2-Formylcinnamaldehyde Formation Yield from the OH Radical

Jul 18, 2012 - Air Pollution Research Center, University of California Riverside, Riverside, California 92521, United States. ‡ Department of Enviro...
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2-Formylcinnamaldehyde Formation Yield from the OH RadicalInitiated Reaction of Naphthalene: Effect of NO2 Concentration Noriko Nishino,†,⊥ Janet Arey,*,†,‡ and Roger Atkinson*,†,‡,§ †

Air Pollution Research Center, University of California Riverside, Riverside, California 92521, United States Department of Environmental Sciences, University of California Riverside, Riverside, California 92521, United States § Department of Chemistry, University of California Riverside, Riverside, California 92521, United States ‡

ABSTRACT: Naphthalene, typically the most abundant polycyclic aromatic hydrocarbon in the atmosphere, reacts with OH radicals by addition to form OH−naphthalene adducts. These OH−naphthalene adducts react with O2 and NO2, with the two reactions being of equal importance in air at an NO 2 mixing ratio of ∼60 ppbv. 2Formylcinnamaldehyde [o-HC(O)C6H4CHCHCHO] is a major product of the OH radical-initiated reaction of naphthalene, with a yield from the reaction of OH−naphthalene adducts with NO2 of ∼56%. We have measured, on a relative basis, the formation yield of 2-formylcinnamaldehyde from the OH radical-initiated reaction of naphthalene in air at average NO2 concentrations of 1.2 × 1011, 1.44 × 1012, and 1.44 × 1013 molecules cm−3 (mixing ratios of 0.005, 0.06, and 0.6 ppmv, respectively). These NO2 concentrations cover the range of conditions corresponding to the OH−naphthalene adducts reacting ∼90% of the time with O2 to ∼90% of the time with NO2. The 2formylcinnamaldehyde formation yield decreased with decreasing NO2 concentration, and a yield from the OH−naphthalene adducts + O2 reaction of 14% is obtained based on a 56% yield from the OH−naphthalene adducts + NO2 reaction. Based on previous measurements of glyoxal and phthaldialdehyde from the naphthalene + OH reaction and literature data for the OH radical-initiated reactions of monocyclic aromatic hydrocarbons, the reactions of OH−naphthalene adducts with O2 appear to differ significantly from the OH−monocyclic adduct + O2 reactions.

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INTRODUCTION Naphthalene is the most abundant polycyclic aromatic hydrocarbon (PAH) in urban atmospheres, being emitted mainly from anthropogenic sources including gasoline engine exhaust and fuel evaporation.1−4 In the lower troposphere, naphthalene is present almost exclusively in the gas phase5 and undergoes atmospheric chemical reactions with hydroxyl (OH) radicals during daylight hours and with nitrate (NO3) radicals during evening and nighttime hours.6 The dominant tropospheric chemical loss process is estimated to be daytime reaction with OH radicals, with a calculated naphthalene lifetime of 6 h for an average OH radical concentration of 2 × 106 molecules cm−3.6 Analogous to the reactions of OH radicals with monocyclic aromatic hydrocarbons, naphthalene and alkylnaphthalenes react with OH radicals mainly by OH radical addition to the aromatic ring to form OH−naphthalene or OH−alkylnaphthalene adducts that in the atmosphere subsequently react with NO2 and O2,6,7 as shown in Scheme 1. Our recent study of nitro-aromatic formation shows that for OH−naphthalene adducts, the NO2 and O2 reactions occur equally in air at an NO2 mixing ratio of ∼0.06 ppmv (1.5 × 1012 molecules cm−3 at 298 K and 760 Torr pressure),7 significantly lower than those for the benzene, toluene, and p-xylene reactions where the corresponding NO2 concentrations are in the range 1.2−5 ppmv.8 Therefore, while the OH−monocyclic aromatic adducts © 2012 American Chemical Society

Scheme 1

Received: Revised: Accepted: Published: 8198

May 9, 2012 July 3, 2012 July 6, 2012 July 18, 2012 dx.doi.org/10.1021/es301865t | Environ. Sci. Technol. 2012, 46, 8198−8204

