Environ. Sci. Technol. 2009, 43, 1349–1353
Formation and Reactions of 2-Formylcinnamaldehyde in the OH Radical-Initiated Reaction of Naphthalene N O R I K O N I S H I N O , † J A N E T A R E Y , †,‡ A N D R O G E R A T K I N S O N * ,†,‡,§ Air Pollution Research Center, University of California Riverside, California 92521
Received September 2, 2008. Revised manuscript received October 30, 2008. Accepted December 13, 2008.
2-Formylcinnamaldehyde [o-HC(O)C6H4CHdCHCHO] is a major product of the OH radical-initiated reaction of naphthalene, the atmospherically most abundant polycyclic aromatic hydrocarbon. Previous studies indicate that 2-formylcinnamaldehyde undergoes photolysis as well as reaction with OH radicals. We have used direct air sampling atmospheric pressure ionization mass spectrometry (API-MS) to monitor 2-formylcinnamaldehyde as its protonated molecular ion during OH radicalinitiated reactions of naphthalene. From the time-dependent behavior of the 2-formylcinnamaldehyde signal, ratios of (2formylcinnamaldehyde removal rate/naphthalene reaction rate) were determined over a range of ∼3 in (OH radical concentration/ light intensity). With an estimated rate constant for the reaction of OH radicals with 2-formylcinnamaldehyde of 5.3 × 10-11 cm3 molecule-1 s-1, the photolysis rate of 2-formylcinnamaldhyde by blacklamps was determined to be approximately equal to that of NO2. Photolysis of 2-formylcinnamaldehyde will be the dominant loss process in the atmosphere, with an estimated lifetime of 2-formylcinnamaldehyde of ∼120 s at a solar zenith angle of 30°. Our data were used to re-evaluate the previous 2-formylcinnamaldehyde measurements of Sasaki et al. (Environ. Sci. Technol. 1997, 31, 3173-3179) and derive a 2-formylcinnamaldehyde formation yield from the OH radical reaction of naphthalene in the presence of NO of 56+15 -10%.
Introduction Naphthalene, the simplest polycyclic aromatic hydrocarbon (PAH), is a minor constituent of gasoline and diesel fuels and is emitted into the atmosphere from vehicle exhaust (1-3), with over 40% of the naphthalene emissions into the atmosphere in Southern California being attributed to gasoline engine exhaust and evaporation (4). Naphthalene is the most abundant individual PAH observed in the atmosphere, and together with the methyl and ethyl-/ dimethylnaphthalenes accounts for typically >90% of the volatile and semivolatile PAH in the atmosphere in southern California (5). Naphthalene is classified by the International * Corresponding author telephone: (951) 827-4191; e-mail:
[email protected]. † Also Interdepartmental Graduate Program in Environmental Toxicology. ‡ Also Department of Environmental Sciences. § Also Department of Chemistry. 10.1021/es802477s CCC: $40.75
Published on Web 02/03/2009
2009 American Chemical Society
Agency for Research on Cancer as a possible human carcinogen (6) and by the California-EPA as a human carcinogen (7). In the lower troposphere, naphthalene and the alkylnaphthalenes undergo gas-phase reaction with OH radicals during daylight hours and with NO3 radicals during nighttime (8, 9). The OH radical reaction is the dominant atmospheric chemical loss process for naphthalene, with an estimated naphthalene lifetime of a few hours (8), and therefore it is important to know the atmospheric chemistry and toxicology of its reaction products. The atmospheric reaction of naphthalene leads to a variety of products, including 2-nitronaphthalene and 1,4-naphthoquinone which have higher genetic toxicity than naphthalene in bacterial and human cell assay systems (10, 11). Sasaki et al. (11) also found genotoxic activity in more polar fractions and suggested 2-formylcinnamaldehyde (Figure 1) as being possibly responsible for this observed activity. 