Environ. Sci. Technol. 1997, 31, 2252-2259
Reaction of Naphthalene and Its Derivatives with Hydroxyl Radicals in the Gas Phase NIGEL J. BUNCE,* LINA LIU, AND JIANG ZHU Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada DOUGLAS A. LANE Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada
Naphthalene is the most abundant polycyclic aromatic hydrocarbon (PAH) found in urban air. It is reactive in the atmosphere under ambient conditions, its chief reaction partner being the hydroxyl radical, OH•. In this work, the reactions of OH• with naphthalene, 1- and 2-naphthol, and 1- and 2-nitronaphthalene were studied in a 9.4 m3 smog chamber. Relative rates of reaction accorded well with previous studies and allowed estimates to be made of the atmospheric lifetimes of these compounds. Numerous oxidation products were identified, and mechanisms proposed for their formation were based on the further transformation of benzocyclohexadienyl radicals formed by addition of OH• to naphthalene. The naphthols and nitronaphthalenes were deduced not to be on the major reaction pathway to the more oxidized products. Because of the high reactivity of PAH in air, we suggest that priority be given to identifying and quantitating their reaction products, some of which may be relatively persistent air toxics.
Introduction Polycyclic aromatic hydrocarbons (PAH) are combustion byproducts, some of which are significant pollutants because of their carcinogenicity. Although forest fires account for about half of the total emissions of PAHs to the Canadian atmosphere (1), the public health concern about them is greatest in urban centers where the population density is higher. Transportation sources produce high levels of PAHs at busy intersections, vehicular subways, and tunnels (2), especially when vehicles are operated with rich fuel:air mixtures (3). In some locations, residential wood heating is a significant source of atmospheric PAHs (4, 5). Significant point sources of PAHs include aluminum smelters (6), coking ovens (7), and in the United States, the open burning of scrap automotive tires (8). At ambient temperatures, PAHs containing two and three rings are found predominantly in the gas phase; those containing six or more rings principally adsorb to particles; and four- and five-ring PAHs are found in both phases (9). Although gas-phase PAHs are difficult to trap on filters, so that their concentrations have often been underestimated, a study of air quality in Canadian cities showed naphthalene to be the most abundant PAH, with concentration often greater than those of all other PAHs combined (10). * Corresponding author phone: 519-824-4120, ext. 3962; fax: 519766-1449.
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Gas-phase PAHs are chemically reactive in the lower troposphere, yielding secondary pollutants that include strongly mutagenic nitro derivatives (11-13), which represent a potential public health concern. Chemical sinks for gasphase PAHs include direct photolysis and reaction with reactive tropospheric gases such as ozone, nitrate radicals, and hydroxyl radicals. Reaction with OH• is usually the most important sink reaction; reaction with NO3• (at night) or with ozone is important for penta-fused compounds such as acenaphthylene, while direct photolysis is an additional possibility for nitro-PAH derivatives (14). Relatively little is known about the oxidation chemistry of even the simplest PAHs (naphthalene, fluorene, phenanthrene, and anthracene) because of the variety and complexity of the products. The half-lives of gas-phase PAH are usually estimated (eq 1) from experimentally-measured second-order rate constants k for reaction with partners such as OH•, for which the secondorder rate constants are generally high (>10-11 cm3 molecule-1 s-1, 15).
t1/2 ) ln(2)/k[OH•]
(1)
Estimated values of [OH•] are usually used, because this quantity is difficult to measure experimentally. [OH•] ) 7.7 × 105 molecule cm-3 (16) (recently updated to 9.7 × 105 molecule cm-3; 17) is a geographical, diurnal, and seasonal average. The commonly cited figure 1.5 × 106 molecule cm-3 is the value from ref 16, adjusted to an average 12-h daylight period; it is therefore most applicable to spring and fall. This value affords half-life estimates of a few hours for many PAHs (14). In practice, the concentration of OH• depends strongly on solar intensity and, therefore, upon latitude, season, time of day, and also on the prevailing concentrations of ozone (a hydroxyl radical precursor) and NO2 (a hydroxyl radical sink). [OH•] can be modeled under specified conditions from which the tropospheric half-life for this reaction channel can be evaluated for a given oxidizable substrate. This method was used to estimate the half-life of naphthalene at the latitude of Toronto, Ontario (43.7° N) in midsummer (2-3 h) and midwinter (50 h) (18). For different days during July 1988, we estimated peak (midday) values of [OH•] as low as 1 × 106 and as high as 1.5 × 107 molecule cm-3 depending on climatic factors and the concentrations of ozone and NOx; experimental midday values of 9 × 105 molecule cm-3 were measured in wet weather at Pullman, WA, in early October 1989 (19). In the present work, we studied the rate and products of the reaction of naphthalene with hydroxyl radicals in air. Most of the work has been carried out using a 9.4 m3 smog chamber, using the photolysis of alkyl nitrites as the source of OH•. Numerous reaction products have been identified, and progress has been made in elucidating the reaction mechanism. Naphthalene is an attractive target for such research because of its high relative abundance in urban air and its simple molecular structure, which limits to some extent the number of reaction products; and a single reaction channel, reaction with OH•, dominates its atmospheric chemistry.