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Scheme 2

and subsequent reaction of the OH−naphthalene−O2• radicals with NO or with HO2 or organic peroxy radicals (Scheme 2, pathway A).9,13 Theoretical studies14,15 indicate that formation of 2-formylcinnamaldehyde is thermodynamically favored through the H-atom shift reaction OH−naphthalene−O2• → • O−naphthalene−OOH followed by decomposition and elimination of OH (Scheme 2, pathway B). While these proposed 2-formylcinnamaldehyde formation mechanisms occur from reaction of the OH−naphthalene adducts with O2, the high measured yield of 2-formylcinnamaldehyde under high-NOx conditions9,10 shows that 2-formylcinnamaldehyde is also formed from reaction of the OH−naphthalene adducts with NO2. To further elucidate the formation pathway(s) of 2formylcinnamaldehyde, in this study we have measured the 2formylcinnamaldehyde formation yield from OH radicalinitiated reactions of naphthalene at different NO2 concentrations, as well as in the absence of NOx.

react almost exclusively with O2 under atmospheric conditions, the reaction of OH−naphthalene adducts with NO2 can be significant in polluted urban air masses. Rate constants for the reactions of OH−benzene, OH−toluene, and OH−p-xylene adducts with NO2 are (2.75−3.6) × 10−11 cm3 molecule−1 s−1 at 300−317 K,8 with the corresponding OH−monocyclic aromatic adduct + O2 rate constants at room temperature being in the range (1.6−8.8) × 10−16 cm3 molecule−1 s−1.8 Based on the magnitude of the OH−monocyclic aromatic hydrocarbon adduct + NO2 rate constants, independent of the specific aromatic hydrocarbon, it is likely that the OH−naphthalene adduct + NO2 rate constants are also ∼3 × 10−11 cm3 molecule−1 s−1 and that the effective rate constant for reaction of O2 with the OH−naphthalene adducts is ∼1 × 10−17 cm3 molecule−1 s−1, an order of magnitude or more lower than those for the OH−monocyclic aromatic hydrocarbon adduct + O2 reactions. Under high NOx conditions where the OH−naphthalene adducts dominantly react with NO2, a ring-opened C10dicarbonyl, 2-formylcinnamaldehyde [o-C6H4(CHO)CH CHCHO], is the major product9 with a formation yield of 56%.10 Analogous to the naphthalene reaction, the major products of the OH radical-initiated reactions of C nalkylnaphthalenes in the presence of part-per-million concentrations of NOx are ring-opened Cn-dicarbonyls.11 Recently, the formation of 2-formylcinnamaldehyde from the naphthalene reaction was observed in the absence of added NOx (≤3 ppbv NOx) by Chan et al.12 and Kautzman et al.13 using chemical ionization mass spectrometry (CIMS) and combined gas chromatography−mass spectrometry. In positive ion CIMS analyses, the m/z 161 ion peak attributed to 2-formylcinnamaldehyde was the dominant ion peak at initial NO2 mixing ratios of 66−289 ppbv as well as at initial NO2 mixing ratios ≤3 ppbv.12,13 The formation of 2-formylcinnamaldehyde from OH + naphthalene has been postulated to arise from decomposition of the OH−naphthalene−O• alkoxy radicals, which could be formed after reaction of O2 with the OH−naphthalene adducts

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EXPERIMENTAL METHODS

Experiments were conducted in a ∼7500 L Teflon chamber at 296 ± 2 K and 735 Torr of dry purified air. The chamber is equipped with a fan for rapid mixing of reactants and two parallel banks of black lamps for irradiation. Naphthalene was introduced into the chamber by flowing N2 gas through a Pyrex tube containing solid naphthalene. For the experiments conducted in the presence of NOx, OH radicals were generated by photolysis of CH3ONO at wavelengths >300 nm and NO was present to suppress formation of O3 and thus of NO3 radicals. The initial naphthalene concentrations were (2.08− 3.01) × 1013 molecules cm−3, and the initial NO and CH3ONO concentrations were ∼2.4 × 1012 or ∼2.4 × 1013 molecules cm−3 each (1 ppmv = 2.40 × 1013 molecules cm−3 at 296 K and 735 Torr pressure). Irradiations were performed at a light intensity corresponding to an NO2 photolysis rate, J(NO2), of 0.140 min−1. Irradiations were carried out for up to 5 min at initial CH3ONO and NO concentrations of ∼2.4 × 1012 molecules cm−3, resulting in up to 8% reaction, and for up to 8199

