Environ. Sci. Technol. 2000, 34, 789-797
Effect of Aerosol Chemical Composition on the Photodegradation of Nitro-polycyclic Aromatic Hydrocarbons ANDERS FEILBERG* AND TORBEN NIELSEN PBK-313, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark
The photodegradation of four ring nitro-PAHs in the presence of organic aerosol constituents has been investigated in model systems to determine their degradation mechanism under ambient air conditions. Light-induced radical chain reactions initiated by organic aerosol constituents, e.g., oxy-PAHs, is most likely to be the dominant degradation pathway for particle-associated nitro-PAH. The photodegradation rates were investigated in a chemical model system simulating the liquid film on particles from diesel exhaust and wood stove stack gases. Relative rates were obtained in pure solutions (direct photolysis) and in the presence of different classes of organic aerosol components comprising PAHs, hydroxy-PAHs, substituted phenols, benzaldehydes, and oxy-PAHs (including polycyclic aromatic quinones) (indirect photolysis). Members of the four latter compound classes are demonstrated to accelerate the photodegradation rate. The mechanisms proposed to explain these effects include both radical chain reactions and photoinduced hydrogen abstraction by nitro-PAHs. PAHs that are known sensitizers of singlet O2 did not accelerate the decay. Certain oxyPAHs strongly accelerate the nitro-PAH decay, whereas others have no effect. This difference is related to the nature of the excited state of the oxy-PAH and the ability to initiate radical chain reactions. The relevance of the model system and the environmental implications of the results are discussed, and the results are compared with relative degradation rates from smog chamber studies with the same compounds.
Introduction Nitropolycyclic aromatic hydrocarbons (nitro-PAHs) comprise a group of potent mutagenic and carcinogenic compounds that potentially pose a threat to human health (16). Nitro-PAHs are present in ambient air both as a result of incomplete combustion, primarily by diesel engines (7), and as a result of atmospheric processes (8-10). The phase distribution of nitro-PAHs in the atmosphere has only been systematically investigated in a few cases. 1- and 2-Nitronaphthalene are mainly present in the gas phase (11), whereas the heavier nitrofluoranthenes and -pyrenes exist almost exclusively in the particle phase (12, 13). In fact, 2-nitrofluoranthene and 2-nitropyrene formed in the gas phase by OH initiated reactions were observed by Fan et al. * Corresponding author phone: (+45) 46 77 42 01; fax (+45) 46 77 42 02; e-mail:
[email protected]. 10.1021/es990566r CCC: $19.00 Published on Web 01/22/2000
2000 American Chemical Society
(13) to condense on airborne particles immediately, so that none of these compounds were detected in the gas phase. To study the fate of the nitro-PAHs consisting of four or more rings it is therefore only relevant to take into account reactions in condensed phases. The presence of four ring nitro-PAHs is important from an environmental health perspective. They are among the most abundant particle associated nitro-PAHs in ambient air (8, 12, 14). Furthermore, the four ring nitro-PAHs appear to be more mutagenic than the others (2). Smog chamber studies (1) have revealed that when sorbed to diesel exhaust and wood smoke particles photolysis is the major degradation pathway for the nitrofluoranthenes and -pyrenes. However, it was observed that the photolysis rates were independent of molecular conformation (1). This is surprising since studies of the photochemistry of pure nitro-PAHs in solution or adsorbed in pure state have shown that the torsion angle of the nitro group relative to the aromatic plane governs the photostability (15-18). Therefore, it is necessary to study the mechanism of photodegradation of nitro-PAHs in model systems simulating diesel soot and wood smoke particles. The photodegradation of unsubstituted PAHs on diesel soot and wood smoke (and possibly other organic aerosols) seems to follow a complex mechanism, that is likely to include other chemical aerosol components (19-21). Combustion generated aerosols are among the major contributors to urban fine particles (22). Several of these particles (including diesel soot and wood smoke) as well as ambient soot particles are believed to consist of an elemental carbon core coated by an organic liquidlike layer (20, 21). Diffusion in the organic liquid film (23) will allow electronically excited molecules to participate in bimolecular reactions with other aerosol constituents. This work describes studies of the photochemistry of representative particle-associated nitro-PAH in a chemical model system consisting of an organic solvent and known constituents of organic aerosols such as diesel exhaust particles and wood smoke particles. The compound classes studied include substituted phenols, PAHs, hydroxy-PAHs, oxy-PAHs, and substituted benzaldehydes.
