Environ. Sci. Technol. 2009, 43, 8554–8560
Yields of Glyoxal and Ring-Cleavage Co-Products from the OH Radical-Initiated Reactions of Naphthalene and Selected Alkylnaphthalenes 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, Environmental Toxicology Graduate Program, Department of Environmental Sciences, and Department of Chemistry, University of California, Riverside, California 92521
Received July 7, 2009. Revised manuscript received September 25, 2009. Accepted October 1, 2009.
Naphthalene and alkylnaphthalenes are the most abundant polycyclic aromatic hydrocarbons present in ambient air and are transformed mainly by chemical reaction with hydroxyl (OH) radicals during daylight hours. To better understand the reaction mechanisms, we have quantified glyoxal from the OH radical-initiated reactions of naphthalene, 1-methylnaphthalene, 1,4-dimethylnaphthalene, acenaphthene, and acenaphthylene as a function of the NO2 concentration and, for the naphthalene reaction, also in the absence of NO2. Glyoxal was formed as a first-generation product from the naphthalene, 1-methylnaphthalene, 1,4-dimethylnaphthalene, and acenaphthene reactions, and its yields were independent of the NO2 concentration over the ranges employed, being 5% in the presence of NO2 and 3% in the absence of NO2 from naphthalene; ∼3% from 1-methylnaphthalene; ∼2% from 1,4-dimethylnaphthalene; ∼10-15% from acenaphthene; and 300 nm) for irradiation. Chemicals were introduced into the chamber by either flushing a Pyrex bulb containing a measured amount of liquid chemical with N2 gas or by flowing N2 gas through a tube containing a solid chemical. In most experiments, OH radicals were generated from the photolysis of CH3ONO in the presence of added NO (9, 14). CH3ONO-NO-alkylnaphthalene-air irradiations were carried out at 20% of the maximum light intensity for 1-15 min, except for one naphthalene experiment where the total irradiation time was 39 min to determine whether glyoxal was also formed as a second-generation product. The initial reactant concentrations (molecules cm-3) were naphthalene,
∼2.4 × 1013, or 1-methylnaphthalene or 1,4-dimethylnaphthalene, ∼1.2 × 1013, or acenaphthene or acenaphthylene, ∼2.4 × 1012; and CH3ONO and NO, (0.48-12) × 1013 each [(0.24-12) × 1013 each for reactions involving naphthalene]. Experiments with naphthalene were also conducted in the absence of NOx, with OH radicals being generated from the dark O3 + 2,3-dimethyl-2-butene reaction (7). The initial 2,3-dimethyl-2-butene and naphthalene concentrations were ∼2.4 × 1013 molecules cm-3 each, and three or four 50 cm3 aliquots of O3 in O2 were introduced into the chamber resulting in up to 19-27% reaction of the initially present naphthalene. A control experiment showed that formation of glyoxal from the O3 + 2,3-dimethyl-2-butene reaction was negligible. The alkylnaphthalenes and their carbonyl products were analyzed by gas chromatography with flame ionization detection (GC-FID). The alkylnaphthalene concentrations were measured by collecting gas samples of 100 cm3 (for naphthalene, 1-methylnaphthalene, and 1,4-dimethylnaphthalene) or 1 L (for acenaphthene and acenaphthylene) volume onto Tenax-TA adsorbent, with subsequent thermal desorption at 250 °C onto a DB-5 megabore column temperature programmed from 0 to 250 °C at 8 °C min-1. Glyoxal was sampled onto a 65 µm polydimethylsiloxane/ divinylbenzene (PDMS/DVB) Solid phase MicroExtraction (SPME) fiber precoated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) (19), by exposing the fiber to the chamber contents for 5 min. The sample was then thermally desorbed at 270 °C onto a DB-1 or DB-5 megabore column, temperature programmed from 40 to 260 °C at 8 °C min-1. The amount of PFBHA on the fiber was in large excess of the sampled carbonyls. Glyoxal was observed only as its dioxime, and its concentrations were derived from the sum of the areas of the three dioxime (syn, syn, anti, anti, and syn, anti) peaks. Replicate measurements of glyoxal at concentrations encountered in the OH radical-initiated reactions typically agreed to within 10%. Glyoxal was calibrated using the OH + 3-methyl-2-butenal reaction, which