Environ. Sci. Techno/. 1995, 29, 2047-2054
Air Toxins from Smoldering Combustion of Biomass LISA M. MCKENZIE,*,+ WE1 M I N HAO,* GEOFFREY N. RICHARDS,'AND DAROLD E. W A R D $ Shafimdeh Center for Wood and Carbohydrate Chemisv, University of Montana, Missoula, Montana 59812, and Intermountain Fire Sciences Laboratory, lntermountain Research Station, Forest Service, US.Department of Agriculture, P.O. Box 8089, Missoula, Montana 59807
Oxygenated organic compounds in condensed (-45 smoke of 29 bench-scale fires of ponderosa pine sapwood, needles, bark, litter, duff, and humus have been identified and quantified under three types of conditions (smoldering, self-sustained smoldering, and flaming). The analyses were performed by gas chromatography/mass spectrometry and gas chromatography/flame ionization detection. The major condensible emissions were acetic acid, 2-furaldehyde, vinyl acetate, acetol, and methanol. The oxygenated organic emissions have been shown to be dependent primarily on fuel chemistry and secondarily on combustion efficiency. Molar ratios of individual compound emissions to CO emissions have been calculated, and exposure levels to these compounds for wildland firefighters have been estimated based on the ratios. Of the compounds measured, none was projected to exceed a toxic level, except for 2-furaldehyde and vinyl acetate, which are suspected carcinogens. OC)
Introduction Wildland fires result in the release of a myriad of compounds, some of which are locally significant due to their toxicity (e.g.,CO and acetic acid). These compounds could be hazardous to firefighters and local residents who are exposed to smoke from wildfires. Some of the compounds are also globally significant as greenhouse gases (e.g., methane and COz), as trace gases that are photochemically reactive in the troposphere (e.g., non-methane hydrocarbons), and as precursors to chlorine and bromine radicals that may destroy stratospheric ozone (e.g.,methyl chloride and methyl bromide), Methane and low molecular weight hydrocarbonshave been detected and quantified in smoke from biomass burning (1). There are strong linear correlations between emitted hydrocarbons and CO ( I ) . Various phenols and other oxygenated organics have previously been detected * E-mail address:
[email protected]. + University of Montana * U.S. Department of Agriculture.
0013-936x/95/0929-2047$09.00/0
@ 1995 American Chemical Society
and quantified in smoke from burning wood (2-6). Emissions of formic and acetic acid have been measured from laboratory fires of straw, hay (7) hardwood, brush, and leaves (8) and in air masses impacted by biomass burning over central and eastern Canada (9).Reinhardt (10) measured the concentrations of benzene, formaldehyde, and acrolein to which firefighters are exposed while fighting wildland fires. He then developed a model to predict the exposure levels of these air toxins based on exposure concentrations to CO. We have previously identified and quantified 26 oxygenated organic compounds in the condensible (-45 "C) fraction of smoke from smoldering combustion of ponderosa pine sapwood (5,s). The major productswere acetic acid, methanol,vinyl acetate, pyruvic aldehyde, and acetol, all of which were attributed to pyrolysis of lignin andlor polysaccharides (6). No polycyclic aromatichydrocarbons (PAHs)were detected in the condensate; therefore, if present, they were at concentrations below the detection limit. Particulate production was also at a low level (6). Both observations along with the fact that the major condensible products were oxygenated organics from the pyrolysis of lignin and/or polysaccharides indicate that the fire temperatures were probably below the temperatures required for extensive formation of polycyclic aromatic hydrocarbons (11). Preliminary exposure levels, based on CO levels, of firefighters to these compounds were calculated. Because of the natural complexity and wide range of biomass fuels, it was important to expand research on the smolderingcombustion of fuels other than wood. For example, under drought conditions when wildfires often occur, 50% or more of the total biomass may be burned through smolderingcombustion of duff and humus layers (defined below). In this paper, we investigate the oxygenated organic emissions from 29 bench-scale fires of various types using individual components derived from ponderosa pine. We then estimate the exposure levels of wildland firefighters to these compounds in smolderingfiresbased on the molar emission ratio of the oxygenated organic compound to CO.