Experimental Section Irradiations were performed with a turntable photoreactor using a 450 W medium-pressure mercury lamp (Ace Hanovia) as light source (20). The lamp was placed in a quartz immersion well with cooling water. The immersion well was surrounded by a Pyrex sleeve to filter out high-energy UV bands (λ > ∼290 nm). Although this does not resemble the spectral distribution of the actinic flux, it does reproduce the wavelengths normally encountered in the atmosphere. The turntable-reactor technique has been introduced to the study of environmental photochemistry by Hoigne´ and co-workers (see for example refs 24 and 25). The photoreactor was positioned in a water bath with constant water circulation. The circulation and temperature in the water bath was maintained with a combined circulation and temperature control system (HETO). All experiments were performed with a temperature of 20 ( 1 °C. Samples of nitro-PAHs with and without cosolutes were irradiated in parallel in 13 × 100 mm quartz reaction tubes in cyclohexane. The reaction tubes were sealed but contained sufficient headspace to keep the solutions saturated with air during the experiment. The reproducibility of the irradiation conditions within experiments was determined by comparing the 1-nitropyrene decay rate constant in parallel samples. The variation was VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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less than 10%. Some variation in the 1-nitropyrene rate constant was observed between different experiments. However, absolute decay rates were only compared within each experiment to determine the effect of the cosolutes. Furthermore, the decay rate constant for 1-nitropyrene in the presence of phenol relative to the rate constant without phenol varied ∼5% for three different experiments, even though the effect of phenol is not very great (a factor of ∼1.3). This shows that the relative effect of the cosolutes can be compared between experiments. A few test experiments were performedwithamonochromaticlightsource(Bausch&Lomb). This setup consists of a SP200 Hg light source with the emission focused into a monochromator to produce light with the desired wavelength. A wavelength of 366 nm was used to selectively produce excited nitro-PAHs and avoid photolysis of the cosolutes. Samples were irradiated in pairs in quartz cuvettes. One sample contained only the nitroPAH, in the other a cosolute was added. To test that the photon flux into the two cuvettes were the same, two tests were performed. First, two solutions of the classic ferrioxalate actinometer (26) were irradiated, and second, identical solutions of 1-NP were irradiated. The production of Fe2+ (complexed with 1,10-phenanthrolin) and the loss of 1-NP were spectrophotometrically determined. The difference between the two cuvettes was less than 5%. The concentrations of nitro-PAHs were ∼10-4 M, and the cosolute concentrations were about 1 × 10-3-5 × 10-3 M. In experiments with polar cosolutes, small amounts of dichloromethane or ethyl acetate were added to all samples in equal amounts in order to dissolve the cosolutes. Samples were removed with a pipet and transferred to brown chromatography vials. The samples were analyzed by HPLC using a Shimadzu HPLC (LC10-AD) equipped with diode array UV-vis detection (SPD-M10A). The column was packed with Nucleosil 50-5 C18. Methanol containing 10 or 20% H2O was used as isocratic eluent. The response of the analytical system was a linear function of concentration (r2 > 0.99). 2-Nitrofluoranthene (2-NF) was synthesized following the procedure of Zielinska et al. (27): Fluoranthene was nitrated with freshly prepared N2O5 (28) in CCl4 at 25 °C. 2-NF was isolated from dinitrofluoranthenes and unreacted fluoranthene by preparative HPLC fractionation using a Nucleosil 50-5 column and a Gilson (model 201) fraction collector connected to the analytical HPLC described above. All other chemicals were commercially available and were used as received.
Results and Discussion Relevance of the Model System. As discussed by McDow et al. (20, 21) there is increasing evidence that at least for some aerosols formed by combustion of organic matter, e.g., diesel and wood smoke particles, the organic fraction is liquid or liquidlike. This evidence includes that the impaction of wood and diesel soot on a metal foil produced a viscous liquid slick that retains no or little particle integrity (20). Furthermore, transmission electron micrographs of ambient soot particles showed that the soot was coated by a liquid film (29). Depth profiles of ambient particulate matter have revealed that the surface of particles collected in polluted urban air was dominated by organic carbon (30). The observation by several authors that absorption is probably the most important sorption mechanism for ambient air (3133) also suggests that an organic liquidlike or amorphous surface layer is involved. Diffusion in the organic fraction of diesel soot and wood smoke particles has been demonstrated by modeling the dynamic behavior of gas-particle partitioning of PAHs (23). The diffusion is retarded by viscosity, tortuosity, and adsorption (23) but will nevertheless allow electronically excited molecules to undergo bimolecular reactions. 790