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10 min at initial CH3ONO and NO concentrations of ∼2.4 × 1013 molecules cm−3, resulting in up to 39% reaction. Average OH radical concentrations during the irradiations were calculated from the naphthalene decay rates to be 1.2 × 107 and 3.5 × 107 molecules cm−3 at initial CH3ONO and NO concentrations of ∼2.4 × 1012 and ∼2.4 × 1013 molecules cm−3, respectively. Average NO2 concentrations were determined by assuming [NO] + [NO2] = constant, where the assumption was derived from a computational simulation using the detailed chemical reaction of a toluene−CH3ONO−NO−air irradiation system.16 For experiments in the absence of NOx, OH radicals were generated from the dark reaction of O3 with 2,3-dimethyl-2butene.7 The initial concentrations of naphthalene and 2,3dimethyl-2-butene were 2.37 × 1013 and 2.4 × 1013 molecules cm−3, respectively. Three 50 cm3 aliquots of O3 in O2 were added to the chamber to generate OH radicals (each addition of O3 in O2 corresponding to ∼7 × 1012 molecules cm−3 of O3 in the chamber), resulting in up to 22% of the initially present naphthalene being reacted. Naphthalene and 2-formylcinnamaldehyde concentrations were measured during the experiments by gas chromatography with flame ionization detection (GC-FID). Gas samples of 100 cm3 volume were collected from the chamber onto Tenax-TA adsorbent for the analysis of naphthalene, while samples for analysis of 2-formylcinnamaldehyde were collected on a 65 μm polydimethylsiloxane/divinylbenzene solid phase microextraction (SPME) fiber, by exposing the fiber to the chamber contents for 20 min with the mixing fan on. In all experiments, 2 prereaction analyses were conducted, followed by 3 reaction periods (irradiation periods or additions of O3/O2 aliquots to the chamber) with Tenax and SPME samples being collected for GC-FID analysis after each reaction period. An additional SMPE sample was collected 40 min after the third (and last) reaction period to obtain the 2-formylcinnamaldehyde dark decay rate. The Tenax samples were thermally desorbed at ∼205 °C onto a 30 m DB-5 megabore column, initially held at 0 °C and then temperature-programmed at 8 °C min−1. The SPME fibers were thermally desorbed at 270 °C onto a DB-5 megabore column, initially held at 40 °C, and then temperatureprogrammed at 8 °C min−1. Calibration of naphthalene was conducted by introducing a known amount of naphthalene into the chamber and conducting several replicate GC-FID analyses. To confirm the identity of the peaks quantified by GC-FID as cis- and trans- 2-formylcinnamaldehyde, analyses of exposed SPME fibers were also carried out by combined gas chromatography−mass spectrometry (GC-MS), using a 60 m DB-5 capillary column in an Agilent 6890N GC interfaced to an Agilent 5975 Inert XL mass selective detector, operated in positive chemical ionization (PCI) mode with methane as the CI gas. Aerosol formation was monitored using a TSI 3936L72 scanning mobility particle sizer (SMPS). Since use of the mixing fan may cause loss of aerosol to the chamber walls, the same series of experiments was repeated for aerosol measurements, but with the mixing fan being turned off after the reactants had been introduced into the chamber. Aerosol formation yields were derived from the mass concentrations of naphthalene reacted and of aerosol formed, assuming that the density of aerosol was the same as that of naphthalene. The chemicals used, and their stated purities, were as follows: naphthalene (98%) and 2,3-dimethyl-2-butene (98%), Aldrich;

and NO (>99%), Matheson Gas Products. CH3ONO was synthesized and stored as described previously,7,16 and O3 was generated using a Welsbach T-408 O3 generator.

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RESULTS GC-MS analyses of SPME fibers exposed to reacted OH + naphthalene mixtures showed the presence of cis- and trans-2formylcinnamaldehyde. Because of the possible interconversion of cis- and trans-2-formylcinnamaldehyde,9 the sum of their concentrations as measured by GC-FID is used here. 2Formylcinnamaldehyde is potentially formed by reactions 1 through 3, and also undergoes chemical reaction with OH radicals and photolysis,9,10 together with loss to the chamber walls10 (reactions 4 through 6, respectively). OH + naphthalene → OH−naphthalene adduct

(1)

OH−naphthalene adduct + O2 → YO2 2‐formylcinnamaldehyde + other products

(2)

OH−naphthalene adduct + NO2 → YNO2 2‐formylcinnamaldehyde + other products (3)

OH + 2‐formylcinnamaldehyde → products

(4)

2‐formylcinnamaldehyde + hν → products

(5)

2‐formylcinnamaldehyde → loss to walls

(6)