Experimental Section Sample Collection. Litter, duff, and humus were collected from a relatively pristine (80years undisturbed) ponderosa pine (Pinus ponderosa) forest at an elevation of 1250 m in northwestern Montana. A 15.2 cm x 15.2 cm square of forest floor was cut from directly beneath a ponderosa pine tree. The square was divided into three horizontal layers. The top litter layer (2.5-5.1 cm) consisted primarily of loose brown needles with lesser amounts of cones, bark, grass, and some occasional fecal material. The reddish brown duff layer (5.1 cm) directly under the litter layer consisted of closely packed decaying needles, cones, bark, and fecal material. The humus layer (5.1 cm) was a gray mat beneath the duff layer with no distinguishable components. Four squares were cut, and each was separated into these three layers. Each of the four layers was then compiled into one sample. Both inner and outer bark was collected from a freshly cut 60-year-old ponderosa pine at an elevation of 1370 m in northwestern Montana. The bark was then loosely
VOL. 29. NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2047
TABLE 1
Mean Emission Facto@ (fig) of Components in Smoke Condensates of Various Fuels needles
wood
compound 1-hydroxy-2-propanone (acetol) 2-cyclopenten-1-one 2-furaldehyde 3-oxobutanoic acid, methyl ester 2-furanmethanol y-butyrolactone (5H-furanone 2-acetylfuran 5-methyl-2-furaldehyde phenol o-hydroxybenzaldehyde 2-methylphenol (@cresol) 3 and/or 4-methylphenol (mlpcresol) 2-methoxyphenol (guaiacol) benzoic acid 2-methoxy-4-methylphenol (4-methylguaiacol) 2-methoxy-4-(l-prop-2enyl)phenol (eugenol) 4-hydroxy-3-methoxybenzaldehyde (vanillin) vinyl acetate pyruvic aldehyde acetic acid formic acid propanoic acid crotonic acid methanol butyric acid hexamethylenetetraamine pyridine benzonitrile
Sb
Fd
(YoRSD)'
(YoRSD)
S
(YoRSD)
bark
S
(YoRSD)
litter
SS'
(YoRSD)
S
(%RSD)
(SaRSD)
(%RSD)
humus
ss
(%RSD)
S (KRSD)
0.07 (53) 0.23 (38) nd
2 (52)
0.19 (27) 0.81 (15) 0.77 (26)
0.0046 (35) 0.14 (8) 0.081 (13) 0.069 (24) 0.11 (14) 0.07 (39) 0.067 (39) 0.061 (8) 0.01 (46) 0.009 (54) 1.6 (13) 1.2 (20) 1.8 (21) 1.1 (10) 0.8 (38) 0.3 (52) 0.43 (10) 0.08 (40) 0.008 (70) 0.48 (20) 0.10 (28) 0.2 (49) 0.22 (25) 0.1 (46) 0.08 (61) 0.075 (17) ndf
0.28 (41) 0.17 (9) 0.22 (14) 0.054 (16) 0.15 (17) 0.11 (3) 0.015 (11) 0.042 (26) 0.044 (20)
0.01 (69) nd nd 0.1 1 (14) 0,011 (29) 0.10 (16) nd 0.06 (11) 0.0074 (34) 0.24 (7) 0.02 (54) 0.35 (16) nd 0.016 (9) 0.003 (48) 0.032 (15) 0.006 (36) 0.067 (21)
0.18 (18)
0.004 (69) 0.063 (18) 0.082 (22) 0.15 (36) 0.14 (9)
0.02 (52) 0.26 (24)
nd 0.16 (25) nd nd 0,005 (102) 0.049 (27) 0.11 (20) 0.3 (47) nd
nd 0.87 (27) nd 0.1 (95) 0.08 (40) 0.06 (98) nd 0.05 (56) 0.087 (34) 0.1 (62) nd 0.07 (54) 0.044 (14) 0.037 (21) 0.046 (13) 0.02 (49) 0.19 (18) 0.5 (29) 0.22 (27) 0.2 (50) 0.29 (22) 0.12 (36) 0.32 (23) 0.07 (57) nd nd 0.019(21) nd 0.03 (14) 0.02 (56) 0.047 (16) 0.02 (80) 0.076 (20) 0.07 (59) 0.081 (19) 0.04 (78)
nd
0.05 (71) nd 0.34 (32) 0.18 (39) 0.022 (41) nd 0.75 (24) nd nd nd nd
0.5 (31) 2.0 (39) 1 (75) nd 1.0 (18) nd 7.6 (32) 5.4 (32) 3.3 (27) 0.43 (17) 0.48 (21) 0.97 (43) 0.39 (39) 0.36 (32) 0.3 (10) 0.037 (28) 0.039 (14) 0.03 (59) 4.9 (30) 1.1 (18) l ( 5 3 ) nd 0.14 (17) nd nd nd nd nd nd nd nd nd nd
a Mean emission factor based on mass of fuel consumed from three separate fires substained smoldering. 'nd, not detected.
