Environ. Sci. Technol. 1993, 27, 2896-2908
Seasonally Dependent Size Distributions of Aliphatic and Polycyclic Aromatic Hydrocarbons in Urban Aerosols from Densely Populated Areas Merce Aceves
Environmental Control Service, Entitat Metropolitana de Barcelona, Carrer 62, 420 Edifici A Zona Franca, 08004-Barcelona, Catalonia, Spain Joan 0. Grlmalt'
Department of Environmental Chemlstry (C.1.D.-C.S.I.C.), Jordi Girona, 18 08034-Barcelona, Catalonia, Spain Aerosol samples corresponding to well-defined cold/warm seasons in the atmosphere of Barcelona have been collected and size-fractionatedduring stable atmospheric conditions. A predominant hydrocarbon pattern derived from gasoline and diesel vehicular traffic emissions has been identified in most particle fractions. In quantitative terms, these hydrocarbons exhibit a predominant occurrence in the submicron fractions, particularly the 202 for the 0.56-0.96- and 7.2 pm, second stage 7.2-3 pm, third stage 3.0-1.5 pm, fourth stage 1.5-0.96 pm, fifth stage 0.96-0.5 pm, and backup filter 3 pm) are those exhibiting lower dispersion in both seasons. The smaller particle-sizefractions (C1.5pm) exhibit a higher dispersion in summer than in winter. This difference is probably related to the slight decrease in TSP and in the proportion of smaller particles that is generally produced in the warm seasons (22). 2898
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humidity, mean (%)
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The impactor stage fractionation shows that Barcelona aerosols can be described by two size-dependent lognormal distributions, one corresponding to the accumulation mode (0.08-2 pm) and the other to mechanical erosion (>2-2.5 pm) (22). Each mode represents about half TSP, and their mass median diameters (MMD) ranges are 0.36-0.40 and 4.6-6.0 pm, respectively (Table 111). These bimodal distributions are currently observed in impactor stage studies from urban areas (e.g., refs. 18and 23-25). The MMD of the lower particle diameter mode are smaller than the 50% efficiency cutoff diameter of the last impactor stage, 0.5 pm, and have therefore been calculated by extrapolation in the cumulative mass plots. Measurements in urban atmospheres with impactors affording higher resolution in the submicron range have shown that the particles 7.2 pm) and finer (C0.5 pm) aerosol fractions are shown in Figure 1. These traces are dominated by n-alkane distributions ranging between C17 and c36 without odd-even carbon number predominance in the c17-c27 range and a slight odd carbon number preference in the c29-c33 range. The c29--c33 distributions are characteristic of higher plant wax sources (26). Other characteristic resolved peaks encompass regular isoprenoids, pristane and phytane, and c27C3417a(H),2l@(H)-hopanes.The GC-resolvedcompounds overlay a UCM of hydrocarbons eluting over all the temperature range of the chromatogram. The n-alkanes without odd-to-even carbon number predominance occurring together with pristane, phytane, 17a(H),21/3(H)-hopanes and a UCM of aliphatic hydrocarbons are characteristic of petroleum residues (27,28). In fact, these chromatographic traces are similar to the aliphatic hydrocarbons resulting from combined exhaust particles of gasoline and diesel-powered vehicles (29).The portion of UCM eluting at higher retention time and the 17a(H),2l@(H)-hopanescorrespond to unburned lubricating oil residues (30, 31). This lubricating oil contribution may originate from leakages in the valve stem seals or piston ring of the engines (32). The coarser particles contain a higher relative proportion of hydrocarbons eluting at higher temperatures, including n-alkanes and UCM. As shown in Figure 2, this is a general trend that results from the strong decrease in hydrocarbon concentration, and particularly the lower chain-length n-alkanes, at increasing size. Thus, the GC profiles of the coarser particles are also enriched in the odd carbon
Table 111. Mass Size Distribution of Aerosol Samples Described in Table I
sample
1
concentration (pglm3) impactor stage 2 3 4 5 filter
MOL, March 25 20 36 31 MOL, June MOL,December 40 38 64 43 PN, May 70 58 PN,December a MMD, mass median diameters.
