Environ. Sci. Technol. 1904, 18, 603-607
mation of the nitriles is not known. One source may be the degradation of amino acids or nucleic acids (or their diagenesis products) under pyrolytic conditions. Other sources may be the reaction of the nucleophilic cyanide anion with aliphatic compounds containing a good leaving group such as a halogen, the addition of hydrogen cyanide to triple bonds (e.g., H C r C H HCN CH,-CHCN) (6))the addition of hydrogen cyanide to an activated double bond (i.e,, one containing an electron-withdrawing group), the conversion of a carboxylic acid or its salt to a nitrile, or the dehydration of amides (RCONH2--• RCN HzO) (7). Another possible mechanism is the dehydrogenation of amines which might also help explain the apparent lack of amines in the offgases. Isonitriles formed under the above conditions would probably isomerize to the corresponding nitriles at retort temperatures (6). The hydrogen cyanide itself may have been formed by a reaction of methane and ammonia in the offgas (CH4+ NHJ HCN + 3H2). To determine the appropriate mechanisms, it will be necessary to perform controlled studies in which the nitrogen content of the oil shales and the retort conditions can be varied systematically.
+
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+
-
Conclusions
(1) Five organic nitrogen species have been identified in the retort offgas of the in situ Rio Blanco Retorts 0 and 1: hydrogen cyanide, acetonitrile, acrylonitrile, propionitrile, and isobutyronitrile. All except hydrogen cyanide were identified in bomb samples from the aboveground Paraho retort. The apparent absence of hydrogen cyanide is probably due to lack of stability. Seven other nitrogen species have been tentatively identified at Rio Blanco Retort 1. (2) The low molecular weight non-ammonia nitrogen species appear to be predominantly nitriles. Low molecular weight aliphatic amines do not appear to be present. (3) The organic nitrogen species were present in amounts on the order of tens of ppm which comprise 1-270 of the
-
gas-phase ammonia nitrogen concentration in the offgas. (4) Most of the nitrogen species except hydrogen cyanide appear to be reasonably stable at least in a qualitative sense for a period of at least several months in a bomb sample. Acknowledgments
We are grateful to John Fruchter for helpful discussions, to Chuck Nelson and Sylvia Downey for logistical support, to Roy Hutson, Don Griffee, and the rest of the Rio Blanco crew, and to Bob Heistand and Paraho personnel for all their help and cooperation. Registry No. HCN, 74-90-8;CH3CN, 75-05-8;H,C-CHCN, 107-13-1; CH,CH&N, 107-12-0; (CH$)&Ii[CN, 78-82-0.
Literature Cited Sklarew, D. S.; Hayes, D. J.; Peterson, M. R.; Olsen, K. B.; Pearson, C. D. Environ. Sci. Technol., preceding paper in this issue. Rinaldi, C. M.; Delaney, J. F.; Hedley, W. H. 1981, EPA600/S7-81-021. Fruchter, J. S.; Wilkerson, C. L.; Sklarew, D. S.; Olsen, K. B.; Ondov, J. M. In “Oil Shale: The Environmental Challenges, Part 111”;Petersen, K. K., Ed.; Colorado School of Mines Press: Golden, CO, 1983; pp 139-164. Supelco, Inc., 1973, Bulletin 737B. Sklarew, D. S. Geochim. Cosmochim. Acta 1979, 43, 1949-1958. Friedrich, K.; Wallenfels, K. In “The Chemistry of the Cyano Group”; Rappoport, Z., Ed.; Interscience: London, 1970; pp 67-122. March, J. ”Advanced Organic Chemistry: Reactions, Mechanisms, and Structures”, McGraw-Hill: New York, 1968; pp 1-1098. Received for review July 5,1983. Revised manuscript received December 5,1983. Accepted January 25,1984. 7% project was supported by the US.Department of Energy, Office of Health and Environmental Research, under Contract DE-ACOG-76RLO 1830.