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Semivolatile organic compounds (SVOC) appear to have higher affinity for aerosols containing organic matter and will preferentially condense on these as opposed to inorganic aerosols (32, 33). Low-volatility nitro-PAHs formed in the gas phase by reactions of the parent PAHs are therefore expected mainly to be absorbed into the organic fraction of ambient particles. Likewise, the heavier nitro-PAHs from combustion sources are expected to be associated with the organic fraction of soot particles. Based on the above discussion, an organic solvent with relevant constituents of combustion aerosols seems a relevant system for studying the photodecomposition of nitro-PAHs associated with urban particulate matter. This methodology has previously been applied to study the photodegradation of unsubstituted PAHs (19-21, 34, 35). Cyclohexane was used as solvent because it is transmissive and inert. In addition the organic fraction of diesel exhaust particles consists largely of branched and linear alkanes (36), although as a whole it is more polar than cyclohexane. It should be noted that ambient organic aerosols such as diesel exhaust might be more viscous than cyclohexane. This physical parameter is probably also important, but in the present study, we only intend to address the effect of chemical composition. The cosolutes chosen are all major components of combustion derived and/or ambient particulate matter. In the experiments the concentration of nitro-PAHs were always much lower than the concentration of cosolutes, which seems the most realistic with respect to ambient air. Most of the experiments were performed with 1-nitropyrene (1-NP) and 3-nitrofluoranthene (3-NF) as representative nitro-PAHs, but 2-nitrofluoranthene (2-NF) and 2-nitropyrene (2-NP) were also studied. The cosolutes chosen in this investigation are listed in Table 1 together with ambient sources. Known constituents of wood smoke or diesel particles were primarily chosen, since these types of particles were used in previous smog chamber experiments with nitro-PAHs (1). However, several of these compound classes are also found on other combustion derived aerosols, e.g., from gasoline vehicles (36). Wood smoke particles are composed of compound derived from combustion of biopolymers. Particularly the occurrence of methoxyphenols (37) arising from combustion of lignin is interesting from a chemical viewpoint (20, 21). The organic phase of diesel particles is believed to be nonpolar, but substituted phenols and hydroxy-PAHs have been detected in diesel standard reference material (38). PAHs, substituted benzaldehydes, and oxy-PAHs are found on diesel particles and wood smoke particles as well as on particles from gasoline vehicles (36, 37). It should be noted that there are still large uncertainties regarding the chemical composition of diesel soot and wood smoke particles (36, 37) as well as other combustion derived particles. Photolysis in the Absence of Cosolutes. 1-NP, 2-NP, 2-NF, and 3-NF were irradiated in cyclohexane solutions in order to investigate the photostability of the individual compounds. For comparison 9-nitroanthracene (9-NA) and 6-nitrobenzo[a]pyrene (6-NBaP) were also irradiated. In the absence of cosolutes, the photodegradation of nitro-PAHs is strongly dependent on the orientation of the nitro-group. In 9-NA and 6-NBaP the nitro-group adapts an approximately perpendicular orientation relative to the aromatic plane (15, 39, 40), and as expected both compounds decayed very fast with less than 1% remaining after 15 min. As for the other nitro-PAHs, 1-NP decayed moderately (see Figure 1), whereas 2-NF, 2-NP, and 3NF were stable toward photolysis within the course of the experiment. In 1-NP the nitro-group is twisted out of the aromatic plane due to a peri-proton (39). In 3-NF the nitro-group is probably twisted slightly but much less than in 1-NP (39) and in 2-NP and 2-NF the nitro-group lies in the same plane as the aromatic rings (39). It has been
TABLE 1. Tested Aerosol Constituents, Their Sources and Effects on Nitro-PAH Photodegradation compound
sourcesa
benzene-1,4-diol (hydroquinone) 4-chlorophenol 4-hydroxybenzoic acid 4-hydroxybenzaldehyde 4-nitrophenol 4-hydroxybiphenyl 1-hydroxynaphthalene (1-naphthol) 2-hydroxynaphthalene (2-naphthol) 2,4,6-trimethylbenzaldehyde 3,4,5-trimethoxybenzaldehyde 4-hydroxy-3-methoxybenzaldehyde (vanillin) 4-hydroxy-3-methoxybenzoic acid (vanillic acid) 4-hydroxy-3-methoxybenzyllic acid (homovanillic acid) 2,6-dimethoxyphenol (syringol) 2-methoxyphenol (guaiacol) 10H-anthracene-9-one (anthrone) 9H-xanthen-9-one (xanthone) 9,10-anthracenedione (anthraquinone) 9,10-phenanthrenedione (9,10-phenanthrenequinone) 9H-fluoren-9-one (fluorenone) 7H-benz(de)anthracene-7-one (benzanthrone) anthracene pyrene chrysene perylene
W B D, P D, P D, P D, P D, P D, P G W W W W W W D, G, NG D, G, NG W, D, G, NG, B, P W, D, G, NG, P D, G, NG, B W, D, G, NG, B D, G, W, B, NG D, G, W, B, NG D, G, W, B, NG D, G, W
effect on nitro-PAH decay accelerated accelerated slightly acc. no effect no effect accelerated accelerated accelerated accelerated slightly acc. acceleratedb acceleratedb acceleratedb acceleratedb acceleratedb highly acc. highly acc.c highly acc. acceleratedc no effect no effect no effect no effect no effect no effect
proposed mechanism H-abstraction H-abstraction H-abstraction H-abstraction H-abstr/rad. react. H-abstr/rad. react. radical react. radical react. H-abstraction H-abstraction H-abstraction H-abstraction H-abstraction radical react. radical react. radical react. radical react.
a D: diesel vehicles (36, 38), G: gasoline vehicles (36), W: wood smoke (37), P: photochemical formation (17, 41, 45, 64), B: boilers (65), NG: natural gas home appliances (66). b 3-NF only, no effect on 1-NP. c Solvent dependent.
