Environ. Sci. Techno/. 1994, 28, 2 147-2 153
Effect of Composition and State of Organic Components on Polycyclic Aromatic Hydrocarbon Decay in Atmospheric Aerosols Stephen R. McDow,' Qlng-rul Sun,t Matti Vartiainen,* Yu-sen Hong,s YI-lln Yao,! Thomas Fister,ll Rong-qi Yao,? and Richard M. Kamens Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27599-7400
Photodegradation of six polycyclic aromatic hydrocarbons (PAHs) was studied in liquids composed of actual organic aerosol constituents using a photochemical turntable reactor. All six PAHs exhibited more rapid decay in a liquid mixture of methoxyphenols, an abundant class of compounds present in wood smoke, than in hexadecane, which is representative of aliphatic hydrocarbons abundant in diesel soot and automobile exhaust. The most rapid decay was observed for benz[alanthracene and benzo[alpyrene in both hexadecane and the methoxyphenols. BenzoEalpyrene decay was approximately seven times faster in the methoxyphenol mixture than in hexadecane. The results indicate that the organic composition of atmospheric particulate matter can influence PAH decay.
Introduction Polycyclic aromatic hydrocarbons (PAHs) associated with soot particles decay rapidly on exposure to light (13). This process is responsible for significant decay of PAHs associated with atmospheric particles (4, 5 ) . In many communities, wood smoke and automobile exhaust are major PAH sources. Particle-associated PAHs from these sources decay rapidly in sunlight under some atmospheric conditions, but are generally stable at night (6, 7). Their reactivity is strongly influenced by atmospheric conditions, including solar radiation intensity, temperature, and atmospheric concentrations of water, ozone, and nitrogen oxides. Sunlight intensity has the strongest influence on PAH decay (6). The surface on which PAHs are adsorbed can also have a strong effect on their rates of degradation. Decay of PAHs on silica thin-layer chromatography plates was noted more than 30 years ago (8, 9). Further studies on glass fiber filters (10, 11) and silica particles (12) confirmed that rapid photodegradation occurs for PAHs adsorbed on silica. In contrast, carbon black (13) and fly ash (12, 14) surfaces appear to have a stabilizing effect. Unburned or partially combusted organic material constitutes a large fraction of the particulate matter emitted from several important atmospheric PAH sources (15). The effect of these substances on PAH decay is also likely to be important. For example, substituted phenols are abundant in wood smoke (16) and react rapidly with PAHs (17). A mechanism for PAH decay in the presence of methoxyphenols was proposed in a previous paper (17).
* Author to whom correspondence should be addressed.
Present address: Department of Technical Physics, Beijing University, Beijing, Peoples' Republic of China. f Present address: Division of Environmental Health, National Public Health Institute, Kuopio, Finland. 8 Present address: Yunnan Environmental Monitoring Station, Lijiang, Yunnan Province, Peoples' Republic of China. 11 Present address: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-7400. f
0013-936X/94/0928-2147$04.50/0
0 1994 American Chemical Society
In this paper, photodegradation rates of PAHs in organic liquids composed of wood smoke and diesel exhaust constituents are compared, and the importance of a liquid organic layer associated with combustion-derived atmospheric particles as a reaction medium is discussed. Experimental Section PAH photochemistry was studied in two organic liquids composed of compounds that represent a significant fraction of the organic mass of combustion aerosols. Hexadecane was chosen because it is representative of aliphatic material, which constitutes the majority of organic material associated with diesel soot. Methoxyphenols were selected because they comprisean important class of compounds found in wood smoke. Analysis. PAHs were analyzed by reverse-phase gradient-elution HPLC using a 15-cm,0.65- mm o.d., 5-mm particle size Supelco LC-PAH column with acetonitrile: water as the mobile phase, excitation at 250 nm, and fluorescence detection at 400 nm. Methoxyphenols were analyzed by capillary gas chromatography with flame ionization