Fluorescence Spectroscopic Study of Reactions between Gaseous Ozone and Surface-Adsorbed Polycyclic Aromatic Hydrocarbons Chlng-Hsong Wu,“ Irvlng Salmeen, and Hlromi Nlkl Research Staff, Ford Motor Company, Dearborn, Michigan 48121
Kinetics of reactions between gaseous ozone (0,) and two polycyclic aromatic hydrocarbons (PAH), perylene and benzo[a]pyrene (BaP),adsorbed on fused silica plates were studied by observation of PAH fluorescence spectra as functions of PAH concentration, reaction time, and 0, concentration. Significant improvement of the fluorescence signal to scattered light ratio was achieved by using small-angle edge-on illumination configuration. In addition, the photoinduced reactions of PAH by excitation light was minimized. The PAH in aggregated states, as inferred from the excimer fluorescence, exhibited a much slower reaction with 0,than those in dispersed forms. With highly dispersed samples, the decays were exponential and the rates proportional to Oj concentrations. From these data, the second-order rate constants were determined with the corresponding half-lives of 132 and 2.6 min in 0.2 ppm of --O3-for_- perylene and BaP,.-respectively. - - -- - - I
Introduction
Despite the great interest in the atmospheric fate and chemical transformations of polycyclic aromatic hydro0013-936X/84/0918-0603$01.50/0
carbons (PAH) associated with airborne particles (1-41, there have been only a few papers describing the kinetics of the reaction between gaseous Ozone (0,) and surfaceadsorbed PAH (5-9). Furthermore, these papers reported very different estimates for the half-life of PAH exposed to 03. For example, in one of the first studies of PAH reactivity, Falk et al. (5) deposited several PAH on filter papers or on soot, exposed the PAH to various atmospheric conditions including a synthetic smog, and analyzed the PAH extractable from the substrates. They found that benzo[a]pyrene (BaP) was one of the most stable PAH studied, with a half-life of several hours at 30 ppm of oxidant (mainly 0,) as determined by the potassium iodide method. More recently, Pitts et al. (6, 7) reported that when BaP was deposited on glass-fiber filters and exposed to 0.2 ppm of O3 in the dark, approximately 50% ofthe BaP decomposed in about 1h and approximately 80% in 4 h. More detailed kinetic studies on 0,reactions with adsorbed PAH were carried out by Katz et al. (8,9). They investigated a series of PAH coated on glass surfaces and on thin-layer chromatography (TLC) plates. The con-
8 1984 American Chemical Society
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centration/time profiles were obtained by exposing the samples to several 0, concentrations for various time intervals and analyzing the unreacted PAH by solvent extraction. Katz et al. (8,9)found that BaP was one of the most reactive PAH with a half-life of 0.6 h in 0.2 ppm of O3in the dark. The wide range in the apparent half-lives of BaP could be due to differing experimental factors such as the substrate materials, manner of sample deposition, and possibly the analytical methods. In all previous studies, the PAH adsorbed on the substrates were analyzed by first extracting from substrates with organic solvents and quantifying the PAH in the extracts by various chromatographic methods or by mass sspectrometry. These analytical procedures may be subject to variability in sample preparation and in the efficiency of PAH recovery by solvent extraction. In addition, analysis requiring extraction is indirect and usually takes a long time. Some of the foregoing experimental uncertainties may be substantially reduced by using a direct analytical method with a fast response time, high sensitivity, and good selectivity for kinetic measurements. One technique with the potential for meeting these requirements is fluorescence spectroscopy. Many PAH are very efficient fluorophores; their emission and excitation spectra and, in some cases, there fluorescence lifetimes have been well characterized in solution and in solids (IO), and fluorescence has been extensively used for identifying and quantifying minute quantities of PAH (11-13). This paper describes an application of fluorescence spectroscopy to a study of the kinetics of the reactions between gaseous O3 and two PAH, perylene and BaP, adsorbed on fused silica plates. The adsorbed PAH were detected directly by observing their characteristic fluorescence spectra. The fluorescence signals were monitored continuously during the course of reactions, and the decays of fluorescencewere studied as functions of sample deposition, reaction time, and O3 concentration. Experimental Section Fluorescence Spectroscopy. PAH fluorescence was detected by using an SLM 8000 spectrofluorometer (SLM-Aminco)and a fluorescence cell which contained the sample plate as diagrammed in Figure 1. A single grating