FIGURE 1. Photodegradation of 3-NF and 1-NP in cyclohexane solution. C0 ) 1 × 10-4 mol l-1. 2-NF and 2-NP were stable over the course of the experiment, analogous to 3-NF. known for many years that a conformation with the nitrogroup out-of-plane relative to the aromatic rings allows photochemical nitro-nitrite rearrangement of the nitro-group followed by further reaction steps, as seen in reaction 1 (15).
Ar-NO2 + hν f Ar-ONO f products
(1)
The observed 1-NP photolysis is therefore ascribed to rearrangement of the nitro-group and subsequent loss of NO and the photostability of the other nitro-PAHs stems from their approximately coplanar structure, which prevents such rearrangement. Our observations are in excellent agreement with those of other studies (1, 15, 16, 18). Since 2-NF, 2-NP, and 3-NF are not decaying, our results suggest that photoautooxidation (reaction with singlet oxygen), which has been observed for PAHs (41), is insignificant for nitro-PAHs, although 1O2 sensitization by triplet state nitro-PAHs should be energetically feasible. This agrees
with previous findings that the photorearrangement of nitroPAHs is independent of the presence of O2 (16, 42, 43). Probably, singlet oxygen is relatively unreactive toward nitroPAHs because the aromatic moiety is deactivated by the electron withdrawing nitro-group. In the case of 1-NP only one product was observed. This product was identified as 1-hydroxypyrene (OH-PY) by comparing with the retention time and UV-vis spectrum of an authentic standard, in agreement with previous studies (16, 42). 1-Hydroxypyrene is formed via reaction 1 followed by loss of NO and hydrogen abstraction by the formed pyrenyloxy radical from the solvent. The Effect of Phenolic Cosolutes. The photodegradation of 1-NP and 3-NF was more or less accelerated in the presence of various phenolic aerosol constituents. The strongest effect was observed for 1- and 2-hydroxynaphthalene, representing hydroxy-PAHs present in diesel exhaust (44) and formed in the atmosphere (45, 46) (see Figure 2A). o-Methoxyphenols, VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Percentage Remaining upon Irradiation for 60 min with and without Phenolic Cosolutes
FIGURE 2. A. Photodegradation of 3-NF and 1-NP in the presence of 1- and 2-hydroxynaphthalene (1- and 2-HN). B. Photodegradation of 3-NF and 1-NP in the presence of methoxyphenols represented by guaiacol (GU) and homovanillic acid (HVA). C0 ) 1 × 10-4 mol l-1. which are important constituents of wood smoke aerosols, were also tested with respect to their influence on nitro-PAH decay. The methoxyphenols tested were vanillin, vanillic acid, homovanillic acid, syringol, and guaiacol (see Table 1). These cosolutes accelerated the decay of 3-NF as expected but surprisingly had no effect on 1-NP. Examples are presented in Figure 2B. Certain para-substituted phenols (e.g., hydroquinone and 4-chlorophenol) were also able to accelerate photodegradation, while para-substituted phenols having an electron attracting substituent (e.g., 4-nitrophenol and 4-hydroxybenzaldehyde) had no or little effect (see also Table 1). Results for phenol and hydroquinone are presented in Table 2 as the percentage of the initial concentration remaining after 60 min irradiation. In experiments with 2-NF and 2-NP, photodegradation was affected by hydroquinone but not by phenol itself. The effect of certain phenolic compounds on the photodegradation of nitro-PAHs can be explained either by (1) abstraction of the phenolic hydrogen by an excited triplet 792
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cosolute
1NP
3NF
2NF
2NP
hydroquinone phenol none
24% 66% 72%
44% 63% 95%
75% 98% 100%
76% 100% 100%
nitro-PAH molecule leading to unstable radical intermediates instead of decay to the ground state or (2) photodissociation of the phenolic O-H bond giving rise to phenoxy radicals and hydrogen atoms. These will subsequently add O2 to produce HO2, which in turn may react with stable molecules such as ground state nitro-PAHs. Triplet state 1-NP has previously been observed to undergo photoreduction by hydrogen abstraction (but not electron transfer) in a nearly diffusion controlled reaction (42). Similarly, 1- and 2-nitronaphthalene have both been observed to undergo photoreduction via hydrogen abstraction (47). Furthermore, Odum et al. (48) proposed that PAHs upon excitation are able to abstract phenolic hydrogen from substituted methoxyphenols. On the other hand, 2-hydroxynaphthalene has been used as a photoinitiator of PAH degradation (49) due to O-H photodissociation and subsequent radical production. As a control, experiments with a monochromatic light source was performed in which 1-NP and 3-NF was irradiated in the presence of 2-hydroxynaphthalene with light of 366 nm. At this wavelength, 2-hydroxynaphthalene does not absorb light, whereas both 1-NP and 3-NF absorb considerably. The decays were observed to be accelerated under these conditions. For example, the 1-NP decay rate constant was ∼1.8 times higher in the presence of 2-hydroxynaphthalene. Thus, the effect is weaker than in the experiments with the full irradiation spectrum (Figure 