Environ. Sci. Techno/. 1995,29,825-832
Factors Affecting Methyl Chloride Emissions from Forest Biomass Combustion TIMOTHY E . R E I N H A R D T * s t A N D DAROLD E. WARD* USDA Forest Service, Pacific Northwest Research Station, 4043 Roosevelt Way NE, Seattle, Washington 98105
Chloromethane (CH3CI or methyl chloride) is a trace gas in the atmosphere that is a characteristic emission of vegetative biomass burning. Measurements of chloromethane emissions from laboratory-scale fires in forest fuels show that CH3CI emission factors vary considerably between flaming and smoldering combustion. Regression analysis indicates that CH3CI emission factors can be predicted from three fire and fuel parameters: combustion efficiency, rate of heat release, and fuel chloride content. The percentage of chlorine in the fine particle emissions from burning forest fuels also varies with combustion conditions. A combustion reaction mechanism is proposed to explain the observed variation in chlorinated emissions.
Introduction Smoke in the atmosphere obscuresvisibility, irritates human respiratory systems, and is harmful at high concentrations. Smoke from burning vegetative biomass is the dominant air pollutant in much of the world. Biomass is burned for heating, cooking, and agriculture. Wildfires and prescribed buming for forest management burn millions of hectares annually. Globally, fire consumes in excess of 6 Pg (Pg = petagram = loi5 9, of biomass per year (1). Assessing the impact of biomass buming on air quality is difficult because of the variability in emission estimates and the lack of emission inventories. Chemical mass balance receptor modeling methods have been used to identify the contributions of various sources of fine particle mass to the atmosphere. In the Pacific Northwest, for example, receptor modeling attributed over 60% of the summertime visibility reduction near class I areas to prescribed burning (2). To apportion the air quality impact at a receptor site among different sources, receptor models require the emissions to be unique and well-defined for each source. For prescribed burning and biomass buming as a whole, the emissions signatures are not adequately defined (3). Chloromethane (methyl chloride, CH3C1) is a minor product of biomass burning. Emissions estimates for CH3C1vary substantially, making estimates of its global impact uncertain (4). Measurements of chloromethane have shown this compound to be a nearly unique tracer gas for air pollutant emissions from burning biomass (5-7). Researchers have shown how CH3Cl can be used in a gaseous tracer type of receptor model to define the impact of fine particles from biomass burning on air quality (8,9). In the gaseous tracer model, the ratio of CH3C1 and fine particle emission factors from a source must be known. These emission factors have been measured for woodstove and fireplace emissions and were found to vary with different fire conditions and fuel types (10). The reasons for this variation were unclear. Receptor modeling of residential wood combustion using the gaseous tracer model may be successful in spite of this variation because many sources are usually operating in a close area. The combined effect of each combustion device operating at different levels of combustion efficiency may produce a well-mixed cloud of combustion products that can be adequatelymodeled using “representative”emissionfactors for fine particles and CH3Cl. However, applying such techniques to estimate the air quality impact of biomass combustion would be inaccurate when the actual ratio of CH3Cl to fine particle emissions differs from the modeled ratio. This study sought to improve the utility of CH3C1 as a tracer of biomass burning by evaluating CH3C1 emissions from forest fuels as a function of key fire and fuel variables. A series of well-defined fuel beds were burned under a combustion hood, and the emissions of CH3C1, fine particles, and other carbonaceous pollutants were mea* Author to whom correspondence should be addressed. t Present address: Radian Corporation, 11711 SE Eighth Street,
Bellevue, WA 98005. Present address: Intermountain Research Station, USDA Forest Service, P.O. Box 8089, Missoula, MT 59807.