produces glyoxal in 40 ( 4% yield (20), as an in situ source of glyoxal. Irradiations of CH3ONO-NO-3methyl-2-butenal-air mixtures were routinely conducted to determine the SPME/GC-FID response of glyoxal as its oxime derivatives, with initial CH3ONO, NO, and 3-methyl-2-butenal concentrations of ∼2.4 × 1013 molecules cm-3 each. Reactions of glyoxal with OH radicals were taken into account (20) to calculate the glyoxal concentrations present in the chamber. The alkylnaphthalenes and Cn-2-dicarbonyls were calibrated by introducing measured amounts into the chamber. The Cn-2-dicarbonyls phthaldialdehyde, 2-acetylbenzaldehyde, and 1,2-diacetylbenzene were sampled by exposing a 65 µm PDMS/DVB SPME fiber (without PFBHA coating) to the chamber contents for 20 min, with subsequent thermal desorption at 270 °C onto a DB-1701 column, initially held at 40 °C and then temperature programmed to 260 °C at 8 °C min-1. Replicate analyses at concentrations in the ranges encountered in the OH radical-initiated reactions agreed to within e5% for phthaldialdehyde and to within ∼10% for 2-acetylbenzaldehyde and 1,2-diacetylbenzene. NO concentrations and the initial NO2 concentrations were monitored during the experiments using a chemiluminescence NO-NO2-NOx analyzer. Because such analyzers read “NO2” as the sum of NO2, CH3ONO, and organic nitrates, the average NO2 concentrations during the experiments were calculated from ([NO] + [NO2]) ) constant, derived from detailed computer calculations of a CH3ONO-NO-NO2-toluene-air irradiation system (21). Chemicals. The chemicals used and their stated purities were naphthalene (98%), acenaphthene (99%), acenaphthylene (95%; GC-FID analyses of samples introduced into the chamber showed the presence of ∼3.5% acenaphthene VOL. 43, NO. 22, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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impurity), 1-methylnaphthalene (99%), 1,4-dimethylnaphthalene (95%), phthaldialdehyde (97%), 2-acetylbenzaldehyde (95%), 1,2-diacetylbenzene (99%), 3-pentanone (99+%), 3-methyl-2-butenal (97%), and 2,3-dimethyl-2-butene (98%), Aldrich; and NO (>99%), Matheson Gas Products. CH3ONO was prepared and stored as described previously (9, 14), and O3 was prepared using a Welsbach T-408 ozone generator.
Results Glyoxal was observed and quantified as a product from all of the alkylnaphthalenes studied, and Cn-2-dicarbonyls were quantified from the reactions of naphthalene, 1-methylnaphthalene, and 1,4-dimethylnaphthalene. Since glyoxal and the Cn-2-dicarbonyls also react with OH radicals, their measured concentrations were corrected for losses by secondary reaction (20), using rate constants for reactions with OH radicals of (in units of 10-11 cm3 molecule-1 s-1): naphthalene, 2.39 (5); acenaphthene, 8.0 (6); acenaphthylene, 12.4 (6); 1-methylnaphthalene, 4.09 (5); 1,4-dimethylnaphthalene, 5.79 (5); glyoxal, 1.1 (7); phthaldialdehyde, 2.3 (22); and 2-acetylbenzaldehyde, 1.7 (22). The OH radical reaction of 1,2-diacetylbenzene was negligible (30% con8556
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FIGURE 2. Plot of the amounts of glyoxal formed, corrected for its secondary reaction with OH radicals, against the amounts of naphthalene reacted, in an irradiation with initial CH3ONO and NO concentrations of 4.8 × 1013 molecules cm-3 each and with a total irradiation time of 39 min resulting in 60% consumption of the initial naphthalene. s, Fit of the data with glyoxal yields from the OH + naphthalene and OH + 2-formylcinnamaldehyde reactions (reactions 4 and 5a, respectively) of β ) 7.3% and γ ) 0%; ----, prediction with β ) 5% and γ ) 10%; and s s, prediction with β ) 5% and γ ) 20%. See text for additional details. The average OH radical concentration during the experiment was 1.6 × 107 molecules cm-3, based on the measured naphthalene concentrations during the course of the experiment, and after 39 min of irradiation 25% of the 2-formylcinnamaldehyde formed is calculated to have reacted with OH radicals (reaction 5a). The error bars of ( 10% are based on replicate analyses of glyoxal in the chamber in the dark, which agreed to within typically 10%.
