Environ. Sci. Technol. 1986, 20, 803-807
Source Emission Characterization of Residential Wood-Burning Stoves and Fireplaces: Fine Particle/Methyl Chloride Ratios for Use in Chemical Mass Balance Modeling S. A. Edgerton," M. A. K. Khalii, and R. A. Rasmussen Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, Beaverton, Oregon 97006
The results of an experiment to determine the ratio of fine aerosol to methyl chloride in residential wood burning are presented. Accurate measurements of this ratio are necessary for receptor models that use methyl chloride as a unique tracer of woodsmoke and for chemical mass balance (CMB) models that include methyl chloride in the wood-burning source composition matrix. It is demonstrated how the values of the fine particle to methyl chloride ratios for various types of wood and burn conditions may be used in a stratified sampling scheme to determine a composite value of the ratio, which is found to be about 0.68 f 0.13 (pg/m3)/parts per trillion (volume) in Portland, OR. The ratio is used in a CMB calculation to estimate the wood-burning contribution to fine particulate concentrations in a residential neighborhood. Ratios of several hydrocarbon gases to C02 in residential wood burning are also presented for use in models that may attempt source reconcilation of hydrocarbon species.
A study was conducted to experimentally measure the ratio of fine particles to gaseous methyl chloride (CH3Cl) in woodsmoke for application in the chemical mass balance (CMB) receptor model. With an accurate value for this ratio and with knowledge of the background CH3C1concentration in a community, one can either use CH3C1as a unique tracer to determine the concentration of fine aerosol in the ambient air from wood-burning stoves and fireplaces (1, 2) or include CH3C1 in the wood-burning source composition matrix of a multicomponent CMB model to help resolve the wood-burning source from other urban sources of fine particles in the air (3). Previous estimates of the source emission ratio of fine particles to methyl chloride from wood burning were estimated by using published values for the fine particle/C02 and the methyl chloride/C02 emission ratios (1). The first direct measurements of the fine particle/methyl chloride emission ratio in residential wood-burningstoves and fireplaces are presented here, and the results are applied in a CMB calculation to demonstrate the use of CH3Cl as a tracer to estimate the concentration of woodsmoke. Experimental Procedures Samples were collected in the cooled diluted plume from home chimneys to approximate as closely as possible the actual conditions of woodsmoke as it enters and becomes dispersed in the urban atmosphere. The inlet of the sample line was directed into the plume about 1-3 m from the chimney. Dilution air was drawn in the same inlet with the plume and allowed to mix in the dilution chamber with a residence time in the chamber n the order of 2-3 min. Carbon dioxide measurements in the flue gas and in the sampled air indicated that the dilution factor was from 50 to 150 during the test runs. A schematic of the sampling apparatus is shown in Figure 1. The gaseous sample was *Address correspondenceto this author at Environmental Physics and Chemistry Section, Battelle Columbus Division, Columbus, OH 43201. 0013-936X/86/0920-0803$01.50/0
Table I. Values for Source Emission Ratio Various Burn Conditions and Wood Types O1,o
01
for CH&l for
x102 g/g
wood type
open damper
closed damper
softwood, fir hardwood, alder/oak fireplace, fir/alder/oak
1.9 & 0.7' 1.2 f 0.6 1.4 f 1.4
4.1 & 1.6 1.5 f 0.7
a a = QJQb = emission rate of fine particles/emission rate of (i) value denotes standard deviation.
gas.