2-Formylcinnamaldehyde has been reported as the major product of the naphthalene reaction (12, 13), and many carbonyls are mutagenic, cytotoxic, and/or clastogenic (14, 15). The OH radical reaction with naphthalene proceeds by initial addition to form an OH-naphthalene adduct which then reacts with O2 and NO2 (8), with the recent study of Nishino et al. (16) showing that in air the reaction with NO2 dominates down to ∼60 ppbv NO2. While Sasaki et al. (13) reported a ∼35% yield of 2-formylcinnamaldehyde from the OH radical-initiated reaction of naphthalene under conditions where the OH-naphthalene adduct reacted with NO2, derivation of this formation yield required use of estimated removal rates of 2-formylcinnamaldehyde because of photolysis and reaction with OH radicals (13). Since 2-formylcinnamaldehyde is a major reaction product of the OH radical-initiated reaction of naphthalene (13), and alkyl homologues of 2-formylcinnamaldehyde appear to be major products of the corresponding reactions of the C1- and C2alkylnaphthalenes (17), there is a need to quantitatively understand the formation yield of 2-formylcinnamaldehyde from the OH + naphthalene reaction and its photolysis rate and OH radical reaction rate constant in order to be able to predict the atmospheric concentrations of 2-formylcinnamaldehyde, and similarly for its alkyl homologues. Wang et al. (17) presented a time-concentration profile, using in situ atmospheric pressure ionization mass spectrometry (API-MS) analyses, for 2-formylcinnamaldehyde during an OH radical-initiated reaction of naphthalene. The modeled fit to the experimental data indicated that 2-formylcinnamaldehyde is highly reactive and that photolysis is rapid, with the (2-formylcinnamaldehyde photolysis rate/NO2 photolysis rate) ratio (kphot/J(NO2)) being ∼1.8 for the blacklamp irradiation used (17), with any wall loss rate of 2-formylcinnamaldehyde being included in the derived photolysis rate. In this work we have carried out a more extensive study using in situ API-MS to more accurately determine the photolysis rate of 2-formylcinnamaldehyde under blacklamp irradiation. Because 2-formylcinnamaldehyde is not commercially available, it was generated in situ from the OH radical-initiated reaction of naphthalene and its time-
FIGURE 1. Structures of phthaldialdehyde and 2-formylcinnamaldehyde. VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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concentration profile monitored in order to obtain kinetic information on its loss processes (18). The resulting ratio of kphot/J(NO2) was then used to re-evaluate the 2-formylcinnamaldehyde formation yield from the Sasaki et al. (13) data.
Experimental Section All experiments were carried out at 296 ( 2 K and 735 Torr total pressure of dry purified air in a 7000 L Teflon chamber interfaced to a PE SCIEX API III MS/MS direct air sampling atmospheric pressure ionization tandem mass spectrometer (API-MS). The chamber is equipped with two parallel banks of blacklamps for irradiation and is fitted with a Teflon-coated fan to ensure rapid mixing of reactants during their introduction into the chamber. OH radicals were generated by the photolysis of methyl nitrite (CH3ONO) in air at wavelengths >300 nm, and NO was added to the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals (13, 16, 17). For four of the six experiments, the initial reactant concentrations (molecule cm-3) were CH3ONO and NO, 2.4 × 1013 each; and naphthalene, ∼5 × 1012. In order to vary the ratio of (OH radical concentration/light intensity), one experiment had initial CH3ONO and NO concentrations of 2.4 × 1013 and 4.8 × 1013 molecule cm-3, respectively, and another had initial CH3ONO and NO concentrations of 4.8 × 1013 and 2.4 × 1013 molecule cm-3, respectively. Irradiations were carried out at 10% or 20% of the maximum light intensity, corresponding to NO2 photolysis rates, J(NO2), of 1.22 × 10-3 s-1 and 2.40 × 10-3 s-1, respectively, for 21-39 min or 19-27 min, respectively, resulting in 49-70% consumption of the initially present naphthalene during the experiments. The naphthalene concentration was measured before and after (two to three replicates each) the single irradiation in each experiment by gas chromatography with flame ionization detection (GC-FID). Gas samples of 100 cm3 volume were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent thermal desorption at ∼205 °C onto a 30 m DB-5 megabore column held at 0 °C and then temperature programmed to 200 °C at 8 °C min-1. Based on replicate analyses in the dark, the GC-FID measurement uncertainties were e3%. Products of the reaction of OH radicals with naphthalene were monitored during the experiments using in situ APIMS analyses in which the chamber contents were sampled through a 25 mm diameter × 75 cm length Pyrex tube at ∼20 L min-1 directly into the API mass spectrometer source. The operation of the API-MS in the MS (scanning) mode has been described previously (13, 17). The positive ion mode was used in this work, with protonated water hydrates (H3O+(H2O)n) generated by the corona discharge in the chamber diluent air being responsible for the protonation of analytes. During each experiment, after prereaction analysis of naphthalene by GC-FID, API-MS spectra were taken every ∼3 min, with each API-MS spectrum consisting of 10 scans requiring 160 s to acquire. After three API-MS spectra of the prereaction mixture had been obtained, the lights were turned on to initiate reaction, with continuing API-MS analyses during the 19-39 min irradiation period. After the lights were turned off, postreaction analyses of naphthalene were conducted and additional API-MS analyses of the reacted mixture were obtained in the dark for 9-15 min, thereby allowing the postreaction dark decays of the product ion peaks to be determined.
Results and Discussion As in the Sasaki et al. (13) and Wang et al. (17) studies, the API-MS spectra of irradiated CH3ONO-NO-naphthaleneair mixtures showed the presence of dominant ion peaks at 135 and 161 u. The 135 u ion peak is attributed to protonated 1350
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FIGURE 2. Plots of signal intensity of the 135 u (0) and 161 u (O) ion peaks attributed to protonated phthaldialdehyde and 2-formylcinnamaldehyde, respectively, during irradiation of a CH3ONO (2.4 × 1013 molecule cm-3)-NO (2.4 × 1013 molecule cm-3)-naphthalene (5 × 1012 molecule cm-3)-air mixture at 10% of the maximum light intensity (corresponding to J(NO2) ) 1.22 × 10-3 s-1). Irradiation time ) 1260 s, ln([naphthalene]t0/ [naphthalene]final) ) 0.674. The lines show calculated fits to the m/z ) 161 ion signals using eq II with values of k2/k1[OH] ) 4, 5, and 6. phthaldialdehyde. Since Sasaki et al. (13) showed, using combined gas chromatography-mass spectrometry, that Z+ E-2-formylcinnamaldehyde accounts for >90% of the four molecular weight 160 products observed, the 161 u ion peak is attributed to protonated 2-formylcinnamaldehyde (see Figure 1 for structures). As also observed by Wang et al. (17), in all experiments carried out here, after turning