8.2 12 19 14 24
6.7 15 21 14 24
8.5 24 31 15 24
35 68 100 54 110
size distribution total
total
fine MMDa
GSDb
total
coarse MMDa
GSDb
104 190 250 205 310
58 120 170 98 180
0.36 0.39 0.36 0.40 0.37
0.64 0.54 0.46 0.70 0.58
60 95 120 135 180
6.0 5.5 4.9 6.0 5.5
0.52 0.51 0.54 0.51 0.46
GSD, geometric standard deviation.
the samples from the spring/summer seasons. Again, this trend appears to be related to the strong decrease of lower chain-length homologues in the warm seasons and is consistent with the lower amount of UCM in spring/ coeff of variation ( % ) equivalent cutoff relative mass ( % ) summer aerosols (Figure 3). However, in terms of absolute diameter (pm) winteP summeP winteP summeP concentrations the changes in odd carbon numbered C29>1.2 16 22 0.62 0.64 C33 n-alkanes are not significant. The small increase in 7.2-3.0 15 18 4.1 1.9 the June sample may reflect a higher emissionby terrestrial 3.0-1.5 7.6 7.3 7.9 8.0 vegetation (33,34). 1.5-0.96 8.4 7.2 2.4 8.0 0.96-0.5 12 10.5 4.2 6.0 An integrated description of these aerosols is obtained C0.5 41 35 1.9 4.3 bv remesentation of the summed size distributions of the n:alk^anes (Figure 4). These accumulated plots allow an a n = 10, estimation of the significance of the size-fractionated air particulates described in Figures 1-3 because they can be numbered n-alkane distributions indicated above. Howcompared with the n-alkane profiles found in the aerosols ever, the absolute concentrations of the higher molecular regularly collected by glass-fiber filtration. Accordingly, homologues, including the vascular plant n-alkanes, are the average n-alkane distributions of the aerosols obtained higher in the smaller aerosol sizes and exhibit the highest in the same stations and in the same months during 1985values in the smallest fraction ( 7.2 pm
I
I
> 7.2 pm
< 0.5 pm
Flgure 1. Representative gas chromatographic profiles of the allphatlc and polycycllc aromatic hydrocarbons In the coarse and fine fractions of Barcelona aerosols. Numbers in allphatlc hydrocarbons refer to +alkane chain lengths. Pr, prlstane. Ph, phytane. Hp, 17a(H),21&H)-hopanes. PAH: A, phenanthrene;B, anthracene;C, fluoranthene;D, pyrene; E, benzo[ghJfluorene;F, 4(H)-cyclopenteno[cdlpyrene; 0,benz[e]anthracene; H, chrysendtrlphenylene; I, benzo[b/dfluoranthene; J, benzo[e]pyrene;K, benzo[a]pyrene;L, lndeno[1,2,3-cd]pyrene; M, benzo[ghJperylene; N, coronene. Sq, squalene. Environ. Sci. Technol., Vol. 27,
No. 13, 1993 2899
MOLINA DECBMBW
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Figure 2. Concentrations of +alkanes in the different particle-size fractionsobtained from the samples descrlbed in Table I. Concentrations in ng/m3.
collected under atmospheric conditions similar to those indicated in Table I. The n-alkane profiles shown in Figures 4 and 5 exhibit a remarkable parallelism in terms of qualitative composition and total concentration, suggesting that the size-fractionated aerosols selected for study are representative of the aliphatic hydrocarbon composition found in the city during the cold and warm seasons. On the other hand, the distributions displayed in Figure 4 show again that the predominance of higher plant hydrocarbons in the samples corresponding to the warm
seasons is essentially due to a decrease of the lower molecular weight n-alkane homologues. The predominant occurrence of these odd carbon numbered C29-C33 n-alkanes in the smallest size fraction (7.2 pm, is a biogenic product representative of microorganism contributions (37). As in the case of the n-alkanes, the compilation of the summed particle-size PAH distributions (Figure 6) provides an integrated description that can be compared with the composition of the airborne particulate PAH collected with glass-fiber filtration. These reference PAH distributions are shown in Figure 7 and result from the averaged PAH composition in the same series of samples used in Figure 5. The similarity between the plots corresponding to the samples obtained by cascade impaction and total filtration is remarkable [note that anthracene and 4(H)cyclopenteno [cd] pyrene were not analyzed in the glassfiber filters], which again supports the significance of the
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IFRflCTION I FRflCTION I I
Figure 3. Concentrationsof the unresolved complex mixture of hydrocarbons in the dlfferent particle-size fractions separated from the Barcelona aerosols. Fractions I and I1 refer to the column chromatography separation described in the Experimental Section. Concentrations in ng/m3. 2900
Environ. Sci. Technol., Voi. 27, No. 13, 1993
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Flgure 4. Summed aerosol size-fractiondistributions of Ralkanes. Concentrations in ng/m3 MOLINA-MARCH
POBLE NOW-DECEMBER
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Flaure 5. AveraQe+alkane disiributions (each . .Olot n = 30-50) from aerosois coilected by glass- fiber filtration during 1985-1992. Concentrations I
in ng/m3.
size-fractionated aerosols selected for study. The examination of selected ratios from the data represented in Figures 6 and 7 provides a better understanding of the relative predominance of different PAH and their origins. These ratios are summarized in Table V. The fluoranthene/(fluoranthene pyrene) ratio generally ranges between 0.40 and 0.45, which is coincident with the values reported for exhausts of gasoline-fueled vehicles (38). In winter, the indeno[l,2,3-cdlpyrene/ (indeno[l,2,3-cdlpyrene + benzo[ghilperylene) ratio is 0.434.44 in PN and 0.36-0.38 in MOL; the first value is close to that reported for emissions from diesel vehicles (39) and the second is intermediate between the ratios observed for gasoline (40)and diesel-powered(39)vehicles.