detection on a 30-m, 0.32-mm i.d. J&W DB-5 column with a 0.25-pm film thickness. Photochemical Reactor. Irradiation experiments were conducted with an Ace Glassware turntable reactor (Ace Glassware, Vineland, NJ) with a 450-W medium pressure Hg lamp and a borosilicate immersion well. Solutions were prepared in 12-mm 0.d. borosilicate reaction tubes, and 15 mL of sample was removed from each reaction tube for analysis by HPLC or GC at regular intervals. The reactor was housed in a small air-cooled aluminum cabinet. Air temperature remained within 2 "C over the course of the experiment. Light intensity was periodicallymeasured with a ferrioxalate actinometer (18). Total UV light intensity was approximately 20 % of the total light intensity and was estimated to be approximately 3 X 10-7 Einstein s-1 sample-l based on the actinometer measurements, the lamp's spectral distribution, and the wavelength-dependent actinometer quantum yield. Stability of the ultraviolet light intensity over time was monitored continuously with an Eppley Ultra-Violet radiometer (Eppley Laboratory, Inc., Newport, RI). Turntable Experiments. A liquid mixture was prepared from 11 methoxyphenol compounds based on proportions found in smoke from hardwood burning (16). The composition of the mixture is described in Table 1. Approximately 100 pg/mL each of benz[al anthracene, chrysene, benzo[al pyrene, benzo[el pyrene, benzo[bl fluoranthene, and benzo[kl fluoranthene were dissolved in the same solution of either hexadecane or the methoxyphenol mixture. These concentrations were chosen because the mass fraction of PAHs in the methoxyphenol mixture is on the same order as that found in wood smoke and diesel exhaust particles (6). The PAHs were selected because they are among the most abundant in combustion source Environ. Scl. Technol., Vol. 28, No. 12, 1994 2147
Benz(a)anthracene
Table 1. Composition of Methoxyphenol Mixture compound
-
mass fraction (74)
syr in go1
guaiacol phenol catechol 4-methylguaiacol 4-methy catecho
3-methoxycatechol o-cresol m-cresol p-cresol cis- and trans-isoeugenol
18.9 17.0 15.8 13.3 6.9 6.3 5.6 4.6 3.7 3.7 4.4
3
3 100
B
v
8
% 3
50
8
8 u U 0
,?
c O b '
and ambient particulate matter, they represent a wide range of photoreactivity, and they are primarily associated with atmospheric particles. The reproducibility of PAH photodegradation rates was investigated by simultaneously irradiating five identical solutions of the six PAHs in the methoxyphenol mixture. Several experiments were carried out using benz[a]anthracene to further investigate the effect of methoxyphenols. (a) The effect of methoxyphenol concentration on benz[al anthracene photodegradation was investigated by irradiating benz [alanthracene dissolved in a mixture of toluene and the methoxyphenol mixture. The mass fraction of the methoxyphenols ranged from 0.01 to 50 % , and benz[alanthracene concentration was 2 pg/mL. (b) Effects of individual methoxyphenols on benz[a]anthracene photodegradation were investigated by irradiating 2 pg/mL benz[alanthracene in toluene solutions containing 1% by mass of guaiacol (2-methoxyphenol), syringol(2,6-dimethoxyphenol),4-methylcatechol(2-hydroxy-4-methylphenol), or isoeugenol(2-methoxy-4-propenylphenol). In a similar experiment, these methoxyphenol compounds were also irradiated in the absence of benz[a]anthracene. (c) Free radical intermediacy ofbenz[alanthracene decay in the methoxyphenol mixture was tested by adding approximately 150 pg/mL BHT (2,6di-tert-butyl-p-cresol)to a 10 pg/mL solution of benz[alanthracene in the methoxyphenol mixture. In this experiment, 5% by weight of water was also added to the methoxyphenol mixture. Chamber Sampling. High concentrations of wood smoke were injected into a 25-m3smog chamber (19)and collected on indium foil in a Sierra Model 236 cascade impactor operated at a flow rate of 566 L/min (20 cfm) for several hours in order to collect a large area of sample on the foil for surface analysis. Results In all experiments, the most rapid decay was observed
'
' ' ' 60 ' '
'
'
'
'
120 ' '
'
'
Benzo(a)pyrene
I -
u
~
OO
60 120 Irradiation Time (minutes)
Flgure 1. Comparison of benz[a]anthracene and benzo[a]pyrene decay In methoxyphenols for flve ldentlcal samples.