drawn from the dilution chamber through an in-line filter by a bellows pump and into an evacuated 0.8-L stainless steel cylinder. The cylinder was taken to the Iaboratory for analysis of volatile components in the woodsmoke plume. Particulate samples were also collected by a parallel set of impactors to determine the size distribution of the particles. These samples were analyzed for organic and elemental carbon and for elemental species by X-ray fluorescence (4). The source sampling experimental design was based on a stratified sampling scheme incorporating burn conditions and wood types typical of the Portland, OR, area. The test woods included local varieties of fir, alder, and oak. Three different box style wood stoves, three different fireplace inserts, and three different fireplaces were tested under the damper open condition and the damper all or partially closed condition. The damper open condition corresponds to a hot burn at a firebox temperature of around 500-600 "C. The damper closed condition corresponds to a cool burn and a firebox temperature of 250-300 OC. The burn rate of the wood, which can be controlled by the positioning of the damper, has been demonstrated to be one of the most important factors determining the chemical characteristics and emission per unit time of aerosol from wood-burning stoves (5-8). Similar procedures for start-up of the fire were carried out during each experiment. Integrated samples of dilute woodsmoke were collected over 20-min periods and analyzed by gas chromatography-electron capture detection (GC-ECD) for CH&l and by gas chromatography-flame ionization detection (GC-FID) for CO, CHI, and other hydrocarbon gases. The sampling began 5-10 min after the start-up of the fire. The particle/gas emission ratio may be higher during the initial start-up by a factor of 2 on the average. These conditions are not representative of the majority of the burn cycle, which typically is 4-5 h, and therefore were not included in the calculations of the gas concentration ratios. Also, it was recently determined in a survey that 11-22% of all woodburners in Portland burn all day (9).The emission ratios, CY = Qa/Qg, where Q, is the emission rate of fine aerosol and Q is the emission rate of CH3C1,for hardwood and softwoodunder hot and cool burn conditions, are presented in Table I. A significant difference is found between the hot and cool burn of fir, but no significant difference is found between the values for CY in the fir, damper open, the hardwood, damper open and closed, and the fireplace [Wilcoxon, y
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 8, 1986
803
= 0.05 (10);y is used to denote the significance level rather than the generally used cy to avoid confusion with the source emission ratio a]. Since fir and other softwoods generally have a higher content of lignin and extractable organic material such as various types of phenolic compounds (10,it is not surprising to find considerably more condensible organic emissions, and therefore higher particle emissions, occurring under starved oxygen conditions during the burning of fir. Other gases analyzed in the smoke plumes were CH4,CO, isoprene, a-pinene, benzene, toluene, ethylbenzene, m+p-xylene, and o-xylene. These gases might be included in a receptor model study of source reconciliation of atmospheric hydrocarbons. The average concentration ratios of these gases to carbon dioxide in the woodsmoke plume along with the concentration ratio of CH3Cl are shown in Table 11. The CH3C1to COz ratio is significantly different between the fir, open damper, and the fir, closed damper, while the ratio between the hardwood, open and closed damper, is only marginally significant (Wilcoxon,y = 0.05). The pattern is similar to that found for the fine partice/CH3C1emission ratio. The ratio of CH3Cl to COz for the fireplaces is not significantly different from the fir, closed damper. The differences between the ratios during the open and closed damper burns should be viewed with caution as the absolute emission rate of C02during a cool burn with the closed damper may be reduced by a factor of 2 or 3 over that during a hot burn. Therefore, increases in the gas/C02 ratio of less than 3 during a cool burn may be due to the decrease in COz rather than to the increase in the absolute emission rate of the gas itself.
