Environ. Sci. Technol. 1997, 31, 2726-2730
Sources of Fine Organic Aerosol. 7. Hot Asphalt Roofing Tar Pot Fumes WOLFGANG F. ROGGE,† LYNN M. HILDEMANN,‡ MONICA A. MAZUREK,§ AND GLEN R. CASS* Environmental Engineering Science Department and Environmental Quality Laboratory, California Institute of Technology, Pasadena, California 91125 BERND R. T. SIMONEIT Petroleum and Environmental Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331
The organic compounds present in fine particulate roofing asphalt tar fumes are characterized using GC/MS techniques. Most of the compound mass identified consists of n-alkanes (73%). Polycyclic aromatic hydrocarbons (PAH) and thia-arenes (S-PAH) contribute nearly 8% of the identified compound mass and account for 0.57% of the total mass emissions. When compared to the PAH mass fraction determined in fine particulate exhaust emitted from catalyst-equipped automobiles (∼0.5%) and heavyduty diesel trucks (∼0.1%), roofing tar pot aerosols show a PAH content similar to that of vehicular exhaust.
Introduction Bituminous materials such as asphalt and coal tar are commonly used for road construction and roof sealing purposes. The bituminous products used in the United States are manufactured principally from crude oils (>99%) and to a minor extent from coal. Nearly 30 million ton of asphalts was consumed in the United States during 1988, roughly onethird of the world production (1). About 15% of the bituminous type material produced is consumed by the building industry for roofing purposes (2) including asphalt roll and asphalt shingle roofing. During the construction of hot-process built-up roofs, hot asphalt tar or coal tar pitch is used as an adhesive. In order to fluidize the asphalt adhesives and sealant at construction sites, small portable heater units are used. These heaters, called roofing tar pots, produce a visible plume of aerosol condensate derived from the hot asphalt batch. Gray (3) estimated that particulate emissions from roofing tar pots contributed roughly 920 kg/ day of fine particulate matter (dp e 2.0 µm) to the 80 km × 80 km heavily urbanized area centered over Los Angeles during the year 1982, while more recent estimates list tar pot emissions that are an order of magnitude smaller (4). Roofing tar fumes that are released during roof construction work might be mutagenic and carcinogenic. For that reason, prior studies of roofing tar fumes have been concerned * To whom correspondence should be addressed. Telephone: 626395-6888; fax: 626-395-2940; e-mail:
[email protected]. † Present address: Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33199. ‡ Present address: Department of Civil Engineering, Stanford University, Stanford, CA 94305-4020. § Present address: Institute of Marine and Coastal Sciences, Rutgers University, P.O. Box 231, New Brunswick, NJ 08903.
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exclusively with the determination of the risk to roofing workers from exposure to such fumes (e.g., refs 5-9). The purpose of the present study is different. A more complete characterization of the organic chemical composition of roofing tar pot aerosol is sought. Since roofing tar pot technology has not changed in recent decades, the information provided on the chemical composition of tar pot fumes is relevant both to present day conditions as well as to interpretation of historical atmospheric measurements and models. These data on aerosol emissions from roofing tar pots combined with parallel studies on the other major urban organic aerosol sources (10-16) can be used to support a comprehensive air quality modeling study of emissions and air quality relationships for urban organic aerosols.
