Environ. Sci. Techno/. 1995, 29,2654-2660
Benz[a]anthracene Photodegradation in the Presence of Known Organic Constituents of Atmespheric MYOSEON JANG AND STEPHEN R . MCDOW'
Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7400 ~~
~
Benz[alanthracene photodegradation rates in toluene solutions containing co-solutes were compared using a photochemical turntable reactor. Each cosolute investigated was found at relatively high concentrations in atmospheric particulate matter. Benz[ a]a nthr a c e ne p hotod e g rad atio n w a s a c c el e r ated by the presence of g,lO-anthraquinone, xanthone, 2-furaldehyde, 2,4-dimethylbenzaldehyde, 9,lOphe nanthrenequinone, 2-ac etylf ur an, and f urfuryI alcohol. Decay was inhibited by 7-benzanthrone. Other compounds had little or no effect on benz[a]anthracene decay. Possible photochemical reaction mechanisms are discussed, and it is concluded that several competing mechanisms may be responsible, including electronic energy transfer followed by reaction from the triplet state, singlet oxygen attack, and radical chain reactions initiated by hydrogen abstraction of aerosol constituents. The results suggest that particle organic composition can strongly influence polycyclic aromatic hydrocarbon photodegradation rates in atmospheric aerosols.
Introduction Polycyclic aromatic hydrocarbons (PAHs) associated with atmospheric particulate matter decay rapidly in sunlight under appropriate atmospheric conditions. Their decay rates are strongly influenced by light intensity,temperature, water vapor concentration, and other atmospheric parameters (1). A number of studies have demonstrated that the nature of a solid particle surface strongly influences PAH decay rates (2-6). In contrast, relatively little attention has been given to the effect of particle-associated organic substances on P A H decay (7, 8). Under atmospheric conditions favorable for PAH phototransformation, PAH containing aerosol particles from common sources such as wood smoke or diesel soot are likely to consist of an elemental carbon core surrounded by a relatively thick, possibly liquid, organic layer of which PAHs represent only a small fraction (9). Organic particle constituents are also
2654 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10, 1995
likely to play an important role in determining atmospheric PAH photodegradation rates (7). In previous experiments, much faster PAH decay was observed in solutions composed 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 (7). Using vanillin, a representative methoxyphenol, and benz[alanthracene,a representative PAH,in toluene solution as a model system, a second-order reaction rate was observed at sufficiently high vanillin concentrations (8). Structure-reactivity relationships indicated that hydrogen abstraction from vanillin was probably the rate-limiting step of benz[alanthracene photodegradation in this system (8).
In further experiments, we investigated the effects of other important classes of organic compounds abundant in wood smoke,diesel soot,or gasoline-poweredautomobile exhaust on benz[alanthracene photodegradation. Preliminary results suggested that some members of four of these classes-methoxyphenols, polycyclic aromatic quinones and ketones, substituted benzaldehydes, and substituted furans-strongly influenced benz[a]anthracene decay (9).In this paper, we report detailed observations of benz[alanthracene decay in the presence of known organic constituents of atmospheric particulate matter and explore possible reaction mechanisms.
Experimental Section Irradiation experiments were conducted using a photochemical turntable reactor (Ace Glassware, Vineland, NJ), The Wlight source was a450-Wmedium-pressure mercury arc lamp, which was placed in a borosilicate immersion well to filter out high energy W bands not encountered in the troposphere. The reactor was submerged in a 70-L water bath, and the temperature was maintained at 16.5 f 0.5 "C by pumpingthe water at 75 mLlmin through exterior copper coils in an ice bath. All solutions were prepared for irradiation in 4 mL of toluene (FisherT291-4, optima grade) or benzene-& (99.5 atom 7% D, Aldrich) in 13 mm x 100 mm quartz reaction tubes designed for use with the turntable reactor (Ace Glassware). Each solution contained approximately M benz[alanthracene and M co-solute to be tested for its effect on photodegradation of benz[a]anthracene. A total of five experiments was conducted, each including three different solutionscontaining benzIa1anthraceneand a co-solute. The composition of each solution before irradiation is described in the first three columns of Table 1. Chemical structures of the dissolved compounds are presented in Figure 1. All chemicals were purchased from Aldrich. A total of 180 pL was removed from the reaction tubes for analysis at regular intervals. A total of five irradiationexperimentswas conducted, with several sample solutions irradiated simultaneously. In each experiment, two reference solutions were also irradiated. One of these was a M benz[a]anthracene (I) solution in toluene. The other was a toluene solution of M benz[a]anthracene and M vanillin (11). The main purpose of these experiments was to determine whether any of the co-solutes strongly accelerated or inhibited benz[a]an-
