Environ. Sci. Technol. 2002, 36, 1442-1451
Chemical Characterization of Fine Particle Emissions from the Fireplace Combustion of Woods Grown in the Southern United States PHILIP M. FINE* Environmental Engineering Science Department, California Institute of Technology, Pasadena, California 91125 GLEN R. CASS† School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 BERND R. T. SIMONEIT College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331
The fireplace combustion of wood is a significant and largely unregulated source of fine particle pollution in the United States. Source apportionment techniques that use particulate organic compounds as tracers have been successful in determining the contribution of wood smoke to ambient fine particle levels in specific areas in California. To apply these techniques to the rest of the United States, the differences in emissions profiles between different wood smoke sources and fuel types should be resolved. To this end, a series of fireplace source tests was conducted on six fuel wood species found in the Southern United States to determine fine particulate emission factors for total mass, ionic and elemental species, elemental and organic carbon, and over 250 individual organic compounds. The wood species tested, chosen for their high abundance and availability in the Southern U.S. region, were yellow poplar, white ash, sweetgum, mockernut hickory, loblolly pine, and slash pine. The differences in the emissions of compounds such as substituted phenols and resin acids help to distinguish between the smoke from hardwood and softwood combustion. Levoglucosan, a cellulose pyrolysis product which may serve as a tracer for wood smoke in general, was quantified in the emissions from all the wood species burned. The furofuran lignan, yangambin, which was emitted in significant quantities from yellow poplar combustion and not detected in any of the other North American wood smokes, is a potential speciesspecific molecular tracer which may be useful in qualitatively identifying particulate emissions from a specific geographical area where yellow poplar is being burned.
Introduction According to U.S. Environmental Protection Agency data for 1995, wood burning in residential fireplaces and wood stoves * Corresponding author phone: (213)740-6896; fax: (213)744-1426; e-mail:
[email protected]. † Deceased. 1442
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accounted for 10% of the total fine particle emissions in the Southern United States (1). Within the individual states of North Carolina, Tennessee, and West Virginia, residential wood combustion contributed 18-20% of overall fine particle emissions (1). Such emissions inventory data, however, may be a poor indicator of the wood smoke contribution to ambient fine particle concentrations as wood burning may occur predominantly at night when atmospheric mixing is poor and also may occur in mountain valleys where the terrain can trap emissions. Source-apportionment techniques that utilize chemical mass balance receptor models (2-4) can be used to calculate the contributions from different fine particle sources to ambient fine particle samples even under complex atmospheric transport conditions. Previous suggestions for wood smoke tracers, such as isotopically “contemporary” carbon and nonmineral potassium (5-7), do not act as unique markers for wood smoke since they are also found in the fine particle emissions from meat cooking (8), refuse incineration (6, 9), and the abrasion products from leaf surfaces (10). However, the numerous particle-phase organic compounds that are unique to the emissions from wood combustion do provide better candidate chemical tracers, some of which have been used in receptor modeling calculations previously (2, 3, 11, 12). The particle-phase organic compound emissions from the fireplace combustion of a few wood species of regional importance have been studied previously (13-22). Furthermore, to apply source apportionment methods based on organic chemical tracers at a national or continental scale, all of the important wood types burned in the United States must be examined. For this reason, an extensive series of source tests was conducted to fully characterize the particulate emissions from the fireplace combustion of the most abundant and available wood species in the United States. This paper presents the results for six wood species found primarily in the Southern United States and is the second of a series of papers (22), each concerning a particular region of the U.S. Since each wood species was burned in a single fireplace source test, information on the uncertainty or variability of emissions from each particular wood species cannot be determined. However, the current fireplace source testing program taken as a whole provides emissions profiles over a wide range of experimental parameters, including varying wood species, that can subsequently be averaged into composite fireplace emission source profiles applicable to a particular region of the U.S. The current work improves upon past speciation studies by quantifying the fireplace emissions of additional compounds, such as the phytosteroids, triterpenoids, and some additional diterpenes. Previous studies that may have identified these compound classes in wood smoke were not sampling under fireplace combustion conditions. In addition to determining the species to species variability in the emission rates of organic wood smoke, this study identifies an additional organic compound that may be specific to the combustion of an individual wood species. Such speciesspecific tracers might then be used to determine the region from which particular wood smoke plumes originated based on the geographical range and burning activity of a particular tree species.