Experimental Methods Sampling. Petroleum-based built-up roofing asphalt (GAF brand) was heated in a typical roofing tar kettle to workable plasticity. The temperature of the tar pot was maintained between 250 and 300 °C. Sample collection was performed using a specially designed dilution source sampling system (17). Hot tar fumes (2 L min-1) were sampled from above a vent opening at the top of the asphalt kettle and diluted 18fold with precleaned air (activated carbon-filtered and HEPAfiltered). The cooled and diluted asphalt fumes were subsequently drawn through an AIHL design cyclone separator that removed particles with an aerodynamic diameter >2.0 µm. The cyclone separator was operated at a flow rate of 27.9 ( 0.3 L min-1 for both sets of tests. Sampling time for each test was 10-15 min. Fine particulate matter was collected in parallel on three quartz fiber filters (Pallflex 2500QAO) and one Teflon filter (Gelman Teflo, 2.0 µm pore size). All quartz fiber filters were annealed at 750 °C for 2-4 h before use to ensure low contamination levels for organic substances. Bulk Chemical Analysis. Trace elements, ionic species, organic carbon (OC), and elemental carbon (EC) have been previously quantified in the roofing tar pot samples. These data on the bulk chemical properties of roofing tar pot emissions are published elsewhere (17). Sample Extraction. The extraction scheme followed originally was developed for ambient fine particulate matter by Mazurek et al. (18) and subsequently has been used in several studies and companion papers (10-16, 19-21). The extraction protocol can be summarized as follows: Prior to sample extraction, perdeuterated tetracosane (n-C24D50), which served as an internal standard, was spiked onto the filter composites. For accurate quantification, the internal standard should be in a concentration range comparable to the organic compounds in the sample. To ensure that n-C24D50 was in a suitable concentration range, the amount to be added as internal standard was estimated using the OC data acquired from EC/OC combustion analysis (22, 23), and replicate samples were extracted and analyzed to ensure that the sample and standard concentrations were matched appropriately (21). The quartz fiber filter samples were extracted together in a sequential procedure that included extraction with hexane (2 × 30 mL) followed by extraction with benzene/2-propanol (2:1 mixture, 3 × 30 mL). Each extraction step was conducted for 10-min and supported by mild ultrasonic agitation, after which the filter extracts were filtered and combined. A two-step rotary evaporation scheme followed by gentle high-purity N2-stream evaporation was employed to reduce the combined sample extract volume to 200-500 µL. One portion of the sample extract was then processed
S0013-936X(96)00525-1 CCC: $14.00
1997 American Chemical Society
with freshly produced diazomethane to convert organic acids to their methyl ester analogues and other compounds with susceptible hydroxy functionalities to their methoxy analogues. Until GC/MS analysis, the sample extracts were stored in the dark at -21 °C. Sample Analysis. A Finnigan 4000 quadrupole mass spectrometer connected to a gas chromatograph and interfaced with an INCOS data system was used for compound identification and quantification. Sample extracts were injected onto a conventional Grob splitless injector (300 °C) that was connected to a 30-m fused-silica DB-1701 column (J&W Scientific, Rancho Cordova, CA). Gas chromatography of the injected sample extract was supported using the following temperature program: (1) isothermal hold at 65 °C for 10 min, (2) temperature increase at 10 °C/min for 21 min, and (3) isothermal hold at 275 °C for another 49 min. The mass spectral data were acquired while operating the mass spectrometer in the electron impact mode (electron energy of 70eV). For previous studies of the same samples, a Varian 4600 high-resolution gas chromatograph (HRGC) with FID detector was used that was operated with the same physical column and temperature program as used during GC/MS analysis (21). Additional information describing the analytical procedure can be found elsewhere (10-12, 18, 19). Quality Assurance. A series of