1, due to deposition at the walls of the reactor. This wall loss factor was used to provide a corrected value of 258 μg m3 for the maximum aerosol mass concentration, which when divided by the mass of o-tolualdehyde reacted (2491 μg m3) gives an aerosol yield of 0.104. SOA yields were also obtained from four different experiments performed in the indoor chamber by converting the measured volume concentrations to mass concentrations using the density of 1.09 g cm3. As shown in Table 1, the SOA yields were found to increase with starting concentration of o-tolualdehyde. This effect has been observed for many other SOA precursors and is consistent with gas/particle partitioning theory.29 The partitioning of semivolatile reaction products to the aerosol phase increases with the amount of available aerosol mass, resulting in higher yields of SOA when greater precursor concentrations are used. It is interesting to note that the yield obtained for the experiment with an initial concentration of 518 ppbV is considerably lower than that determined in the equivalent experiment at EUPHORE. The semivolatile reaction products would be expected to undergo a higher degree of wall loss in the smaller indoor chamber, resulting in less partitioning to the particle 9654
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Figure 4. Aerosol mass (TEOM) and volume (SMPS) concentrations measured during the photolysis of o-tolualdehyde at EUPHORE on 3 July 2003. The inset contains the plot of mass versus volume concentration used to determine the density of the aerosol.
phase and lower SOA yields.30 In addition, aerosol formation is slightly slower in the indoor chamber and this suggests that the photolysis rate may also influence SOA yield. A similar rate effect has also been observed in previous studies of the photooxidation of aromatic hydrocarbons, where various factors including OH precursor type and concentration,31,32 light intensity33 and relative humidity34 have been shown to affect OH levels, oxidation rate, and SOA mass yield. The results obtained here suggest that direct photolysis of otolualdehyde may contribute to SOA formation in chamber studies of o-xylene photooxidation35 and in the ambient atmosphere. Information on the chemical composition of the SOA can provide insights into the key species and processes involved in aerosol formation and also help to identify molecular markers for specific sources of ambient organic aerosol.29 The only carbonyl products identified in filter samples collected in the indoor chamber experiments were o-phthalaldehyde (approximate yield of 10%) and glyoxal (not quantified). The same carbonyl products were identified in particles collected at EUPHORE, along with o-toluic acid, a hydroxyl-containing compound with MW 120, tentatively attributed to benzocyclobutenol, phthalaldehydic acid, phthalide, and phthalic anhydride, Supporting Information Table S2. The latter three compounds were also detected in filter samples of SOA generated from the photooxidation of o-tolualdehyde, where direct photolysis and reaction with OH were both responsible for loss of the aromatic compound.36 Interestingly, a number of oxygenated polycyclic aromatic compounds were also identified in this study, indicating that intermolecular reactions could be involved in SOA formation and growth. There is evidence to suggest that biradical (Criegee) intermediates are involved in addition reactions with peroxy radicals leading to SOA formation during the ozonolysis of alkenes,29,37 and it is possible that the biradical species produced during the photolysis of o-tolualdehyde may also be involved in similar types of reactions leading to polycyclic compounds with low volatility and hence SOA formation.