YO2 and YNO2 are the yields of 2-formylcinnamaldehyde from the reactions of the OH−naphthalene adducts with O2 (reaction 2) and NO2 (reaction 3), respectively. Since 2formylcinnamaldehyde is removed by reaction with OH radicals, photolysis, and dark decay (reactions 4−6), corrections need to be made to account for the effects of these loss processes on the measured concentrations of 2formylcinnamaldehyde. In correcting for secondary reactions of 2-formylcinnamaldehyde, two sets of OH radical reaction rate constant (k4) and photolysis rate (k5) were used. One set [case (a)] used the rate constant and photolysis rate reported by Nishino et al.,10 of k4/k1 = 2.22 (derived from k1 = 2.39 × 10−11 cm3 molecule−1 s−1 6 and the estimated17 rate constant of k4 = 5.30 × 10−11 cm3 molecule−1 s−1) and k5/J(NO2) = 1.0.10 In the second set of rate constants [case (b)], we assumed that photolysis of 2-formylcinnamaldehyde does not occur or is very slow, with k5/J(NO2) = 0 (based on the observations of Kautzman et al.13 in the presence of particles which suggested that photolysis of 2-formylcinnamaldehyde is slower than we derived previously10). The rate constant ratio k4/k1 = 4.8 was then obtained from a least-squares analysis of the experimental data of Nishino et al.10 assuming that photolysis did not occur. OH radicals were generated from the photolysis of CH 3 ONO in the presence of NO at average NO 2 concentrations of 0.60 and 0.06 ppmv, and from the dark O3 + 2,3-dimethyl-2-butene reaction in the absence of added NOx, with a measured NO2 concentration in the chamber of 0.005 ppmv. Corrections for wall losses were taken into account using the measured dark decay rate of 2-formylcinnamaldehyde after each experiment, these being 2.81 × 10−3, 1.54 × 10−3, and 4.65 × 10−3 min−1 at average NO2 concentrations of 0.60, 0.06, and 0.005 ppmv, respectively. Over the 94−106 min durations of the experiments (i.e., from initiation of the reaction to the 8200

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midpoint of the first sampling period after the third reaction period), these dark decay rates would correspond to 14−37% loss of initially present 2-formylcinnamaldehyde. Each reaction period was treated individually to derive the amount of 2formylcinnamaldehyde formed during that reaction period (taking into account dark decays during the intervals between reaction periods and sampling). The multiplicative correction factors, F, to account for photolysis, OH radical reaction and wall losses during the individual irradiation periods or reaction periods ranged from 1.04 to 1.75, being highest for case (a) which included photolysis of 2-formylcinnamaldehyde as a loss process. In all cases, during the reaction periods wall loss was minor (≤6% of the total 2-formylcinnamaldehyde loss rate) and for case (a) the ratio of (2-formylcinnamaldehyde photolysis rate/reaction rate with OH radicals) was 2.6−4.4 at 0.06 ppmv NO2 and 1.1−1.4 at 0.6 ppmv NO2. Plots of the cumulative amounts of 2-formylcinnamaldehyde formed throughout the reactions against the amounts of naphthalene reacted are shown in Figure 1 for case (a). The slight curvature in the plots at 0.06 and 0.6 ppmv NO2, and the resulting negative intercepts (which were within one standard

deviation of zero) may be due to the fact that the NO2 concentration (and hence the 2-formylcinnamaldehyde yield) increased during the experiments in which OH radicals were generated from the photolysis of CH3ONO, because of NO-toNO2 conversion from the reactions of NO with HO2 and organic peroxy (RO2•) radicals. Analogous plots for case (b) (not shown), which assumed no photolysis of 2-formylcinnamaldehyde but a more rapid reaction with OH radicals, were of equal or better quality of fit. A preliminary series of experiments had been carried out with similar initial reactant concentrations and with irradiation or reaction periods identical to those described above. However, in this preliminary series of experiments dark decays of 2formylcinnamaldehyde were not measured after the third and last irradiation period or reaction period, and hence the 2formylcinnamaldehyde data could not be corrected for dark decay. The raw data (i.e., the 2-formylcinnamaldehyde GC-FID area counts and amounts and percentages of naphthalene reacted) were in good agreement with those from the experiments in which 2-formylcinnamaldehyde dark decay rates were measured and corrected for, and the significant decrease in 2-formylcinnamaldehyde formation yield with decreasing NO2 concentration was clearly evident. A separate set of experiments was conducted to measure aerosol formation using an SMPS. Aerosol formation was observed in all of the experiments, and the yields at the end of experiments were 14% and 1% at initial NO mixing ratios of 1 ppmv (43% naphthalene reacted) and 0.1 ppmv (10% naphthalene reacted), respectively, and 46% in the absence of NO (23% naphthalene reacted). The lower aerosol yield at 0.1 ppmv initial NO concentration was probably due to the small extent of reaction in this experiment. The aerosol yields obtained in our experiments were lower than those reported by Chan et al.,12 and this is likely explained by the absence of seed particles and the lower extents of reaction in the present experiments compared to those of Chan et al.12 (Chan et al.12 observed that under their conditions the aerosol yield (defined as aerosol formed/naphthalene reacted) increased as the reaction proceeded). The aerosol yield was the highest in the absence of NO, consistent with previous aerosol studies,12 and this can be explained by the occurrence of RO2• + RO2• reactions to form peroxides and other low volatility species.12,13 Interestingly, the measured dark decay rates of 2- formylcinnamaldehyde correlated with the aerosol mass concentration, suggesting that the loss of 2-formylcinnamaldehyde to surfaces was slightly enhanced by the formed aerosols. Because of its relatively high vapor pressure, the contribution of 2formylcinnamaldehyde to aerosol formation and growth should be minor.