*
0013-936W95/0929-0825$09.00/0
1995 American Chemical Society
VOL. 29. NO. 3, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
825
sured. Based on these measurements, multiple regression techniques were used to predict the emissions of CH3Cl as a function of fuel chloride content, rate of heat release (reaction intensity), and combustion efficiency (11). Chemistry of Methyl Chloride Formation. A series of steps are involved in wood combustion (12): (1)fuel heating and drying, (2) solid fuel pyrolysis to volatiles and char, (3) pyrolysis and oxidation of the volatiles, and (4) oxidation of the char. Pyrolysis of the solid fuel occurs via two distinct pathways (13). The first pathway operates below 300 "C. It is characterized by bond scission between individual units in the cellulosic polymers, elimination of water, formation of free radicals, carbonyl, carboxyl, and hydroperoxide groups. This leads to decarboxylation and decarbonylation reactions that generate C02 and CO. Above 300 "C, the second pathway dominates, breaking molecules apart by transglycosylation, fission, and disproportionation reactions. Reactive carbonaceous char and noncombustible volatiles are the major products of the first pathway. Low molecular weight volatiles and tarry anhydro sugars are the products of the second pathway. Pyrolysis and oxidation of reactive carbonaceous char are characteristic of glowing and smoldering combustion. Pyrolysis and oxidation of volatiles constitute flaming combustion. In flaming combustion, free radicals dominate the reaction mechanisms, rapidly breaking apart larger molecules into simpler compounds and atoms. Self-sustaining and branching chain reactions generate the radicals which propagate the flame:
+ 0, - OH' + 0' OH' + CH, - H,O + CH,' H' + CH, H, + CH,' 0' + CH, OH' + CH,' H'
+
-r
(1)
(2) (3)
(4)
Reactions 1-4 are typical of the high-temperature oxidation of methane in flaming combustion (14). Reactions 1 and 4 are important chain-branching mechanisms. In flames, small amounts of HCl are effective flame inhibitors via reaction 5: H'
+ HC1-
H,
+ C1'
(51
The CP radical will eventually terminate the chain via recombination with H'or with another CP radical in the following reaction (15): Cl'+ Cl'+ M - C1,
+M
(6)
The heterocatalytic low-temperature reaction between gaseous methanol and dissociated HCl over activated charcoal has been studied as a method of industrial CH3Cl production (16): CH,OH
+ HCl - CH,CI + H,O
(7)
The reactants for this are present during biomass combustion. Activated charcoal is the reactive char formed during the low-temperature pathway of pyrolysis associated with glowing and smoldering combustion. Gaseous methanol is an important product of wood pyrolysis-up to 2% of the wood mass is converted to methanol during pyrolysis 826
ENVIRONMENTAL SCIENCE Ei TECHNOLOGY / VOL. 29. NO. 3, 1995
(13). Finally, chlorine is incorporated into woody fuels from local sources and atmospheric deposition of salt (17). Reaction 5 has a lower activation energy than reaction 7. A study of the occurrence of chlorinated compounds in an oxygen-methane flame found that HC1 can compete effectively for H' radicals at the low temperatures found in the early part of a flame (18). At the higher temperatures later in a flame, reaction 5 loses its advantage because sufficient energy is available to overcome the activation energy barrier of reaction 7. The oxygen-methane flame study found that CH3Cl was formed in the early part of the flame zone, apparently at the expense of formaldehyde. Chloromethane was subsequently destroyed within the flame, leaving only HCl in the emissions. From these observations, it would appear unlikely that CH3C1emissions are from a flaming combustion mechanism. It is possible that reaction 7 is responsible for chloromethane production during wood combustion.