2A), where the 1-NP decay rate constant is ∼3 times higher in the presence of 2-hydroxynaphthalene, but it indicates that H-abstraction do occur. However, it is likely that O-H photodissociation initiates radical reactions (49) so that with the full light spectrum two mechanisms occur simultaneously. The photodissociation mechanism may also explain why the effects of 1- and 2-hydroxynaphthalene are comparable (Figure 2A) despite the O-H bond energy is higher in 2-hydroxynaphthalene (50). On the other hand, phenol itself accelerated the degradation of both 3-NF and 1-NP as shown in Figure 3 and Table 2, a fact which cannot be ascribed to photodissociation since phenol does not absorb light above ∼290 nm. In addition, the effect of o-methoxyphenols cannot be explained by photodissociation because this would lead to degradation of both 1-NP and 3-NF. As mentioned previously this was not the case in the experiments. It can be speculated that the difference is due to steric effects; the methoxy-group is in the ortho-position relative to the hydroxy-group, and as mentioned previously, in 1-NP the nitro-group is twisted out of the plane. Consequently it may be difficult for the nitro-group in 1-NP to approach the phenolic hydrogen. A compound like vanillin may possibly also initiate radical chain reactions via hydrogen abstraction by the aldehyde group (see later). However, this can be ruled out since it would lead to degradation of both 1-NP and 3-NF. Experiments with 2-NF and 2-NP indicate that the triplet states of these compounds are less reactive toward H-donors. There is no effect of phenol and the effect of hydroquinone is less pronounced than in the case of 1-NP and 3-NF (see Table 2). Not all para-substituted phenols have an effect on the photodegradation although they absorb light at higher wavelengths. For example, 4-nitrophenol and hydroxybenzaldehyde had very little effect indicating that these com-
FIGURE 3. Photodegradation of 3-NF, 2-NF and 2-NP in the presence of phenol. C0 ) 1 × 10-4 mol l-1. Without cosolutes, 3-NF, 2-NF, and 2-NP were stable over the course of the experiment.
FIGURE 4. Hammet plot for 3-NF. Only phenols with no or very little absorption above 300 nm (black circles) have been included in the regression (full line). Results for phenols with significant absorption above 300 nm (open diamonds) are also shown. pounds are not efficient H-donors and do not photodissociate to a significant degree. To further investigate the mechanism for the effect of phenol on the nitro-PAH photolysis it was attempted to obtain Hammet plots for the photodegradation of 3-NF and 1-NP. This was done by plotting the logarithm (base e) to the photolysis rate constant for each nitro-PAH/ substituted phenol sample relative to that of unsubstituted phenol (ln(ksubst/kphenol)) versus the Hammet σ parameter. σ is a relative measure of a substituents electron donoracceptor properties. A good correlation for 3-NF may be obtained, but only if those substituted phenols that absorb light above ∼300 nm are excluded from the plot, as seen in Figure 4. This is a strong indication that H-abstraction by triplet state 3-NF from phenolic compounds do occur since σ correlates very well with the O-H bond strength (51). For example, De Lucas and De Netto-Ferrera (52) obtained good Hammet correlations for H-abstraction by triplet state oxyPAH from a series of para-substituted phenols. However, it is also evident from Figure 4 that the situation is more complex if the phenols absorb light in the same wavelength region as the nitro-PAHs. In that case, both light shielding and O-H photodissociation may obscure the picture. We could not obtain a reasonable Hammet correlation for 1-NP even when the light-absorbing phenols were excluded. The reason for this is not clear. It is possible that the phenols will
affect the unimolecular photolysis of 1-NP in addition to being H-donors so that the overall mechanism becomes more complicated than for 3-NF. A reaction sequence involving hydrogen abstraction by excited nitro-PAHs is proposed below:
Ar-NO2 + hν f 1Ar-NO2* f 3Ar-NO2* 3
(2)
Ar-NO2* + Ph-OH f Ar-NO2H• + Ph-O• f products (3)
Interestingly, only phenolic hydrogen donors are able to accelerate the photodegradation. Other potential hydrogen donors such as toluene or 2-propanol with relatively weak C-H bonds do not have any effect. This suggests that hydrogen abstraction (reaction 3) may proceed via a partially hydrogen-bonded exciplex. In the case of 1-NP, a 1-hydroxypyrene peak was observed both in the control samples and in samples containing H-donors. In all experiments with efficient H-donors, several early eluting peaks were observed to build up with time in the HPLC chromatograms, both in experiments with 1-NP and 3-NF. These were not seen in the control samples without cosolutes. The early eluting peaks presumably results from formation of polar reaction products, perhaps via ringVOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Photodegradation of 1-nitropyrene in the presence of anthraquinone (AQ), chrysene (CHR), fluorenone, anthrone, and 2,4,6trimethylbenzaldehyde (TMB). C0 ) 1 × 10-4 mol l-1.
