Environ. Sci. Technol. 1990, 24, 1581-1585
Woodburning as a Source of Atmospheric Polycyclic Aromatic Hydrocarbons Diana J. Freeman and Frank C. R. Cattell"
Graduate School of the Environment, Macquarie University, New South Wales, 2 109, Australia Airborne particulate matter containing polycyclic aromatic hydrocarbons derived from burning natural vegetation and paper products in a variety of ways was collected and analyzed by HPLC. Similar profiles of compounds resulted from most of the combustion sources that do not involve fossil fuels and that are likely to contribute to Sydney's atmospheric particulates. In addition, the profiles did not change markedly as a result of reactions occurring in the atmosphere or from reactions occurring on the filter after collection. Concentrations of benzo[alpyrene and coronene were higher in the polycyclic aromatic hydrocarbons derived from bush fires than from other sources. Bush fires are likely to be a significant source of exposure of the Sydney population to polycyclic aromatic hydrocarbons. ~
~
Introduction Interest in the carcinogenic effects of combustion products dates back at least 200 years, when Sir Percival Pott noted an increase in scrotal cancer among chimney sweeps in London ( I ) . Association of the cancer with a chemical was strengthened in 1933 when the polycyclic aromatic hydrocarbon (PAH) benzo[a] pyrene (BaP) was isolated from chimney soot. Much of the interest in PAHs in air since then has been focused on production from the incomplete combustion of fossil fuels and on the single PAH, BaP, which has been linked with cancer in laboratory studies and among certain occupations (2). Much less work has been carried out to determine the contribution of woodburning to the atmospheric burden of PAHs, although concerns over the price and availability of fossil fuels together with environmental concerns have resulted in increased use of wood as a fuel in many developed countries, including Australia, since the early 1970s. Woodburning can generate high concentrations of PAHs; total concentrations of 3000 pg m-3 and concentrations of 60 pg m-3 for BaP have been measured in flue emissions from small residential stoves compared with ambient PAH concentrations, which are usually of the order of a few nanograms per cubic meter (3). As a result, residential wood combustion can be a very significant source of atmospheric aerosols containing PAHs, particularly in the winter months ( 4 , 5 ) . Greenberg et al. have shown that up to 98% of the BaP in New Jersey during winter could arise from residential woodburning (5, 6 ) . In Australia, in major cities as well as regional centers, the impact of woodburning on air quality has been underestimated in the past. In the state of New South Wales, during 1983,193400 households (11% of the total households) used wood or another solid fuel as the major heating source and this figure had increased to 225000 (14.6% of houses) 2 years later (7). In Sydney, Australia's most populous city, other important potential sources of PAHs that also do not arise from fossil fuels are commercial and private incinerators, combustion of domestic and garden waste in largely uncontrolled situations and, on occasions, bush fires, which although very intermittent in nature can have a very marked effect on air quality for periods of a few days. Recently an estimate for the urban region of Sydney of the contribution of burning of vegetation to air pollution has 0013-936X/90/0924-1581$02.50/0
been obtained based on carbon 14 isotope measurements. These measurements indicate that as much as 91% of particulate carbon arises from nonfossil fuel sources in some leafy suburbs of Sydney, largely from the burning of domestic litter (8). The high concentrations are consistent with estimates that combustion of a tonne of plant waste in a typical backyard burn can generate about 9 kg of particulate matter (9). Even in the central business district of Sydney, one-third of the carbon content of the suspended particulate matter was of recent (i.e., nonfossil fuel) origin, indicating the importance of wood and paper burning within the builtup confines of the city (IO). The only other major sources of particulate material containing carbon in Sydney are emissions from gasoline- and diesel-fueled motor vehicles (11). The purpose of this study was to examine the PAHs produced in Sydney from burning wood and other vegetation and to see if the patterns of PAHs produced from each of the nonfossil fuel sources were sufficiently similar to each other but sufficiently different from the fossil fuel sources to allow a source reconciliation to be carried out on the observed proportions of PAHs in ambient air.
