Research Speciation of Gas-Phase and Fine Particle Emissions from Burning of Foliar Fuels MICHAEL D. HAYS, CHRISTOPHER D. GERON, KARA J. LINNA, AND N. DEAN SMITH* National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711 JAMES J. SCHAUER Department of Civil and Environmental Engineering, University of WisconsinsMadison, Madison, Wisconsin 57306
Fine particle matter with aerodynamic diameter 70% by mass) PM2.5 are chemically speciated by gas chromatography/mass spectrometry. Expressed as a percent of PM2.5 mass, emission ranges by organic compound class are as follows: n-alkane (0.12%), polycyclic aromatic hydrocarbon (PAH) (0.02-0.2%), n-alkanoic acid (1-3%), n-alkanedioic acid (0.060.3%), n-alkenoic acid (0.3-3%), resin acid (0.5-6%), triterpenoid (0.2-0.5%), methoxyphenol (0.5-3%), and phytosterol (0.2-0.6%). A molecular tracer of biomass combustion, the sugar levoglucosan is abundant and constitutes a remarkably narrow PM2.5 mass range (2.83.6%). Organic chemical signatures in PM2.5 from open combustion of fine fuels differ with those of residential wood combustion and other related sources, making them functional for source-receptor modeling of PM. Inorganic matter [PM2.5 - (organic compounds + elemental carbon)] on average is estimated to make up 8% of the PM2.5. Wavelength dispersive X-ray fluorescence spectroscopy and ion chromatography identify 3% of PM2.5 as elements and water-soluble ions, respectively. Compared with residential wood burning, the PM2.5 of fine fuel combustion is nitrate enriched but shows lower potassium levels. Gasphase C2-C13 hydrocarbon and C2-C9 carbonyl emissions are speciated by respective EPA Methods TO-15 and TO11A. They comprise mainly low molecular weight C2-C3 compounds and hazardous air pollutants (48 wt % of total quantified volatile organic carbon).
Introduction Epidemiological studies indicate a link between atmospheric fine particle (PM2.5) mass concentrations and morbidity and mortality (1, 2). With rising public health concerns (3-6), recent environmental initiatives in the United States seek to 10.1021/es0111683 CCC: $22.00 Published on Web 04/18/2002
2002 American Chemical Society
establish a scientific rationale for regulatory decision making related to airborne PM2.5. Area combustion sources, including biomass open burning (wild and prescribed forest fires), are estimated to represent 37% of the direct emissions of PM2.5 mass to the atmosphere in the United States (7). Wild and prescribed forest fires in North America represent the largest area source of fine PM emissions, exceeding total PM emissions from all other sources combined in some areas such as the southeastern United States (∼1 × 106 metric tons/year) (8). Therefore, accurate characterization of the chemical composition and physical properties of primary PM2.5 and reactive gas PM2.5 precursor emissions from biomass combustion sources is needed for source characterization/attribution and health effects assessments. Fire is a natural ecological process; it controls insects and disease and is necessary for natural succession of plant communities. Foliage, litter, and herbaceous matter are typically the predominant fuel components consumed in many prescribed and wildfire settings. Even though foliage may be a fairly minor component of many fuel beds by mass, it has much higher consumption efficiencies (>80%) than heavier woody fuels >10 cm in diameter ( 1 µm) particles. For each biofuel tested, a gravimetrically measured coarse component (2.5 µm < da < 10 µm) is evident, making up 80%). The 1.2 factor approximates the average molecular weight per carbon for organic compounds comprising the OC. Other factors ranging from 1.2 to 2 were suggested by others and are dependent on the emission source OC composition (47). The narrow range of EC/total carbon ratios (1-5%) can indicate a precise combustion environment (48) and falls within the lower end of a broader range reported by others (1-28%) (49-51). Possible factors contributing to the rather large range would include differences in combustion conditions, fuel structure (fine), and instrumental methods (15, 25, 52, 53). Inorganic Speciation. All mass not classified as either organic compounds or EC is considered to be inorganic matter [PM - (OC + EC), labeled in Figure 3 as other], which on average makes up 8% of the PM2.5 mass. Measurements to determine PM2.5 water content were not performed. P. ponderosa shows no inorganic fraction because the calculated organic compound mass (OC × 1.2) exceeds the PM2.5 mass (Figure 3). This is likely due to a well-known positive sampling artifact (54), which affects aerosol partitioning by ascribing gas-phase, filter-adsorbed OC to the particle phase. PM