+
The indeno [1,2,3-cdl pyrene/ (indeno[ 1,2,3-cdlpyrene + benzo[ghilperylene) values slightly decrease in summer, 0.42 and 0.314.35 in P N and MOL, respectively, which may reflect a lower contribution from minor combustion sources involving higher thermal processes such as domestic beating (41-43). This difference between the two seasons is unlikely due to photochemical reactions enhanced by higher ambient temperatures. Benzo[ghilperylene is morereactive than indeno[l,2,3-cdlpyrene (44, 45),and a photochemically induced change would involve an index increase, not a decrease. The benzo[elpyrene/(benzo[elpyrene + benzo[alpyrene) ratio is also seasonally dependent, with values of 0.45-0.50 in winter and 0.644.73 in spring/summer. The Environ. Sci. Technol., VoI. 27. NO.
13. 1993
2901
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Figure 6. Summed aerosol size-fraction distributions of poiycyclic aromatic hydrocarbons. Leners as in Figure 1. Concentrations In nglm3.Letter asslgnmenls as in Figure 1. MOLINA-DECEMBER
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Flgure 7. Average PAH dlstrlbutions (each plot n = 30-50) lrom aerosols cOllected by glassfiber filtration during 1985-1992. Concentratlons In nglm3. Lener assignments as in Figure 1.
former are characteristic of emissions from gasoline (38) and diesel (46) vehicles. Contributions from higher thermal sources would involve lower values. The spring/ summer ratios are too high when compared to the values characteristic of vehicular traffic emissions (38). These high ratios are in fact a consequence of the preferential degradation of benzo[alpyrene with respect to henzo[elpyrene after emission to the atmosphere (47). The fast decay of some specific PAH during the warm seasons is also reflected in the decrease of henz[alanthracene with respect to chrysene + triphenylene and in thedisappearanceof4(H)-cyclopenteno[cdlpyrene (47, 48). A similar seasonaltrend has been observed in astudy 2902
Environ. Scl. Techmi.. Voi. 27. No. 13. 1993
performed in New Jersey (49). 4(X)-Cyclopenteno[cdlpyrene is characteristicof gasoline-fueledvehicles (50,5I), although it is also a significant compound in the PAH composition of diesel vehicular exhausts (46). According to these ratios, and particularly the fluoranthene/(fluoranthene + pyrene) and the indeno[l,2,3-cdlpyrene/(indeno[l,2,3-~dlpyrene+ benzo[ghilperylene) ratios, the atmospheric PAH in MOL and P N stations essentially originate from emissions of gasoline and diesel vehicles. This is also in agreement with the aliphatic hydrocarboncomposition. F-kherinsightintotherhative composition of these two sources in the two stations can be obtained from the joint evaluation of a group of the less
Table V. Total Hydrocarbons and Representative PAH Ratios in Aerosol Samples Described in Table I
sample MOL, March MOL, June MOL, December PN, May PN, December
total concentrationsa n-alkanes UCM PAH 180 (140)d 165 (160) 350 (400) 295 (320) 830 (880)
3600 (3200) 1600 (1600) 6100 (5300) 4300 (4800) 12000 (13000)
32 (31) 19 (17) 310 (270) 10 (12) 240 (220)
representative PAH ratios C/(C+D)c G/(G+H) J/(J+K) 0.42 (0.41) 0.42 (0.41) 0.40 (0.38) 0.45 (0.44) 0.42 (0.40)
0.31 (0.30) 0.36 (0.33) 0.35 (0.34) 0.34 (0.34) 0.37 (0.39)
0.72 (0.73) 0.67 (0.64) 0.48 (0.50) 0.65 (0.67) 0.45 (0.47)
GC 8.3 (7.4) 5.2 (4.9) 7.8 (8.4) 11 (10) 11 (14)
normalized signature of major PAHb H J K L M 18 (17) 9.3 (9.8) 14 (16) 21 (21) 19 (21)
16 (16) 17 (16) 14 (12) 16 (17) 14 (18)
6.4 (6.0) 8.1 (9.2) 15 (12) 6.6 (8.4) 18 (14)
15 (16) 17 (17) 15 (14) 16 (15) 13 (14)
29 (28) 32 (32) 25 (25) 23 (22) 17 (18)
N 7.5 (11)
10 (11) 8.7 (12) 6.4 (6.1) 8.2 (7.1)
a Concentration in ng/ma. b PAH with molecular weight higher than 226. Letters refer to Figure 1. Values in parentheses refer to the averaged distributions from the data set obtained during 1985-1992 by glass-fiber filtration.