monochromator was used for emission and a double monochromator for excitation. Both monochromators were tunable from 200 to 900 nm and were equipped with aberration-corrected concave holographic gratings. The stray light rejection ratios for the excitation and emission monochromators were and lo4 nm-l, respectively. The detector was cooled photomultiplier (EM1 9659 QA) operated in the photon counting mode for low dark current and high gain stability. Data acquisition, processing, and storage were performed on a microcomputer (HP85) interfaced with the detectors and monochromator drives. The cylindrical fluorescence cell (4 cm diameter X 6 cm height) was fabricated from optical quality quartz for UV transmission. A removable stopper on the top of the reactor was used for suspending the sample plate. Tubes were provided for gas inlet and outlet. The inlet tube was extended to the bottom of the reactor to promote gas mixing. The reactor was mounted on a four-way ( x , y, z, and angular) adjustable base for fine tuning the position and the direction of illumination on the sample plate. Sample Preparation and Gas Handling. The rectangular optically polished fused silica plates (Suprasil 11; 5 X 1.3 X 0.16 cm) were used as a model substrate upon which the PAH were deposited. The cleanliness of the surface was very critical for achieving homogeneous sample deposition and for reducing scattered light and background 804
Environ. Sci. Technol., Voi. 18,
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Flgure 1. Schematic diagram of the spectrometer and the fluorescence cell. (1) Xenon arc lamp (450 W), (2) excltatlon monochromator, (3) module containing focusing and beam splitting optics, (4) beam splitter for reference slgnai, (5) fluorescence cell, (8) emission moncchromator, (7) emission photomultlpiler, (8) reference photomultlpiier, (9) auxlllary photomultiplier (not used here), (10) monochromator controller, (11) data acquisition, and (12) data processor. The fluorescence cell is shown enlarged and rotated by 180' to illustrate the position and direction of the excitation radlation and the portion of emlsslon collected.
fluorescence from the surface. Thus, the plates were cleaned before use by washing with a series of solvents, i.e., phosphate-free cleaning solution (RBS-pf, Pierce Chemical), distilled water, 2-propanol (Baker, reagent grade), distilled water, and occasionally 2% HF solution (Baker, reagent grade), and distilled water in an ultrasonic bath for 30-60 min each. The plates were air-dried at room temperature. After such treatment, the silica plates showed no trace of fluorescence over the range 300-700 nm, and the solvent spread uniformly over the surface when applied as a drop. Both perylene and BaP (Aldrich, gold label 99+ %) and the solvent, n-hexane (Bardick & Jackson Laboratories, UV grade), were used without further purification. Stock solutions of PAH in n-hexane (concentration between 1 and 10 pg/mL) were prepared freshly for every experiment. n-Hexane was used as the solvent because it has low surface tension and good surface wettability. Thin layers of PAH were prepared on the fused silica plates by depositing a known quantity (20-200 pL) of the stock solution with a microsyringe and spreading the solution evenly on the plate surfaces. The solvent was evaporated at the ambient conditions. The average surface coverage ranged from 10 to lo3 A2/molecule as estimated from the geometric surface area of the plate and total weight of PAH deposited. The homogeneity of the deposited layer was spot checked with a fluorescence microscope (Leitz Ortholux I1 with epilumination), and in most cases, uniform intensity was observed throughout the surface, indicating no significant formation of microclusters. Ozone up to 10 ppm concentration in air was generated by passing ultrapure air (Air Monitoring Lab) through a quartz tubing irradiated with a 10 in. long Pen-ray light (Ultraviolet Products). The volume of irradiation was adjusted to produce the desired ozone concentration. The air flow, as measured with a calibrated mass flowmeter (Teledyne Hastings-Raydist), was maintained at 210 mL/min at near atmospheric pressure and at room temperature. The ozone-containing air stream passed through a three-way valve to direct the flow either into or around
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IO
1 I
Well Dispersed Samples A E-----
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Before Ozone Exposure A f t e r Ozone Exposure
Aggregated Samples I
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Figure 2. Fluorescence spectra of perylene adsorbed on fused slllca plates: A (-), welldispersed sample before ozone reactlon; B (- -), after reaction for 10 mln at [03] = 10 ppm; C (-4,aggregated sample after reaction for 30 min at [O,] = 10 before ozone reaction; D PPm.