benzo[blfluoranthene, and benzo[elpyrene, coefficients of variation between the five samples were larger, and poorer fits were observed for first-order decay, probably in part because of slower reaction rates for these compounds. No significant decay was observed for any of the six PAHs in samples subjected to identical conditions but without irradiation. In Figure 2, benz[al anthracene and benzo[alpyrene decay is compared between PAH solutions in hexadecane and in the methoxyphenol mixture for two experiments. For both compounds, much faster decay was observed in the methoxyphenol solution than in the hexadecane solution. Good fits were also observed in these experiments in the methoxyphenol mixture for fist-order decay of benz[alanthracene (r2= 0.971 on March 6 and 0.999 on March 18) and benzo[alpyrene (r2 = 0.967 on March 6 and 0.997 on March 18). For hexadecane solutions, generally poorer fits for first-order behavior were observed. A good fit was observed in both experiments only for benzo[alpyrene (r2 = 0.888 on March 6 and 0.993 on March 18). The firstorder fits for the March 18 experiment are shown in Figure
for benz[alanthracene and benzo[a]pyrene. This is
3.
consistent with smog chamber studies of PAH decay in sunlight (6). In the simultaneously irradiated identical samples, first-order decay was observed for the three most reactive compounds: benz[alanthracene (r2 = 0.9920.999), benzo[alpyrene (r2= 0.992-0.998), and benzo[klfluoranthene (r2= 0.964-0.998). Coefficients of variation for the rate constants were 4.4% for benz[alanthracene, 4.9% for benzo[alpyrene, and 9.2 5% for benzo[klfluoranthene. Figure 1compares benz[alanthracene and benzo[alpyrene decay. Average first-order rate constants were 1.168* 0.051 X 10-2min-lforbenz[alanthraceneand 1.140 f 0.055 X min-1 for benzo[alpyrene. For chrysene,
Table 2 directly compares benzo[alpyrene photodegradation rates between hexadecane solution and the methoxyphenol mixture. First-order rate constants were approximately seven times greater in the methoxyphenol mixture than in hexadecane, with good agreement between experiments. Good agreement between first-order rate constants was also observed between these two experiments in both methoxyphenols and hexadecane, and reasonable agreement was observed between the rate constants in methoxyphenols in these experiments and the average rate constants reported previously for the data in Figure 1.
2146
Envlron. Scl. Technol., Val. 28, No. 12, 1994
Benzo(a)pyrene (3/6/91)
Benz(a)anthracene (3/6/91)
I
I
100%
50%
0 " " " " " 0
0
200
0
400
Benzo(a)pyrene (3/18/91)
Benz(a)anthracene (3/18/91)
-
100%
400
200
I
.-9 0
. I
8
-s
50%-
.3
LE
0
0
400 200 Irradiation Time (minutes) -A-
in Hexadecane
0
c"
0
-e-
I "
"
'
" ' 400 200 Irradiation Time (minutes)
in Methoxyphenols
Flgure 2. Comparison of benz[a]anthracene and benzo[a]pyrene decay in hexadecane and the methoxyphenolmlxture.
0.8 I
Table 2. First-Order Rate Constants for Benzo[a]pyrene
in methoxyphenols in hexadecane
ratio
Time (minutes) 0
In Hexadecane 0.2
0.1
0
0
200
400
Time (minutes) Flgure 3. First-order rate constant fits for benzo[a]pyrene decay In hexadecane and the methoxyphenolmlxture on March 18, 1991.