on the lights the m/z 161 ion signal attributed to 2-formylcinnamaldehyde increased rapidly in intensity to a maximum and then decreased more slowly, as shown for a representative experiment in Figure 2. In contrast, the intensity of the m/z 135 signal attributed to phthaldialdehyde increased throughout the irradiation period (Figure 2), except for one experiment where it attained a peak just prior to the end of the irradiation. The initial lag in the formation of phthaldialdehyde (Figure 2) was observed in all six experiments, indicating some secondary formation of phthaldialdehyde. It appears that phthaldialdehyde is both a first- and second-generation product of the OH radical reaction with naphthalene (13, 17, 19), with formation from 2-formylcinnamaldehyde being the likely second-generation pathway (13, 17). We therefore focus on the time-behavior of the m/z 161 ion peak attributed almost entirely to Z- plus E-2-formylcinnamaldehyde (13). The API-MS analyses cannot distinguish between the Z- and E-isomers, and while Z-/Ephotoisomerization of 2-formylcinnamaldehyde appears to be important (13) this was of no consequence here since we are interested in the photolytic removal rate of 2-formylcinnamaldehyde (Z + E). The reactions forming and removing 2-formylcinnamaldehyde are expected to be naphthalene + OH f R 2-formylcinnamaldehyde 2-formylcinnamaldehyde + OH f products 2-formylcinnamaldehyde + hν f products 2-formylcinnamaldehyde f wall
(R1) (R2a) (R2b) (R2c)
where R is the yield of 2-formylcinnamaldehyde from reaction 1 and k1, kOH, kphot, and kw are the rate constants for reaction 1 and reactions 2a through 2c, respectively. Assuming a
TABLE 1. Experimental Conditions and Results for the Experiments Carried Out, Where k1 Is the Rate Constant for the Reaction of OH Radicals with Naphthalene and k2 Is the Removal Rate of 2-Formylcinnamaldehydea 103 × J(NO2) (s-1)
irradiation time (s)
104 × k1[OH] (s-1)
k2/k1[OH]b
103 × k2 (s-1)
103 × (kOH[OH] + kphot) (s-1)
kphot/J(NO2)c
1.22 1.22 2.40 2.40 2.40 1.22
1260 2340 1440 1620e 1140 1440f
5.35 4.43 8.40 5.27 8.14 7.73
5 ( 1d 6(1 5(1 7.5 ( 1.5 5(1 4.75 ( 0.75
2.68 ( 0.54 2.66 ( 0.45 4.20 ( 0.84 3.95 ( 0.80 4.07 ( 0.82 3.67 ( 0.58
2.43 ( 0.54 2.41 ( 0.45 3.95 ( 0.84 3.70 ( 0.80 3.82 ( 0.82 3.42 ( 0.58
0.98 ( 0.44 1.14 ( 0.37 0.84 ( 0.35 1.04 ( 0.33 0.81 ( 0.34 1.35 ( 0.48
a Initial CH3ONO and NO concentrations were 2.4 × 1013 molecule cm-3 each, unless noted otherwise. b Estimated overall uncertainty based on fits of predicted versus experimental data (see Figure 2 for example). c Calculated using kOH/k1 ) 2.3 (see text). d Data and fits shown in Figure 2. e Initial CH3ONO and NO concentrations of 2.4 × 1013 and 4.8 × 1013 molecule cm-3, respectively. f Initial CH3ONO and NO concentrations of 4.8 × 1013 and 2.4 × 1013 molecule cm-3, respectively.
constant OH radical concentration during the CH3ONONO-naphthalene-air irradiation, then (18): [2-formylcinnamaldehyde]t ) R[naphthalene]t0k1[OH] (k2 - k1[OH])
(e-k1[OH]t - e-k2t) (I)
where [2-formylcinnamaldehyde]t is the 2-formylcinnamaldehyde concentration at time t, [naphthalene]t0 is the initial naphthalene concentration, k2 ) (kOH[OH] + kphot + kw), and [OH] is the OH radical concentration. The photolysis rate of CH3ONO with blacklamp irradiation is 0.244 J(NO2), (20) and hence the CH3ONO lifetimes were 56 min at J(NO2) ) 1.22 × 10-3 s-1 and 28 min at J(NO2) ) 2.40 × 10-3 s-1. While the OH radical concentrations decrease with irradiation time, the average OH radical concentrations were not significantly different from the initial concentrations in our experiments, being calculated to