quality control and monitoring steps were followed. The major steps include field and laboratory blank testing, solvent testing to monitor possible contaminants, recovery experiments for a large set of polar and nonpolar standard compounds, replicate sample analyses, dilution air testing, and more. For a more detailed discussion, the reader is referred to accompanying source and ambient fine organic particle studies published earlier (10-12, 19, 21, 24). Compound Identification and Quantification. Compound identification was conducted using the National Institute of Standards and Technology (NIST) mass spectral library accessed by the INCOS Data System, the NIST/EPA/ NIH mass spectral database (PC Version 4.0) distributed by NIST, and by reference to authentic standards injected onto the GC/MS system used here. Compound identification was conducted accordingly: (a) positivessample mass spectrum and authentic standard mass spectrum compared well and showed identical retention times; (b) probablessame as before, except no authentic standards were available, but the NIST or NIST/EPA/NIH library mass spectrum and the sample mass spectrum agreed well; (c) possiblessame as above except that the sample spectrum contained information from other compounds but with minor overlap; (d) tentativesthe sample spectrum contained additional information from possibly several compounds (noise) with overlap. The compound quantification process was based on the application of n-C24D50 as internal standard and 1-phenyldodecane as co-injection standard. To correct for detector response to compounds having different structures and retention times, sets of known standard compounds were injected onto the analytical system to monitor their specific MS response. For more information, the interested reader is referred to Rogge et al. (10, 11). Standard Compounds. Confirmation and quantification of organic compounds was obtained through the use of more than 150 authentic standards, see Rogge et al. (11). The following standard mixtures were injected onto the GC/MS system: (1) normal alkanes ranging from n-C10 to n-C36; (2) normal alkanoic acids as methyl esters ranging from n-C6 to n-C30; (3) unsaturated aliphatic acids such as oleic acid and linoleic acid as methyl esters; (4) normal alkanols ranging from n-C10 to n-C30; (5) several phenolic compounds, benzaldehydes, and substituted aromatic acids; (6) a suite of 39 aromatic and polycyclic aromatic hydrocarbons (PAH); (7) 10 polycyclic aromatic ketones and quinones; (8) a set of eight aromatic and polycyclic aromatic nitrogen- and sulfur-
FIGURE 1. Mass balance for elutable organic matter in the fine particle emissions from roofing tar pot fumes. substituted compounds; (9) steroids including cholesterol and cholestane; (10) a set of four phytosterols; (11) several natural resins; (12) plasticizers; (13) a suite of 11 aliphatic dicarboxylic acids (C3-C10); and (14) one suite containing seven aromatic di- and tricarboxylic acids (all as methyl ester analogues), and (15) other compounds.
Results For hot-process built-up roofs, coal tar pitch and asphalt tar products are typically used as an adhesive (6, 7). Coal tar pitches are byproducts resulting from the pyrolysis of coal, often termed carbonization. Typically the carbonization process is conducted in the high-temperature range of 9001200 °C (2). At those temperatures coal tars are distillated from the bituminous coal leaving solid coke as a residue. Asphalt, although it occurs naturally, is mainly obtained as a byproduct from crude oil refining. Depending on the application and crude oil quality, asphalt is manufactured by straight reduction, air-blowing, propane deasphalting, and thermal cracking (2). During the refining process, crude oil is injected into a fractionation column at temperatures ranging from 340 to 400 °C. After removing the lighter crude oil fractions by distillation, the residue is commonly called straight-reduced asphalt. Straight-reduced asphalt is mainly used for road pavements. For roof surface coatings, an asphalt is desired that is more viscous and less resilient. To remove the more volatile organic fraction and to improve the quality of the asphalt by partial