Although significant decay of benz[alanthracene was also observed in hexadecane,there was poor reproducibility between the two experiments, and the data were poorly fit to a first-order decay curve, so it was not possible to determine a precise first-order rate constant. It was also not possible to obtain reliable rate constants for the other
March 6 (min-1)
March 18 (min-1)
av (min-1)
1.05 X 10-2
1.06 X 10-2 1.57 X lo3 6.74
1.05 X 10-2 1.53 X 10-3
1.48 X 103 7.11
6.92
PAHs in hexadecane because very little decay was observed even after 6 h of irradiation. However, Figures 4 and 5 show that decay of these PAHs in methoxyphenols was also much faster than in hexadecane. Figure 6 shows that benz[a]anthracene decay is dependent on the amount of methoxyphenols. Even a relatively small amount of the methoxyphenol mixture in a toluene solution caused much more rapid benz[a]anthracene decay than in pure toluene. Figure 7 shows benz[alanthracene photodegradation was also significantly faster in toluene solutions of guaiacol, syringol, 4-methylcatechol, and isoeugenol than in pure toluene. The fastest rate was observed in the isoeugenol solution, with approximately 80 % decay observed when the first sample was analyzed after 5 min of irradiation. Following isoeugenol in order of decreasing benz[alanthracene photoreactivity were 4-methylcatechol, syringol, and guaiacol. The same reactivity order was observed for the photodegradation of the methoxyphenols themselves in toluene when benz[a]anthracene was not present, as shown in Figure 8. Ambient sampling experiments have also suggested that methoxyphenols decay in sunlight and that compounds related to syringol (substituted 2,6dimethoxyphenols) in general decayed more rapidly than compounds related to guaiacol (substituted 2-methoxyphenols) (20). The same study reported apparent decay of isoeugenol and formation of vanillin. Vanillin is a known product of isoeugenol photodegradation (21). Figure 9 sho& that decay of benz[alanthracene in the methoxyphenol mixture was only slightly inhibited by the Envlron. Sci. Technol., Vol. 28, No. 12, 1994 2149
Chrysene
Benzo(b)fluoranthene
7
.
100%
50%
ob^'''"'" 200 400
0
0
Benzo(k)fluoranthene
o
~
'
'
200 '
'
'
400
Benzo(e)pyrene
'
400'
0 '
'
0
Irradiation Time (minutes) 4
200
in Hexadecane
200
400
Irradiation Time (minutes) 4
in Methoxyphenols
Flgure 4. Comparisonof chrysene,benzo[b]fluoranthene,benzo[k]fluoranthene,and benzo[e]pyrenedecayin hexadecaneand the methoxyphenol mixture.
addition of the free radical inhibitor BHT (2,6-di-tertbutyl-4-methylphenol). Benz[alanthracene decay appeared to follow first-order kinetics both in the presence and in the absence of BHT, and the calculated benz[alanthracene half-life was 70% greater in the presence of BHT. Much greater inhibition would be expected for a typical free radical chain reaction. The original purpose of collecting wood smoke particles by impaction was to obtain information on surface chemical composition by X-ray photoemission spectroscopy. This was not possible because the sample clearly formed a viscous liquid tar on the impactor foil. Analysis of such a sample was considered potentially damaging to the instrument. However,the observation of the physical state of collected wood smoke particles was considered valuable information.