covered with kraft paper to protect it from light and air dried. Brown needles were collected from the ground a few days after they had fallen. The needles, bark, and litter were ground to pass a 1-mm sieve in a Wiley mill. The duff and humus were manually forced through a 5.6-mm wire sieve, and the rocks were removed. The ground samples were stored at -20 "C in a freezer. Fires. Three replicates each of needles, litter, duff, and humus were allowed to smolder for 40 min, and two bark fires were allowed to smolder for 45 min using a radiant electric heater as previously described (67. These fires are defined as smolder fires. Three replicates each of bark, litter, and duff fires were lit with matches and allowed to smolder without supplementaryheating until visible smoldering ceased. These fires are defined as self-sustained smolder fires. Three fires of ponderosa pine wood sticks were ignited with matches and allowed to flame. These fires were extinguished when the flaming ceased. These are defined as flaming fires. Analyses of Condensibles, Noncondensibles, and Inorganics. The procedures for analyses of condensible and noncondensible compounds by GUMS and GClFID have been described previously (6). The samples were analyzed for metals by inductively coupled argon plasma emission spectrometry (ICAPES)on a Jarrell-AshAtomComp Series 800 ICAPES using an external standard method. 2048 IENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8,1995
nd 0.044(12) nd nd nd 0.1 (51) 0.02 (44) 0.014 (18) 0.06 (55) 0.2 (13) 0.2(15) 0.08 (15) 0.01 (44) nd 0.028 (21) 0.013 (13) 0.051 (12) 0.028 (23)
nd nd nd nd 0.02 (47) 0.04 (36) nd nd nd
0.1 (59)
0.07 (53) 0.14 (10) nd
0.07 (33) nd 0.11 (7) 0.2 (72)
nd nd nd 0.04 (55) 0.096 (19) nd
0.08 (65) nd
0.076 (16) 0.002 (74) 0.027 (18) 0.037 (30) 0.2 (67) 3 (51) 2.0 (13) 4.4 (29) 0.97 (24) 0.25 (22) 0.068 (10) 4.3 (37) nd nd nd nd
0.3 (41)
S
2.0 (18)
0.068 (14) nd
0.47 (23) 0.27 (27) 0.55 (48) 0.2 (41)
duff
ss
nd
nd
nd
nd
0.033 (31) 0.2 (88) 0.012 (31) 0.067 (16) nd 0.7 (27) 0.3 (43) 2.4 (34) l(55) 0.3 (43) 0.04 (43) 2.0 (17) 0.08 (51) nd 0.03 (39) 0.03 (24)
0.5 (40) 0.15 (19) 0.3 (34) nd nd nd nd nd 1.3 (35) 0.85 (8) 0.66 (22) 0.19 (24) 0.2 (41) 0.038 (9) nd nd 0.1 (92) 0.11 (14) 0.075 (21) nd nd nd nd nd 0.6 (69) 0.69 (13) 0.55 (34) 0.09 (33) 0.08 (66) 0.035 (34) 0.039 (36) nd nd 0.28 (22) 0.02 (64) 0.059 (38) 0.017 (25) 0.2 (52) 0.011 (4) 0.01 (24) 0.06 (80) 0.050 (48) 0.024 (16) 0.005 (47)
Smoldering. Relative standard deviation. Flaming. e Self-
The various fractions within the smoke collected from the fires have been described previously (6). No attempt was made to identify or quantify products, such as formaldehydeand levoglucosan that were not amenable to our gas chromatography conditions.
Results and Discussion Condensible Emissions. The mean emissionfactors of the condensible products from three smoldering fires of ponderosa pine sapwood, which were reported previously (61,and from the 26 additional fires described in this paper are presented in Table 1 and Figure 1. The emission factors are based on the dry mass of fuel consumed. The major condensible compound emitted from all the fires, except the humus fires, was acetic acid. It was the second highest emission in the humus fires. A probable major source of acetic acid is the acetate groups known to exist in xylan hemicelluloses (11, 12). The smoldering needle fires gave the highest yield, while the flaming wood and smoldering humus fires emitted the least acetic acid. In flaming fires, many initially produced pyrolysis products are expected to be at least partially oxidized in the flame. Methanol was the second most abundant condensible compound emitted from the smoldering wood, needles, litter, and duff fires and from the self-sustainedsmoldering duff fires, with smolderingwood and needles emitting two
7r *
TABLE 2
Molar Ratios of Nitrogen to Carbon and Total Metal Content molar ratio N E
fuel
Vinyl Acebte ~
,
char from total metals self-substained char from unburned smoldering smoldering fuel ( O g )
sapwood 5.1 x 10.' needles 3.1 x 10.'