TABLE 3. Photochemical Classification of Oxy-PAHs compound
excited-state charactera O (1O2)b
Reactive Oxy-PAHs anthraquinone (cyclohexane) nπ* anthraquinone (MeOH) nπ* anthrone nπ* xanthone nπ*c 9,10-phenanthrenequinone nπ*c benzanthrone xanthone (MeOH) fluorenone a
Unreactive Oxy-PAHs ππ* ππ* ππ*
Indicates the nature of the lowest excited triplet state. yield, from Gorman et al. (53). c Solvent dependent.
0.2 0.2 0.25 0.33
9
ArCO + hν f 1ArCO* f 3ArCO* 3
0.33 0.7 b 1O quantum 2
opening reactions, since the corresponding UV spectra do not resemble those of pyrene or fluoranthene derivatives. To summarize, several different phenolic compounds, including hydroxy-PAHs and methoxyphenols, are able to accelerate the photodegradation of nitro-PAHs. This effect is partly ascribed to reaction 3, but for compounds which are able to undergo phenolic O-H photodissociation upon irradiation, for example 2-hydroxynaphthalene (49), radical reactions initiated by this photodissociation are also important and perhaps dominating. Influence of Photosensitizers. As discussed above, nitroPAHs do not appear to undergo photooxidation. However, a number of potential sensitizers of reactive singlet oxygen (1O2) are present on combustion aerosols, most notably PAHs and oxy-PAHs. It was therefore important to investigate if 1O formation could influence the photodegradation of nitro2 PAHs. Experiments were performed in which 1-NP and 3-NF were irradiated in the presence and absence of PAHs and oxy-PAHs. All of these compounds except perylene are efficient 1O2 sensitizers (53). Examples of the decays are shown in Figure 5, and the experiments are summarized in Table 1. As can be seen PAHs had no observable effect on nitroPAH decays. Furthermore, only those oxy-PAHs with the lowest triplet state possessing (nπ*) character had an effect (see Table 3). The effect of (3nπ*) oxy-PAHs is discussed in a subsequent section. The ability to sensitize 1O2 is clearly 794
not related to the nitro-PAH photodegradation. 1O2 is undoubtedly formed in the presence of the sensitizers and the lack of effect on the nitro-PAHs can be explained by the following: (a) the lifetime of 1O2 in cyclohexane (τ ) 23 µs (53)) is too short for it to be formed by energy transfer, diffuse, encounter, and react with another molecule (nitro-PAH), (b) 1 O2 does not react with nitro-PAHs to a significant degree as previously discussed, or (c) the reaction of 1O2 with the sensitizer itself is very efficient, so that 1O2 does not escape the solvent cage after its formation. It has been observed that the decay of benz[a]anthracene was inhibited by benzanthrone in toluene (τ ) 29 µs (53)) due to energy transfer but was accelerated in benzene-d6 where the lifetime of 1O2 is much longer (19). The lifetime of 1O2 in organic aerosols is expected to be comparable to the lifetime in toluene or cyclohexane. It thus seems that reactions of sensitized 1O2 with nitro-PAHs are unlikely to play an important role. Some PAHs and oxy-PAHs may also sensitize triplet nitroPAHs (42) if they possess sufficiently high triplet energies (e.g., chrysene and anthraquinone), but this process alone does not lead to accelerated decay, since both PAHs and oxy-PAHs should have an effect. Radical Formation by Carbonyl Compounds. As noted above oxy-PAHs with an (nπ*) triplet state configuration strongly accelerates the nitro-PAH decay (Figures 5-7). It is well-known that oxy-PAHs with this triplet state configuration efficiently abstracts hydrogen from donors, e.g., cyclohexane, whereas oxy-PAHs with a (ππ*) triplet state configuration usually does not abstract hydrogen (54). The only reasonable explanation for the effect of oxy-PAHs is that hydrogen abstraction by an oxy-PAH triggers radical chain reactions (see reactions 4 and 5 below, where ArCO denotes an oxyPAH), which ultimately lead to nitro-PAH degradation.
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 5, 2000
•
(4)
•
ArCO* + RH f ArCOH + R f radical chain reactions (5)
The hydrogen abstraction reaction initially produce ketyl radicals (ArCOH•) and alkyl radicals (R•), which in the presence of O2 will react further to give reactive products including HO2 (49, 55, 56) and possibly OH radicals (49). Other products include alcohols and hydroperoxides (49). A further confirmation of the proposed mechanism was achieved by irradiating 3-NF in the presence of xanthone both in cyclohexane and in methanol. As can be seen in Figure 6, the nitro-PAH photodegradation in cyclohexane was very fast, whereas in methanol there was no effect of xanthone. The reason for this solvent effect is that the triplet state of xanthone is inverted from the reactive (nπ*) state in nonpolar cyclohexane ((dielectric constant) ) 2.02) to the unreactive (ππ*) state in polar methanol ( ) 32.66) (54). The same experiment was performed in ethyl acetate ( ) 6.05) with the same result as in cyclohexane. As a control the experiments were performed with anthraquinone for which solvent induced state inversion does not occur (57) and as expected the nitro-PAH decay was accelerated in all three solvents. Another experiment to verify the mechanism was performed in which 1-NP and 3-NF was irradiated in CCl4 in the presence of anthraquinone. The results of this experiment are presented in Table 4. No effect of anthraquinone was observed in CCl4 for either 1-NP or 3-NF. Additionally, anthraquinone itself decayed slower in CCl4 (91% remaining after 90 min.) compared to experiments in cyclohexane where normally less than ∼60% remained. Clearly, the ability of the oxy-PAH to abstract hydrogen from the surroundings determines its effect on nitro-PAH photodegradation but also seems to govern its own photostability. 2,4,6-Trimethylben-
FIGURE 6. Photodegradation of 3-NF in the presence of xanthone in methanol (MeOH) and cyclohexane and in the presence of 2,4,6trimethylbenzaldehyde (TMB). C0 ) 1 × 10-4 mol l-1.