Discussion Particle Structure and Composition. Consideration of available information about combustion particle structure and chemical composition for aerosols from common PAH sources was essential for designing a relevant experimental approach. Particulate matter in wood smoke and in automobile exhaust from both gasoline and diesel engines consists mostly of elemental carbon and organic material. The sum of organic and elemental carbon mass typically accounts for more than 80% of the total particulate mass emissions in automobile exhaust from both diesel and gasoline-powered engines (22,23) and more than 50% of the total particulate mass in wood smoke (15, 24). If total organic mass instead of organic carbon mass is considered, the fraction is even greater due to the contribution of hydrogen, oxygen,and other elements. This is especially true for wood smoke, which contains highly 2150
Envlron. Scl. Technol., Vol. 28, No. 12, 1994
oxygenated organic compounds. Approximately 70-90 % of the wood smoke particle mass is extractable in organic solvents (6). I t can be concluded that each of these sources consists almost entirely of elemental carbon and organic material. PAHs associated with coal fly ash (25), and organics associated with other combustion sources (26) are mostly in the gas phase at stack temperatures, but adsorb on to particles during cooling. I t is routinely assumed that organic compounds adsorb on solid particle surfaces after particle formation in the combustion zone (2, 25, 27). A thermodynamic model predicts that PAHs remain primarily in the gas phase until stack temperatures drop below 150 O C (28). It follows that organic material is likely to sorb onto the surface of elemental carbon after combustion to form an organic layer. Ross et al. (29) estimated that in typical diesel soot samples this organic coating is approximately 4 nm thick. Given the greater organic to elemental carbon ratios generally observed for wood smoke than for diesel soot, an even thicker organic layer would be expected. In some wood smoke particles, elemental carbon is not observed at all (30). These observations suggest that the existence of a well-defined organic phase is a strong possibility in both diesel soot and wood smoke. The observation of aviscous liquid after collection of wood smoke particles by impaction suggests that at least in wood smoke this phase could be a liquid. The exact nature of the sorption of organic material to aerosol particles from common combustion sources is poorly understood. It should be emphasized that there is considerable uncertainty about whether well-defined organic layers exist on such particles and if so whether they form a mobile phase conducive to collisions between potential reactants. However, the available observations
Decay After 90 minutes March 18, 1991
2
.-C
.-
80%
E
60%
.-8
.-u
40 %
E
20%
p:
14
0
"
8 c
40%
p: 0
20
30
Flgure 7. Effect of four different methoxyphenolsfoundin wood smoke on benz [alanthracene decay.
I
80% .-2 .-C 60%
10
Time (min)
Decay After 240 minutes March 6, 1991
I
0
\paiacol
. I e
E 20%
L
0
0 in Methoxyphenols EB in Hexadecane Flgure 5. Fraction remaining after simultaneous irradiation of PAHs in hexadecane and the methoxyphenols mixture.
100%
"
'
0'
0
40 Time (min)
80
Flgure 8. Decay of methoxyphenois in toluene solutions.
aJl
C d cd
*3
100%
.3
E
2d
50%
0
.3 U
50%
: 0
0
0
10 20 30 Irradiation Time (minutes)
Figure 6. Comparisonof benz [alanthracenedecay in toluene solutions with various amounts of the methoxyphenoi mixture added.
0 0
30
60
90
Irradiation Time (minutes) suggest that the existence of a mobile organic phase which contains PAHs is a strong possibility. Based on these observations of combustion particle structure, an experimental approach was selected that focused on photodegradation of dilute solutions of PAHs in organic liquids composed of actual. combustion particle components. Aerosol Chemical Composition. The total particle mass accounted for by PAHs in diesel exhaust (23) and wood smoke (6)is generally less than 1% . Consequently, the organic fraction in these sources is clearly dominated by other classes of organic compounds. Very little is known about the composition of particle-associated organic material. With current analytical techniques, more than 80% of the organic mass associated with automobile
Figure 9. Effect of radical inhibitor BHT on benz[a]anthracenedecay in the methoxyphenol mixture.
exhaust (23)or ambient particulate matter (31)cannot be structurally identified. However, some idea of the chemical characteristics of the organic fraction is available from bioassay-directed fractionation studies in which particulate organic extracts are eluted through silica columns with successively more polar solvents (32-34). For example, the least polar hexane fraction generally contains only aliphatic hydrocarbons, followed by polycyclic organic matter in the second fraction, and more polar compounds in the following fractions (33). Envlron. Sci. Technol., Vol. 28, No. 12, 1994
2151
DIESEL EXHAUST
(Schuetzle et al. 1981)
WOODSMOUE
(Kamens et al. 1985)
'"Elemental"Carbon Extractable Orzanics: 0 Elution in Hexane or Hexane:MeC12 Mixture Elution in Methylene Chloride Elution in Methanol or Acetonitrile % Figure 10. Particle composition of diesel soot under test condibns which Included Idles. cruises, and accelerations (fromrefs 22 and 33) and wood smoke from an oxygen starved cool burn (from refs 24 and P3
34).