0013-936W95/0929-2654$09.00/0
0 1995 American Chemical Society
TABLE 1
Solution Composition and Benz[a]antkracene Decay Rate Constants benz[e]anthraceneconcn (MI
co-solute
Experiment 1 10-5 10-5 10-5 10-5 10-5
9,lO-anthraquinone (111) xanthone (IV) oleic acid (V) vanillin no co-solute
1.15 1.15 1.15 x 1.15 1.15 x
y-butyrolactone (VI) 2-furaldehyde (VII) 2-naphthaldehyde (VIII) vanillin no co-solute
Experiment 2 1.20 x 10-5 1.20 x 10-5 1.20 10-5 1.20 10-5 1.20 10-5
4-hydroxybenzoic acid (IX) 2,4-dimethylbenzaldehyde (X) 9,lO-phenanthrenequinone (XI) vanillin no co-solute
1.20 1.20 1.20 1.20 1.20
7-benzanthrone (XII) isoquinoline (XIII) N,N-dibutylformamide (XIV) vanillin no co-solute
1.00 x 1.00
1.00 1.00 1.00
Experiment 5 1.10 10-5 1.10 10-5 1.10 x 10-5 1.10 10-5 1.10 10-5
2-acetylfuran (XV) furfuryl alcohol (XVI) 2,3-benzofuran (XVII) vanillin no co-solute
& /
*
/
G CHO IIO C H 3
111
I
& IV
QO
VI
6 CO,H
+
CH,(CH,),CH = CH(CH?)&02H
V
a,u xv
0.99 x 1.11 x 1.00 x 1.34
10-3 10-3 10-3 10-3
1.00 x 10-3 1.00 10-3 1.17 x 10-3 1.11 x 10-3 1.02 x 10-3 1.00 x 10-3 1.11 10-3 1.03 x 10-3
1.00
10-3
1.00 10-3 1.00 10-3 1.06
10-3
1.42 x 10-3 1.00 x 10-3 1.00 x 10-3 10-3
1.01
kDb(min-'1
rz
fast fast 0.0093 fast 0.0088
0.966
0.0102 fast 0.0085 fast 0.0106
0.957 0.969 0.981 0.962
0.0087 fast fast fast 0.0100
0.951
0.0009 0.0082 0.0094 fast 0.0107
0.91 1 0.963 0.91 1
fast fast 0.0100 fast 0.0100
0.922
0.967
0.992 0.973
decay in the absence of any co-solutes. 4-Hydroxybenzoic acid did not completely dissolve,so the initial concentration cannot be considered accurate. To explore the possibility that the reaction involves singlet oxygen, two solutions, one containing M benz[a]anthracenein the presence of M 7-bemanthrone and the other containing M benz[a]anthracene only were prepared in deuterated benzene. Quantitative analysis was carried out with a HewlettPackard 5890 gas chromatograph interfaced to a 5971A mass selective detector (MSD). Samples were introduced by splitless injection using a Hewlett-Packard 7673 autoinjector. The column was a 30 m, 0.32 mm i.d., J&W DB-5 fused-silica capillary column with a 0.25pm film thickness. The temperature program was as follows: 120 "C for 2 min, 120-250 "C at 15 "C/min, 250-300 "C at 8 "Clmin, and hold for 3 min at 300 "C. The MSD scanned from 50 to 550 amu at 1.2 stands. Chrysene-dlzwas added to each sample immediately after removal from the reaction tube for use as an internal standard in quantitative analysis.
acHo Vlll
@ XI
IX
XI1
x x x x
Experiment 3 10-5 10-5 10-5 10-5 10-5 Experiment 4 10-5 10-5 10-5 10-5 10-5
co-solute concn (M)
Selection of Reactants XI v
m XVll
FIGURE1. Chemical structures of benz[a]anthracene and co-solutes used in irradiation experiments.
thracene decay. Experimental conditions were optimized for the determinationof rate constantsof bem[alanthracene
Table 2 lists reported ambient concentrations and emission sources for the co-solutes used in this study. Only a small fraction of the organic mass associated with atmospheric particulate matter can be accounted for by structurally identified and quantified organic compounds (10). Most quantitative data available concerning particle-associated organic species are generally presented as ambient concentrations of individual compounds. There have been few attempts to quantify the fraction of total particulate organic mass accounted for by important classes of organic compounds. A series of papers by Rogge et al. (10-131,in VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2655
TABLE 2
Obsenations of Irradiation Experiment Co=Solateson PIlziculate Matter compound
9,lO-anthraquinone (111)
9-xanthone (IV) oleic acid (V)
y-butyrolactone (VI) 2-furaldehyde (VII) 2-naphthaldehyde (VIII) 4-hydroxybenzoic acid (IX) dimethylbenzaldehydes (e.g., X) 9,lO-phenanthrenequinone (XI) 7-benzanthrone (XII)
isoquinoline (XIII) N,N-di butylformamide (XIV) 2-acetylfuran (XV) furfuryl alcohol (XVI) 2,3-benzofuran (XVII)
particle type
amount
ref
ambient air diesel soot gasoline soot wood smoke other sources diesel soot gasoline soot other sources ambient air diesel soot gasoline soot other sources wood smoke wood smoke gasoline soot ambient air diesel soot gasoline soot diesel soot gasoline soot other sources ambient air diesel soot gasoline soot wood smoke other sources ambient diesel soot gasoline soot diesel soot gasoline soot wood smoke wood smoke wood smoke
0.22-1.89 nglm3 23.5 pglkm 4.4-24.3 pglkm 0.5-50 pglg 0->10 mglg 2.7 pglkm 2.6-29.4 pglkm 0->1 mglg 100pg/g I mgig