TABLE 4. Percentage Remaining after 90 mina
a
irr conditions
1NP
3NF
AQ
CCl4 CCl4 + AQ
26% 38%
96% 97%
91%
Irradiation in CCl4 with and without AQ.
zaldehyde was similarly able to accelerate the degradation of 1-NP and 3-NF (see Figures 5 and 6). This is also ascribed to radical formation following hydrogen abstraction by the triplet state aldehyde. 3,4,6-Trimethoxybenzaldehyde had only a small effect, which is not unexpected since strongly electron donating substituents (methoxy-groups) on a carbonyl can lead to triplet state inversion or at least mixing of the (nπ*) and (ππ*) triplet states resulting in a lowered reactivity (54). Alkyl-substituted benzaldehydes are significant constituents of exhaust particles from gasoline vehicles and are present on diesel particles (36), whereas methoxysubstituted benzaldehydes are constituents of wood smoke (37). The decay rates of 1-NP and 3-NF in the presence of reactive carbonyl compounds were comparable, although 3-NF did appear to decay slightly faster. In an experiment in which samples of 1-NP, 2-NF, and 3-NF were irradiated in an excess of anthraquinone, 2-NF decayed a little faster than 1-NP and 3-NF (see Figure 7A). In a similar experiment with 2-NF and 2-NP, 2-NP was observed to decay faster than 2-NF (see Figure 7B). These experiments suggest that the reactivity of radical species toward nitro-PAHs decrease in the order 2-NP > 2-NF > 3-NF ∼ 1-NP, although the differences are minor. The effect of oxy-PAHs and alkyl-benzaldehydes is not specific for nitro-PAHs but is a general degradation pathway for all organic compounds associated with aerosols that contain oxy-PAHs. Anthraquinone, phenanthrenequinone, xanthone, and 2,4-dimethylbenzaldehyde have previously been observed to accelerate the photodegradation of benz[a]anthracene (19). The photostability of the individual oxy-PAHs is also strongly dependent on the ability to abstract hydrogen. The
FIGURE 7. Comparison of the effect of anthraquinone on the photodegradation of different nitro-PAHs. A. CAQ ) 6 × 10-4 mol l-1, C0 ) 1 × 10-4 mol l-1. B. CAQ ) 2 × 10-3 mol l-1, C0 ) 1 × 10-4 mol l-1. hydrogen atom acceptors (nπ*) were observed to decay rapidly compared to the unreactive oxy-PAHs (ππ*). The results obtained in this work allow a straightforward classification of oxy-PAHs with respect to (a) the radical formation potential and thus the effect on other aerosol constituents and (b) the photostability of the individual oxyPAHs (see Table 3). Comparison with Smog Chamber Experiments. To assess the environmental significance of the types of mechanisms proposed to occur in the organic phase of combustion aerosols, it is natural to compare our results with the detailed smog chamber study of nitro-PAH photostabilities by Fan et al. (1). In that study it was found that, associated with diesel particles, the photolysis rate constants of 1-NP, 2-NP, 2-NF, and 3-NF were not very different (knitro-PAH ) (0.022-0.035) × kNO2). The same was observed when the compounds were associated with wood smoke particles, but the decay was significantly faster (knitro-PAH ) (0.045-0.050) × kNO2). When the relative nitro-PAH photolysis rates in pure solution (see Figure 1) are compared with smog chamber experiments with the same compounds associated with wood smoke or diesel particles (1), it is clear that simple unimolecular photolysis is an unimportant pathway for nitro-PAHs VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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on these types of particles, although for 1-NP it may contribute to the overall decay. Since we found that the nitro-PAHs were degraded more rapidly in the presence of phenolic aerosol constituents, reaction 3 or radical production via phenolic photodissociation could be responsible for the reactivities. However, hydrogen abstraction (reaction 3) would probably be more important for 3-NF than for 1-NP on wood smoke particles since some of the potential H-donors are o-methoxyphenols, which as mentioned previously have no effect on 1-NP photodegradation. Wood smoke particles, however, also contain other phenols such as hydroquinone (37) which accelerates both 1-NP and 3-NF photodegradation strongly. Nevertheless, if reactions 2 and 3 were the dominant process on wood smoke particles, 3-NF would be expected to decay somewhat faster than 1-NP, which was not the case in the smog chamber experiments (1). Of course, it cannot be ruled out that this reduced reactivity of triplet 1-NP is compensated for by monomolecular photolysis. In contrast to 1-NP and 3-NF the photodegradation of 2-NF and 2-NP was not accelerated in the presence of phenol and was accelerated to a lesser extent in the presence of hydroquinone (see Figure 3 and Table 2). Based on the above discussion it seems clear that the effect of phenolic aerosol constituents alone cannot explain the smog chamber observations, although it is possible that radical production via phenolic O-H photodissociation may contribute to the decay on ambient aerosols, especially in the