Figure 10shows a comparison of wood smoke and diesel soot particle composition using data frombioassay-directed fractionation along with data for organic and elemental carbon ratios (22, 24, 33, 34). There are considerable differences between the two sources. Aliphatic and aromatic hydrocarbons constitute more than half of the extractable organic matter associated withdiesel soot (33). This is also confirmed by other studies (23, 35,36). On gas chromatograms,most of this material is present as a broad band of unresolved branched and cyclic hydrocarbons (23). Organic material associated with wood smoke is much more polar than diesel soot organic material, and only a small percentageof the organic extract is accounted for by hydrocarbons (34). This difference reflects the different nature of organic material in the two fuels. Because wood is composed mainly of polysaccharides and lignin, wood smoke is likely to be composed mainly of combustion products of these two highly oxygenated polymers. High concentrations of methoxyphenols, the basic structural units of lignin in wood, have been identified in wood smoke (16)as well as ambient atmospheric particles in regions where wood smoke was a major source (20). Possible polysaccharide combustion products, including levoglucosan (37) and substituted furans (38),have also been observed in wood smoke. Similarly, the predominance of the aliphatic hydrocarbon fraction in diesel exhaust arisesbecause diesel fuel consists mainly of aliphatic hydrocarbons (23). The selection of the methoxyphenol mixture and hexadecane as reaction media as well as the selection of the PAH concentrationsused in the photodegradation experiments were based on these considerations. Reactivity of Benz[a]anthracene and Methoxyphenols. Two possible explanations for the observation that benz[alanthracene photodegradationwas more rapid in the methoxyphenol mixture than in hexadecane are as follows: (a) The methoxyphenol mixture is more polar than hexadecane, and benztalanthracene decay might be influenced by solvent polarity. (b) A chemical reaction involving methoxyphenols might constitute a part of the benztalanthracene decay mechanism. It is unlikely that the difference in benz[alanthracene decay rate is due to polarity of the medium. Figure 6 shows that the rate of 2152
Envlron. SCI. Technol., Vol. 28. NO. 12. 1994
benz[alanthracene decay is strongly influenced by methoxyphenol concentration at concentrations of 0.01-1 7% , which are probably too low to strongly affect solvent polarity. In Figure 7, benztalanthracene decay was much faster in toluene solutions of 1%isoeugenol than 1% guaiacol. The substitution of a propenyl group in isoeugenol for a hydrogen in guaiacol is unlikely to change the polarity of the medium enough to account for the large difference in decay rates. There ismorecompellingevidence that methoxyphenols participate as reactants in the benz[olanthracene decay mechanism. Odum et al. (I7) used a model system of benztalanthracene and vanillin to investigate the kinetics of benztalanthracene decay. The reaction clearly followed second-order kinetics, depending on the concentration of both vanillin and benzLalanthracene. Phenols can react by direct photolysis to form phenoxy radicals after absorbing light (39). Hydrogen abstraction by excited triplet state molecules or by singlet oxygen also readily occurs because the0-H bondenergyin phenols is relatively weak, especially in highly substituted phenols (40). While substituted phenols are among the best antioxidants known for preventing peroxy radical production, they can also act as initiators of radical reactions after hydrogen abstraction. 1-Naphthol is a good example of a phenolic compound that is used commercially as an antioxidant but is an important free radical source in environmental photochemistry (41). The investigation of benz[al anthracenedecay inhibition by BHT was carried out with 5% water present in the methoxyphenol mixture as a part of an investigation to explain the effect of combustion aerosol water content on the effect of PAH decay. Results of the study have been reported elsewhere (42). Because little inhibition was observed and BHT inhibition was considered unlikely to he affected by the presence of water, similar experiments were not repeated in the methoxyphenol mixture without water. Instead, to further study of the benz[alanthracene decay mechanism, structure-reactivity relationships were determined for benztalanthracene decay in the presence of a variety of substituted methoxyphenols. Results of these mechanistic studies have been reported in detail elsewhere (17). The results suggested that hydrogen abstractionof the phenolic hydrogen by excited state benz[alanthracene was the rate-limiting step for benz[alanthracene decay in the presence of methoxyphenols. This process wouldlead to the formationofsubstitutedphenoxy radicals as products of benz[alanthracene decay, but they are apparently too stable for a free radical chain reaction to be important in further decay of benz[alanthracene. This is consistent with the observations of Figure 9 and is not surprising because substituted phenoxy radicals are among the most effective inhibitors of free radical chain reactions. On the basis of these results, it appears likely that PAH reactivity in wood smoke is partly due to the chemical properties of methoxyphenolic compounds, which constitute a significant fraction of the organic mass of wood smoke. Theresultsdonot explain therapiddecay of PAHs observed in diesel soot or gasoline-powered automobile exhaust. In particles from these sources, it seems more likely that some minor constituents of the organic layer are responsible for PAH decay. For example, anthraquino. ne and xanthone as well as other carbonyl substituted aromatic compounds that are good photosensitizers have
been identified in diesel exhaust (23). Direct photolysis or effects of surface properties also cannot be ruled out.