Figure 1. Plots of the 2-formylcinnamaldehyde GC-FID peak areas, corrected for wall loss and for reaction with OH radicals and photolysis (with, in this case, k4/k1 = 2.22 and k5/J(NO2) = 1.0), against the amounts of naphthalene reacted with OH radicals, at average NO2 concentrations of 0.005, 0.06, and 0.6 ppmv. OH radicals were generated by the photolysis of CH3ONO at average NO2 concentrations of 0.06 and 0.6 ppmv, and by the dark reaction of O3 with 2,3-dimethyl-2-butene in the absence of added NOx at an average NO2 concentration in the chamber of 0.005 ppmv. Note that the data at 0.06 ppmv average NO2 concentration are plotted using the upper X-axis and right-hand side Y-axis (both of which are scaled by a factor of 6 from the bottom and left-hand side axes, respectively).

Table 1. Measured Formation Yields of 2-Formylcinnamaldehyde Relative to That at 0.6 ppmv NO2 for Two Sets of Rate Constant Ratios k4/k1 and Photolysis Rates k5/J(NO2) 2-formylcinnamaldehyde formation yielda,b at average NO2 (ppmv) k4/k1

k5/J(NO2)

0.005

0.06

0.6

2.22 4.8

1.0 0

0.30 ± 0.04 (15 ± 2%) 0.39 ± 0.04 (20 ± 3%)

0.77 ± 0.13 (40 ± 7%) 0.66 ± 0.06 (34 ± 3%)

1.0 (52%) 1.0 (52%)

a

Indicated errors are two least-squares standard deviations. bThe relative yields are placed on an absolute basis using a 2-formylcinnamaldehyde yield at 0.6 ppmv NO2 of 52% (see text), and are given in parentheses. 8201

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would be formed by RO2• + R′O2• → RO• + R′O• + O2 and RO2• + HO2 → RO• + OH + O2 in competition with the molecular channel of the RO2• + R′O2• reaction (i.e., that to form an alcohol and carbonyl) and with RO2• + HO2 → ROOH + O2. Any formation of OH−naphthalene−O• radicals (and 2-formylcinnamaldehyde) from the precursor OH− naphthalene−OO• radical may therefore be less efficient in the absence of NO than in the presence of NO where the RO2• + NO reaction dominates. At present, the detailed chemical mechanism of the OH− naphthalene adduct + NO2 reactions, leading to 2-formylcinnamaldehyde and other products including 1- and 2-nitronaphthalene in 0.35% and 0.60% yield, respectively,7 is not known. Indeed, it is possible that the precursor intermediate to 2-formylcinnamaldehyde in the OH−naphthalene adduct + NO2 reaction is not that shown in Scheme 1.18 In contrast to high-NO2 conditions where most of the products from the OH radical-initiated reaction of naphthalene (i.e., from the OH− naphthalene adduct + NO2 reaction) appear to be accounted for9,10 even though their formation pathways are not understood, a large fraction of products formed from the OH− naphthalene adducts + O2 reaction have not yet been quantified. Chan et al.12 and Kautzman et al.13 reported that more ring-retaining gas-phase products were observed in the absence of NOx than in the presence of NOx, and the CIMS mass spectra showed a high relative abundance of ion peaks attributed to naphthol and naphthoquinone.12,13 The observed ring-retaining gas-phase products could also result from RO2• + RO2• or RO2• + HO2 reactions to form peroxides, contributing to higher aerosol yields, but the measured peroxide fractions in the aerosol were similar in the presence (∼26%) and absence (∼28%) of NOx.13 Analogous to the OH radical reaction of monocyclic aromatic hydrocarbons,19−22 the naphthalene reaction can potentially form naphthols, ring-retaining products, from one pathway of the reaction of OH−naphthalene adducts with O2 (Scheme 1). Another possible reaction pathway of the O2 reaction with the OH−naphthalene adducts involves cyclization to form a bicyclic radical (Scheme 2, pathway C) which would then react further to form phthaldialdehyde [o-HC(O)C6H4CHO] plus glyoxal. The reaction pathway involving cyclization of the OH− aromatic−O2• radical to form a bicyclic radical dominates for the monocyclic aromatic hydrocarbons,21−23 noting that reaction of the OH−aromatic−O2• radical with NO to form the OH−aromatic−O• alkoxy radical can become competitive with the cyclization reaction at NO concentrations ≥1 ppmv for benzene (see refs 20, 22 and the Supporting Information to ref 23). However, the glyoxal formation yields from OH + naphthalene have been measured to be 5.2 ± 2.8% in the presence of NOx, independent of NO2 concentration, and 2.8 ± 2.0% in the absence of NOx,24 and phthaldialdehyde and glyoxal are formed in equal yields from OH + naphthalene in the presence of NOx.24 The low formation yields of glyoxal and phthaldialdehyde from OH + naphthalene 24 therefore suggests that pathway C is of minor importance for the OH− naphthalene−O2• radicals, and that if bicyclic radicals are formed to a significant extent in the naphthalene reaction they do not undergo ring cleavage to form glyoxal and phthaldialdehyde. The observations of low formation yields of glyoxal + phthaldialdehyde24 are consistent with the theoretical calculations of Zhang et al.,15 which predicted that the most favorable cyclization is that to form the bicyclic radical shown