presence of hydroxyPAHs. As mentioned previously 2-NF and 2-NP (as well as 1-NP and 3-NF) decayed rapidly in the presence of the radical precursor, anthraquinone (see Figure 7A,B). This and the reduced effect of phenolic compounds indicate that radical chain reactions are the main degradation pathways for 2-NF and 2-NP. This is strongly supported by the fact that 2-NP decayed slightly faster than 2-NF both in our experiments (in the presence of anthraquinone, see Figure 7B) and on diesel particles, where the relative rates were k2-NP ) (0.0300.035) × kNO2 and k2-NF ) (0.022-0.025) × kNO2 (1). We propose that radical chain reactions initiated by reactions 4 and 5 is the dominant mechanism for nitro-PAH photodegradation on both diesel particles and wood smoke particles. In addition, hydrogen abstraction by the excited triplet state (reactions 2 and 3) may contribute to the degradation of 1-NP and 3-NF. It is feasible to explain the faster decay on wood smoke observed for all nitro-PAHs by a higher concentration of radical precursors or more efficient radical formation in the more polar organic phase of wood smoke. Oxy-PAHs and substituted benzaldehydes are present on both diesel particles (36) and wood smoke particles (37, 58). A number of other aromatic carbonyl compounds are also present on wood smoke particles, e.g., substituted acetophenones and phenylacetones (37). In addition to oxyPAHs emitted by a variety of combustion sources (see Table 1), these are also expected to be produced by gas and particle phase reactions of the parent PAHs (35, 41, 59). Therefore, oxy-PAHs are expected to be present in the same environments as the ubiquitous PAHs. The occurrence of the radical precursors and the radical yields are, however, highly uncertain and subject to large variations. Environmental Implications. In ambient polluted air semivolatile organic compounds (SVOCs) have been observed to be absorbed (rather than adsorbed) in organic combustion aerosols (32). These aerosols could be due to diesel vehicles, wood burning, gasoline vehicles, and other combustion sources (60). In addition, secondary organic aerosol represents an important sorption medium for organic compounds in the atmosphere (32). In this work, we have demonstrated that the fate of particulate nitro-PAHs will be highly de796
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pendent on the chemical composition of the aerosols. Members of several different compound classes are able to accelerate the nitro-PAH decay rates significantly. A further level of complexity is introduced when it is taken into account that the chemical composition will change with time due to chemical reactions and gas-particle conversions. We propose that an important factor governing the overall aerosol chemistry is the presence of reactive carbonyl compounds such as certain oxy-PAHs, which can trigger radical chain reactions and ultimately change the chemical composition. We have demonstrated this to be the case for nitro-PAH degradation, and consequently, the photodegradation of nitro-PAHs may be faster on aged organic aerosols than on freshly emitted aerosols. Radical reactions on organic aerosols may also change the hygroscopic properties of the aerosols through the production of polar compounds. Adsorbed on coal fly ash 1-NP has been reported to be stable toward photolysis (61). The same has been observed for unsubstituted PAHs (62), especially for the carbonaceous fraction of the coal ash. It was speculated by these authors that this was due to light shielding, but energy transfer from the excited molecule to the graphitic surface to which it is attached also provides a plausible explanation. Coal fly ash is a complex inhomogeneous mixture characterized by a relatively high mineral content (61, 62) and a low content of organic carbon (e.g., less than 1% (63)), whereas diesel soot and wood smoke consist almost exclusively of organic and elemental carbon (21). However, elemental carbon in the atmosphere is probably under most conditions coated with organic carbon and water (21, 29, 30) and condensation of organic vapors is therefore not expected to occur directly to graphite surfaces in the atmosphere. Nevertheless, because of diffusion through the organic coating a certain fraction of a given SVOC will be adsorbed onto elemental carbon with time (23). This fraction will probably be less susceptible to direct photolysis but may still be subject to radical reactions such as those outlined in this paper.
Acknowledgments The authors are most grateful to Prof. Richard M. Kamens, University of North Carolina at Chapel Hill (U.S.A.), for lending us the photochemical turntable setup applied in this work. This work has been supported by a grant from the Danish Research Academy and by the Danish Natural Science Research Council.
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Received for review May 18, 1999. Revised manuscript received November 8, 1999. Accepted December 6, 1999. ES990566R
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