naa na
bark
2.2
litter
Acetol
p,o
unburned fuel
duff
humus a
4.3 x 10-3 1.2 x 10.' 2.0 x 2.5 x
10-3
1.6 x
2.4 x l o 2
na
1.2 x 1.4 x 3.4 2.0 x 3.3 x 3.5 x
10-3 103
lo2 10.' 102
1.7 8.9 3.9 9.9 34 190
Not applicable.
ziesi0 has three times the polysaccharide content of the needles and six times that of the bark (I 7). Thus, the high
L .e
Formic Acid
fin and Flrl hp
11
,
Z-Puraldehyde
I
nn .ad r d rIp
FIGURE 1. Major condensible emissions ( g k g of fuel consumed)vs fuel and fire types: 1. smoldering wood; 2. flaming wood 3, smoldering needles; 4smoldering bark:5. self-sustained smoldering bark; 6, smoldering liner; 1. self-sustained smoldering liner; 8. smoldering ddf; 9. self-sustainedsmoldering duff; 10, smoldering humus.
times more methanol than the other fires. Methanol is a known pyrolysis product of wood and is thought to arise from pyrolysis of 0-methyl uronic acid units in hemicelluloses and from methoxybenzene units in lignin (13).If there were a higher percentage of methoxybenzeneunits and/or uronic acid units in wood and needles than in the other fuel types, it could account for the high methanol yields. As expected (see above), the lowest methanol emissions were from the flaming wood fires. 2- Furaldehyde,known to be a pyrolysis product ofxylan hemicelluloses (11,12),was also a major emission in all of
thefires,withthehighestyieldsfromthesmolderingneedle and bark fires and the self-sustainedsmolderingbark fires. Again, the lowest emissions were from the flaming wood fires. Lipari et al. (14) have previously observed 2-furaldehyde in wood stove emissions. Acetol and vinyl acetate were major emissionsin all the fires except for the smolderinghumus fires, in which they were not detected. The smolderingwoodfiresemitted four times more acetol and 1.5 times more vinyl acetate than any other fire. Acetol is known to be produced in the pyrolysis ofpolysaccharides(includinghemicelluloses and cellulose) (13,and its emission is likely to be dependent on the polysaccharide content of the fuel. The amount of polysaccharide in a tree varies with location, height, and season (16). The wood of Douglas fir (Pseudotsuga men-
acetol emissions in the smoldering wood fires may be attributed to a higher percentage of polysaccharides in sapwood than other parts of the tree. The same may be m e for p y ~ v i aldehyde c (2-oxopropanal),a known polysaccharide pyrolysis product (13, whichwas a major emission from the smolderingfires of wood, needles, and litter but was not detected in any other fire. Vinyl acetate has been detectedinthecombustionofIhois bituminouscoal(l@, but prior to the present report, it had not been detected in other types of biomass burning. Formic acid was a major emission in all the fires except for the self-sustained smolderingduff fires in which it was not detected. The smoldering wood fires and the selfsustained bark fires emitted twice as much formic acid as any other fire. Formic acid is a known wood pyrolysis product (19). Phenol, 5-methyl-2-furaldehyde, and 2-cyclopenten-lone were emitted in all the fires. Methyl 3-oxobutanoate, the cresols, guaiacol, 4-methylguaiacol, vanillin, and propanoicacidwereeminedinallofthefireswiththeexception of the smoldering humus fires. The fires of litter, duff, and humus were unique in that they emitted nitrogen compounds. These materials have markedly higher nitrogen to carbon ratios than bark and sapwood, as shown in Table 2. Needles contain the highest nitrogen to carbon ratio, and yet no nitrogen-containing compoundswere detected in the emissions from the needle fires. However, the nitrogen to carbon ratio was higher in the chars than the original material of the litter, duff, and humus, whereas itwaslower in thecharthantheunburned needles. Thus, nitrogen is concentrated in the chars ofthe floor layers and diluted in the chars of