Photolysis of aromatic carbonyls in the solid and liquid phases has been shown to generate a wide range of polycyclic aromatic products,24,25 and another intriguing possibility is that some of the photochemically active compounds present in the SOA, for example, o-phthalaldehyde, may undergo light-induced intermolecular reactions with other aromatic species to produce the oxygenated polycyclic aromatic compounds. This work is one of the first studies to demonstrate that direct photolysis of a volatile organic compound can produce SOA. The formation of aerosol from photolysis of 2,4-hexadienedial38 and ortho-nitrophenols39 has been noted, although not investigated in detail. More recently, Kessler et al.40 used the photolysis of alkyl iodides to generate single organic radical precursors and proposed that this technique could be used as a simplified experimental approach to investigate SOA formation from alkanes. Similarly, the photolysis of o-tolualdehyde could also be considered as a good model system to investigate SOA formation mechanisms. The possible range of reaction pathways is considerably less complex than in OH-initiated photooxidation systems and does not involve the use of OH precursors. Although beyond the scope of the present work, a systematic study of the various parameters affecting aerosol production from direct photolysis of o-tolualdehyde could prove to be beneficial in elucidating key processes responsible for SOA formation. Atmospheric Implications. The rate coefficient for the sunlight photolysis of o-tolualdehyde can be used to calculate the tropospheric lifetime with respect to photolysis (τp) from the relationship: τp = 1/j. Using the values of j(o-tolualdehyde) obtained from the EUPHORE experiments yields photolysis lifetimes in the range 1.3 1.7 h. The average value of j(o-tolualdehyde)/j(NO2) = 2.53 102 can be used to provide an estimate of the photolysis lifetime under a variety of other solar irradiation conditions. The other atmospheric loss processes for o-tolualdehyde are reaction with OH and NO3, which have lifetimes of 13.6 and 56.7 h respectively.10 Thus photolysis by sunlight is clearly the dominant atmospheric loss process for 9655
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Environmental Science & Technology o-tolualdehyde. The short photolysis lifetime indicates that if released or formed in the atmosphere, it will readily photolyze in the troposphere and could contribute to the production of ozone and secondary organic aerosol. Atmospheric degradation of the reaction products could also contribute to the formation of ozone and other oxidants. The major atmospheric fate of benzocyclobutenol is likely to be gas-phase reaction with OH radicals, with an estimated lifetime of around 24 h,41 whereas ophthalaldehyde can undergo both photolysis and reaction with OH radicals.28 Finally, the kinetic and mechanistic information obtained for the photolysis of o-tolualdehyde could be included in photochemical degradation models, such as the Master Chemical Mechanism,42 that are used to predict secondary pollutant formation. It is envisaged that the inclusion of this chemistry will help to improve the prediction capability of the models.
’ ASSOCIATED CONTENT
bS
Supporting Information. Photolytic loss plot, FTIR spectra, GCMS data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +353 21 4902454; fax: +353 21 4903014; e-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Higher Education Authority in Ireland and the European Commission through the research project EUROCHAMP 2 (contract number 228335). The Instituto Universitario CEAM-UMH is partly supported by Generalitat Valenciana, Fundaci on Bancaja, and the projects GRACCIE (Consolider-Ingenio 2010) and FEEDBACKS (Prometeo - Generalitat Valenciana). ’ REFERENCES (1) Feng, Y.; Wen, S.; Chen, Y.; Wang, X.; L€u, H.; Bi, X.; Sheng, G.; Fu, J. Ambient levels of carbonyl compounds and their sources in Guangzhou, China. Atmos. Environ. 2005, 39, 1789–1695. (2) L€u, H.; Cai, Q.-Y.; Wen, S.; Chi, Y.; Guo, S.; Sheng, G.; Fu, J. Seasonal and diurnal variations of carbonyl compounds in the urban atmosphere of Guangzhou, China. Sci. Total Environ. 2010, 408, 3523–3529. (3) Obermeyer, G.; Aschmann, S. M.; Atkinson, R.; Arey, J. Carbonyl atmospheric reaction products of aromatic hydrocarbons in ambient air. Atmos. Environ. 2009, 43, 3736–3744. (4) Kean, A. J.; Grosjean, E; Grosjean, D.; Harley, R. On-road measurement of carbonyls in California light-duty vehicle emissions. Environ. Sci. Technol. 2001, 35, 4198–4204. (5) Jakober, C. A.; Robert, M. A.; Riddle, S. G.; Destaillats, H.; Charles, M. J.; Green, P. G.; Kleeman, M. J. Carbonyl emissions from gasoline and diesel motor vehicles. Environ. Sci. Technol. 2008, 42, 4697–4703. (6) Calvert, J. G. A.; Becker, K. H.; Kamens, R. M.; Seinfeld, J. H.; Wallington, T. J.; Yarwood, G., The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons; Oxford University Press: Oxford, UK, 2002. (7) Atkinson, R; Aschmann, S. M.; Arey, J. Formation of ringretaining products from the OH radical-initiated reactions of o-, m-, and p-xylene. Int. J. Chem. Kinet. 1991, 23, 77–97.