loadings from burns with P. ponderosa are heavy, perhaps exacerbating the artifact. As Figure 3 suggests, the artifact appears to have had less effect on the remaining filters of the biofuel data set. Generally, lower mass concentrations of elements (calcium, magnesium, potassium, and chlorine) and ionic species (nitrate ion and ammonium ion) in the PM2.5 of aged litter combustion are observed compared with that from the combustion of live foliage (Table 2). Nutrients are leached or translocated to other plant tissues during foliage senescence (55), and combustion volatilizes ions and releases gaseous oxides and reduced forms of nitrogen, chlorine, and sulfur to the atmosphere. T. heterophylla occurs primarily in the coastal mountain regions of the Pacific Northwest. Relatively high emissions of chlorine and sodium from its combustion likely reflect its proximity to a marine environment (56). Litter accumulation in understories of southeastern U.S. savannahs frequently burn due to ignition by lightning during dry seasons and drought episodes and increased anthropogenic and silvicultural activities. During dry conditions, even low-intensity fires in these ecosystems consume upper organic soil horizons in addition to litter and live fuel. Positive identification of the heavy metal elements titanium, iron, zinc, and cadmium was made from a single filter of FPSP. Many plants are VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Representative fine particle size distributions of aerosols from biofuel burning as determined by a scanning mobility particle sizer. hyperaccumulators of metals in their aboveground shoots, so metals are not necessarily from soils (57). Many Florida forests are located near anthropogenic combustion/incineration sources, and it is possible that metal deposition to those forests may contribute to measured levels. Gas- and Particle-Phase Semivolatile Organic Compounds. Tables 3 and 4 show particle- and gas-phase emissions data for 127 individual semivolatile organic compounds from the combustion of six biofuels. Mean and standard deviation values from replicate combustion tests of Loblolly pine, Western hemlock, Ponderosa pine, and MHFF are provided. n-Alkane, PAH, oxy-PAH, n-alkanoic acid, n-alkanedioic acid, n-alkeneoic acid, resin acid, sugar, meth2286
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oxyphenol, and phytosterol compound classes are represented. Data for FPSP and WGLP represent a single test. Past work indicates an uncertainty (RSD) of ∼20% (26). Particle-phase data (Table 3) for individual compounds are expressed as a mass fraction of PM2.5 in accordance with CMB profile input requirements. Normalizing to PM2.5 mass generally reduces the variability induced by total PM emissions. Semivolatile gas-phase emissions are expressed as milligrams of compound per kilogram of wet biomass burned (Table 3). Figure 4 constructs a mass balance of the organic compound fraction of the particle phase. Unresolved complex mixture (UCM) accounts for 53-61% by mass of GC/MS eluted organic compounds and, on average, for 28% of the
FIGURE 3. PM2.5, organic compound, and elemental carbon mass emission rates from burning of foliar fuels. PM2.5 mass. Assuming the mass of organic compounds to be =OC × 1.2 (36), 44-62% of the particle-phase organic compound fraction elutes. High levels of UCM, believed to consist of branched, cyclic, and naphthenic hydrocarbons of higher molecular mass (62), are also characteristic of wood (14) and cigarette smoke (58) elutables. Even without tentatively identified species nearly 80 wt % of GC/MS elutables, which include UCM, are accounted for. Because the organic compounds resolved and identified in this work typically are present at high concentrations, they are more precisely quantified and thus more likely to