the needles during burning. This would suggest that nitrogen lost from the needles during the burn is in a form($ that we have not detected, e.g., NH3. Pyidine and benzonitrile were emitted from all of the fires of litter, duff, and humus. Hexamethylenetetramine was a major condensible emission from the fires of dufi and humus. Hexamethylenetetramine is used in fuel pellets, plastics, and pesticides (20);therefore, the possibility that it was a contaminant in the duff sample was investigated, although there were no documented pesticide sprayings at the sampling site. Another duff sample was collected from adifferent locationwithinthe same sampling site. The smoldering fire from that sample also emitted hexamethylenetetramine. A sample (20 9,of this duff was extracted with 200 mL of HzO, which was adjusted with VOL. 29. NO. 8.1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY. 2049
1.00
- 1
Carbon Dioxide
combustion efficiencies below 9046,which is common for smolderingfires (1). with the bark fires having the lowest values. The mean combustionefficiency for the smoldering humus fires was 92 2% (n= 3). which was higher than expected for a smoldering fire. The inorganic content of the humus howeverwas7746,andthis may have influenced its combustion behavior. Inorganica. The results of the I W E S analysis are listed in Table 2 (complete results are contained in supporting Table 1). The humus had the highest concentration of all the inorganic elementsmeasured,followed by the dufflayer and then the Litter layer. Needles had more inorganics than wood or bark, as was expected since needles are more metabolically active. Chemishyof SmolderingCombustion. The chemistry of smoldering combustion of lignocellulosics has been extensivelystudied,especiallyinrelation to fires in building insulation. The studies of effects of heat flow geometry in layers of smolderingnewsprint insulation (e.g., ref 24) and oftheinhenceofinorganiccompoundsonthecombustion ofwood (25,26)are very relevant to smolderingin the duff layer during a forest fire. The emissions from this phase of a fire are especially imponant in relation to their impact onwildlandfirefightersduringmoppingupoperations. Our results reported earlier (5,6) and in this paper show that such condensible emissions contain predominantly compounds known to be pyrolysis products (as opposed to combustionproducts) ofpolysaccharidesand lignin. Thus, for example, acetol has been shown to be a major product from the pyrolysis of a range of polysaccharides, especially in presence of inorganic components (27).and guaiacol is a major product of the pyrolysis of lignin (26). The fact that smolderingcombustion produces pyrolysis products is not surprising. The smoldering of a fuel such as duff occurs by way of the slow advance of a glowing combustion front. As this front advances, the biomass adjacent to the front becomes heated and convened to char. This process produces water as the major product and occurs in a relatively oxygen-deficient atmosphere hecause of the oxidative processes in the adjacent glowing combustion front. Thus, the char formation phase produces volatile pyrolysis products. Subsequently,the char layer is further heated as the front advances and reaches temperatures in the region of 450-500 "C at which temperature oxygen chemisorption on the char is maxmized, especially in the presence of inorganic components (22, 23). This chemisorption is the major exothermic
+
Carbon Monoxide
nul .od RR T m
Methane
M .od
mR
T,~
FIGURE 2. Noncondensible emissions l m g of fuel consumed) va fuel and fire type: 1,smoldering wood:Z,fIaming wood: 3, smoldering needles: 4. smoldering bark: 5, self-sustained smoldering bark, 6. smoldering liner; 7. self-sustained smoldering liner; 8, smoldering dun; 9, self-sustained smoldering duff; 10. smoldering humus.