oxidation, straight-reduced asphalt is heat-treated by continuously bubbling hot air through the asphalt at temperatures from 200 to 275 °C. “Air-blown” asphalt is then used to produce asphalt impregnated paper rolls and asphalt shingles. For hot-process built-up roofs, asphalt tar is the adhesive used most widely in oil-producing countries. Before application, asphalt tar (or alternatively coal tar pitch) is reheated to reach the desired workability. The recommended kettle temperature is 200-270 °C (6). Commonly, the kettle temperature is poorly controlled, and kettle temperatures above 600 °C have been measured (25). At such high temperatures and in the absence of oxygen, pyrolysis of asphalt containing precursor constituents leads to PAH-type compound synthesis, resulting in increased mutagenic and carcinogenic activity of the hot tar fumes (6, 26). Here, fine particulate roofing asphalt tar fumes are examined on a molecular level to provide a fine particulate source profile that can be used to estimate the contribution of organic matter released from hot-process built-up roofing to the urban atmosphere. Mass Balance for Fine Particulate Organic Tar Pot Fumes. A material balance constructed for the extractable organic portion of fine particulate matter released from roofing tar pot operations is shown schematically in Figure 1. Of the
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TABLE 1. Mass Concentration of Organic Compounds in the Fine Aerosol Emissions from Hot Asphalt Roofing Tar Pot Fumes emission rates (µg g-1)
compounda ID
n-tetradecane n-pentadecane n-hexadecane n-heptadecane n-octadecane n-nonadecane n-eicosane n-heneicosane n-docosane n-tricosane n-tetracosane
3788.0 5827.0 5936.0 9659.0 6231.0 3319.0 3890.0 3042.0 2536.0 1888.0 2396.0
a a a a a a a a a a a
n-heptanoic acid n-octanoic acid n-nonanoic acid n-decanoic acid n-undecanoic acid n-dodecanoic acid n-tridecanoic acid n-tetradecanoic acid (myristic acid) n-pentadecanoic acid n-hexadecanoic acid (palmitic acid)
87.1 1043.0 1312.0 2043.0 1263.0 1081.0 537.1 894.9 362.4 2492.0
n-Alkanes n-pentacosane n-hexacosane n-heptacosane n-octacosane n-nonacosane n-triacontane n-hentriacontane n-dotriacontane total class emission rate
n-Alkanoic Acidsb a n-heptadecanoic acid a n-octadecanoic acid (stearic acid) a n-nonadecanoic acid a n-eicosanoic acid a n-heneicosanoic acid a n-docosanoic acid a n-tricosanoic acid a n-tetracosanoic acid a a total class emission rate
338.8
n-Alkenoic Acidb a total class emission rate
3,5-dimethylbenzoic acid 2-methylbenzoic acid 3-methylbenzoic acid 4-methylbenzoic acid
97.2 38.9 124.6 477.7
Other Organic Acidsb b benzeneacetic acid b b total class emission rate a
dimethylbenzo[b]thiophenes dibenzothiophene methyldibenzothiophenes
244.6 257.8 629.9
dimethylnaphthalenes phenanthrene anthracene methyl(phenanthrenes, anthracenes) dimethyl(phenanthrenes, anthracenes) fluoranthene pyrene benzacenaphthylene 2-phenylnaphthalene methyl(fluoranthenes, pyrenes)
Polycyclic Aromatic Hydrocarbons 604.6 a benzo[a]fluorene/benzo[b]fluorene 462.3 a benzo[ghi]fluoranthene 130.3 a benz[a]anthracene 844.5 b chrysene/triphenylene 751.8 b benzo[k]fluoranthene 71.6 a benzo[b]fluoranthene 136.9 a benzo[e]pyrene 11.4 b benzo[a]pyrene 46.2 b 124.9 b total class emission rate
20S&R-5R(H),14β(H),17β(H)-cholestanes 20R-5R(H),14R(H),17R(H)-cholestane 20S&R-5R(H),14β(H),17β(H)-ergostanes
113.9 101.4 96.1
cis-9-octadecenoic acid (oleic acid)
22,29,30-trisnorneohopane 17R(H),21β(H)-29-norhopane 17R(H),21β(H)-hopane 22S-17R(H),21β(H)-30-homohopane a
34.1 54.8 47.2 21.7
Thia-Arenes a dimethyldibenzothiophenes a b total class emission rate
Regular Steranes b 20S&R-5R(H),14β(H),17β(H)-sitostanes a b total class emission rate Pentacyclic Triterpanes b 22R-17R(H),21β(H)-30-homohopane b b total class emission rate b
emission rates (µg g-1)
compounda ID
2171.0 1040.0 496.5 228.4 100.6 64.6 35.8 17.9
a a a a a a a a
52666.8
139.6 823.4 64.2 89.8 28.5 17.0 7.3 8.3
a a a a a a a a
12293.6 338.8 62.6
a
801.0
1170.3
b
2302.6 61.2 4.3 24.8 64.9 11.7 6.7 6.0 5.8
a b a a a a a a
3369.9 34.0
b
345.4 9.5
b
167.3
For more details see text: a, positive; b, probable; c, possible; d, tentative. b Detected as methyl ester.