ARTICLE
(8) Atkinson, R.; Arey, J. Atmospheric degradation of volatile organic compounds. Chem. Rev. 2003, 103 (12), 4605–4638. (9) Thiault, G; Mellouki, A.; Le Bras, G. Kinetics of gas phase reactions of OH and Cl with aromatic aldehydes. Phys. Chem. Chem. Phys. 2002, 4, 2194–2199. (10) Clifford, G. M.; Th€uner, L. P.; Wenger, J. C.; Shallcross, D. E. Kinetics of the gas-phase reactions of OH and NO3 radicals with aromatic aldehydes. J. Photochem. Photobiol., A 2005, 176, 172–182. (11) Thiault, G.; Mellouki, A.; Le Bras, G.; Chakir, A.; SokolowskiGomez, N.; Daumont, D. UV-absorption cross sections of benzaldehyde, ortho-, meta-, and para-tolualdehyde. J. Photochem. Photobiol., A 2004, 162, 273–281. (12) Volkamer, R.; Platt, U.; Wirtz, K. The European Photoreactor (EUPHORE), 3rd Annual Report 2000; Barnes, I., Sidebottom, H., Eds.; Bergische Universit€at Wuppertal: Wuppertal, Germany, 2001, 1. (13) Thiault, G.; Mellouki, A.; Le Bras, G.; Wirtz, K. The European Photoreactor (EUPHORE), 4th Annual Report 2001; Barnes, I., Ed.; Bergische Universit€at Wuppertal: Wuppertal, Germany, 2003, 29. (14) Klotz, B.; Sørensen, S.; Barnes, I.; Becker, K. H.; Etzkorn, T.; Volkammer, R.; Platt, U.; Wirtz, K.; Martín-Reviejo, M. Atmospheric oxidation of toluene in a large-volume outdoor photoreactor: In situ determination of ring-retaining product yields. J. Phys. Chem. 1998, 102, 10289–10299. (15) Martín-Reviejo, M; Wirtz, K. Is benzene a precursor for secondary organic aerosol?. Environ. Sci. Technol. 2005, 39, 1045–1054. (16) Wenger, J. C.; Le Calve, S.; Sidebottom, H. W.; Wirtz, K.; Martín-Reviejo, M; Franklin, J. A. Photolysis of chloral under atmospheric conditions. Environ. Sci. Technol. 2004, 38, 831–837. (17) Sellevag, S. R.; Kelly, T.; Sidebottom, H.; Nielsen, C. J. A study of the IR and UV-Vis absorption cross-sections, photolysis and OHinitiated oxidation of CF3CHO and CF3CH2CHO. Phys. Chem. Chem. Phys. 2004, 6, 1243–1252. (18) O’Connor, M. P.; Wenger, J. C.; Mellouki, A.; Wirtz, K.; Mu~ noz, A. The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal. Phys. Chem. Chem. Phys. 2006, 8, 5236–5246. (19) Borras, E.; Tortajada-Genaro, L. A. Determination of oxygenated compounds in secondary organic aerosol from isoprene and toluene smog chamber experiments. Int. J. Environ. Anal. Chem. 2011in press. (20) Th€uner, L. P.; Bardini, P.; Rea, G. J.; Wenger, J. C. Kinetics of the gas-phase reactions of OH and NO3 radicals with dimethylphenols. J. Phys. Chem. A. 2004, 108, 11019–11025. (21) Temime, B.; Healy, R. M.; Wenger, J. C. A denuder-filter sampling technique for the detection of gas and particle phase carbonyl compounds. Environ. Sci. Technol. 2007, 41, 6514–6520. (22) Healy, R. M.; Wenger, J. C.; Metzger, A.; Duplissy, J.; Kalberer, M.; Dommen, J. Gas/particle partitioning of carbonyls in the photooxidation of isoprene and 1,3,5-trimethylbenzene. Atmos. Chem. Phys. 2008, 8, 3215–3220. (23) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. In Chemical Kinetics and Photochemical Data for use in Stratospheric Modelling, JPL Publication 97-4; Jet Propulsion Laboratory, Pasadena, CA, 1997. (24) Wagner, P. J.; Subrahmanyam, D.; Park, B.-S. Mechanism for the photocyclization of o -alkyl ketones to cyclobutenols. J. Am. Chem. Soc. 1991, 113, 709–710. (25) Moorthy, J. N.; Mal, P.; Natarajan, R.; Venugopalan, P. Efficient photocyclization of o-alkylbenzaldehydes in the solid state: Direct observation of E-xylylenols en route to benzocyclobutenols. J. Org. Chem. 2001, 66, 7013–7019. (26) Gebicki, J.; Krantz, A. Trapping of photoenols from o-tolualdehydes in gas matrices. Dependence of photoenol formation on the nature of the carbonyl function. J. Chem. Soc. Perkin Trans. 2 1984, 1623–1627. (27) Yang, N. C.; Yang, D-D. H. Photochemical reactions of ketones in solution. J. Am. Chem. Soc. 1958, 80, 2913–2914. (28) Wang, L.; Arey, J.; Atkinson, R. Kinetics and products of photolysis and reaction with OH radicals of a series of aromatic carbonyl compounds. Environ. Sci. Technol. 2006, 40, 5465–5471. 9656