Experimental Section 3-L Chamber. Preliminary experiments were carried out using a 3-L flow-through vessel machined from solid Teflon and capable of disassembly for cleaning. Gas lines equipped with flow meters brought together the following three streams: naphthalene in air, equilibrated with solid naphthalene at fixed temperature; 15 ppmv NO in N2; and ethyl nitrite in N2, obtained by passing dry N2 gas over cooled liquid
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1997 American Chemical Society
ethyl nitrite. The concentrations of naphthalene and ethyl nitrite were determined in control experiments without photolysis for fixed temperatures of the naphthalene and ethyl nitrite reservoirs and fixed gas flow rates, as reported previously (20). A 4.6-W low-pressure mercury arc lamp was used for the photolysis of ethyl nitrite. Reaction products were collected in a cold trap on the exit side of the reactor. Smog Chamber. The smog chamber was a cylindrical, 9.4 m3, flow-mode design, constructed from Teflon film with 2 m diameter Teflon-coated aluminum end plates. A full description of the chamber has been published elsewhere (21). The lamps were a combination of Phillips fluorescent UV-A and Sylvania blacklights, having λmax ∼ 375 nm and J(NO2) ) 1.2 × 10-3 s-1 at λ > 350 nm with all the lamps on. This system was designed to optimize the production of hydroxyl radicals rather than to simulate sunlight. Ports were fitted on the end plates of the chamber to allow the entry and exit of gases, and the contents of the chamber were mixed by means of an interior fan. The chamber was flushed with pure dry air for 2 days prior to each experiment, with the lamps on at maximum intensity, in order to avoid contamination from previous experiments. An on-line HewlettPackard Model 5890 Series II gas chromatograph with a packed 8 ft × 1/8 in stainless steel packed column (OV17 on Chromosorb W) and flame ionization detector was used to quantitate the initial and unreacted amounts of reactant during kinetic runs. Samples were introduced into the GC by means of a 5-mL sampling loop. Off-line Analysis. Samples to be analyzed at the University of Guelph were condensed out of the chamber using a proportional flow meter and liquid nitrogen-cooled impinger; by recording the flow rate and collection time, the concentration of material in the cold trap could be related back to the concentration in the chamber. The contents of the trap were rinsed out using CH2Cl2 (for GC/MS analysis) or methanol (for HPLC analysis). The solvent (7-9 mL) was blown down with ultrapure nitrogen before analysis, and the residue was made up to 100 µL in a V-shaped vial before analysis. GC/MS was carried out using a Hewlett-Packard Series II GC equipped with 30 m J&W DB-5MS column with film thickness of 0.25 µm, interfaced to a Hewlett-Packard Model 5971 mass selective detector operating in the EI mode. The internal standard was 1-naphthylbutyrate, which was added in weighed amount to a weighed quantity of the coldtrapped product mixture. The standard was chosen to elute late in each run so as not to interfere with product peaks. Later, a Hewlett-Packard Model 5972 mass selective detector was used to allow off-line GC/MS, and the column was replaced by a capillary 25 m × 0.2 mm column coated with 5 µm of phenylmethylsilicone oil (5% phenyl). The following were typical temperature programs: DB-5MS column, 100 °C (11.0 min), then 3 °C min-1 to 170 °C, then 10 °C min-1 to 250 °C, then 20 °C min-1 to 290 °C; phenylmethylsilicone column, 100 °C for 3 min, then 5 °C min-1 to 250 °C, then hold for 5 min at 250 °C. HPLC was carried out using a Gilson HPLC equipped with binary pumps, Model 805 manometric module, Model 811B dynamic mixer, Waters Model 441 UV absorbance detector (254 nm), Waters Model 710 injector, and Varian Model 4290 integrator. The column was a Waters µBondpak reverse phase C18 of dimensions 3.9 mm × 300 mm. Solvents were HPLC grade, filtered through a 0.45-µm Nylon 66 membrane and degassed before use. Retention times were recorded relative to that of cinnamic acid. Early experiments were carried out using mixtures of water and methanol; the results in Table 2 were obtained using methanol (solvent A) and CH3CO2H/ CH3CO2NH4 buffer, pH 3.6 (solvent B) using the following program: 25% A with linear gradient to 1:1 A:B over 6 min; hold at 1:1 A:B for 10 min, then linear gradient to 100% A over 14 min.