Application The emission ratios of fine particles to CH3C1 from wood-burningstoves and fireplaces are used in a stratified sampling scheme to predict the fine particulate concentrations in the air from woodsmoke by using the gaseous tracer CH3C1. The Oregon Department of Environmental Quality in 1981 surveyed 1108 households in the Portland area (9) and found that 37% of the wood burned was burned in fireplaces, and for those who burn in wood stoves 19% of the population generally burn with an open damper while 81% burn with the damper partially or completely closed. The survey also estimates the number of woodstoves, fireplaces, and fireplace inserts in use and the type of wood generally used. A weighting scheme is applied according to the percentages of the population burning under each condition to construct a composite a,acOmp, for the Portland area. The filter data from the source sampling experiments with similar dilution enable an approximate estimate to be made of the fine particulate mass contribution weighting factor relative to the softwood, cool burn, for the other burn experiments. For example, the hardwood, cool burn, fine particulate emission factor is only 42% of the emission factor for softwood, cool burn. The probability that a woodsmoke particle in the ambient air has been emitted from any burn category is determined by multiplying the percentage of the population burning in each category, i, by the appropriate mass weighting factor and normalizing the sum of all probabilities such that Cpi = 1. This is shown in Table 111. Then the mean value of ace,,, and its uncertainty can be determined by using the stratified sampling statistics procedure (12),which breaks down the variation of a variable into its components. By use of the survey results, our calculations, which are discussed in detail in the Appendix, show that cycomp = 0.68 f 0.13 (pg/m3)/parts per trillion (volume) (pptv) for Portland, OR. 804
Envlron. Sci. Technol., Val. 20, No. 8, 1986
d
d
n
m II
d
d
n
m II
v
r:
Y
N
N
O P-
0
ti
c:
u?
n
d
\
\
DILUTIOM CHAMBER
,
-4
J'
TEFLON FLTER
1
SMOKE PLUME SAMPLINQ SYSTEM
Flgure 1. Smoke plume sampling system showing gaseous collection system (left) and aerosol collectlon system ( 4 ) (right).
Table 111. Contribution of Each Burn Category to the Composite Emission Ratio for Fine Particle/CH,Cl Emissions from Wood Burning"
burn category i
% of wood burned in Each Category (lop
mass emission ratio, category i to softwood cool category
mass weighting factor
Pi
softwood, cool softwood, hot hardwood, cool hardwood, hot fireplace
26 5 26 5 37
1.00 0.13 0.42 0.06 0.07
0.595 0.077 0.250 0.036 0.042
0.65 0.01 0.27 0.01 0.06
"piis the probability that an ambient woodsmoke particle was emitted from category i. bAssuming 50% of the wood burned is softwood and 50% is hardwood.
Table IV. Values for Composite Emission Ratio, aoOmp [(bg/m3)./pptv], Assuming Different Percentages of the Population Burning Softwood and Hardwood and Hot and Cool Burn % burning softwood
% burning
cool
40
50
60
60 81
0.61 0.63
0.65 0.68
0.69 0.72
species
% compn
species
% comp"
Si c1
0.051 i 0.204
Pb CH&I OC EC
0.010 f 0.021 0.310 f 0.030 49.780 5.584 8.700 f 6.163
sum
59.593
K Zn Br
It is important to note that knowledge of the wood types and burn conditions that are used in a community is necessary information for determining the appropriate local value of acomp (Table IV). The concentration of wood burning to the local fine aerosol concentration, C,, at a receptor site, assuming the dispersion of the gas from the source is similar to the dispersion of the fine particles from the same source emitted simultaneously, is C, ( ~ g / m ~ = )aACg (PPtv)
Table V. Wood-Burning Source Profile Developed in This Work and Used in the CMB Calculations
(1)
0.228 0.472 0.037 0.005
f 0.674 f 1.412 f 0.094 f 0.001
"Percent composition of fine particles in source. Value for CHICl is ratio of CHICl to fine particles in emissions.
where AC, is the elevated concentration of the gas CH3C1 at the receptor and a is the emission ratio of fine aerosol to CH3Cl in the source. The uncertainty in the value of a has been shown to be a much smaller portion of the uncertainty in the C, than the uncertainty in the background CH&l concentrations at the concentration level typically found in a residential neighborhood; nevertheless, the wood-burning source can generally be estimated to within 20-40% with CH,Cl as a unique tracer (3). When Environ. Sci. Technol., Vol. 20, No. 8, 1986
805
Table VI. Contribution of Woodstoves (in gg/m8) to the Fine Particulate Concentrations in Hillsboro, OR, Calculated with the CMB Modela sample period (start date)
set 1
set 2
set 3
set 4
set 5
total mass
1/17/84,6 p.m.-6 a.m. 1/19/84,6 p.m.-6 a.m. 1/26/84, 6 p.m.-6 a.m. 1/27/84, 6 p.m.-6 a.m. 2/2/84,6-9 p.m. 2/3/84,6-9 p.m. 2/3/84,9 p.m.-6 a.m.