Experimental Section Collection and Extraction of Organic Material. Airborne particulate matter was collected on 20 X 25 cm glass fiber filters (Pallflex Products Co.) for 30 min with a conventional high-volume (hi-vol) sampler having a flow of 1.0 m3/min. The glass fiber filters were equilibrated at constant relative humidity and weighed before and after exposure, and the collected material was extracted with cyclohexane-methylene chloride 40:60 (100 mL) in an ultrasonic bath for 30 min, followed by 2 X 10 min periods with 2 X 50 mL of solvent. The crude combined extracts were partially evaporated under vacuum in a water bath at 40 "C. The remainder of the solvent was evaporated under a stream of purified nitrogen at room temperature. The sample was made to 100 pL with methylene chloride prior to analysis. Analysis of PAHs. The equipment used in this work comprised a Waters HPLC system consisting of two 6000 A pumps, a U6K injector, a Model 450 variable-wavelength optical absorbance detector set at either 254 or 287 nm, a radial compression module, and a radial PAH PAK cartridge (polymeric C-18). The Model 840 data and chromatography control station was interfaced with a Hitachi F-1000 variable-wavelengthfluorescent spectrometer set at an excitation wavelength of 300 nm and an emission wavelength of 400 nm. Routinely, an aliquot of 20 pL was used and eluted by a linear gradient from 40:6G9010 of acetonitrile-water at a constant flow of 2.0 mL/min for 30 min. Identification of the PAHs and calibration was effected by reference to responses of known standards (Supelco Inc.). The validity of the PAH determination in the extract was established by enrichment with known PAHs and analysis at 254 and 287 nm. The response ratios at these two wavelengths were compared with standards. In this study 10 compounds from the standard EPA PAH mix of 16 PAHs were assayed as well as coronene (COR). The EPA standards assayed were fluoranthene (FLU), pyrene (PYR), benz[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF),
@ 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 10, 1990
1581
Table 1. PAA Concentrations (rglg) of Particulate Matter from the Burning of Wood and Other Vegetation sourceD
PAH
1
2
3
4
5
6
7
8
9
10
FLU PYR BaA CHR BbF BkF BaP DbA BghiP IP COR
87.6 102 38.9 63.4 30.2 21.3 27.3 1.2 5.8 10.0 0.7
182 222 13.3 32.3 10.5 4.9 26.2 7.7 11.0 14.0 0.7
54.1 240 10.8 75.6 9.0 7.2 37.2 8.0 6.0 12.3 0.6
70.1 68.1 60.1 64.3 6.2 10.0 70.5 4.3 10.2 20.3 1.0
200 128 38.1 125 6.0 20.7 194 12.2 8.2 19.5 26.0
27.2 38.7 3.9
66.5 73.5 5.5 13.0 1.5 1.3 5.5 0.7 1.5 5.0 1.3
53.2 59.1 8.0 40.0 2.2 1.4 2.2 0.8 2.0 4.0 1.2
31.0 28.0 14.0 28.0 4.9 2.9 21.0 3.5 3.5 14.0 2.8
130 116 43.5 66.7 8.9 7.3 50.8 14.5 14.5 29.0 5.9
6.0
1.1 1.1 6.0 1.0 1.5 3.0 0.1
"Sources: 1. Open wood fire burning Australian native wwd. 2. Open barbecue burning of Australian native plants. 3. Open barbecue cooking of meat with Australian native plants as fuel. 4. Large-scale backyard bum of both native and introduced species and some cardboard products. 5. Largescale bush fire, consumingnative plants only. 6. Leaves from Australian native vegetation burnt on an open fire. 7. Wood from Australian native vegetation bumt on an open fire. 8. Commercial incinerator burning paper and cardboard products. 9. Cigarette mainstream and sideatream smoke. 10. Cigarette sidestream smoke.