FIGURE 3. Plot of glyoxal formation yields against the average NO2 concentration: O, in the presence of NOx; b, in the absence of added NOx. The error bars are two least-squares standard deviations and the solid line is the average glyoxal formation yield in the presence of NOx of β ) 5.2 ( 2.8% (see text). sumption of naphthalene were not used in deriving the glyoxal yield, β. Irradiations of CH3ONO-NO-naphthalene-air mixtures were carried out at the initial CH3ONO and NO concentrations and with the average NO2 concentrations and extents of reaction listed in Supporting Information (SI) Table S1. The glyoxal formation yields obtained from least-squares analyses of data such as those shown in Figure 2 are plotted against the average NO2 concentration in Figure 3, and are listed in SI Table S1. In addition, experiments were carried out in the absence of added NOx, by generating OH radicals from the dark O3 + 2,3-dimethyl-2-butene reaction, and the glyoxal formation yields from three replicate experiments are included in Figure 3 and SI Table S1. Since the O3 reaction
FIGURE 4. Plots of the measured amounts of glyoxal (O, 0, 4) and phthaldialdehyde (b, 9, 2) formed, corrected for secondary reaction with OH radicals (see text), against the amounts of naphthalene reacted, at average NO2 concentrations (molecules cm-3) of: O, b, 5.7 × 1013; 4, 2, 6.7 × 1012; and 0, 9, 4.2 × 1012. To minimize any effect from the formation of second-generation products, data at >30% reaction of the initial naphthalene are not included. Linear least-squares fits: s, glyoxal; ---, phthaldialdehyde. The error bars of ( 10% for glyoxal and ( 5% for phthaldialdehyde are based on replicate analyses of glyoxal and phthaldialdehyde in the chamber in the dark, which agreed to within typically 10% and to within e5%, respectively. rate constant for 2,3-dimethyl-2-butene (7) is a factor of ∼500 higher than that estimated for 2-formylcinnamaldehyde (using the rate constant for trans-cinnamaldehyde (24)), any formation of glyoxal from O3 + 2-formylcinnamaldehyde was negligible. The relatively large uncertainties in the individual measured glyoxal yields are due to the small amounts of glyoxal formed and of naphthalene reacted, the latter being especially important at the lower initial CH3ONO and NO concentrations needed to attain low NO2 concentrations. In the presence of NOx, there is no discernible trend in the glyoxal formation yield with NO2 concentration, with glyoxal yield (%) ) (5.3 ( 1.1) - {(0.10 ( 0.69) × 10-13 [NO2]}, where the indicated errors are two least-squares standard deviations and [NO2] is in molecules cm-3. The average glyoxal formation yields obtained in the presence and absence of NOx were 5.2 ( 2.8% and 2.8 ( 2.0%, respectively, where the indicated errors are two standard deviations. Although indistinguishable within the experimental uncertainties, a lower glyoxal yield in the absence of NOx would be expected if glyoxal is formed from an alkoxy radical intermediate, with formation of the alkoxy radical from ROO• + ROO• (absence of NOx) being less efficient than from ROO• + NO (presence of NOx), as previously observed for formation of biacetyl from the OH + o-xylene reaction (11). Since the reactions of OHnaphthalene adducts with NO2 and O2 are equally important at NO2 concentrations of ∼60 ppbV (∼1.4 × 1012 molecules cm-3) (9), our results suggest that the reactions of OHnaphthalene adducts with NO2 and O2 both form glyoxal in similar yield. Measurement of Glyoxal and Its Potential Coproducts. CH3ONO-NO-air irradiations of naphthalene, 1-methylnaphthalene, and 1,4-dimethylnaphthalene were carried out at initial CH3ONO and NO concentrations of 4.8 × 1012, 2.4 × 1013, and 1.2 × 1014 molecules cm-3 each. Plots of the amounts of glyoxal and its potential coproduct formed, corrected for their reactions with OH radicals, against the amounts of alkylnaphthalenes reacted at the various average NO2 concentrations are shown in Figures 4 and 5, and the results are given in Tables 1 and SI S2. To minimize any effect of secondary formation of glyoxal and its coproducts,