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e),
the fluorescence cell. The concentration was measured with a UV absorption ozone analyzer (Dasibi, Model 1003 AH) after a 10-fold dilution with pure air. Experimental Procedures. The freshly prepared sample plate was placed vertically at the center of the fluorescence cell. Proper alignment and orientation of the sample plate relative to the optical paths were essential for achieving the strongest fluorescence signal and for reducing the reflected light. The excitation light excited from the monochromator through a 2 mm wide slit and was focused with a 5 cm focal length lens to form a narrow beam with a cross section of -0.05 X 0.5 cm a t the edge of the sample plate. The alignment was performed by fine tuning the sample plate position with the four-way adjustable base of the fluorescence cell while the detector output was monitored at the fluorescence peak and at a valley on the short wavelength side of the fluorescence peak. The maximum ratio of peak to valley signals was observed when the sample plate was orientated nearly parallel ( 15') to the excitation beam, so that the exciting light was incident on the edge of the plate as shown in Figure 1. In this arrangement, the exciting light was totally internally reflected, the PAH fluorescence was excited by the surface evanescent wave (14), the reflected light was minimized, and the edge intercepted the entire beam. In addition, the PAH fluorescence decay in pure air attributable to photoinduced chemical reactions was reduced because the incident light was diffused over a large area by this configuration. Before each run, a fluorescence spectrum in the wavelength range 350-650 nm was recorded when the sample was in pure air. The intensity of a selected fluorescence band was then monitored as a function of time before and after the ozone was added. At the end of the run another complete spectrum was recorded to examine the base-line drift and to search for a new fluorescence features attributable to the reaction products. N
Results and Discussion Ozone-Perylene Reaction. (1) Fluorescence Spectra of Adsorbed Perylene. Perylene was studied first since this compound has a high fluorescenceyield (13). In Figure 2, curve A is a typical fluorescence spectrum of perylene well dispersed on a fused silica plate with the average surface coverage estimated to be 150 A2/molecule. This corresponds to about one-third of a monolayer on the bask of the molecular dimensions calculated from the bond
length data obtained by X-ray diffraction (15). The excitation wavelength at 410 nm was chosen because it corresonds to one of the strongest absorption peaks and is sufficiently separated from the main fluorescence band (442 nm) to further reduce interference due to the scattered light. Three major bands, i.e., 442,470, and 503 nm, are identical with those observed in dilute n-hexane solution, indicating no significant perturbation due to substrate interaction. The fluorescence intensities were found to be highly sensitive to the presence of oxygen (02)in air. For example, in a N2atmosphere, more than a 5-fold increase in intensity was observed. This is consistent with the previous observations (16, 17) that O2 is an efficient quencher of PAH fluorescence. Spectrum B was recorded after the same perylene sample corresponding to that of spectrum A had been exposed to 10 ppm of O3for 10 min. The fluorescence signals due to perylene disappeared almost completely, and the residual spectrum consisted mainly of background light from scattering and reflection. No spectral feature attributable to reaction products was observed. Curve C shows a fluorescence spectrum of perylene adsorbed on a sample plate with a surface concentration increased &fold. Compared with curve A, the intensities at 442, 470, and 503 nm (monomer emission) decreased, and a new broad fluorescence signal with a maximum near 565 nm was observed. This fluorescence spectrum may be attributed to the formation of excited dimers (excimers) via the Forster-Kasper mechanism (18), Le., by the combination of