volatile PAH (benz[a]anthrace>e, chrysene/triphenylene, benzo [e]pyrene, benzo[a] pyrene, indeno[1,2,3-cdlpyrene, benzo[ghi]perylene, and coronene) and from the comparison with the fingerprint characteristic of each source in a chemical mass balance model. With this purpose, the normalized source signature model described by Li and Kamens (46) has been adapted to the data set obtained in MOL and PN, including the use of photochemical degradation rate constants (44, 45). The normalized composition of the seven PAH mentioned above is given in Table V. Again, no major discrepancies are observed between the samples collected by cascade impaction and the reference-averaged set obtained by total filtration. The chemical mass balance of these data shows that the PAH content in MOL reflects a proportion of 30-40% and about 60% for the gasoline and diesel vehicular exhausts, respectively. Conversely, in PN, the PAH distributions correspond to 100% of emissions from diesel vehicles. These results are in agreement with the aboveindicated differences in vehicular traffic characteristic of each station. The accumulated percent of heavy vehicles and diesel-fueled cars gives traffic rates of 15% and 60 % diesel-propelled vehicles in MOL and PN, respectively. The higher proportion of vehicular diesel-sourced PAH with respect to the traffic rate figures is likely a consequence of the higher output of diesel vehicles than gasoline cars [about 30-100 times higher in terms of particulate emission among equivalent sized powered engines (52)l. Size Distribution. In terms of quantitative composition, both the aliphatic and aromatic hydrocarbons are strongly size-dependent (Figures 2-4 and 6). For instance, the PAH concentrations are generally more than 50 times higher in the backup filter than in the first stage. A systematic description of the size dependency of these hydrocarbons is provided by the Lundgren plots. These plots have been calculated taking 0.08 and 30 pm as the lower and upper limits (size diameters). The lower limit is based on the results of urban aerosol studies in which size-fractionation devices with resolution power of 0.10.01 pm were used (23, 24). The second limit is taken according to data reported for aerosols of different origins (53). Another upper limit commonly used in these diagrams is 20 pm (18). The use of this value would represent a 1.4 times increase in the intensity of the upper size fraction (although the total area would not change). Representative examples, samples from MOL station in March and December, of TSP, total n-alkanes, odd carbon numbered C29-C33 n-alkanes, UCM, and total PAH distributions are shown in Figure 8. Similar Lundgren
DECEMBER
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P
n TSP
1
10.
2%
?#-ALKANES
n-ALKANES
1.0
TSP
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m
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a
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UCM 3Dx)
W
3Dx)
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I
2%
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1
PAH
Figure 8. Lundgren size distributions of aerosol collected at MOL station.
diagrams are obtained for the other samples included in Table I. According to Figure 8plots, despite the bimodal particlesize distributions of the aerosol particles, all hydrocarbons exhibit a predominant occurrence in the size fraction lower than 0.5 km. This occurrence is also generally observed at the individual compound level, with no major differences between low or high molecular weight homologues. One question to be considered concerns whether the proportion of hydrocarbons retained in the CO.5-pm fraction could be enhanced by sampling artifacts such as adsorption of gas-phase compounds onto filters (54, 55) or collected Envlron. Sci. Technol., Vol. 27, No. 13. 1993
2903
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’*-----n-
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e
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/
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A n-CZb
alcohol
e “430 alcohol
2~
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,
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r A
Cholcrt-5-en-
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e R-Sltortcrol
n
i l r ;
34
Flgure 9. Concentrations of rrhexacosan-1-01, n-triacontan-1-01, cholest-5-en-3@-01, and @-sitosterol in the different size fractions separated from the Barcelona aerosols. Concentrations in nglm3.
particles (56). However, this artifact effect is minimal for individual n-alkanes >C25 and PAH with molecular weight Torr) (54, >230 (subcooled liquid vapor pressure 57), and these compounds also exhibit a predominant occurrence in the smallest size fraction (C0.5 pm). This hydrocarbon predominance in the C0.5 pm fraction is consistent with the occurrence of a mode of aerosol particles with 0.36-0.40 MMD, indicating that the whole hydrocarbon distribution essentially reflects the gas-toparticle condensation in the submicron range after hydrocarbon emission to the atmosphere and cooling. The proportion of hydrocarbon mass in the fraction