Methods The carbon mass balance method was used exclusively to measure the equivalent weight of fuel burned for the emission factor determinations (19). This method relies on the relatively consistent carbon content of forest fuels and the assumption that all carbonaceous emissions sampled (in excess of background levels) are derived from the carbon in the fuel burned. The amount of fuel burned can then be calculated from the sum of the carbon in the emissions. Fuel Preparation. Three different types of fuel beds were burned on a 0.5-m2burning platform at the University of Washington, College of Forest Resources: Douglas fir timber harvest residues (logging slash), Douglas fir fuel sticks, and duff beds (partially decomposed litter from the forest floor). Most of the fuel beds were constructed of Douglas fir (Pseudotsugu menzesii [mirb.] Franco) lumber sawn into fuel sticks of homogeneous size and composition (1.2 cm x 1.2 cm x 20 cm). The fir sticks were arranged into fuel beds having a constant packing ratio (ratio of fuel bed array density to fuel particle bulk density), but the mass of fuel used per bed was varied to produce different reaction intensities during combustion (20). The fir sticks were naturally low in chlorine content. To evaluate the effect of fuel chloride concentration on chloromethane production, some of the fuel beds were made of fir sticks that had been soaked in aqueous solutions of 1.0 and 2.0 mg/L sodium chloride which increased the fuel chloride content. The duff fuel beds were composed of undisturbed litter and humus layers extractedfrom the forest floor underneath a mixed stand of Western redcedar (Thuja plicutu [Donn exD. Don])and Western hemlock (Tsuguheterophylla [Raf.] Sarg.). The logging slash fuel beds were constructed of Douglas fir branches between 0.6 and 7.5 cm in diameter and 0.4 m in length. Samples were taken from all fuel beds for determining their moisture content and chloride content prior to burning. Emissions Sampling. All of the emissions from the fire were drawn by a fan through a 40 cm diameter stainless steel exhaust duct. Emissions were isokineticallysampled from this duct through an EPA Method 5-type sampling train. Velocity profiles in the duct were measured using a type-S pitot tube. An inertial impactor (Andersen Instruments) was used in the sampling train to exclude particles larger than 2.0 pm in aerodynamic diameter. The fine
TABLE 1
Fire Characteristics burn combustion ID phase 5 5 6 6 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23
flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering flaming smoldering
reaction intensity fuel type
untreated untreated 1.0 mg/L 1.0 mg/L untreated untreated duff duff duff duff 1.0 mg/L 1.0 mg/L 1.0 mg/L 1.0 mg/L untreated untreated 2.0 mg/L 2.0 mg/L untreated untreated duff duff 2.0 mg/L 2.0 mg/L 2.0 mg/L 2.0 mg/L duff duff slash slash slash slash slash slash
(kW
combustion fuel efficiency chloride s-') ratio ( O h ) bds)
82.3 65.4 NA 65.6 NA 57.2 52.8 33.2 38.6 NA 16.5 21.7 51.6 14.6 22.7 3.1 39.8 3.3 40.7 12.5 30.8 7.1 60.5 14.4 NA 12.7 46.9 11.9 16.5 57.2 16.7 6.8 134.1 34.4
98.3 93.7 NA 86.9 NA 91.4 75.1 79.6 77.0 NA 97.5 96.4 98.0 90.9 97.7 92.4 97.8 92.1 98.2 87.7 94.9 67.4 98.2 85.7 NA 84.5 94.8 66.7 86.2 96.5 94.9 85.6 98.4 90.1
96 96 NA 48 NA 15 130 130 125 NA 83 83 29 29 0 0 195 195 0 0 118 118 153 153 NA 102 111 111 0 0 14 14 13 13
particles (PM2.0) were then collected on 47-mm Zefluor Teflon filters (Gelman). Finally, gases were cooled and collected in five-layer gas sampling bags (Calibrated Instruments) that were inert to the compounds of interest. Reagent ethanol was used to ignite the fuel beds. Once the ethanol had burned off, integrated sampling was conducted over two distinct phases of each fire-flaming and smoldering. The flaming phase of a fire was defined by flames over at least 75% of the fuel bed surface. The smoldering phase was defined by only smoldering and glowing combustion processes over 75% of the fuel bed. Chemical Analyses. Milled 1.5-goven-dried fuel samples were shaken for 1h with 50 mL of 0.1 M sodium nitrate and then filtered. Chloride in the resulting fuel sample extract was measured by chloride ion electrode (Orion) (21, 22). Standards were prepared from dilutions of a sodium chloride stock solution adjusted to the sample's ionic strength with sodium nitrate. Carbon dioxide (C02) and carbon monoxide (CO) in emissions samples were analyzed by infrared absorbance spectrometry (Horiba). Methane (CHI) and non-methane hydrocarbons (NMHC in propane equivalents) were analyzed by gas chromatography (Baseline Industries) at 50 "C on a 50/80 mesh Porapak N column with flame ionization detection. Standardswere prepared by volumetric dilutions of a certified gas standard. Chloromethanewas determined by gas chromatography (Perkin-Elmer)at 72 "C on an 80/ 100mesh PorasilB column with electron capture detection. Nitrogen/oxygen carrier
gas (2090 ppm oxygen) enabled detection of ambient atmospheric levels ( ~ 0 . ppb) 8 of chloromethane (23, 24). Standards were made by volumetric dilutions of a primary standard generated by a permeation tube (VICI Metronics) (25).