Experimental Methods Wood Selection. A previous paper describes the methodology for determining the most commonly available wood species in the United States (22). Briefly, a national availability ranking of each wood species was calculated based on state-level wood burning activity data and forestry surveys of existing 10.1021/es0108988 CCC: $22.00
2002 American Chemical Society Published on Web 03/01/2002
TABLE 1. Southern United States Wood Species Selected for Use in This Study tree species
scientific name
moisture content of tested wood (dry basis) (%)
Loblolly Pine Yellow Poplar White Ash Sweetgum Mockernut Hickory Slash Pine
Pinus taeda Liriodendron tulipifera Fraxinus americana Liquidambar styraciflua Carya tomentosa Pinus elliottii
12 33 11 14 12 13
U.S. range
national availability ranking
from NJ to TX including entire Southeastern U.S. East Coast from N. FL to MA and west to Mississippi River entire Eastern U.S. excluding S. FL from NJ to TX including entire Southeastern U.S. from MA to IL south to E. TX and east to N. FL from W. LA to S. SC south through FL
2 8 11 12 20 21
tree stands within each state. Samples of these wood species were supplied by commercial organizations and forestry research groups. Six of these wood species were chosen for their importance to the Southern U.S. region (Table 1), and the results from fireplace source tests conducted for these woods are presented here. Table 1 also provides the scientific names, geographic ranges, average moisture contents of the woods tested, and the national availability ranking of the six Southern U.S. woods. Source Tests. A previous paper describes the source testing apparatus and procedures used here in detail (22). For each test, the same residential masonry fireplace was used to burn wood samples of between 5 and 12 kg per test for durations of between 81 and 202 min. Cross contamination between tests, due to the condensation of material onto the masonry and the potential evaporation of the material during the next test, was a concern. Since previous investigations showed that this was more of an issue for the softwood emissions, the hardwoods were burned first. Furthermore, an examination of the data from several sets of subsequent tests showed that for compounds present at high levels in the first test but not expected to be present in the second test the carry-over contamination was less than 5%. The smoke was sampled from a port 4 m above the fire using the dilution source sampler of Hildemann et al. (8, 22, 23). This sampling apparatus is designed to obtain an accurate representation of the gas/particle partitioning of organic compounds at ambient conditions by diluting the sample with particle-free air and allowing the smoke to cool as organic vapors condense into the particle phase in a residence time chamber. Sampling began just prior to ignition and continued throughout the burn cycle through smoldering until a very low level of particles were observed with the electronic particle sizing instrumentation. The configuration of the cyclone separators and the filters deployed for fine particle collection is also described in a previous paper (22). Samples of the woods were removed and weighed the same day of the source tests for moisture content measurements using an ovendrying method (24). Subsequent analyses of the fine particle samples include gravimetric mass, organic carbon (OC) and elemental carbon (EC) (25), ionic species by ion chromatography (IC) (26), elemental composition by X-ray fluorescence (XRF) analysis (27), and organic compound speciation by gas chromatography/mass spectrometry (GC/MS). Organic Chemical Analyses. Organic compound analysis is performed using the methods of Mazurek et al. (28) and Rogge et al. (29) as outlined in a previous paper in this series (22). After the addition of a suite of deuterated compounds for use as internal recovery standards, source samples collected on quartz fiber filters are sonicated in solvent washes twice in hexane (Fischer Optima Grade) and then three times in a 2:1 benzene/2-propanol mixture (benzene: E&M Scientific; 2-propanol: Burdick & Jackson). The solvent extracts are then combined and reduced in volume to approximately 1 mL. After splitting the extract into two separate fractions, one portion is derivatized with diazomethane to convert organic acids to their methyl ester
analogues which are more amenable to quantification by GC/MS. All GC/MS analysis is performed on a HewlettPackard GC/MSD (GC model 5890, MSD model 5973) with a 30 m × 0.25 mm diameter HP-5MS capillary column (Hewlett-Packard). A co-injection standard of 1-phenyldodecane is used to gauge overall instrument response for all injections. Identification and quantification of individual organic compounds is achieved via comparison with a set of prepared authentic standard mixtures containing hundreds of organic compounds found in wood smoke and other source effluents. When a particular standard compound cannot be obtained, identification and quantification are based on similar compounds for which standards are available. Comparison with mass spectral libraries as well as fundamental interpretation of mass spectra are also used to aid in compound identification.