excited singlet molecules with those in the ground state. The formation of excimers is to be expected when the molecules are close together as in concentrated solutions, crystals, and aggregated films. Curve D is the spectrum of the sample corresponding to that of spectrum C after a 30-min exposure to 10 ppm of 0,. The portion of the fluorescence due to monomers had decreased substantially (70%) while the excimer fluorescence intensity decreased only slightly (15%). This indicates that aggregated perylene reacts much more slowly with 0,. Presumably, the PAH molecules at the surface of the aggregates were available to react first with the 03, and the oxidation products did not leave the surface but remained to form a diffusion barrier preventing the access of O3to the bulk. Thus, the observed reaction rate of the bulk molecules was reduced. (2) Kinetics. The kinetics of the reaction between gaseous O3and adsorbed perylene were studied by monitoring the fluorescence intensity of the monomeric perylene at 442 nm as functions of reaction time and 0, concentration. In preliminary experiments using higher concentration samples (50 A2/molecule or less), upon exposure to O3the fluorescence signal showed an initial fast decay followed by a slower decrease. As the samples became more dispersed such as the sample corresponding to spectrum A (Figure 2), the fast decay portion became predominant and the observed rate became reproducible from to run. The fast decay is attributed to the reaction of O3with surface perylene, and the slow process is due to the diffusion-limited reaction with bulk molecules. Figure 3 shows some fluorescence decay curves of typical well-dispersed perylene samples reacting with gaseous O3 at various concentrations. The photoinduced decay rates were noticeable but were slow as shown by the data for [O,] = 0. In the presence of O,, the decays were exponential over 80% of signal reduction. A plot of the exponential decay rates vs. ozone concentration up to 10 ppm for 1 2 independent measurements was constructed, and a linear relationship was observed. Envlron. Scl. Technol., Vol. 18, No. 8, 1984
605
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ABefore Ozone Exposure 8------After Ozone Exposure
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WAVELENGTH I N M I
Flgure 4. Fluorescencespectra of BaP adsorbed on fused silica plates: A (-), weildlspersed sample before ozone reaction; B (---), after reaction for 5 min at [O,] = 1.5 ppm; C (-. -), aggregated sample before ozone reaction; D (-), after reaction for 20 min at [O,] = 1.5
PPm.
These results can be expressed by the second-order rate equation -d[perylene] /dt = k[perylene] [O,]
(1)
where [perylene] = perylene deposition density (molecule/cm2), [O,] = gaseous ozone concentration (pprn), and k = rate constant of the ozone-perylene reaction (ppm-' min-'). Under the flow reactor conditions where [O,] is constant, eq 1 becomes -d In [perylenel/dt = k[031
(2)
This equation is consistent with the experimental observations, in that the decays of perylene are exponential at a given [O,] and that the first-order decay rates are proportional to [O,]. The rate constant is independent of the initial perylene concentration. Therefore, it is not subject to variability of sample preparation. A rate constant, k = 0.026 f 0.006 (217) ppm-' min-l at 298 K, was determined from the slope of the least-squares fitted straight line. The corresponding half-life [0.693/ (k[O,])] of the adsorbed perylene is equal to 132 rnin in 0.2 ppm of 03. Ozone-BaP Reaction. (1) Fluorescence spectra of adsorbed BaP. Figure 4 shows four fluorescence spectra (367-nm excitation) of BaP adsorbed on fused silica plates. The conditions of curves A-D were similar to those described for perylene in Figure 2 with the exception that curves B and D were taken after exposure to only 1.5 ppm 606
2 3 TIME ( m l n )
4
5
Figure 5. Fluorescence intensity decay of BaP as a function of time at various ozone concentrations.
Flgure 3. Fluorescence intensity decay of peryiene as a function of time at various ozone concentrations.