Conclusions Combustion-derived atmospheric particles are likely to be coated by a multimolecular organic layer, of which only a small fraction is polycyclic organic matter. The results reported here indicate that chemical composition of this organic layer might have an important effect on the reactivity of PAHs and possibly other compounds. In particular, PAH decay is strongly accelerated by the presence of methoxyphenols, which are abundant in wood smoke. When PAH decay was compared between the methoxyphenol mixture and hexadecane, all six of the PAHs studied decayed much more rapidly in the methoxyphenol mixture. The most rapid decay was observed for benz[a]anthracene and benzo[alpyrene, which is consistent with chamber studies of PAH decay on wood smoke particles in sunlight. Methoxyphenols are only one of many classes of organic compounds associated with atmospheric particulate matter that might initiate or otherwise participate in reactions leading to the decay of PAHs in wood smoke. Further study of these compound classes is required as more information on atmospheric aerosol composition becomes available, before conclusions can be drawn about the importance of methoxyphenols in comparison to other possible pathways of PAH decay. Acknowledgments This research was partially funded by the U.S.Environmental Protection Agency Office of Exploratory Research (Grant R-816678), the EPA Air Research and Exposure Assessment Laboratory (Contract CR-815002), the Ford Motor Company, and the Finland-US. Educational Exchange Commission. We are grateful for the support of William Wilson and Denis Schuetzle and for helpful conversations with Bruce Faust, Steve Hawthorne, and Jay Odum. Literature Cited Tebbens, B. D.; Mukai, M.; Thomas, J. F. Am. Ind. Hyg. ASSOC. J. 1966, 27, 415-422. Thomas, J. F.; Mukai, M.; Tebbens, B. D. Environ. Sci. Technol. 1968,2, 33-39. Tebbens, B. D.; Mukai, M.; Thomas, J. F. Am. Ind. Hyg. ASSOC. J. 1971, 32, 365-372. Nielsen, T. Atmos. Enuiron. 1988, 22, 2249-2254. Greenberg A. Atmos. Environ. 1989, 23, 2797-2799. Kamens, R. M.; Guo, Z.; Fulcher, J. N.; Bell, D. A. Enuiron. Sci. Technol. 1988, 22, 103-108. Kamens, R. M.; Fan, Z.; Yao, Y.; Vartiainen, M. Chemosphere, 1994, 28, 1623-1632. Kortum, G.; Braun, W. Ann. N.Y. Acad. Sci. 1960, 632, 104-115. Inscoe, M. N. Anal. Chem. 1964,36, 2505-2506. Cimberle, M. R.; Bottino, P.; Valerio, F. Chemosphere 1983, 12, 317-324. Peters, J.; Seifert, B. Atmos. Environ. 1980, 14, 117-119. Behymer, T. D.; Hites, R. A. Enuiron. Sci. Technol. 1988. 20, i m - 1 3 1 9 . (13) Daisey, J. M.; Lewandow, C. G.; Zorz, M. Enuiron. Sci. Technol. 1982, 16, 857-861.
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Received for reuiew March 2, 1994. Revised manuscript received August 11, 1994. Accepted August 15, 1994." Abstract published in Advance ACS Abstracts, September 15, 1994.
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