58 f 8 45 f 6 46 f 6 36 f 5 103 f 15 46 f 7 41 f 5
59 f 8 48 f 7 49 f 8 37 f 6 108 f 20 51 f 5 55 f 9
53 f 8 40 f 7 41 8 35 f 6 94 f 15 54 f 5 37 f 9
53 f 13 40 f 13 41 f 13 33 f 13 94 f 17 58 f 13 37 12
47 f 76 43 f 70 111 f xxx 108 f xxx 127 f xxx 108 f xxx 108 f xxx
66 f 3 55 f 3 48 f 2 32 f 2 106 f 11 51 f 5 53 f 3
*
*
In set 1, the species OC, EC, Pb, Si, C1, K, Zn, Br, and CHaClwere used. In sets 2 and 3, CH3Cl and OC were deleted respectively. In set 4 only CH3Clwas used, and in set 5 OC, EC, and CH3C1were omitted in the calculations. The comparison shows the consistency of OC and CH3Cl as tracers of wood combustion and the occasional inconsistency of relying on the K concentrations as in set 5. OC is organic carbon, EC is elemental carbon, and the (f)value represents the uncertainty calculated in the effective variance solution by propagating the uncertainties in the source and ambient measurements. The xxx indicates that the uncertaintv is greater than 100 d m 3 .
included in a multicomponent CMB model, the mass balance of CH&l is written as C, = ajSj (2) where Sj is the contribution of wood burning to the fine particles and aj = l/a. The uj is included in the source composition matrix with the other species used to characterize each source. Data from Hillsboro, OR, a small mostly residential community about 25 km from downtown Portland, is used to illustrate the use of methyl chloride in receptor modeling apportionment of residential wood-burning air pollution. Ambient gaseous and particulate samples were analyzed for 6 days in the wintertime of 1984. The aerosol was collected on Teflon filters by a cyclone with a 2.5-pm cut point. Elemental concentrations were determined by X-ray fluorescence, and organic and elemental carbon concentrations were determined by the thermal-optical method of carbon analysis developed a t the Oregon Graduate Center (13). The gas samples were collected by an integrated gas sampling system (14) and analyzed for CH3C1by GC-ECD (15). Gaseous and particulate samples were collected concurrently during the evening hours when wood-burning emissions were expected to be the greater. The chemical mass balance (CMB) least-squares fitting procedure with the effective variance weighting scheme (16) was used to estimate the contribution to fine particle pollution from woodstoves and fireplaces, automobiles, and urban dust. The calculations were carried out by using the QSAS I11 software package (17). The woodstove survey used to construct the composite source profile included the Hillsboro area. The source profile used for residential wood combustion in the model is shown in Table V. The source profiles used for automobiles and for urban dust have been developed previously for the Portland area (18). Residential woodstoves and fireplaces were found to contribute 73-100% of the total mass of fine particles in the air on winter evenings in Hillsboro; they ranged from 32 to 106 pg/m3 during the sample collection period. The results of the CMB calculations are shown in Table VI. In a simple community with few other sources of organic carbon, both the CH3C1and the organic carbon (OC) are key species in determining the wood-burning contribution. In the first set of calculations, the species used included organic carbon (OC), elemental carbon (EC), Pb, Si, C1, K, Zn, Br, and CH3C1. In the next two sets of calculations, CH3Cl and OC were deleted respectively from the calculations. In set 4,the unique CH3C1tracer approach was applied. The calculations of the first four sets agree well, indicating the consistency of the organic carbon and the methyl chloride measurements. In set 5, OC, EC, and CH&l are deleted from the CMB calculations. In this set, 806
Envlron. Sci. Technol., Vol. 20, No. 8, 1986
the K and the C1 are key species in determing the woodburning contribution. The large uncertainties of set 5 are a reflection of the large variability of K and C1 in the source emissions. The wood-burningcontributions on half of the evening samples are overpredicted by a factor of 2-3 and are much larger than the total measured mass. These results provide evidence that the large variability in the K emissions from wood combustion make it an unreliable tracer for woodsmoke in the ambient air. In communities where there are other sources of organic carbon, the inclusion of CH3Cl in the CMB model provides estimates of fine particle pollution from residential wood burning with greater precision than if only the particulate species were used (19). The emission ratios of fine particles to CH,C1 reported in this paper are useful in urban source apportionment studies where it is suspected that woodstoves and fireplaces may contribute significantly to air pollution. This ratio has also been applied in local studies of air pollution from backyard burning (2) and regional studies of haze from slash and agricultural burning (20) and may also find application in global studies on the contribution of biomass combustion to tropospheric gases and aerosols.