benzo[k]fluorantbene (BkF), benzo[a]pyrene (BaP), dibenz(a,h)anthracene (DbA), benzo[ghi]perylene (BghiP), and indeno[cd]pyrene (IP).The more volatile PAHs were deliberately excluded from the study since they can be removed from the filter during collection. Description of Combustion Types Sampled. (1) Open Wood Fire Burning Australian Native Wood. The domestic open wood fire consisted of a grate approximately 10 cm above the floor level upon which wood was rested. The grate was completely open on three sides, with the hack of the fireplace approximately 50 cm from the rear edge of the grate. A copper hood conducted smoke away out of the room and up the chimney. This fire consumed approximately 10 kg of wood per hour and is typical of fires used for heating Sydney homes. (2) Open Barbecue Cooking of Meat with Australian Native Plants as Fuel. The burning of Australian wood with and without meat to stimulate a barbecue took place on a small portable cast-iron barbecue with the supporting grid approximately 20 cm above the base plate, and open on all four sides as well as from above. (3) Large-Scale Backyard B u m of Both Native and Introduced Species and Some Cardboard Products. The large-scale backyard burn consisted of a mass of predominantly old vegetation and some cardboard, 5 m high and approximately 4 m wide and deep. Once the fre had started, it was continuously fed with more vegetation and burnt for about 2 b. This mix of vegetation and cardboard fuels is typical of burns in the gardens of suburban homes. (4) Large-Scale Bush Fire, Consuming Native Plants Only. The large-scale bush fire took place in a National Park on the outskirts of Sydney and represented total uncontrolled burning of native vegetation. (5) Leaves and Wood from Australian Native Vegetation Burnt on a n Open Fire. Leaves and wood were derived from native plants and burnt as described in (2) above. (6) Commercial Incinerator Burning Paper and Cardboard Products. Thii incinerator was typical of one employed on commercial premises. The dimensions of the incinerator body, which consisted of a closed chamber, were approximately 2 X 1X 1m3. Access to the chamber was obtained by opening the casbiron door at ground leveL The incinerator body was fitted with a chimney approximately 5 m high. For all of the collections the sampling period was less than 1h and the sampler, which was typically 3 m from the combustion murce, was moved to ensure that sampliig 1582 Environ. Sci. Technol.. VoI. 24. No. 10. 1990
't iI 14
4
LL BbF
BXF
BaP ObA BghiP
IP
COR
Flgure 1. Profile of PAHs from the burning of wocd and other vege-
tation.
was always from within the plume. Results and Discussion The PAH profiles in the emissions from various combustion processes were measured and the results are shown in Figure 1and Table I. Results given in Figure 1have been normalized such that the peak for indeno[cdlpyrene (IP) is given the value unity to facilitate comparison between samples containing different amounts of PAHs. IP
Table 11. Correlation Coefficients between Logarithmically Transformed PAH Concentrations from Different Sources
source0
1
2
3
4
5
6
7
8
9
10
1 2 3 4 5 6 7 8 9 10
1.000
0.813 1.000
0.831 0.943 1.000
0.868 0.848 0.873 LOO0
0.578 0.644 0.678 0.731 1.OOO
0.845 0.983 0.949 0.918 0.695 1.OOO
0.767 0.876 0.818 0.788 0.812 0.887 1.OOO
0.785 0.831
0.769 0.834 0.846 0.899 0.836 0.876 0.901 0.858 1.OOO
0.728 0.907 0.876 0.901 0.821 0.936 0.921 0.875 0.957 1.OOO
0.800
0.759 0.713 0.834 0.949 1.OOO
OSources: 1. Open wood fire burning Australian native wood. 2. Open barbecue burning of Australian native plants. 3. Open barbecue cooking of meat with Australian native plants as fuel. 4. Large-scale backyard burn of both native and introduced species and some cardboard products. 5. Large-scale bush fire, consuming native plants only. 6. Leaves from Australian native vegetation burnt on an open fire. 7. Wood from Australian native vegetation burnt on an open fire. 8. Commercial incinerator burning paper and cardboard products. 9. Cigarette mainstream and sidestream smoke. 10. Cigarette sidestream smoke.