NaOH to a pH of 9, and no hexamethylenetetraminewas detected in the extract. Both formaldehydeand ammonia, which are precursors to hexamethylenetetramine(201, are known emissions of biomass combustion (10, 21). It is feasible,therefore, that hexamethylenetetramineis formed in the smolder zone or the smoke plume when formaldehyde and ammonia emissions are high. Noncondenslble Gases. The results for the noncondensible compounds are presented in Figure 2. The selfsustained duff fires and the smoldering humus fires produced the most C02, perhaps because the duff and humus had the highest inorganic content which would catalyze the chemisorption of oxygen on the char in the smolderzone,th~smaximizingC0~emissions (22,23).The smoldering bark fires emitted the most CO. Similar amounts of methane were emitted in the smolderingwood, needles, bark, and litter and the self-sustained litter and duff fires. The greatest amount of ethene and ethane were emitted in the smoldering needle fires. The combustion efficiency is defined as the molar emission ratio of C02 to the s u m of C 0 2 and CO above ambient levels (1). The mean combustion efficiency for the flaming fires in our experimentswas 95 1% (n= 3), which is consistent withthose derivedfromflamingphases of forest and savanna fires ( I ) . AU the smolderingand selfsustained smoldering fires, except the humus fires, had
+
2050. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 8.1995
processinthesmolderingcombustionofthecharandwith suitable fuel geomeuy results in the generation of heat more rapidly than the removal of heat, with consequent "runaway" combustion (Le..the glowing phase) (2.31,which is the major source of CO and C 0 2 and probably of hydrocarbons. Relationships between Edaslons. Factor analysis of
alltheemissionsmeasuredfromallthefiresusingavarimax rotation (29)revealedfivedistinctfactors that are presented in Table 3. The first factor is dominated by known hemicellulose pyrolysis products such as 2-furaldehyde, 2-acetylfuran, acetic acid, and phenol (30). Phenol, hydroxybenzaldehyde, methane, CO, and cresol are also known lignin pyrolysis products (31, 32). When just the compoundslisted in factor 1are analyzed byfactoranalysis, thefirstfourcompoundsinthelist dominate the first factor, while the next six compounds dominate the second factor, leaving ethane and CO in a third factor. This suggests that
TABLE 3
Results of Factor Analyses’ factor 1, hemicellulosas/lignin 2-furaldehyde 2-acetylfuran propanoic acid acetic acid phenol hydroxybenzaldehyde benzoic acid cresol methane ethene ethane
factor 2, lignin
factor 3, nitrogen compds
factor 4, polysaccharides
guaiacol 4-methylguaiacol eugenol vanillin 5-methylfuraldehyde butyric acid
benzonitrile pyridine hexamethylenetetraamine
2-furanmethanol butyrolactone acetol cyclopenten-1-one oxobutanoic acid, methyl ester P(5Wfuranone vinyl acetate formic acid methanol crotonic acid pyruvic aldehyde
factor 5
co2
pn LU
a These are the five major factors as they were determined by factor analysis of the complete data set. The names assigned to each factor have been assigned based on the compounds that load most heavily into a given factor.
0.6
0.1
lo t
r
ddb
0
0.4
\
t
0 .o 60
0 I
06
.
1
.
70
l
76
.
l
00
.
I
66
.
l
90
96
1
FIGURE 3. 2-Furaldehyde vs combustion efficiency.
the first four compounds arise primarily from the pyrolysis of hemicelluloses, whereas the next six compounds are pyrolysis products of hemicelluloses and lignin. The second factor is dominated by lignin pyrolysis products. Guaiacol,4-methylguaiacol,eugenol,andvanillin are known to be produced in pyrolysis of lignin (30, 31). Their strong correlationswith each other (supportingTable 2 contains the Spearman correlation (33)coefficients)along with their factoring together suggest that they may have a common precursor within the fuel and form under similar conditions and mechanisms. 5-Methylfuraldehyde has been reported previously as a hemicellulose pyrolysis product (30)and has not been detected as alignin pyrolysis product (31). It correlatesmost stronglywith2-furaldehyde ( r = 0.89, n = 29) and also correlates well with cresol ( T = 0.72, n=29),methylguaiacol (r=O.73, n=29), andvanillin (r= 0.73, n = 29). It maybe that while 5-methylfuraldehyde is a pyrolysis product of hemicelluloses,its formationduring pyrolysis is also closelylinked with the lignin content of the fuel. The third factor contains the three nitrogen compounds. The fourth factor is dominated by pyrolysis products of polysaccharides. AU of the compounds in this factor, with the exception of methyl 3-oxobutanoate,vinyl acetate, and