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Environ. Sci. Technoi., Voi. 18, No. 8, 1984
of O3 for 5 and 20 min, respectively. Three major fluorescence bands, namely, 403,430, and 455 nm, correspond to emission from monomeric BaP (12), and the broad band at 485 nm corresponds to the excimer emission. As was observed for perylene (Figure 2) these spectra show that the aggregated BaP reacted with ozone much more slowly than the well-dispersed (monomeric) one, In addition, the reaction rates of BaP were much faster than those for perylene. (2) Kinetics. The decay of the fluorescence intensity at 403 nm was studied for various ozone concentrations as shown in Figure 5. The photoinduced decay rate for BaP in ure air was about a factor of 5 faster than that of perylene for the same excitation monochromator slib. To reduce this rate further, the excitation slits were reduced to half of those used for the perylene study. Upon exposure to ozone, the fluorescence intensity again showed a fast initial decay rate, which increased with the increase of ozone concentration, and a slower decay rate independent of ozone concentration in the later stages. The slow decay processes became dominant after the fluorescence intensity was reduced by about 70% from the initial value, which was sooner than that observed for perylene. Also, the fluorescence spectra recorded after 0,reactions (see curve B in Figure 4) showed some residual bands near 405 and 433 nm which disappeared slowly during prolonged O3 exposure. The source of these fluorescence bands has not yet been identified, but they may be due to either the BaP trapped inside the clusters or the intermediate reaction product. A plot of the initial logarithmic decay rates vs. O3 concentrations show a linear relationship from 0 to 1.5 ppm of 0,concentration. From the slope of the least-squares fitted straight line, a rate constant of 1.3 f 0.20 (2a) ppm-' min-l was determined. The corresponding half-life of BaP adsorbed on fused silica plate and exposed to 0.2 ppm of 0,was 2.6 min, which is about 50 times shorter than that of perylene. The half-life of BaP determined here is at least 10 times shorter than the values reported in the literature (4-9). The discrepancy could be partially attributed to the difference in extent of sample aggregation and the time resolution of the analytical methods. Among the previous studies of BaP-O3 reactions, the work by Lane and Katz (8) is by far the most extensive. In their study the shortest interval over which the sample was exposed to ozone was estimated to be several minutes during which substantial amounts (-60%) of BaP had already reacted and the fast initial decay portion might not have been observed. Most recently, Cape and Kalkwarf (19)reported on studies of gaseous ozone reactions with some PAH coated
on fly ash and on glass surfaces using solvent extraction and HPLC for PAH analysis. The ratio of the rate constant of BaP to that of perylene was calculated to be 20 which is in reasonable agreement with the value of 50 determined in the present study. Whle details of their study were not reported, their data, nevertheless, indicated a high reactivity of BaP despite the difference in substrates and analytical methods used. An attempt was made to interpret the rate constant, k , in terms of the reaction probability when the gaseous O3 molecules collide with the adsorbed PAH molecules. The collision frequency, N , of the gaseous molecules with a concentration, C (molecule/cm3), and an arithmetic average velocity, v, (cm/s), striking a unit area can be approximated by the equation (20)
N = Cv,/4
(3)
Substituting v, = [8RoT/(nM,)]1/2 into eq 3, one obtains
N = 6280OC/M,1l2
(4)
at 298 K , where Ro is the gas constant, T is the absolute temperature (K), and M, is the molecular weight. The reaction probability, P, for the adsorbed molecule with the collisional cross section, A , can be calculated by using the equation
P = k/(NA) (5) Substituting the values of M , = 48, C = 2.45 X 1013molecule/cm3 for l pprn of 03,k = 0.026 ppm-l min-l or 4.3 X lo4 ppm-’ s-l and A = cm2 into eq 4 and 5, one obtains P = 1.9 X lo-’ for perylene. Similarly, the reaction for BaP. probability, P, is 1.04 X Conclusions Fluorescence spectroscopy has been used to study the kinetics of heterogeneous reactions between gaseous ozone and PAH adsorbed on fused silica plates. The technique permits direct PAH measurements with a fast response time and reasonable selectivity and high sensitivity. The extent of aggregation of PAH adsorption on surfaces can be inferred from the excimer emission. Gaseous O3reacted much slower with bulk PAH molecules than with those on the surface. With well-dispersed samples, only monomer fluorescence was observed. The monomeric fluorescence intensities decay exponentially over more than 70% of the reaction, and the rates increase linearly with O3 concentrations. The O3 reaction rate constants for perylene and BaP were determined. The half-life of BaP in O3 determined in this study is even lower than the lowest value of those reported previously. Part of the reasons can be attributed to the well-dispersed PAH samples and the analytical technique with high time resolution used here. The atmospheric chemical lifetime of BaP is expected to vary, widely depending on various factors which remain to be determined. In general, the PAH associated with airborne particulate matter may occur in a wide range of aggregated states and in association with a variety of substrates. These PAH may encounter various chemical reactions including photooxidation, reactions with ozone, and reactions with other
reactive constituents in the atmosphere. To quantitatively assess the chemical lifetimes of PAH in the atmosphere, more kinetic studies are needed on the reactions of PAH deposited in various degrees of aggregation on different substrates and exposed to solar radiation and to reactive atmospheric compounds. The application of the fluorescence spectroscopic technique to the reactions of other PAH with various gaseous constituents in the atmosphere is under way. Registry No. BaP, 50-32-8;Os,10028-15-6; perylene, 198-55-0; silica, 7631-86-9.