FIGURE 5. Plots of the measured amounts of glyoxal (O, 0, 4) and 2-acetylbenzaldehyde (from 1-methylnaphthalene) or 1,2-diacetylbenzene (from 1,4-dimethylnaphthalene) (b, 9, 2) formed, corrected for secondary reaction with OH radicals (see text), against the amounts of 1-methylnaphthalene (top) and 1,4-dimethylnaphthalene (bottom) reacted, at average NO2 concentrations (molecules cm-3) of: O, b, (3.6-4.0) × 1013; 4, 2, (0.98-1.0) × 1013; and 0, 9, (2.1-2.4) × 1012. To minimize any effects from the formation of second-generation products, data at >30% reaction of the initial 1-methylnaphthalene or 1,4-dimethylnaphthalene are omitted. Linear least-squares fits to the data at the lowest extents of reaction: s, glyoxal; ---, coproduct. The error bars of ( 10% for glyoxal, 2-acetylbenzaldehyde and 1,2-diacetylbenzene are based on replicate analyses of glyoxal, 2-acetylbenzaldehyde and 1,2-diacetylbenzene in the chamber in the dark, which agreed to within typically 10%. only data at 30% and all of the data obtained were therefore used in the analyses. As observed in the naphthalene reaction, within the experimental uncertainties the glyoxal formation yields were independent of the average NO2 concentration over the ranges used. The plots in Figure 6 show an increasing glyoxal yield with increasing extent of reaction, suggesting some second-generation formation of glyoxal. The first-generation glyoxal yields derived from the initial slopes of second-order regression fits to the data are also given in Table 1. The acenaphthylene introduced into the chamber contained ∼3.5% acenaphthene impurity, and hence a significant fraction of the small amount of glyoxal observed from the acenaphthylene reaction may have been formed from this acenaphthene impurity. FIGURE 6. Plots of the amounts of glyoxal formed, corrected for its secondary reaction with OH radicals, against the amounts of acenaphthene and acenaphthylene reacted at average NO2 concentrations (in molecules cm-3) of O, (3.1-5.6) × 1013; 4, (0.7-1.2) × 1013; and 0, (1.6-4.0) × 1012. s, Fit of the data using a first-order (linear) regression: ---, fit to the data using a second-order regression. The error bars of ( 10% are based on replicate analyses of glyoxal in the chamber in the dark, which agreed to within typically 10%. ylnaphthalene and 1,4-dimethylnaphthalene reactions, the initial yields of 2-acetylbenzaldehyde and 1,2-diacetylbenzene are a factor of ∼2 greater than the corresponding glyoxal yields, possibly due to problems with the SPME/GC-FID calibrations for 2-acetylbenzaldehyde and 1,2-diacetylbenzene. As noted above, phthaldialdehyde, 2-acetylbenzaldehyde and 1,2-diacetylbenzene were calibrated by introducing known amounts of the dicarbonyls from a Pyrex bulb into the chamber. Less than 100% efficiency of introduction into the chamber, an effect likely to be more pronounced for 2-acetylbenzaldehyde and 1,2-diacetylbenzene because of their lower volatilities, would lead to an underestimation of their SPME/GC-FID response factors and to an overestimation of their concentrations (and formation yields) during the reactions. Acenaphthene and Acenaphthylene. CH3ONO-NO-air irradiations of acenaphthene and acenaphthylene were conducted with initial CH3ONO and NO concentrations of 4.8 × 1012, 2.4 × 1013, and 1.2 × 1014 molecules cm-3 each. Plots of the amounts of glyoxal formed, corrected for reaction with OH radicals, against the amounts of acenaphthene and 8558
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Discussion Glyoxal is formed as a first-generation product from the OH radical-initiated reactions of naphthalene, 1-methylnaphthalene, 1,4-dimethylnaphthalene, and acenaphthene, but is formed in