Fine particle mass was measured by gravimetry using a microbalance (Cahn). Chlorine content of the fine particle mass on the filters was determined by an independent laboratory using X-ray fluorescence (NEA Inc., Beaverton, OR). Emission Factor Calculations. Emission factors are defined here as the amount of a given pollutant emitted per kilogram of fuel burned. For both flaming and smoldering phases of each burn, emission factors were calculated at standard temperature and pressure according to the following equations (26): (1) The carbon mass balance equation (eq 8) to determine the mass of fuel that contributed to the emissions sample:
ccn w,= R where W, is the fuel contributing to the emissions from each phase of combustion (flaming = f, smoldering = s) (g/m3),C, is the carbon mass fraction of the emissions (g/m3),rz is the carbonaceous compound (CO, Con,CHI, NMHC, PM2.0), and R is the carbon fraction of woody fuels (0.497). (2) The emission factors are calculated from
EF,, = Eip/Wp where EFi, is the emission factor for compound i (in g or mg emissions/kg of fuel) in combustion phase p, and Ei, is the net concentration of any compound i in the emissions from phase p (g/m3). Calculation of Fuel and FireVariables. The rate of fuel consumption through the duct samplingplane is calculated from the fuel consumption and the cross-sectional average duct gas velocity:
W=W,
(10)
where W is the rate of fuel consumption (g m-2 s-l), and Vsis the average duct gas velocity (m/s). Reaction intensity (the net heat release rate per unit area of fuel bed) for each phase of combustion is approximated by the following equation, which relies on an estimate of the net heat yield of combustion of 15.12 kJ/g of fuel, which is appropriate for small fires and low fuel moistures (27):
(11) where Iris the reaction intensity (kW/m2),A, is the crosssectional area of the stack (m2),Af is the fuel bed area (m2), and H, is the net heat yield of combustion (kJ/g). Combustion efficiency is calculated as the actual carbon dioxide emission factor divided by the carbon dioxide emission factor for theoretically complete combustion (1835gof Con/ kg of fuel). Chlorine Mass Balance. Apartial chlorine mass balance was derived for all burns with data for fuel bed chloride, chloromethane emissions, and chlorine in fine particles. VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
827
140
cn
scn
120
L
0 0
100
IL
80
v
.4-
m
c
.-0
.-
60
W
40
v) v)
E
9
2a
20
0
5 6 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Burn Identification Number
1 0Flaming phase
Smoldering phase
-
[
FIGURE 1. Fine particle emission factors. h
En
5
E
v
100
L
0 c
m
I'
80
LL S
.-0 v) .-In W E
60
40
0 C
5
E -c0 0
L
20
0
5
6
0
9 12 13 14 15 16 18 19 21 22 23
Burn Identification Number
FIGURE 2. Chloromethane emission factors.
This neglects chlorine that is emitted as HCI and chlorine in coarse uarticles, hut is useful to determine how much fuel chloriie is converted to fme particle chloride and CHJCI. The conversionefficiencyisthemass ofchlorine found in the emissions divided by the chloride found in the fuel. Aweiehted average of chlorine emissions for the two ohases of theyfire was c&dated by weighting the sum of chorine 828. ENVIRONMENTAL SCIENCE &TECHNOLOGY i VOL. 29, NO. 3.1995
phase by the amount of fuel consumed
I
where Wand W,are the fuel contributing to the emissions
500
fl
1
400
300 200
100 0
10
11
20
17
Burn Identification Number
1 0Flaming phase FIGURE 3. Chloromethane emission factors from duff fuel beds.
i
7 500
Chloromethane Ratios
fir sticks and slash fir sticks and slash duff duff
1
m
TABLE 2
fuel bed type
Smoldering phase
I
CH$I/CO2 phase (ppm x 10-5/ppml F
S F S
+ 0.3
0.2 1.2 6.2 26.0 zt
+ 1.5 + 6.8 5.8
EF PMZ.O/EF CHaCl (glgl
: i.
2400 Y
6365 j, 9360 9640 15235 1209 zt 697 276 zt 69
+
for flaming and smoldering, respectively (g/m3),X l t and X I s are the chlorine fraction sum of the flaming and smoldering emissions (mg of CUkg of fuel), andClb is the chloride fraction of the fuel bed (mg of CUkg of fuel).