LC/MS. A Hewlett-Packard Series II Model 1090 liquid chromatograph was interfaced to a VG Quattro II mass selective detector operating with MassLynx software. The mobile phase was a gradient of methanol and acetic acid/ ammonium acetate buffer, pH 3.6, and was used for both positive and negative ion electrospray LC/MS. Reagents. HPLC grade methanol, dichloromethane, isooctane, spectranalyzed grade THF, ACS reagent grade anhydrous ethyl ether, 1,4-dioxane, tetrahydrofuran, isopropyl alcohol, acetic acid (99.8%), and sulfuric acid (Fisher) were used as received, except that THF was purified by keeping it overnight over potassium hydroxide pellets, followed by distillation in a flame-dried apparatus over sodium metal under nitrogen. Authentic standards and reagents for organic synthesis were obtained from Aldrich. 1,4-Naphthoquinone, benzoic acid, phthalide, phthalaldehyde, and 2,4-dinitro-1-naphthol were purified using sublimation and had purities >99% by GC/MS or HPLC. 2-Nitronaphthalene was recrystallized from ethanol, mp, 72-74 °C, lit. (22) 79 °C, purity >99.9% by GC/MS. Syntheses. Isopropyl Nitrite (23, 24). To a mixture of 30 mL of water and 40 mL (0.75 mol) of concentrated sulfuric acid in a 250-mL Erlemeyer flask, cooled to 0 °C, was added 115 mL (1.5 mol) of ACS grade (97%) isopropyl alcohol. The solution was allowed to warm to room temperature, transferred to an equalizing-pressure funnel with ground glass stopper, added dropwise over 2 h into a solution of sodium nitrite (114 g, 1.65 mol) in 500 mL of water in a three-neck flask, and cooled to -5 °C in an ice/salt bath. The mixture was stirred in the cold for a further 1 h, after which time the upper (yellow) layer was separated, dried over Na2SO4, and distilled (bp 39-40 °C). The product was kept, foil-wrapped, in the refrigerator. The synthesis of ethyl nitrite was similar, except that the product (bp 16-17 °C) was simply separated from the reaction mixture and used without distillation. 2,3-Epoxy-2,3-dihydro-1,4-naphthoquinone (25). Ten milliliters of 5.25% sodium hypochlorite (NaOCl) was added to a solution of 1.0 g of 1,4-naphthoquinone in 20 mL of 1,4dioxane in a water bath at 0 °C. After 3 min, the reaction was quenched by adding 35 mL of water. The crude product was filtered and recrystallized from ethanol, mp 132-133 °C (lit. (25) 134-136 °C), 99.9% pure by GC/MS; m/z (%): 74 (29), 76 (54), 77 (34), 89 (60), 105 (100), 146 (38), 173 (51), 174 (M+, 1); 1H NMR (CDCl3) 400 MHz: δ 7.895 (dd, J ) 6.0, 3.2 Hz, 2H), 7.762 (dd, J ) 6.0, 3.2 Hz, 2H), 4.012 (s, 2H). (E)-2-Formylcinnamaldehyde. 2-Chlorocarbonyl-transcinnamyl chloride was prepared from 2-carboxy-transcinnamic acid (2 g, 10 mmol) and PCl5 (4.5 g, 21 mmol) as reported by Elvidge and Jones (26). The mixture was heated at 100 °C until evolution of hydrogen chloride ceased, and 10 mL of petroleum ether (bp 80-100 °C) was added. The acid chloride crystallized over night and was dried in vacuo to give 0.93 g (4.5 mmol, 45%) of needle-shaped crystals that were 97% pure, as determined by GC/MS; mp 63-65 °C (lit. (27) 70 °C); GC/MS mass spectrum m/z (%): 228 (M+), 195 (33), 193 (98), 167 (34), 165 (100), 157 (37), 130 (24), 129 (26), 102 (50), 101 (41), 75 (25). The crude 2-chlorocarbonyl-trans-cinnamyl chloride was reduced to the aldehyde by the method of Herbert and Subba Rao (28). The acid chloride (0.93 g, 4.5 mmol) was dissolved in 5 mL of dried THF and placed in a flame-dried, N2-flushed 25-mL flask fitted with a stirrer and 10-mL syringe, and cooled to -78 °C. Lithium tri-tert-butoxyaluminohydride (9 mmol in 9 mL of dried THF) was added over a period of 15 min, avoiding any major rise in temperature. The mixture was allowed to warm to room temperature, and ice water (4 mL) was injected into the flask. After several extractions with ether and removal of the ether in vacuo, the product was purified by TLC (silica gel, using 1.5:1, ether:isooctane) to give 22 mg (14%) of 2-formylcinnamaldehyde (29), Rf ) 0.46, mp 57-58 °C; GC/MS m/z: 77 (23%), 103 (31), 131 (100), 160 (M+,