extractable and chromatographically elutable organic matter, close to 16% is chromatographically resolved as single compound peaks. More than 50% of the chromatographically resolved compound mass could subsequently be identified. The overwhelming majority of the elutable organic mass that could be identified consists of n-alkanes (73%), carboxylic acids such as n-alkanoic acids (17%), and benzoic acids. Polycyclic aromatic hydrocarbons (PAH) and thia-arenes (S-PAH) comprised 7.9% of the identifiable mass. A quantitative assessment of individual compounds and compound classes is given in Table 1.
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Alkanes. Figure 2a depicts the n-alkane concentration pattern found in the roofing asphalt fume particulate matter. n-Alkanes have been identified ranging from C14 to C32 with C17 as the most prominent member. Since the heating of tar is analogous to a distillation process, the lower molecular weight (more volatile) species will tend to be emitted preferentially, yielding lower emission rates for the larger nalkanes. At typical ambient concentrations and temperature, in the urban atmosphere, n-alkanes up to C18 are mainly found in the gas phase (27), while higher molecular weight n-alkanes are preferably found in the particle phase (27, 28).
FIGURE 3. Partial mass fragmentogram (m/z 191) showing the hopane marker distribution in asphalt roofing tar pot emissions. [C27 neo ) 22,29,30-trisnorneohopane, C27r ) 22,29,30-trisnorhopane, C28 ) 17r,18r,21β(H)-28,30-bisnorhopane, C29r ) 29-norhopane, C30r ) hopane, C31r ) 22S and 22R homohopanes, all with the 17r(H) configuration.]
FIGURE 2. Emission profiles for fine aerosol from roofing tar pot fumes: (a) n-alkanes and (b) n-alkanoic acids. Adsorption of gas-phase organics onto the filter material is a known cause for the presence of some lower molecular weight organic compounds in aerosol samples; however, for this source test, less than 1% of the OC collected was attributed to gas-phase adsorption (17). The relatively high concentration of the C14-C18 n-alkanes found here is more likely caused by high asphalt fume concentrations, which even after dilution would cause formation of some particle-phase lower molecular weight n-alkanes as a result of the partitioning between the gas and particle phase that occurs as the tar pot vapors are cooled. Niemeier and co-workers (6) pointed out that the carcinogenic activity of asphalt fumes cannot be explained solely on the basis of their PAH content and hypothesized that co-carcinogenic effects of aliphatic hydrocarbons could be responsible for part of the carcinogenic activity, as was suggested earlier by other researchers (29-31). Carboxylic Acids. n-Alkanoic acids (C8-C24) have been found in concentrations that are typically several times lower than for the n-alkanes. The n-alkanoic acids concentration profile in Figure 2b shows highest concentrations for hexadecanoic acid, which is commonly found in the emissions of fossil fuel burning sources (11). In addition, several methylsubstituted benzoic acids and benzeneacetic acid have been identified. Of the benzoic acid-type compounds, 4-methylbenzoic acid is most abundant (see Table 1). Polycyclic Aromatic Hydrocarbons. Because of their mutagenic and carcinogenic potential and the resulting health risk to roofing workers, the PAH content of asphalt and coal tar pitch fumes has been extensively investigated in the past. PAH concentrations have been measured in the raw material itself, in fume condensates, and as ambient samples in the breathing zone of roofing workers (6, 7, 32, 33). Roofing coal tar pitches have been found to contain PAH levels several orders of magnitude higher than that of roofing asphalt tar. For example, benzo[a]pyrene concentrations have been