DISCUSSION The slopes of the plots shown in Figure 1 are proportional to the formation yields of 2-formylcinnamaldehyde, with the proportionality constant being the SPME/GC-FID response factor for 2-formylcinnamaldehyde (in area counts ppb−1). The formation yields of 2-formylcinnamaldehyde obtained from least-squares analyses of the data shown in Figure 1 and of analogous plots for case (b) are listed in Table 1, relative to the 2-formylcinnamaldehyde yield at 0.6 ppmv NO2. Since the OH−naphthalene adducts react equally in air with O2 and NO2 at an NO2 concentration of 0.06 ppmv,7 ∼90% of the OH− naphthalene adducts reacted with NO2 and ∼10% reacted with O2 at an average NO2 concentration of 0.6 ppmv. Assuming that 2-formylcinnamaldehyde is formed and lost by reactions 1−6, its formation yield is then given by 2‐formylcinnamaldehyde yield = (YNO2k 3[NO2 ] + YO2k 2[O2 ])/(k 3[NO2 ] + k 2[O2 ]) (I)

where YO2 and YNO2 are the 2-formylcinnamaldehyde yields from the reactions of the OH−naphthalene adducts with O2 and NO2, respectively, and k2 and k3 are the corresponding rate constants. For YNO2 = 56%,10 our data at 0.005 and 0.6 ppmv NO2 result in YO2 = 14% and a calculated 2-formylcinnamaldehyde yield at 0.6 ppmv NO2 of 52%. We therefore normalize our relative 2-formylcinnamaldehyde yields to this value, and the resulting yields at the various NO2 concentrations and reaction rate scenarios [cases (a) and (b)] are also given in Table 1. While the derived formation yields of 2-formylcinnamaldehyde given in Table 1 for NO2 concentrations of 0.005 and 0.06 ppmv show some variation depending on the rate constants k4 and k5 used to correct the measured concentrations of 2formylcinnamaldehyde for reaction with OH radicals and photolysis, these variations are relatively minor and do not affect the trend of the 2-formylcinnamaldehyde formation yield with NO2 concentration. Clearly, the 2-formylcinnamaldehyde yield deceases with decreasing NO2 concentration, and 2formylcinnamaldehyde is still formed from OH + naphthalene at sufficiently low NO2 concentrations that ≥90% of the OH− naphthalene adducts react with O2. Our data therefore show that 2-formylcinnamaldehyde is formed from both the OH− naphthalene adduct + O2 and OH−naphthalene adduct + NO2 reactions, with a 4-fold higher yield from the OH−naphthalene adduct + NO2 reactions than from the OH−naphthalene adduct + O2 reactions. The 2-formylcinnamaldehyde formation yield at a given NO2 concentration can be calculated from eq I with YNO2 = 56%, YO2 = 14%, and k2[O2]/k3 = 60 ppbv (1.44 × 1012 molecules cm−3 at 296 K and 735 Torr air). As noted above and shown in Scheme 2, formation of 2formylcinnamaldehyde from reaction of the OH−naphthalene adducts with O 2 has been proposed to occur from decomposition of the OH−naphthalene−O• alkoxy radical9,12,13 and/or from isomerization of the OH−naphthalene−O2• radical to •O−naphthalene−OOH followed by decomposition and elimination of OH14,15 (Scheme 2, pathways A and B, respectively). Our present study does not distinguish between these two pathways. If 2-formylcinnamaldehyde is formed after decomposition of the alkoxy radical via pathway A in Scheme 2, then in the absence of NO (as in our experiment at 0.005 ppmv NO2 where OH radicals were generated from O3 + 2,3-dimethyl-2-butene) the alkoxy radical 8202