was chosen because it is present in reasonable quantities and is involatile and unreactive. Although the PAH profiles from burning a variety of vegetation material in a number of ways appear broadly similar, Table I does show also that there are some significant differences. The correspondence between the emissions from the various sources was tested by a calculation of the correlation between the logarithmically transformed PAH concentrations for the different sources. A logarithmic transformation was used because the concentrations more closely fitted a log normal than a normal distribution. The logarithmic transformation has the effect of reducing the influence of extreme values on correlations. Table I1 shows the correlation between the PAHs for the emission sources in the present study. In general, correlation coefficients between the various sources are greater than 0.8 although the correlations between some sources and bush fires are a little worse than this. Emissions of fluoranthene and pyrene from cigarettes are low compared with most other sources. This was not a significant problem for source reconciliations of outdoor air quality, since cigarettes are not a major source of PAHs in outdoor air and the more volatile PAHs were not incorporated into the modeling scheme, but it could be important for the source apportionment of indoor air quality. The BaP and coronene concentration for bush fires is high; in fact, the coronene concentration from the bush fire is larger than that from motor vehicles. The coronene to IP ratio is very variable for all the sources compared with the other PAHs. The high coronene concentration in bush fires is very important because coronene is often used as a marker for motor vehicles, particularly gasoline-fueled ones (12). Although the samples for the bush fire were taken close to a road, the high coronene concentrations cannot be explained by contamination from motor vehicle emissions since the particle concentration during the bush fire was very much larger than normal and the concentration of lead, which is an additive to gasoline in Australia, was very low in the collected samples. Particularly large amounts of BaP were also found in the bush-fire sample with concentrations of 194 pg/g compared with other collections, which varied from 2.2 pg/g for paper incineration to 71 pg/g for the backyard burn. High concentrations of BaP seem to arise from low-temperature combustion. The emissions from bush fires is compatible with this interpretation since the intensity of a fire can vary enormously, and although peak temperatures in a bush fire are very high, much of the combustion takes place at lower temperatures. The concentrations found in this study are very much less variable than those reported by Cooke et al. (13). The
correlation coefficients, estimated similarly to those in Table I1 for the emissions reported by Cooke et al., ranged from -0.045 to 0.885 and averaged 0.399. The results both in terms of concentrations and variability are, however, much more consistent with ambient concentrations reported by Sexton et al. ( 4 ) for Warterbury, VT, where between 46 and 73% of the particles arose from residential wood combustion. The major difference is that the relative concentrations of BaP reported in the present paper are higher by about a factor of 2.5 than those found by Sexton et al. Relative concentrations of BghiP and COR may also be somewhat lower than those reported for ambient samples by Sexton et al., which is consistent with a small input from vehicles in the ambient samples, but the difference is not statistically significant. (1) Degradation of PAHs. Accurate source reconciliation requires that the relative amounts of PAHs do not change markedly between emission and analysis. Changes can occur through reactions in the atmosphere prior to collection, or through reactions on the filter or if the collection efficiencies are not equal for all the PAHs, as could occur if the more volatile components were removed from the filter. Measurements made in the present study showed that while up to 50% of PHE and ANT could be lost from the filter, due to volatilization, during a 24-h sampling period losses of the other PAHs were very much less. There was no evidence of loss due to reactions occurring between gases in the air and the filtered material at the concentrations of ozone, nitrogen dioxide, or sulfur dioxide that occur in Sydney air. This observation is consistent