dx.doi.org/10.1021/es2026533 |Environ. Sci. Technol. 2011, 45, 9649–9657
Environmental Science & Technology
ARTICLE
(29) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, T. F.; Monod, A.; Prev^ot, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J. The formation, properties and impact of secondary organic aerosol: Current and emerging issues. Atmos. Chem. Phys. 2009, 9, 5155–5236. (30) Matsunaga, A; Ziemann, P. J. Gas-wall partitioning of organic compounds in a Teflon film chamber and potential effects on reaction product and aerosol yield measurements. Aerosol Sci. Technol. 2010, 44, 881–892. (31) Ng, N. L.; Kroll, J. H.; Chan, A. W. H.; Chhabra, P.; Flagan, R. C.; Seinfeld, J. H. Secondary organic aerosol formation from mxylene, toluene, and benzene. Atmos. Chem. Phys. 2007, 7, 3909–3922. (32) Song, C.; Na, K. S.; Warren, B.; Malloy, Q.; Cocker, D. R. Secondary organic aerosol formation from m-xylene in the absence of NOx. Environ. Sci. Technol. 2007, 41, 7409–7416. (33) Warren, B.; Song, C.; Cocker, D. R. Light intensity and light source influence on secondary organic aerosol formation for the mxylene/NOx photooxidation system. Environ. Sci. Technol. 2008, 42, 5461–5466. (34) Healy, R. M.; Temime, B.; Kuprovskyte, K.; Wenger, J. C. The effect of relative humidity on gas/particle partitioning and aerosol mass yield in the photooxidation of p-xylene. Environ. Sci. Technol. 2009, 43, 1884–1889. (35) Song, C.; Na, K.; Warren, B.; Malloy, Q.; Cocker, D. R. Secondary organic aerosol formation from the photooxidation of pand o-xylene. Environ. Sci. Technol. 2007, 41, 7403–7408. (36) Webb, P. J.; Hamilton, J. F.; Lewis, A. C.; Wirtz, K. Formation of oxygenated-polycyclic aromatic compounds in aerosol from the photooxidation of o-tolualdehyde. Polycyclic Aromat. Compd. 2006, 26, 236–252. (37) Sadezky, A.; Winterhalter, R.; Kanawati, B.; R€ompp, A.; Spengler, B.; Mellouki, A.; Le Bras, G.; Chaimbault, P.; Moortgat, G. K. Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers: New evidence for the central role of the Criegee Intermediate as oligomer chain unit. Atmos. Chem. Phys. 2008, 8, 2667–2699. (38) Klotz, B.; Barnes, I.; Becker, K. H. Kinetic study of the gas-phase photolysis and OH radical reaction of E,Z- and E,E-2,4-hexadienedial. Int. J. Chem. Kinet. 1999, 31, 689–697. (39) Bejan, I.; Abd El Aal, Y.; Barnes, I.; Benter, T.; Bohn, B.; Wiesen, P.; Kleffmann, J. The photolysis of ortho-nitrophenols: A new gas phase source of HONO. Phys. Chem. Chem. Phys. 2006, 8, 2028–2035. (40) Kessler, S. H.; Nah, T.; Carrasquillo, A. J.; Jayne, J. T.; Worsnop, D. R.; Wilson, K. R.; Kroll, J. H. Formation of secondary organic aerosol from the direct photolytic generation of organic radicals. J. Phys. Chem. Lett. 2011, 2, 1295–1300. (41) US Environmental Protection Agency. Estimation Programs Interface (EPI) Suite v.3.20, 2007, http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm (accessed October 7, 2011). (42) Bloss, C.; Wagner, V.; Jenkin, M. E.; Volkamer, R.; Bloss, W. J.; Lee, J. D.; Heard, D. E.; Wirtz, K.; Martin-Reviejo, M.; Rea, G.; Wenger, J. C.; Pilling, M. J. Development of a detailed chemical mechanism (MCMv3.1) for the atmospheric oxidation of aromatic hydrocarbons. Atmos. Chem. Phys. 2005, 5, 641–664.
9657
dx.doi.org/10.1021/es2026533 |Environ. Sci. Technol. 2011, 45, 9649–9657