have been 28% lower than the initial OH radical concentrations for a 39 min irradiation at 10% light intensity and 35% lower than the initial OH radical concentrations for a 27 min irradiation at 20% light intensity. These estimates agree with OH radical concentrations derived from organic decay rates in irradiated CH3ONO-NOorganic-air mixtures, with the Sasaki et al. (13) naphthalene decays showing an average OH radical concentration over 27 min irradiation at 20% light intensity (similar to that used here) 30-35% lower than the initial value. Hence to a good approximation, eq I is valid for the conditions employed here. Equation I can be simplified (18) to [2–formylcinnamaldehyde]t ) A(e-x - e-Bx)
(II)
where A ) R[naphthalene]t0k1[OH]/(k2 - k1[OH]), B ) k2/ k1[OH], and x ) ln([naphthalene]to/[naphthalene]t). The value of ln([naphthalene]to/[naphthalene]t) at which the 2-formylcinnamaldehyde concentration is a maximum, [2-formylcinnamaldehyde]max, depends only on the ratio k2/ k1[OH], and is given (18) by ln(k2/k1[OH])/[(k2/k1[OH]) - 1] ) lnB/(B - 1). Measurement of the 2-formylcinnamaldehyde concentration as a function of the extent of reaction during each OH radical-initiated reaction of naphthalene therefore allows the ratio k2/k1[OH] to be determined for that reaction. A representative time-concentration profile of 2-formylcinnamaldehyde, as monitored using the 161 u ion peak signal intensity from the API-MS analyses, is shown in Figure 2. The light intensity, in terms of the NO2 photolysis rate J(NO2), and measured naphthalene reaction rate k1[OH] (obtained from ln([naphthalene]initial/[naphthalene]final)/irradiation time) for each experiment are listed in Table 1. In all six experiments, the 2-formylcinnamaldehyde signal increased rapidly to a maximum and then decayed at a rate similar to that of the naphthalene reaction rate, as in Figure 2. Ratios of k2/ k1[OH] were obtained from visual fits of eq II to the
FIGURE 3. Plot of (kphot + kOH[OH])/J(NO2) against k1[OH]/J(NO2). The dashed line is the unweighted least-squares fit, with a slope of b ) 3.2 ( 1.1 and an intercept of a ) 0.66 ( 0.46, where the indicated errors are two least-squares standard deviations. The solid line is a fit with b ) 2.3 (21, 22) and a ) 1.0 (see text). experimental data, using (k2/k1[OH]) ratios derived from values of ln([naphthalene]t0/[naphthalene]t) at which the 2-formylcinnamaldehyde concentration was a maximum as an initial input. Figure 2 shows fits of eq II to the experimental data for one experiment for three values of k2/k1[OH], and the resulting best-fit ratios k2/k1[OH] are given in Table 1 together with the estimated uncertainties. The resulting values of k2 ) (kOH[OH] + kphot + kw) are also given in Table 1. For the six experiments, the dark decays rates of 2-formylcinnamaldehyde derived from the API-MS data after the lights were turned off were in the range (4.7 ( 15.5) × 10-5 s-1 to (4.5 ( 1.5) × 10-4 s-1 (errors are two standard deviations), with an average of 2.5 × 10-4 s-1 which was used for all experiments to obtain (kOH[OH] + kphot). Since kphot+kOH[OH] ) aJ(NO2) + bk1[OH]
(III)