Results
i
6
65%
70%
75%
80%
85%
90%
95%
fOO%
Combustion Efficiency
~
-
Firsicks
0
Slash
-
DuffEleds~
Fire Characteristics. Reaction intensity, combustion ef-
FIGURE4. Chloromethaneemissionfactorsvscombustionefficiency.
ficiency, and fuel chloride for the test burns are listed in Table 1. These reactionintensities are typicalofprescribed burns. Smoldering combustion phases have a lower rate ofconversionoffuel to emissions than flaming combustion. Apparent exceptions to this rule are due to the eruption of flames in unhurnt fuel during a smoldering phase (in the case of burn 12) or a poor ignition during a flaming phase (for burn 21). Fuel bed chloride concentrations (on a dry fuel weight basis) arefoundinTable 1. Soakingfirsticksinthesodium chloride solutions was effective at raising their salt content above the levels found in the untreated fir sticks and the slash branches. The levels of chloride found to occur naturally in the duffsamples were relativelyhomogeneous and higher than in the other forest fuels. Emission Factors. Flaming and smoldering phase emission factors for fine particles, chloromethane (exclud-
ingdufFheds),andchloromethanefromduffbedsareshown in Figures 1-3 , respectively. The mean emission factors for both PM2.0 and CHEI are significantly higher for smoldering combustion than for flaming combustion, Smoldering combustion dominated for the duff fuel beds. CH3CIRatios. The ratios ofCH3CIand key combustion products are listed in Table 2 for comparison with measurements by other researchers. The CH3CI/C02ratios agree with some previous data (10).but are a factor of 10 lower than other data, with the exception of smoldering duff beds ( 3 . Our ratio data for PM2.0 and CH&l are an order of magnitude higher than previous data, again with theexceptionofsmolderingduffheds (10). Ourratios show morevariationamonghuns,whichmight beexpectedfrom experiments aimed at examining the range of CH&l emissions. Some previous data came from measurements of woodstove and fireplace emissions. In those devices, VOL. 29, NO. 3,19951 ENVIRONMENTAL SCIENCE &TECHNOLOGY -829
TABLE 3
- 4001 , Y m
.
* I
,? 300
z
Y
Chloromethane Regression Equationsa data set xo x, 4 x,
,
all fuel beds 1359 0.371* fir sticks 316 0.199* firsticks and slash 243 0.197'
200 1
0.3 -4731* 108* -1125' 95* -864*
0
40 60 80 100 120 Reaction Intensity (kWIMZ-s)
20
-
140
Partial Chlorine Conversion Efficiency burn IO
Firsticks
D
Slash
A
Duff Beds1
5
fuel bed type
panial chlorine conversion efficiency I%)
untreated fir
1 6 23 13
FIGURE 5. Chloromethane emission factors vs reaction intensity.
u,
0.78 30 0.72 17 0.67 23
*An asteriskl*lindicatesa numberissignificantat95%confidence level.
~
$50 E
n
TABLE 4
-6
-
Z
-
8
'
L ~
I 150
200
100 Fuel Chlorlde Concentration (us$) FIGURE 6. Chloromethane emission factors vs fuel chloride.
the fuels are consistently hand-sized for ease of handling, and the combustion conditions are relatively controlled. These factors may result in fundamental differencesin the
7
..
ld
11 17 20
duff duff duff
69 70 90
PM2.0/CH3Clratiosfrom the combustionconditions typical of prescribed burns, which have a range of fuel sizes and heterogeneous fuel distributions. Regression Analysis. The data were analyzed using multiple regression techniques. Although these data are not ideally suited to regression analysis because they are not evenly distihutedacross the ranges forthe independent variables, it is informative to develop estimates of the equation parameters. Scatterplots of EF CH3CI versus combustion efficiency,reaction intensity, and fuel chloride are presented in Figures 4-6, respectively. Nonlinear relationships were apparent for CH3CI emission factors vs reaction intensity (I,) and EF CHsCl vs r) (combustion
7
0.6
0.5 0.4
0.3
0.2 0.1 0
5 11 12 13 14 15 16 17 18 19 20 21 22 23 Burn Identification Number
I U Flaming ' Phase FIGURE 1. Chlorine percentage in fine particle emissions.
830. ENVIRONMENTAL SCIENCE &TECHNOLOGY i VOL. 29. NO. 3.1995
Smoldering Phase
1
+
efficiency), so the transformed variables 1/I, and log(? 1)were used in the multiple regression. The combination of these three variables was found to explain 67-78% of the variance in the observed EF CH3C1, depending on the subset of data used. The regression is shown in eq 13, and the coefficients are presented in Table 3.