Acknowledgments We thank John Rau for assistance with sample collection and Patricia Quinn, Don Steams, and Robert Watkins for laboratory analyses. We also thank Robert Stevens of the US. EPA for the use of a cyclone sampler and John Cooper of NEA, Inc., Beaverton, OR, for the use of source apportionment software.
Appendix If a population can be divided into k subpopulations, the mean and variance of each subpopulation can be determined separately. The mean value for the population is p = P l p l -k P Z h + + Pkpk (AI) where p i is the probability that the random variable will lie in the ith subpopulation. The variance of the mean in a stratified sampling system is **a
k
~1'
= l / n ( C ) p i [ u ?+ (pi - 1111' 1
(-42)
where 5 is the estimated mean value, p is the population mean, pi and ui are the mean and standard deviation of category i , and n is the sample size. Since there was no statistically significant difference found between the average values of a for fireplace burning, hot and cool hardwood burning, and hot softwood
Environ. Sci. Technol. 1986, 20, 807-810
burning, the individual values for these types of burns are averaged together to give af= (1.5 f 0.8) X lo2 (A31
(6)
where ctf is the average value for the ratio of all fireplace, hardwood hot and cool burns, and softwood hot burns, and the (A)value is the standard deviation. The value of aC, which is the ratio for softwood cool burn, is cy, = (4.1 f 1.6) X 10’ (A4)
(7)
(8) (9)
Therefore, two populations are used to determine the value of cymmp, the softwood cool burn, cyc, and the ctf given above. By use of the survey results, cicompwould be calculated as follows: ctcOmp= [0.65 (4.1) 0.35 (1.5)] X 10’ (A5)
(10) (11)
+
1982; pp 746-771. (12) Hoel, P. G. Introduction to Mathematical Statistics; Wiley: New York, 1971. (13) Johnson, R. L.; Shah, J. J.; Huntzicker, J. J. Presented at the Symposium on Chemical Composition of Atmospheric
where 65% of the mass is contributed from softwood, cool burning, and 35% is contributed from all other categories. Values for ct &g/m3)/pptv are given in Table IV, assuming 40%, 50% and 60% softwood burning and 60% and 81% cool burning. A 5% correction for the fraction of large particles (>2.5 pm) found in the smoke samples was made as determined from the impactor collection. The value for the emission ratio ctcomp and the uncertainty of the ratio using the survey results to estimate wood type usage and typical burn conditions for the Portland, OR, area is ctmmP(pg/m3)/pptv= 0.68 f 0.13 (-46)
Aerosols: Source/Air Quality Relatioships, Second Chemical Congress of the North American Continent, Las Vegas, NV, 1980. (14) Edgerton, S. A.; Khalil, M. A. K.; Rasmussen, R. A. J . Environ. Sci. Health 1985, A(20),563-581. (15) Rasmussen, R. A.; Rasmussen, L. E.; Khalil, M. A. K.; Dalluge, R. W. J . Geophys. Res. 1980, 85, 7350-7356. (16) Watson, J. G.; Cooper, J. A.; Huntzicker, J. J. Atrnos. Environ. 1984, 18, 1347-1355. (17) NEA, Inc. “QuantitativeSource Apportionment System 111”, 1984. (18) Watson, J. G. Ph.D. Dissertation,Oregon Graduate Center, Beaverton, OR, 1979. (19) DeCesar, R. T.; Edgerton, S. A,; Khalil, M. A. K.; Rasmussen, R. A. Chemosphere 1985, 14, 1495-1501. (20) Khalil, M. A. K.; Rasmussen, R. A.; Edgerton, S. A. J . Air Pollut. Control Assoc. 1985, 35, 838-840.