Literature Cited Biologic Effects of Atmospheric Pollutants “Particulate Polycyclic Organic Matters”; National Academy of Sciences: Washington, DC, 1972. Fox, M. A,; Olive, S. Science (Washington,D.C.) 1979,205, 582-583.
Butler, J. D.; Crossley, P. Atmos. Environ. 1981,15,91-94. Tebbens, B. D.; Mukai, M.; Thomas, J. F. Am. Ind. Hyg. ASSOC. J. 1971, 32, 365-372. Falk, H. L.; Markul, I.; Kotin, P. AMA Arch. Ind. Health 1956,13, 13-17. Pitts, J. N., Jr.; Van Cauwenberghe, K.; Grosjean, D.; Schmid, J. P.; Fitz, D. R.; Belser, W. L.; Knudson, G. P.; Hynds, P. M. Science (Washington, D.C.) 1978, 202, 515-519. Pitts, J. N., Jr.; Lokensgard, D. M.; Ripley, P. S.; Van Cauwenberghe, K.; Van Vaeeh, L.; Schaffer, S. D.; Thilly, A. J.; Pelser, W. L., Jr. Science (Washington,D.C.) 1980, 210,1347-1349. Lane, D. A.; Katz, M. Adv. Environ. Sci. Technol. 1977, 8, 137-154. Katz, M.; Chan, C.; Tosine, B.; Sakuma, T. Polynucl. Aromat. Hydrocarbons,Int. Symp. Chem. Bio1.-Carcinog. Mutagen. 1979, 171-189. Birks, J. B. “Photophysics of Aromatic Molecules”; Wiley-Interscience: New York, 1969. Lee, M. L.; Novotny, M. V.; Bartle, K. D. “Analytical Chemistry of Polycyclic Aromatic Hydrocarbons”;Academic Press: New York, 1981. Sawicki, E.; Hauser, T. R.; Stanley, T. W. J. Air Pollut. 1960,2,253-272. Berlman, I. B. “Handbook of Fluorescence Spectra of Aromatic Molecules”,2nd ed.; Academic Press: New York, 1971. Thompson, N. L.; Burghardt, T. P.; Axelrod, D. Biophys. J. 1981, 33, 435-454. Camerman, A.; Trotter, J. Proc. R. SOC.London, Ser. A 1964, A279, 129-146. Ishida, H.; Takahashi, H.; Tsubomura, H. Bull. Chem. SOC. Jpn. 1982,43, 3130-3136. Burinovich, G. P.; Salokhiddinov, K. I. Chem. Phys. Lett. 1982,85,9-11. Forster, T.; Kasper, K. 2.Elektrochem. 1955,59,976-980. Cope, V. M.; Kalkwarf, D. R. “Abstracts of Papers“, 184th National Meeting of the American Chemical Society, Kansas City, MO, Sept 1982;American Chemical Society: Washington, DC, 1982; Abstr. PHYS 79. Dushman, S. “Scientific Foundations of Vacuum Technique”; Lafferty, J. M., Ed.; Wiley: New York, 1962; p 14. Received for review July 6, 1983. Revised manuscript received January 19, 1984. Accepted February I, 1984.
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