be implemented for source apportionment. Derived from GC/MS analysis, the sections following are organized by compound class. n-Alkanes. As a class, the straight-chain n-alkanes comprise between 0.1 (P. ponderosa) and 2% (MHFF) of PM2.5 mass, corresponding to emission rates of 30-195 mg/kg. Analysis of particle-phase neutral extracts for the C16-C40 n-alkane hydrocarbon emissions demonstrates odd/even carbon number predominance, which is attributable to preservation of the lipid monomers covering each biofuel species’ protective epiticular layer (63). The Pinaceae fuel family consistently shows a C27 carbon maximum (Cmax), whereas Cmax positions of the remaining litter composite fuels are associated with higher carbon numbers (Cmax ) C29 for MHFF and FPSP and Cmax ) C35 for WGLP). Fuel class ordinarily regulates Cmax emissions, although the C35 homologue as Cmax may indicate combustion of the organic soil layer. In addition to its apparently unique waxy surface composition, aged MHFF demonstrates the highest n-alkane emission quotient, possibly due to the thickness of the waxy surface. It is unlikely that this fuel’s loose packing and low moisture content contributed to this result. Because the n-alkane (and n-alkanoic acid) whole-range carbon preference index (CPI) should remain a robust feature throughout phenological stages of a foliar fuel, at least between conifers and hardwoods, we can use it as a means of distinguishing among certain fuel types and sources. Observed CPIs of 1.6-4.4, calculated as described by Simoneit (61) (n-alkane CPI ) ∑ concentrations of odd carbon number/∑ concentrations of even carbon number homologues), are realistic for natural fire smoke (CPI ) 1.2-10) but distinct from those computed for other closely related sources such as fireplace combustion emissions (conifer CPI ) 0.91-1.25 and broad-leaf deciduous CPI ) 0.95-1.87) and
TABLE 2. Mass Emission Rates of PM2.5 and Organic Carbon, Elemental Carbon, Water-Soluble Ions, and Elements in PM2.5 biomass type (g/kg) Pinaceae
PM2.5 mass organic carbon elemental carbon mass fraction of PM2.5 (%) water-soluble ions chloride nitrate sulfate potassium ammonium magnesium calcium elements sodium magnesium silicon phosphorus sulfur chlorine potassium calcium titanium iron zinc cadmium
Loblolly pine
Western hemlock
Ponderosa pine
Aceraceae/ Fagaceae MHFF
Palmae/ Pinaceae FPSP
Poaceae/ Pinaceae WGLP
28.4 ( 11.6 19.7 ( 9.1 1.3 ( 0.3
11.2 ( 0.7 8.0 ( 0.3 0.4 ( 0.1
33.5 ( 10.5 27.8 ( 9.9 0.4 ( 0.1
10.8 ( 3.9 8.5 ( 2.9 0.2 ( 0.01
14.6 11.5 0.2
27.2 19.3 0.4
NDa 0.6 ( 0.5 ND NQb NQ ND NQ
1.68 ( 0.20 1.07 ( 0.03 ND 0.69 ( 0.14 0.37 ( 0.06 ND ND
NQ 1.17 ( 0.94 NQ 0.16 ( 0.08 0.341 NQ NQ
0.20 ( 0.14 1.59 ( 0.42 ND NQ NQ NQ NQ
0.76 4.05 ND NQ NQ ND NQ
0.21 0.94 ND NQ 0.26 ND ND
ND ND ND NQ 0.01 ND ND ND ND ND ND ND
0.24 ND ND ND 0.21 ( 0.07 1.71 ( 0.01 0.75 ( 0.21 ND ND ND ND ND
ND ND ND ND 0.14 ( 0.01 0.10 ( 0.03 0.14 ( 0.08 NQ ND ND ND ND
ND NQ NQ ND 0.19 0.11 0.06 ND ND ND ND ND
0.03 ND 0.01 ND 0.10 0.28 0.11 NQ 0.13 0.08 0.41 0.94
ND ND 0.01 ND 0.11 0.27 0.05 ND ND ND ND ND
a ND, not detected. b NQ, detected but below quantifiable limits. Additional notes: For XRF, elements with concentrations 70% by mass for all cases). For each fuel, pyrogenic PAHs comprise 300 °C) and persists in air and in sediment cores where it can reveal a fire record over geological time (74). Levoglucosan contains an intramolecular glycosidic linkage and is identified and quantified in foliar fuel smoke extracts as its trimethylsilyl ether derivative. Despite the considerable diversity of biofuels in the fire experiments discussed, it is consistently the most abundant molecular marker and constitutes an especially narrow PM2.5 mass range (2.8-3.6%), which is ∼15-21 wt % of the components resolved and identified by GC/MS. Wood smoke particles in general are slightly more enriched with levoglucosan and can show PM2.5 mass concentrations as high as 23% (63). Resin acids occur naturally in conifer wood, bark, and foliage