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Naphthalene distributions and human exposure in Southern California. Atmos. Environ. 2005, 39, 489−507. (5) Bidleman, T. F. Atmospheric processes. Environ. Sci. Technol. 1988, 22, 361−367. (6) Atkinson, R.; Arey, J. Mechanisms of the gas-phase reactions of aromatic hydrocarbons and PAHs with OH and NO3 radicals. Polycyclic Aromat. Compd. 2007, 27, 15−40. (7) Nishino, N.; Atkinson, R.; Arey, J. Formation of nitro products from the gas-phase OH radical-initiated reactions of toluene, naphthalene, and biphenyl: effect of NO2 concentration. Environ. Sci. Technol. 2008, 42, 9203−9209. (8) Koch, R.; Knispel, R.; Elend, M.; Siese, M.; Zetzsch, C. Consecutive reactions of aromatic-OH adducts with NO, NO2 and O2: benzene, toluene, m- and p-xylene, hexamethylbenzene, phenol, mcresol and aniline. Atmos. Chem. Phys. 2007, 7, 2057−2071. (9) Sasaki, J.; Aschmann, S. M.; Kwok, E. S. C.; Atkinson, R.; Arey, J. Products of the gas-phase OH and NO3 radical-initiated reactions of naphthalene. Environ. Sci. Technol. 1997, 31, 3173−3179. (10) Nishino, N.; Arey, J.; Atkinson, R. Formation and reactions of 2formylcinnamaldehyde in the OH radical-initiated reaction of naphthalene. Environ. Sci. Technol. 2009, 43, 1349−1353. (11) Wang, L.; Atkinson, R.; Arey, J. Dicarbonyl products of the OH radical-initiated reactions of naphthalene and the C1- and C2alkylnaphthalenes. Environ. Sci. Technol. 2007, 41, 2803−2810. (12) Chan, A. W. H.; Kautzman, K. E.; Chhabra, P. S.; Surratt, J. D.; Chan, M. N.; Crounse, J. D.; Kürten, A.; Wennberg, P. O.; Flagan, R. C.; Seinfeld, J. H. Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: implications for oxidation of intermediate volatility organic compounds (IVOCs). Atmos. Chem. Phys. 2009, 9, 3049−3060. (13) Kautzman, K. E.; Surratt, J. D.; Chan, M. N.; Chan, A. W. H.; Hersey, S. P.; Chhabra, P. S.; Dalleska, N. F.; Wennberg, P. O.; Flagan, R. C.; Seinfeld, J. H. Chemical composition of gas- and aerosol-phase products from the photooxidation of naphthalene. J. Phys. Chem. A 2010, 114, 913−934. (14) Qu, X.; Zhang, Q.; Wang, W. Mechanism for OH-initiated photooxidation of naphthalene in the presence of O2 and NOx: a DFT study. Chem. Phys. Lett. 2006, 429, 77−85. (15) Zhang, Z.; Lin, L.; Wang, L. Atmospheric oxidation mechanism of naphthalene initiated by OH radical. A theoretical study. Phys. Chem. Chem. Phys. 2012, 14, 2645−2650. (16) Atkinson, R.; Aschmann, S. M.; Arey, J.; Carter, W. P. L. Formation of ring-retaining products from the OH radical-initiated reactions of benzene and toluene. Int. J. Chem. Kinet. 1989, 21, 801− 827. (17) Kwok, E. S. C.; Atkinson, R. Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: an update. Atmos. Environ. 1995, 29, 1685−1695. (18) Zimmermann, K.; Atkinson, R.; Arey, J. Effect of NO2 concentration on dimethylnitronaphthalene yields and isomer distribution patterns from the gas-phase OH radical-initiated reactions of selected dimethylnaphthalenes. Environ. Sci. Technol. 2012, No. DOI: 10.1021/es3009826. (19) Volkamer, R.; Klotz, B.; Barnes, I.; Imamura, T.; Wirtz, K.; Washida, N.; Becker, K. H.; Platt, U. OH-initiated oxidation of benzene. Part I. Phenol formation under atmospheric conditions. Phys. Chem. Chem. Phys. 2002, 4, 1598−1610. (20) Klotz, B.; Volkamer, R.; Hurley, M. D.; Sulbaek Andersen, M. P.; Nielsen, O. J.; Barnes, I.; Imamura, T.; Wirtz, K.; Becker, K.-H.; Platt, U.; Wallington, T. J.; Washida, N. OH-initiated oxidation of benzene. Part II. Influence of elevated NOx concentrations. Phys. Chem. Chem. Phys. 2002, 4, 4399−4411. (21) Glowacki, D. R.; Wang, L.; Pilling, M. J. Evidence of formation of bicyclic species in the early stages of atmospheric benzene oxidation. J. Phys. Chem. A 2009, 113, 5385−5396. (22) Birdsall, A. W.; Elrod, M. J. Comprehensive NO-dependent study of the products of the oxidation of atmospherically relevant aromatic compounds. J. Phys. Chem. A 2011, 115, 5397−5407.