EF CH3C1= X,
+ X,[Cl-] + x* - + X3 log(q + 1) 4
(13)
For graphic representation in Figure 6, a weightedaverage CH3Cl emission factor was calculated for each fire by weighting the observed emission factor for each combustion phase by the amount of fuel (W,) contributing to the sample, as was done for chlorine in eq 12. However, the individual emission factors for each phase are used as separate observations for the regression equations. The regression coefficients are generally consistent among subsets. A hierarchical decomposition of the regression model was made using F-ratios to test the difference of the regression coefficients from zero for each data set. The hierarchical approach was used because of the interrelationships among these parameters: fuel sodium chloride affects reaction intensity, which in turn is related to combustion efficiency. Significance of coefficients is indicated by an asterisk in Table 3. Percentage of Chlorine in PM2.0. Figure 7 shows the percentage of chlorine found in the fine particle emissions samples that were analyzed for trace elements by X-ray fluorescence. The difference in the percentage of chlorine in PM2.0 between flaming and smoldering phases is significant at the 95% confidence level. Chlorine Conversion Efficiency. The efficiency for conversion of chlorine in fuel to chlorine in emissions (consideringonlythe chloride emitted in fine particles and CH3C1) is summarized in Table 4. Most of the fuel chloride is converted to chloride in fine particles and CH3Cl for the duffburns. The rest of the burns had conversion efficiencies ranging between 1 and 23%. The balance of the chlorine may be in the unmeasured conversion of fuel chloride to HC1.
Discussion Based on our observations, we can summarize several points: (1) The ratios of other combustion products (such as fine particles or carbon dioxide) to chloromethane are not constant for these modeled forest fuels. (2) Reaction intensity is a key variable determining the amount of chloromethane emitted from a biomass burn. Lower intensity fires (below 60 kW m-* s-l) will emit proportionately more CH3C1. (3) Combustion efficiency is another important factor controllingthe emissionsof CH3Cl. Inefficient combustion, typical of smoldering and glowing processes, is associated with high rates of CH3Cl production. Efficient flaming combustion produces very little chloromethane. (4)The amount of chloride in the fuel influences the production of CH3Cl. Inorganic chloride in the fuel is an important source of the chlorine in CH3Cl. (5) The equilibrium distribution of chloride between CH3Cl and chlorine in PM2.0 depends on combustion conditions. Flaming combustion results in the highest percentages of chlorine in the fine particle mass. The high percentages cannot be explained by a decrease in the percentage of organic carbon in particles from higher
intensity combustion, because this is offset by an increase in elemental carbon (28). If gaseous HC1 is the primary chlorinated product of flamingcombustion, some of it may be rapidly adsorbed into water vapor, which condenses onto the fine particles comprising PM2.0. Once in contact with the particulate matter, alkaline compounds or metal oxides present in the particles may react with the HCl to form stable salts. Our data identlfyfactors that are important in controlling CH3Clproduction from woody fuel combustion. Combustion conditions (which vary substantially) and, to a lesser degree, fuel chloride (which varies greatly between geographiclocation and plant portions (21))must be considered when estimating CH3C1 emission factors from different biomass combustion sources. In view of the variability of CH3Cl/pollutant ratios, the adequacy of available ratio estimates should be evaluated for their effect on the results of a specific application. Field measurements of these parameters will lead to better emission factor estimates for CH3C1and ratios to other pollutants. These ratios will in turn contribute to more accurate and precise receptor modeling results. Global estimates of chloromethane emissions may be improved by using data about biomass combustion conditions and applying the emissions model we have developed.
Acknowledgments The authors would like to thank Dr. Michael Pilat of the Department of Civil Engineering and Dr. Bjorn Hrutfiord and Dr. StewartPickford of the College of Forest Resources, University of Washington, for their valuable comments. The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S.Department ofAgriculture of any product or service to the exclusion of others that may be suitable.
literature Cited (1) Crutzen, P. J.; Andreae, M. 0. Science 1990, 250, 1669-1678.