where a ) kphot/J(NO2) and b ) kOH/k1, then a plot of (kOH[OH] + kphot)/J(NO2) against k1[OH]/J(NO2), noting that k1[OH] is the naphthalene reaction rate, should have an intercept of a and a slope of b. Such a plot is shown in Figure 3, with the positive slope and nonzero intercept showing that 2-formylcinnamaldehyde does indeed react with OH radicals and undergo photolysis. The dashed line in Figure 3 is the unweighted least-squares fit, with a ) 0.66 ( 0.46 and b ) 3.2 ( 1.1 (errors are two least-squares standard deviations). Since the uncertainties associated with each data point in Figure 3 are large (∼(20%), an alternative approach is to use VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Plots of the amounts of 2-formylcinnamaldehyde (Z + E) formed, corrected for photolysis and reaction with OH radicals, against the amounts of naphthalene reacted with OH radicals. The measured concentrations are from Sasaki et al. (13), with irradiations carried out at a light intensity corresponding to J(NO2) ) 2.3 × 10-3 s-1. O, b: 2-Formylcinnamaldehyde data corrected as done by Sasaki et al. (13) with kphot ) 7.5 × 10-4 s-1 and kOH/k1 ) b ) 1.9 (kOH ) 4.1 × 10-11 cm3 molecule-1 s-1 and k1 ) 2.16 × 10-11 cm3 molecule-1 s-1). 4, 2: 2-Formylcinnamaldehyde data corrected using kphot ) 2.3 × 10-3 s-1 and kOH/k1 ) b ) 2.3 (kOH ) 5.3 × 10-11 cm3 molecule-1 s-1 and k1 ) 2.30 × 10-11 cm3 molecule-1 s-1); data displaced vertically by 1.0 × 1012 molecule cm-3 for clarity. b, 2: Average naphthalene reaction rate (k1[OH]) during the irradiation was 2.28 × 10-3 s-1, O, 4: average naphthalene reaction rate during the irradiations was 1.20 × 10-2 s-1. an estimated room temperature rate constant for the reaction of OH radicals with Z-2-formylcinnamaldehyde of kOH ) 5.3 × 10-11 cm3 molecule-1 s-1 (21), which combined with k1 ) 2.30 × 10-11 cm3 molecule-1 s-1 (22) leads to b ) 2.3. Using this value of b, values of kphot and kphot/J(NO2) can be calculated for each experiment (Table 1), with an average of kphot/J(NO2) ) a ) 1.03 ( 0.40 (simple average, two standard deviations) or 1.00 ( 0.16 (weighted average and associated uncertainty). The solid line in Figure 3 is calculated with a ) 1.0 and b ) 2.3, showing a good fit to the experimental data within the uncertainties. Clearly, kphot/J(NO2) ∼1.0 and the photolysis rate of 2-formylcinnamaldehyde is similar to that of NO2, at least for blacklamp irradiation. Assuming this ratio is also applicable to atmospheric conditions, then in the atmosphere photolysis of 2-formylcinnamaldehyde will be the dominant loss process, with a lifetime of 2-formylcinnamaldehyde approximately that of NO2 and hence ∼120 s at a solar zenith angle of 30° (23). The values of kOH and kphot/J(NO2) derived here, which are not independent of one another because we measured (kOH[OH] + kphot + kw), can then be used to re-evaluate the 2-formylcinnamaldehyde formation yield from the OH radical-initiated reaction of naphthalene using the Sasaki et al. (13) data. Sasaki et al. (13) carried out their experiments in a Teflon chamber similar to that used here, with blacklamp irradiation at a light intensity corresponding to J(NO2) ∼ 2.3 × 10-3 s-1. To correct for OH radical reaction and photolysis of 2-formylcinnamaldehyde during their experiments, Sasaki et al. (13) used kOH ) 4.1 × 10-11 cm3 molecule-1 s-1, k1 ) 2.16 × 10-11 cm3 molecule-1 s-1, and kphot ) 7.5 × 10-4 s-1. A plot of the amounts of 2-formylcinnamaldehyde formed, corrected for secondary reactions using these parameters, against the amount of naphthalene reacted is shown in Figure 4, leading to the 2-formylcinnamaldehyde formation yield 1352