below, further reactions of which are unlikely to lead to glyoxal + phthaldialdehyde.

The second-most abundant product observed by Sasaki et al.9 under their high-NOx conditions was a molecular weight 176 hydroxy-epoxy-carbonyl (see Scheme 2 for structure). While Kautzman et al.13 proposed that this product is formed from the bicyclic radical (i.e., though pathway C in Scheme 1), Zhang et al.15 proposed that formation of this molecular weight 176 product occurs via cyclization of the OH−naphthalene−O• radical formed in pathway A, as shown in Scheme 2. Clearly, although the initial step in the reaction of OH radicals with naphthalene and alkylnaphthalenes is analogous to that for the monocyclic aromatic hydrocarbons, the subsequent reaction mechanism involving reaction of OH−naphthalene adducts with O2 appears to be significantly different from the reactions of OH−monocyclic aromatic adducts with O2. The products and mechanism of the OH radical-initiated reactions of naphthalenes and alkylnaphthalenes need further investigation under conditions where the OH−naphthalene and OH−alkylnaphthalene adducts react with O2 and NO2, given the low NO2 concentrations at which the O2 and NO2 reactions with the OH-adducts are equally important.7

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; telephone: (951) 827-3502 (J.A.). E-mail: [email protected]; telephone: (951) 827-4191 (R.A.). Present Address ⊥

Department of Chemistry, University of California, Irvine, CA 92697. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Environmental Protection Agency (Grant R833752) and the University of California Agricultural Experiment Station for supporting this research. While this work has been supported in part by the U.S. Environmental Protection Agency, the results and the contents of this publication do not necessarily reflect the views and the opinions of the Agency.

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REFERENCES

(1) Zielinska, B.; Sagebiel, J. C.; Harshfield, G.; Gertler, A. W.; Pierson, W. R. Volatile organic compounds up to C20 emitted from motor vehicles: measurement methods. Atmos. Environ. 1996, 30, 2269−2286. (2) Fraser, M. P.; Cass, G. R.; Simoneit, B. R. T. Gas-phase and particle-phase organic compounds emitted from motor vehicle traffic in a Los Angeles roadway tunnel. Environ. Sci. Technol. 1998, 32, 2051−2060. (3) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A.; Miguel, A. H.; Hering, S. V.; Hammond, S. K. Characterization of polycyclic aromatic hydrocarbons in motor vehicle fuels and exhaust emissions. Environ. Sci. Technol. 1999, 33, 3091−3099. (4) Lu, R.; Wu, J.; Turco, R. P.; Winer, A. M.; Atkinson, R.; Arey, J.; Paulson, S. E.; Lurmann, F. W.; Miguel, A. H.; Eiguren-Fernandez, A. 8203

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Environmental Science & Technology

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(23) Nishino, N.; Arey, J.; Atkinson, R. Formation yields of glyoxal and methylglyoxal from the gas-phase OH radical-initiated reactions of toluene, xylenes, and trimethylbenzenes as a function of NO2 concentration. J. Phys. Chem. A 2010, 114, 10140−10147. (24) Nishino, N.; Arey, J.; Atkinson, R. Yields of glyoxal and ringcleavage co-products from the OH radical-initiated reactions of naphthalene and selected alkylnaphthalenes. Environ. Sci. Technol. 2009, 43, 8554−8560.

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