Results Table 2 lists the emission factors for bulk chemical analysis measured during the fireplace combustion of Southern U.S. woods. Included are data on emissions for fine particle mass, organic and elemental carbon, ionic species, and the most abundant chemical elements in the smoke. Uncertainties in the table are based solely on analytical and measurement errors. The fine particle (dp < 2.5 µm) mass emission factor for fireplace combustion of Southern U.S. wood species averaged 4.3 g of particulate matter emitted kg-1 of wood burned with a range from 1.6 to 6.8 g kg-1 over all six wood species tested. These results correspond well to the 5.3 g kg-1 average fine particle mass emission rate from combustion of the Northeastern U.S. woods reported previously (22) and are comparable to several previous studies on fireplace wood combustion (21, 30-32). All of the more recently published emission rates for PM2.5 are lower than the present USEPA emission factor for fireplace wood combustion of 17.3 g PM2.5 per kilogram wood burned [1996 #33]. The recent advances in dilution sampling technology, which provide for gas-particle partitioning under conditions approximating the atmosphere downwind of a source, are a possible reason for the discrepancy. Table 2 also indicates that most of the fine particulate mass emitted from the fireplace combustion of the wood species tested is comprised of organic carbon which makes up over 74% of the fine particle mass. To convert the mass of organic carbon to an estimate of organic compound mass, it must be multiplied by a scale factor (generally 1.2-1.4 for atmospheric samples (33)) to account for the oxygen, hydrogen, and other elemental content of the organic compounds present. In the present study, a scale factor of 1.4 is used owing to the substantial number of oxygenated species in wood smoke. The resulting mass overbalance seen in Table 2 for several of the Southern U.S. wood species can most likely be explained by increased organic vapor adsorption onto the quartz fiber filters used for organics collection relative to the Teflon filters from which gravimetric mass measurements were taken (34). The mass overbalance was more pronounced for the softwood smoke samples suggesting a greater degree of organic vapor adsorption for softwood VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Fine Particle Mass Emission Rates and Chemical Composition for the Fireplace Combustion of Selected Southern U.S. Wood Speciesb hardwoods Yellow Poplar fine particle emissions rate (g kg-1 wood burned) elemental and organic carbon (wt % of fine particle mass) organic carbon (OC)a elemental carbon (EC) ionic species (wt % of fine particle mass) chloride nitrate sulfate ammonium elemental species (wt % of fine particle mass) silicon sulfur chlorine potassium zinc calcium bromine rubidium lead
White Ash
Sweetgum
softwoods Mockernut Hickory
Loblolly Pine
Slash Pine
6.8 ( 0.8
3.3 ( 0.3
3.5 ( 0.4
6.8 ( 0.9
3.7 ( 0.4
1.6 ( 0.3
84.9 ( 5.1 3.4 ( 0.4
76.8 ( 5.4 6.4 ( 0.9
78.8 ( 6.0 2.7 ( 0.6
74.2 ( 6.4 1.2 ( 0.2
100.4 ( 6.4 17.9 ( 1.6
100.6 ( 6.5 14.2 ( 1.7
0.15 ( 0.01 0.32 ( 0.02 0.36 ( 0.02 0.04 ( 0.01
0.46 ( 0.03 0.65 ( 0.04 0.77 ( 0.05 0.07 ( 0.01
0.27 ( 0.03 0.63 ( 0.04 0.50 ( 0.04 0.13 ( 0.01
0.17 ( 0.01 0.26 ( 0.01 0.18 ( 0.01 0.06 ( 0.01
0.019 ( 0.002 0.040 ( 0.004 0.027 ( 0.004 0.008 ( 0.001 0.101 ( 0.004 0.261 ( 0.010 0.167 ( 0.004 0.081 ( 0.002 0.124 ( 0.005 0.512 ( 0.013 0.254 ( 0.008 0.164 ( 0.003 0.726 ( 0.006 1.751 ( 0.013 0.797 ( 0.009 0.200 ( 0.003 0.006 ( 0.001 0.049 ( 0.001 0.038 ( 0.001 0.027 ( 0.001