resins. They are hydrophobic and can be directly volatilized (pimaric, sandaracopimaric, isopimaric, and abietic acids) or pyrolitically synthesized (dehydroabietic acid) during combustion (12). Their use as tracers requires further investigation because recent work shows that dehydroabietic acid degrades in water after being irradiated with ultraviolet (UV) light (λ ) 254 nm) (75). In all biofuels tested, resin acids in emissions occur primarily as C17C20 diterpenoids identified as their methyl esters. The work of Simoneit et al. (12) provides a complete discussion of their molecular structure and mass to charge (m/z) data. Dehydroabietic acid (C21) is the most abundant diterpenoid emitted from burning foliar fuels (38-1482 mg/kg). Sitostenone and β-sitosterol are C29 phytosterols, found in all foliar fuel smoke extracts, and specific to biomass burning in general. Both constituents are detected in tropospheric organic matter derived from biogenic detritus and on average in less quantity in wood smoke (14, 61). Sitostenone is a thermal dehydrogenation product of β-sitosterol, which is a natural constituent of vascular plant wax
FIGURE 5. Mass emission rates by class of the NMVOCs and carbonyls from the combustion of foliar fuels. and mostly dominant here. Associated triterpenoic C30 biomarkers, R-amyrin (urs-12-en-3β-ol) and β-amyrin (olean12-en-3β-ol), are detected in the smoke from every composite fuel and occur naturally in angiosperm gums and mucilage, explaining their absence from gymnosperm smoke extracts. Apparently, R-amyrin and β-amyrin occur naturally in the Palmae (Sabal palmetto) and Poaceae families (Aristida stricta). To our knowledge, this is the first report of the association between these taxa and R-amyrin and β-amyrin combustion emissions. Qualitatively, terpenes and sterols should be more specific to individual species, or at least families, and should not vary seasonally. Within seasonal burning windows, quantitative variability should be small (45). C2)C13 Hydrocarbons. Emission data for 78 individual C2-C13 hydrocarbons include n-alkane, n-alkene, n-alkyne, methyl- and ethyl-branched alkane, alkene, cyclic, and aromatic compound classes (Table 5). For each biofuel, n-alkene class emissions exceed those of the n-alkane and VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 6. Mass Emission Rates of Carbonyls from Biomass Burning As Measured by EPA Method TO-11Aa biomass type (mg/kg of biomass burned) Pinaceae
formaldehyde acetaldehyde acetone propionaldehyde crotonaldehyde butyraldehyde benzaldehyde isovaleraldehyde valeraldehyde m-tolualdehyde p-tolualdehyde hexaldehyde diacetyl methacrolein 2-butanone glyoxal acetophenone methylglyoxal octanal nonanal
Aceraceae/Fagaceae
Palmae/Pinaceae
Poaceae/Pinaceae
Loblolly pine
Western hemlock
Ponderosa pine
MHFF
FPSP
WGLP
523.4 ( 161.8 510.4 ( 156.8 1059.5 ( 966.5 104.3 ( 7.9 49.8 ( 30.6 67.5 ( 29.0 38.8 ( 15.0
440.0 ( 83.3 399.7 ( 46.6 1090.0 ( 979.5 161.2 39.4 ( 3.6 72.5 ( 9.8 32.6 ( 7.9 31.0 ( 6.1
840.0 ( 154.2 1146.6 ( 90.1 564.9 ( 70.0 207.7 ( 1.2 73.7 ( 3.3 123.4 ( 11.1 47.6 ( 2.7 71.6 ( 2.4 67.0 ( 34.2 17.8 ( 5.0 3.5 ( 5.0 14.6 ( 2.2 280.1 ( 8.3 171.8 ( 2.0 177.1 ( 29.1 1549.0 ( 106.4 84.3 ( 14.2 944.7 ( 87.9 6.2 ( 1.5 15.7 ( 2.7
304.9 ( 95.7 358.0 ( 20.7 848.2 ( 0.0 76.2 40.1 ( 4.5 43.0 ( 12.0 19.9 ( 7.4
399.4 528.5 359.9 80.1 39.5 93.3 40.7
1138.1 1071.8 530.2 121.0 74.9 106.6 72.0
10.8 18.3 ( 9.0 8.8 ( 12.4 9.9 ( 1.1 51.3 ( 0.3 32.7 ( 10.3 51.8 ( 43.5 986.5 ( 134.0 41.3 ( 14.3 520.6 ( 218.7 1.6 ( 0.8 5.7 ( 3.5
52.6 40.7
175.8
1.2 90.9 69.4 83.7 1058.2 63.4 808.3 1.2 8.1
2.9 233.4 121.0 74.9 3284.6 123.9 1385.9 2.9 11.2
0.6 39.6 ( 29.5 0.6 12.9 ( 5.0 61.2 ( 39.8 82.8 ( 30.7 129.1 ( 89.5 1438.7 ( 399.1 49.0 ( 16.2 881.2 ( 262.2 5.5 10.2 ( 5.1
17.0 ( 0.9 14.3 12.4 ( 2.2 67.4 ( 17.1 46.9 ( 5.5 70.2 ( 3.7 516.0 ( 17.9 44.8 ( 5.4 581.7 ( 17.2 3.0 ( 0.2 6.7 ( 0.8
a Notes: For Pinaceae fuels and MHFF, mean and standard deviation values for duplicate combustion tests are reported. No reported standard deviation denotes that the compound was detected for a single combustion test only. Observe that 2,5-dimethylbenzaldehyde and o-tolualdehyde were not detected.