where the (f)value indicates the 95% confidence limits. Registry No. CH3Cl, 74-87-3.
Literature Cited Khalil, M. A. K.; Edgerton, S. A.; Rasmussen, R. A. Environ. Sci. Technol. 1983, 17, 555-559. Edgerton, S. A.; Khalil, M. A. K.; Rasmussen, R. A. J . Air Pollut. Control Assoc. 1984, 34, 661-664. Edgerton, S. A. Ph.D. Dissertation, Oregon Graduate Center, Beaverton, OR, 1985. Rau, J. A. Ph.D. Dissertation, Oregon Graduate Center, Beaverton, OR, 1985. Hubble, B. R.; Stetter, J. R.; Gebert, E.; Harkness, J. B. L.; Flotard, R. D. In Residential Solid Fuels; Cooper, J.
A,; Malek, D., Eds.; Oregon Graduate Center: Beaverton, OR, 1982; pp 79-138. Muhlbaier, J. L. In Residential Solid Fuels; Cooper, J. A.; Malek, D., Eds.; Oregon Graduate Center: Beaverton, OR, 1982; pp 164-187. Barnett, S. G.; Shea, D. In Residential Solid Fuels: Cooper, J. A.; Malek, D., Eds.; Oregon Graduate Center: Beaverton, OR, 1982. Cooke, W. M.; Allen, J. M.; Hall, R. E. In Residential Solid Fuels; Cooper, J. A.; Malek, D., Eds.; Oregon Graduate Center: Beaverton, OR, 1982. Oregon Department of Environmental Quality “Portland Wood Heat Survey”. C. Cummings, 1984. Snedecor, G. W.; Cochran, W. G. Statistical Methods; The Iowa State University Press: Ames, IA, 1980. Shafizadeh, F. In Residential Solid Fuels; Cooper, J. A.; Malek, D., Eds.; Oregon Graduate Center: Beaverton, OR,
Received for review September 12, 1985. Revised manuscript received January 24, 1986. Accepted March 17, 1986. Parts of this project were supported by a grant from the W.S. EPA (R810090-01-0). Additional support was provided by the Biospherics Research Corp. and the Andarz Co.
Aqueous Solubilities of Six Polychlorinated Biphenyl Congeners at Four Temperatures Rebecca M. Dickhut,” Anders W. Andren, and David E. Armstrong
Water Chemistry Program, University of Wisconsin, Madison, Wisconsin 53706 Aqueous solubilities of six polychlorinated biphenyls (PCBs) were measured at 25 “C and three other temperatures by using a slightly modified generator column method. Solubility increased exponentially with temperature in the range 0.4-80 “C for all six PCBs investigated, as well as for biphenyl and 4-chlorobiphenyl. Enthalpies of solution for the PCBs and biphenyl were determined, ranging from 28.5 (4-chlorobiphenyl) to 66.6 kJ/mol (decachlorobiphenyl). These values can be used to interpolate solubilities in the experimental temperature ranges, with average errors of