with those of Grosjean et al. (14). To minimize errors arising from the loss of the more volatile compounds, concentrations of PHE, ANT, FLU,and PYR were not used in the source reconciliation procedure. PAHs bound to particles can be transported large distances although most of the particles collected in the urban environment will originate from nearby sources. Korfmacher et al. (15) have shown that PAHs adsorbed on fly ash are much less susceptible to photolysis than the pure compounds. Lunde and Bjorseth (16) have shown that PAHs generated in the United Kingdom can be transported to Southern Norway without major degradation. On the other hand, Kamens et al. (17, 18) showed that rapid photolytic degradation of PAHs on wood soot can occur with reaction half-times of less than an hour. This would not necessarily lead to much change in the product spectrum, however, since the removal half-times were very similar for most of the PAHs. If photolytic decomposition of PAHs leads to a change in the relative amounts of each of the PAHs present there Environ. Sci. Technol., Vol. 24, NO. 10, 1990
1583
Table 111. Relative Concentrationsaof
PAHs as a Function of Time of Day time of day
PAH
4-6
6-8
PHE ANT
1.66 0.02 2.90 1.83 0.28 0.83 0.75 0.33 0.51 0.17 1.50 1.00 1.34
1.25 0.04 3.11 1.27 0.20 0.89 0.84 0.42 0.65 0.18 1.40 1.00 1.31
1.26 0.04 2.64 1.17 0.31 0.89 0.75 0.47 0.58 0.26 1.56 1.00 1.49
1.82 0.02 2.38 1.71 0.24 0.82 0.78 0.47 0.53 0.20 1.49 1.00 1.20
13.22
12.56
12.42
12.67
FLU PYR BaA CHR BbF BkF BaP DbA BghiP IP COR total a
8-10
10-12
12-14
14-16
16-18
18-20
2.21 5.75 1.32 0.21 0.64 0.89 0.36 0.50 0.29 1.04 1.00 1.21
2.47 0.04 2.24 1.13 0.18 0.58 0.62 0.20 0.42 0.18 1.33 1.00 1.31
1.76 0.04 2.61 1.31 0.30 0.73 0.71 0.29 0.47 0.24 1.00 1.00 1.50
2.66 0.05 2.84 1.31 0.18 0.87 0.72 0.31 0.44 0.26 1.15 1.00 1.20
15.43
11.71
11.99
13.03
n.d.
All concentrations are relative to the concentration of IP.
DbA E , PAHxlf3 0 Pb
.- -
1:
I 4
Table I V Changes in PAH Concentration during the Year
3.0
0
\
1
I
I
I
I
t
I
6
8
10
12
14
16
P
18
I 20
month
lead," pg m-3
TSP, pg m-3
total PAH ngm-3
PAH/Pb
January February March April May June July August September October November December
0.2 0.3 0.6 0.8 1.1 1.0 0.9 0.9 0.5 0.4 0.5 0.4
22.8 24.9 33.3 49.7 81.0 85.3 64.6 35.2 35.7 44.5 30.6 22.3
3.56 4.25 5.72 7.79 32.2 37.5 34.2 24.3 17.3 14.9 7.30 3.69
0.018 0.014 0.010 0.010 0.029 0.037 0.038 0.027 0.035 0.037 0.015 0.009
"Lead concentrations reported by lution Control Commission (19).
- .JW South Wa.,s
State Pol-
Time - ESST (h)
Figure 2. Diurnal variation in PAH and lead concentrations during summer.
should be a change in the relative concentrations with time of day on a sunny summer day when insolation is high. Samples of suspended particulate matter were collected for 2-h periods from 4 a.m. until 8 p.m. on a typical summer day in January, when in fact most of the PAHs arise from motor vehicles rather than burning of vegetation. As indicated in Table 111, these measurements showed that the total concentration of PAHs in the well-illuminated period from 10 a.m. to 4 p.m. were lower by a factor of 2.0 than those observed between 4 a.m. and 10 a.m. and 4 p.m. and 8 p.m. As indicated by Table 111, the ratio of concentrations of each of the PAHs to that of IP does not seem to change throughout the day, suggesting that photochemical activity and other reactions do not have a major effect on product ratios. Over the same period, as shown in Figure 2, the particulate lead concentration, which arises from the use of lead as an additive to gasoline, varied in a similar manner to that of the PAHs. Further evidence on the importance of atmospheric reactions in determining PAH concentration can be obtained by comparing ambient concentrations in the winter months with those in the summer months since removal of the PAHs is likely to be more rapid at the higher temperatures which occur in summer rather than winter. During the period of the study the New South Wales Pollution Control Commission (SPCC) collected high-volume air samples, which were analyzed for lead (19) at a site at which we were carrying out ambient air monitoring. As shown in Table 1584 Environ. Sci. Technol., Vol. 24, No. 10, 1990