aromatic classes, the next two highest hydrocarbon emission classes, by average factors of 2.4 and 2.9, respectively. The C5 isoprene structure constitutes the framework from which several cyclic terpenoids form. These are often stored within vessels, trichomes, and oil glands within plant tissues (45). Isoprene and 1,3-butadiene emissions are likely due to thermal decomposition of these terpenoids. Unaltered cyclic C10 monoterpenes, R-pinene and β-pinene, are detected (βpinene is not detected in Western hemlock emissions) and quantified for all taxa. P. ponderosa emits the greatest quantities of terpenoids, isoprene, and 1,3-butadiene. This result is not surprising as P. ponderosa fuel was highly resinous and fresher than most fuels tested, and its open burning also releases the largest total amount of C2-C13 hydrocarbons on a mass-per-unit-fuel-burned basis, 4.7 g/kg. Figure 5 shows total measured VOC emissions by compound class. Total measured VOCs sum the C2-C13 hydrocarbon (SUMMA canister) and C2-C9 carbonyl mass emission factors (DNPH cartridges), which are discussed later. Ethane, ethene, and propene (C2 and C3) dominate gas-phase hydrocarbon emissions for virtually all of the fuels tested. This is supported by other studies of combustion of biomass in the open (49) and wood in fireplace and woodstove appliances (15, 36, and references cited therein). McDonald et al. (15) report an ethene emission factor from fireplace combustion tests with Ponderosa and pinion pines that is within 10% of our mean value from softwood foliage open burns. Schauer (36) evaluates only fireplace combustion emissions from Pinus sp. and reports a higher value (1120 mg/kg) than the average given here for foliage burns (629 mg/kg). Of the 78 C2-C13 hydrocarbons listed (Table 5), 10 (hexane, 1,3-butadiene, 2,2,4-trimethylpentane, benzene, toluene, ethylbenzene, o-, m-, p-xylene, and styrene) are Hazardous Air Pollutants (HAPs) as defined in Section 112 of the Title III Clean Air Act Amendments. These 10 C4-C8 HAPs on average comprise 18 wt % of the total released C2C13 hydrocarbons. C2)C9 Carbonyls. Mass emission rates of gas-phase C2C9 carbonyls from aliphatic, olefinic, aromatic aldehyde, dicarbonyl, and aliphatic ketone classes are given in Table 6. Five of the 22 C2-C9 carbonyls (acetaldehyde, acetophe2294
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none, formaldehyde, 2-butanone, and propionaldehyde) analyzed in smoke extracts as their DNPH derivatives are HAPs and on average account for 30 wt % of the total carbonyls and 18 wt % of the total VOCs (C2-C13 hydrocarbons and C2-C9 carbonyls) quantified. Generally, carbonyl mass emissions data (Table 6; Figure 5) agree well with the results of others (15, 36) and confirm low molecular weight carbonyls (formaldehyde, acetaldehyde, acetone, glyoxal, and methylglyoxal) as the most abundant. These compounds can account for as much as 87 wt % (WGLP) of the total carbonyls quantified. Similar production rates of carbonyls are observed among taxa, symptomatic of the thermal alteration of plant cellulose. Burns of WGLP and P. ponderosa foliage release total carbonyl amounts of 8.5 and 6.4 g/kg, respectively. These emission totals are >2 times those of the other biomass species tested. As expected, these two fuels also yield the highest HAP emission levels.
Acknowledgments We are grateful for the assistance provided by Yuanji Dong of Arcadis, Geraghty & Miller. The Eastern Research Group (Morrisville, NC) provided analysis of the volatile organic carbons including the carbonyls. Gene Stephenson and Mike Bowling (Arcadis, Geraghty & Miller) assisted in conducting burns, acquiring fuels, and collecting trace gas data. Greg Nall and George Custer (USDA Forest Service) also assisted in collecting the fuels.
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(42) (43) (44) (45) (46) (47) (48) (49)
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Received for review July 30, 2001. Revised manuscript received January 15, 2002. Accepted March 6, 2002. ES0111683 VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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