determined in roofing asphalt ranging from 1 to 8 ppm, whereas roofing coal tar pitches revealed benzo[a]pyrene concentrations from 2 000 to 11 000 ppm (6, 7, 33). Accordingly, ambient PAH-levels in the work environment of roofers differ drastically depending on the type of hot adhesive used (7). During the course of the present study, 17 individual PAH and alkyl-PAH have been identified in the roofing asphalt fumes. Phenanthrene and anthracene and their methyl- and dimethyl-alkylated analogues accounted for about 65% of the PAH mass concentration identified. In contrast benzo[a]pyrene, a PAH typically monitored in roofing tar fumes (6, 7, 34), is found to contribute only 0.2% of the PAH mass identified. Similar distribution patterns have been found in condensed asphalt tar fumes and in ambient air at roofing sites (6, 7). In addition, heterocyclic aromatic hydrocarbons containing sulfur (S-PAH), such as dibenzothiophene and its methyl-substituted homologues, have been identified at concentration levels similar to that of phenanthrene and its alkylated homologues. S-PAH show mutagenic potentials similar to other PAH, but due to their slightly increased polarity they are more soluble in water and more prone to aquatic bioaccumulation (35, 36). PAH emissions in the Los Angeles area in the 1980s are displayed by Schauer et al. (37) and Rogge et al. (38). The major sources of PAH are seen to be older noncatalyst gasolined-powered motor vehicles and natural gas combustion, while the PAH emissions from catalyst-equipped motor vehicles and diesel trucks are much smaller. The relative PAH and S-PAH content of roofing tar pot emissions (0.57% of the total mass emissions) is comparable to the mass fraction of PAH emitted from catalyst-equipped automobiles (∼0.5%) and heavy-duty diesel trucks (∼0.1%) (11). Sterane and Triterpane Hydrocarbons. Fossil fuels such as coal and crude oil are the products of biogenic organic matter transformed in sedimentary rocks by diagenesis and catagenesis over millions of years (39-42). In geochemistry, steranes and triterpanes are commonly used as marker compounds to assess the maturity of crude oil and to trace its migration from the source rocks to the crude oil reservoirs (40, 41). Sterane and hopane triterpanoids have been identified in lubricating oils, vehicular exhaust, road dust, tire wear, and in airborne suspended particles (11, 12, 28, 39, 43-45). Among the spectrum of steranes and hopanes present in petroleum, only those previously reported by Rogge et al. (11, 12) have been quantified in the fine particulate asphalt tar fume samples. The results are summarized in Table 1. As shown by Rogge et al. (11), steranes and hopanes can be used to trace the presence of motor vehicle exhaust in the urban atmosphere. Given the hopane and sterane emissions from roofing tar pots quantified in Table 1, the magnitude of possible interferences from roofing tar pot emissions can be determined when conducting a vehicular tracer analysis. The concentrations of the sterane and hopane
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markers are significant in the tar pot sample studied here, confirming petroleum as the source of this tar. However, the hopane distribution in the tar pot emissions (Figure 3) differs from that reported previously for Los Angeles vehicular emissions (11) and urban aerosol (37). The tar pot sample contains a significant amount of 17R,18R,21β(H)-28,30bisnorhopane, a common component of many southern California crude oils (37, 41), while the vehicular exhaust and atmospheric samples are not enriched in this compound. The implication is that there are crude oils from many geographic areas that contribute to Los Angeles motor vehicle exhaust aerosol, while the petroleum tar studied here is much more enriched in the products of southern California oil fields.
Acknowledgments We thank Ed Ruth for his assistance with the mass spectrometry analysis. This research was supported by the California Air Resources Board under Agreement A932-127. Portions of the work benefited from research supported by the U.S. Environmental Protection Agency under Agreement R-813277-01-0 and by the South Coast Air Quality Management District. Partial funding also was provided by the U.S. Department of Energy under Contract DE-AC02-76CH00016. The statements and conclusions in the report are those of the contractor and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products. This manuscript has not been subject to the EPA’s peer and policy review, and hence does not necessarily reflect the views of the EPA.
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Received for review June 17, 1996. Revised manuscript received June 2, 1997. Accepted June 16, 1997.X ES960525K X
Abstract published in Advance ACS Abstracts, August 1, 1997.