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of 35% reported by Sasaki et al. (13). A similar plot, but using kOH ) 5.3 × 10-11 cm3 molecule-1 s-1, k1 ) 2.30 × 10-11 cm3 molecule-1 s-1, and kphot ) 2.3 × 10-3 s-1 (i.e., a ) 1.0 and b ) 2.3) from the present work is also shown in Figure 4, with a 2-formylcinnamaldehyde formation yield of 56 ( 5%, where the error is two least-squares standard deviations. With a ) 0.66 and b ) 3.2 (corresponding to the dashed line in Figure 3), the resulting 2-formylcinnamaldehyde formation yield becomes 61 ( 6%. Taking into account a ( 50% estimated overall uncertainty in a (i.e., in kphot) while maintaining b ) 2.3 leads to a 2-formylcinnamaldehyde formation yield of +15 %. Combined with the other products observed and 56-10 quantified from the OH radical reaction with naphthalene (glyoxal plus phthaldialdehyde, 5 ( 1% (19); 1,4-naphthoquinone, 1.0 ( 0.3% (13); two other molecular weight 160 products, 1.4 ( 0.3% and 1.5 ( 0.3% (13); 1-naphthol, 2.9 ( 1.5% (13); 2-naphthol, 3.8 ( 1.1% (13); 1-nitronaphthalene, 0.35% (16); 2-nitronaphthalene, 0.60% (16); 1-hydroxy-2nitronaphthalene, 1.1 ( 1.1% (13); molecular weight 174 product, ∼5% (13) and molecular weight 176 product, 13 ( 3% (13), we can account for ∼92 ( 15% and hence essentially all of the reaction products. Clearly, given the high formation yield of 2-formylcinnamaldehyde and its short photolysis lifetime, product and mechanistic studies of the photolysis of 2-formylcinnamaldehyde are needed. While it is possible that the OH radical-initiated reaction of 2-formylcinnamaldehyde, which is important in laboratory studies of OH + naphthalene but not in ambient air, leads in part to the formation of phthaldialdehyde + glyoxal, the photolysis products are as yet unknown but may be mainly radical species. We can use the formation yield for 2-formylcinnamaldehyde from reaction 1 of R ) 0.56 and the values of kphot and kOH derived here to estimate the concentrations of 2-formylcinnamaldehyde in the atmosphere. Using a 12-h average daytime OH radical concentration of 2.0 × 106 molecule cm-3 (24) and a 12-h average daytime NO2 photolysis rate of 5.2 × 10-3 s-1 (25), photolysis of 2-formylcinnamaldehyde is calculated to dominate over its reaction with OH radicals by a factor of ∼50. Hence only reactions 1 and 2b need to be considered. Since photolysis of 2-formylcinnamaldehyde is rapid, 2-formylcinnamaldehyde should be at steady-state, with: [2-formylcinnamaldehyde] ) Rk1[OH][naphthalene]/kphot (IV) Hence [2-formylcinnamaldehyde] ∼ 5 × 10-3 [naphthalene]. With recent ambient daytime naphthalene concentrations measured in Southern California (5, 26) of 127-1211 ng m-3 in downtown Los Angeles and 21.5-535 ng m-3 in Riverside, the predicted 2-formylcinnamaldehyde concentrations are in the range 0.1-6 ng m-3. Assuming similar formation yields and photolysis rates of alkyl-substituted 2-formylcinnamaldehydes formed from the C1- and C2alkylnaphthalenes would increase the ambient concentration of ∑(2-formylcinnamaldehyde + alkyl-substituted 2-formylcinnamaldehydes) by a factor of ∼2. These concentrations are comparable to the ambient concentrations of particlephase PAHs in Southern California and Mexico City (27, 28), and therefore the genotoxicity of 2-formycinnamaldehyde and its alkyl-homologues should be evaluated.
Acknowledgments The authors thank the U.S. Environmental Protection Agency (Grant R833752) for supporting this research. While this work has been supported by this Agency, the results and content of this publication do not necessarily reflect the views and opinion of the Agency. The authors also thank William D. Long for measurements of the NO2 photolysis rates and Dr.
William P. L. Carter for providing the photolysis rate ratio of methyl nitrite versus NO2 appropriate for blacklamp irradiation. N.N. thanks the University of California Toxic Substances Research & Teaching Program for partial support, and J.A. and R.A. thank the University of California Agricultural Experiment Station for partial salary support.
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