2-cyclopenten-1-one (although several substituted cyclopenten-1-ones have been reported), are known pyrolysis products of polysaccharides (11, 15,30). The fifth factor is dominated by COz. Most of the emitted compounds are strongly weighted into factors 1, 2, or 4, and since woody materials are primarily composed of three types of macromolecules(viz., cellulose, hemicelluloses,and lignin),these results suggest that the emissions are strongly influenced by the chemical composition of the fuel. Emissions v8 Combustion Efficiency. All of the emissions, exc*pt C02, are negatively correlated linearly with the combustion efficiency. All the emissions tend to increase as the combustion efficiencydecreases. A strong negative linear correlation coefficient was found between the emission of 2-furaldehyde and the combustion efficiency ( r = -0.86, n = 29) as shown in Figure 3. This negative correlation supports the postulate that 2-furaldehyde is a product of pyrolysis rather than combustion. 2-Furaldehyde is most probably derived from pentose units of hemicelluloses present in roughly equal proportions in all of the fuels. The linear regression equation presented in Figure 3 could be used to predict the emissions of 2-furaldehyde from a fire primarily of softwoods, such as VOL. 29, NO. 9, 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
205f
2.6
I
2.0
3 1 .o 0
[Add]
-0.017
\
0 '
0.6
0.0 00
70
06
80
76
06
ob
- -
96
1
0
canbPr(l0s UfrCira*
FIGURE 4. Acetol vs combustion efficiency. TABLE 4
Molar Ratios to Carbon Monoxide main molar ratio to COa (x 10-31
compound 2-furaldehyde 5-methylfuraldehyde 2-acetylfuran phenol o-cresol m/pcresol guaiacol 4-methylguaiacol vanillin acetol vinyl acetate 2-cyclopenten-1-one acetic acid formic acid propanoic acid 3-oxobutanoic acid, methyl ester methanol methane ethane ethene
mean molar ratios to CO from other studies (x W)
1.5 f 0.93 0.30 f 0.19 0.33 f 0.16b 0.32 i 0.20 0.27 f 0.13b 0.52 f 0.25b 0.17 f 0.O8lb 1.0 f 0.83b 0.5 f .57b 1.2 f 1.7b 1.7 f 2.16 0.2 f 0.13 7.4 f 6.2 8.7 f 6.1,c 1.6 f 2.4,* 8.0 f 4.0,8 3.2 f 0.4,' 2.6 f 6.8s 1.5 f 1.5b 2.6 f 2.0,c0.17 f 0.27,d 20,e 35 f 22s 0.39 f 0.1gb 0.41 f 0.44b 11 f 9.0 29 i 11 2.5 f 1.2 12 f 9.0
45 f 13,h 55,' 140 f 93; 58 f 18,k71,' 91 f 3.1,'" 76 f 13" 4.0 f 1.4,h 6.8 f 5.2"' 17 f 9.1,h 12 f 8.7"'
*The mean of the molar ratio of the compound listed to carbon monoxide over the 26 smoldering fires f 1 SD. The three flaming wood fires are excluded. Not detected in humus fires; therefore, the meanisfrom23fires. Hayandstraw(7). dHardwood, brushandleaves (8). DECAFE(9).'Guri,Venezuela (7). g ABLE-3B (9). African savanna (38). ' Brazilian savanna (39). Fuelwood (42). Florida wetlands (37). 'Temperate and boreal forests(4l). Agricultural residues (34)." Sagebrush and forestry slash (40).
*
ponderosa pine, given the combustion efficiency, which often has been measured in field experiments ( 1 ) . Acetol was chosen to illustrate the dependence of the emissions on the type of fuel burned. Some correlation between the combustion efficiency and the emission of acetol is reflected in the Spearman coefficient (r = -0.54, n = 291,although no linear trend was apparent. Linear relations became apparent after the wood fires were 2052
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separated from the rest of the fires. Figure 4 clearly shows that the emission of acetol is dependent not only on the combustion efficiency but also on the polysaccharide content of the fuel, with wood having the highest polysaccharide content (17). Hence, unless the chemical composition of the fuel is known and can be considered in the regression equation, modeling the emissions of oxygenated organics against the combustion efficiency with linear regression is not very predictive, except in the case of 2-furaldehyde. Molar Ratios to CO. Several investigators have found good positive correlations between incomplete combustion products and CO, whereas these products are weakly correlated with COz (9, 10,341. This implies that constant ratios may exist between products of incomplete combustion. Carbon monoxide concentrations in smoke are often determinedin studies of biomass burning, and the exposure levels of firefightersto CO also have been directlymeasured (10). Therefore,the molar ratios of the emissions of various compounds to the emission of CO should allow for comparisonwith other studies and can be used to estimate the exposure levels of firefighters to these compounds. Hence for each fire, the molar emission ratio of each compound to CO was calculated. The mean molar ratios were then determined, and a Duncan's means comparison test (35) was carried out to compare these ratios. Only the mean value of flaming wood was found to be significantly different from the means of other fires for the majority of the emissions. Therefore, the mean molar emission ratio of each compound to CO was calculated from the smoldering and self-sustained smoldering fires (Table 4). Emissions of formic and acetic acid from the combustion of biomass and the molar ratios to CO determined from previous studies are presented in Table 4. The ratios from laboratory fires of straw and hay (7)agree well with our results. The ratios from laboratoryfires of hardwood, brush, and leaves (8)however are 10 times smaller than our ratio for formic acid and about six times smaller than our ratio for acetic acid. These differences appear too large to be due to differences in fuel chemistry between ponderosa pine and hardwood fuels, and they may be associated more with fuel geometry controlling access of air during the fires.