IV, the ratio of the total concentration of PAHs to lead was 0.017 with a standard error of 0.004 for the six warmtemperature months (October to March) compared with a value of 0.029 for the cooler months. Burning of vegetation is estimated from source reconciliation to provide 6% of the PAHs in the warmer months and 24% during the cooler months, with motor vehicles providing the bulk of the remainder. Since vehicular traffic is fairly uniform throughout the year, the PAH to lead concentration ratio suggests that in the warmer months about 28 f 8% of the PAHs could be removed from the air before collection. This estimate is reasonably consistent with changes in the PAH to lead ratio that were measured during the day in summer. The small amount of removal of PAHs that appears to have occurred is not inconsistent with the reaction halftimes of less than an hour that have been reported for photolytic destruction of PAHs. Hanna (20) has shown that in a city ambient concentrations of primary pollutants are determined largely by sources within a few kilometers, even for unreactive pollutants. (2). Impact of Vegetative Combustion on Atmospheric PAH Levels. The contribution of PAHs derived from combustion of vegetation depends very much on the season. PAHs resulting from the burning of vegetation normally make a small minor contribution to the ambient concentration in summer but provide about 30% of the PAHs during winter. This pattern can be distributed by major events such as bush fires. As shown in Figure 3, a bush fire 50 km distant from the sampling site greatly affected suburban air by producing high concentrations
Relativemnc. Of PAH
r
l2
8
0summer profile I Winferpmlile
?zz Bushlire influence on Summer plolile
/I
0Bushlire Backvard Burn
4
2
0 FLU PVR
Flgure 3.
Ban
Typical profiles of
CHR BbF BkF BaP ObA BghiP IP COR
PAHs in amblent
air.
of total suspended particulates (TSP) as well as elevating the PAH burden, in particular for BaP, but also for FLU, PYR, CHR, and DbA both absolutely and relative to IP. This sample, which was collected during summer, can be compared with the more typical summertime collection, which is also shown. In Sydney, elevated concentrations of particulate matter from bush fires often occur for 3 or more days. One hush fire, therefore, could conceivably give rise to 5% of the total exposure to BaP over the course of 1 year. A normal suburban collection, unaffected by bush fires or local combustion sources, has a large motor vehicle component, as indicated by significant contributions of BghiP and COR to the overall PAH profile. In wood burns, however, the concentrations of these PAHs relative to IP are smaller but those of FLU, PYR, BaP and DbA are much greater. During winter, wood is burnt for domestic purposes, which results in elevating the total atmospheric PAH burden and particularly the concentration of BaP. This can be seen in Figure 3, which shows typical summer and winter profiles. In Sydney atmospheric concentrations increase during the cooler months, not only because of the additional sources, but also because of a combination of meteorological conditions, such as strong radiation inversions during the night, which effectivelytrap low-level emissions close to the ground and also low wind speeds occurring during drainage flow along the river valleys. The shorter daylight hours and reduced temperatures would also retard volatilization and chemical destruction of PAHs. A combination of these factors therefore produces higher concentrations of all PAHs and in particular BaP. Conclusion With the exception of those resulting from bush fires, profiles of polycyclic aromatic hydrocarbons arising from the combustion of nonfossil fuel material, which are likely
to be significant sources of particulate matter in Sydney, are very similar to each other but different from profiles arising from combustion of fossil fuels in motor vehicles. Bush fires lead to high concentrations of benzo[a]pyrene and also corouene, which is often used as an indicator of polycyclic aromatic hydrocarbons derived from gasolinefueled motor vehicles. Bush fires, which are common in the Sydney region during summer, could contribute very significantly to exposure of the population to polycyclic aromatic hydrocarbons in general and benzo[a]pyrene in particular. Registry No. COR, 191-07-1;FLU, 206-44-0; PYR, 129-00-0; BaA, 56-55-3; CHR, 21801-9; BbF, 205-99-2; BkF, 207-08-9; BaP, 50-32-8; DbA, 53-70-3; BghiP, 191-24-2; IP, 193-39-5. Literature Cited (1) Pott, P. Chirurgical Obseruations; Hawes, Clarke and Cullings: London, 1775. (2) International Agency for Research on Cancer. Polynuclear Aromatic ComDounds Part 1: IARC MonoeraDhs on the Evaluation of