(2) Pacific Northwest Regional Aerosol Mass Apportionment Study; R. W. Beck Associates: Seattle, WA, 1986. (3) Ward, D. E.; Core, J. E. Presented at 21st Annual Meeting, Air Pollution Control Association, Pacific Northwest International Section, Portland, OR, 1984, Paper 84-19. (4) Crutzen, P. J.; Heidt, L. E.; Krasnec, J. P.; Pollock, W. H.; Seiler, W. Nature 1979,282, 253-256. (5) Khalil, M. A. K.; Rasmussen, R. A. Environ. Sci. Technol. 1983, 17 (3),157-164. (6) Khalil, M. A. K.; Rasmussen, R. A.; Edgerton, S. A. 1.Air Pollut. Control Assoc. 1985, 35 (E), 838-840. (7) Tassios, S.; Packham, D. R. J. Air Pollut. Control Assoc. 1985,35 (11, 41-42. (8) Khalil, M. A. K.; Edgerton, S. A.; Rasmussen, R. A. Environ. Sci. Technol. 1983, 17 (9), 555-559. (9) Davidson, C. I.; Lin, S. F.; Osborn, J. F.; Pandey, M. R.; Rasmussen, R. A.; Khalil, M. A. K. Environ. Sci. Technol. 1986, 20 (6), 561567. (10) Edgerton, S. A.; Khalil, M. A. K.; Rasmussen, R. A. Environ. Sci. Technol. 1986, 20 (€9,803-807. (11) Reinhardt, T. E. Master’s Thesis, University ofwashington, 1987. (12) Tillman, D. A. Wood Sci., 1981, 13 (41, 177-184. (13) Shafizadeh, F. In Chemistry ofSolid Wood; American Chemical Society: Washington, DC, 1984; Chapter 13. (14) Barnard, J. A.; Bradley, J. N. Flame and Combustion, 2nd ed.; Chapman and Hall: New York, 1985; p 158. (15) Semenoff, N. Chemical Kinetics and Chain Reactions; Oxford Press: Oxford, 1935; p 102. (16) Sop, C. S.; Jin, P. H.; Ryon, K. Y. Choson Minjujuii Inmin Konghwaguk Kwahagwon Tongbo 1980, 3, 16-19. (17) Likens, G. E. Chem. Eng. News 1976, 44, 29-40.
VOL. 29, NO. 3, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
831
(18) Wilson, W. E.; O’Donovan,I. T.; Fristrom, R. M. Flame Inhibition by Halogen Compounds. Twelfth International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1969; pp 929-941. (19) Nelson, R. M., Jr.An Evaluation ofthe Carbon Balance Technique for Estimating Emission Factors and Fuel Consumption in Forest Fuels; Research Paper SE-231; U.S. Department of Agriculture, Forest Service, Southeastem Forest Experiment Station: Asheville, NC, 1982. (20) Rothermel, R. C.AMathematicalMode1forPredictingFireSpread in Wildland Fuels; Research Paper INT-115; U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station: Missoula, MT, 1972. (21) Webb, W. R.; Aylward, P. J. Aust. Pulp Pap. Ind. Tech. Assoc. J. 1983, 36 (4), 293-297. (22) Gaines, T. P.; Parker, M. B.; Gascho, G. J. Agron. J. 1984, 76, 371-374. (23) Rasmussen, R. A.; Rasmussen, L. E.; Khali!, M. A. K.; Dalluge, R. W. 1. Geophys. Res. 1980, 85 (ClZ), 7350-7356. (24) Grimsrud, E. P.; Miller, D. A. Anal. Chem. 1978, 50 (8), 11411145.
832 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3, 1995
(25) Singh, H. B.; Salas, L.; Lillian, D.;Arnts,R. R.; Appleby, A. Environ. Sci. Technol. 1977, 11 (5), 511-513. (26) Ward, D. E.; Hardy, C. C. Presented at the Air Pollution Control Association, 77thAnnual Meeting, San Francisco, CA,1984, Paper 84-36.3. (27) Byram, G. M. In Forest Fire: Control and Use;Davis, K. P., Ed.; McGraw-Hill: New York, 1959; pp 61-89. (28) Ward, D. E.; Hardy, C. C. Advances in the Characterization and Control ofEmissionsfrom Prescribed BroadcastFires of Coniferous SpeciesLoggingSlash on Clearcut Units;Final Report to the Pacific Northwest and Alaska Bioconversion Policy Group and Biomass Utilization Task Force. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: Sfattle, WA, 1986; pp 96-97.
Received for review November 16, 1994. Accepted November 30, 1994. @
ES940107K @Abstractpublished in Advance ACS Abstracts, January 1, 1995.