Formic and acetic acid ratios to CO have also been determined in plumes from biomass burning (Table 4). The average acetic acid ratio observed during the Dynamique et Chimie de 1’Atmosphereen Foret Equatoriale (DECAFE) experiment in 1988 (36) is in good agreement with our mean ratio. The mean ratio for acetic acid observed in a forest fire plume from Guri, Venezuela (7), is about halfthat of our ratio, while the mean ratio of acetic acid to CO observed during the NASAArctic Boundary Layer Expedition (ABLE)-3Bover the Canadian subarctic in 1990 (9) was more than double our ratio. The mean ratios of formic acid to CO observed in DECAFE (36) and ABLE-3B (9) are at least ten times higher than our observations, suggesting some oxidation of formaldehyde to formic acid in the plume (a similar general postulate has been made by Lefer et al. (9)). If this postulate is correct, the ratios determined in the laboratoryfires would be more applicable (at least for formic acid) in assessing exposures of wildland firefighters, who are close to the source of combustion. The ratios determined from plume measurements would be more applicable in assessing exposures of residents in the vicinityof wildfires and also the contribution of biomass fires to the global source of formic acid. Our ratios of ethene and ethane to CO in Table 4 agree quite well with previous studies (34,381. Methane and CO have been measured in both plumes from wildland fires (34, 37-41) and cooking fires (42). The mean molar emission ratio of methane to CO (Table 4) from our study is lower than all those previously reported, although the high end of our range overlaps with the low end of the ranges from the African savanna and the Florida wetlands. This effect may again be due to the relatively small amount of fuel burned (20 g). The formation of methane is favored at higher pyrolysis temperatures (43) such as may occur when larger quantities of fuel are burned in the field. There are more possibilities for air to accessthe aerialcomponents of the fuel and for the formation of insulated heat trapping areas, e.g., in duff layers. Exposure Levels. The concentrations of condensible compounds would be difficult to measure directly at the point of exposure for firefighters. However, CO exposure levels have been measured with sampling devices carried by firefighters (10). In a previous paper, we determined preliminary estimates of exposure levels of some of the oxygenated organic compounds based on their molar emission ratios to CO and an average CO exposure of 40 ppm within 8 h/day (6). We can now use the extended molar emission ratios determined in this study to refine our previous estimates by employing Reinhardt’s(10)peak CO exposure level of 54 ppm. These exposure levels are presented in Table 5 along with the allowable NIOSH time weighted averages (TWAs) (8 h/day, 40 hlweek) for each compound when available and the resultant toxicological effects upon inhalation. Reinhardt used this method to determine exposure levels of wildland firefighters to formaldehyde (0.5 pprn), acrolein (70 ppb), and benzene (60 ppb) (10). This method has also been used for benzene emissions from biomass fires in the United States (44) and globally ( 4 5 ) . All of the exposure levels of condensible compounds produced in smoldering combustion of ponderosa pine fuels, which were quantified in this study, are well below the allowable TWAs based on a peak exposure to CO of 54 ppm. No single compound is present at a hazardous level except for vinyl acetate and 2-furaldehyde, which are
TABLE 5
h p w m levels exposure compound
levelsa (ppb)
2-furaldehyde
81 f 50
5-methylfuraldehyde 2-acetylfuran phenol @cresol m/pcresol guaiacol 4-methylguaiacol vanil Iin acetol
16 f 10 18 f 8.6 17 i~11 15f7 28 f 14 9.2 f 4.4 54 f 45 27 f 31 65 f 92
vinyl acetate
92 f 110
2-cyclopenten-I-one acetic acid formic acid propanoic acid 3-oxobutanoic acid, methyl ester methanol
M A b (ppb) end effecr
11 f 7 400 f 330 81 f 81 21 f 10 22 f 24
2000, suspected carcinogen, irritant irritant irritant 5000, irritant 2000, irritant 2000, irritant irritant irritant irritant no toxicological data given suspected carcinogen, irritant irritant 10 000, irritant 5000, irritant 10 000, irritant irritant
590 f 490
200 000, irritant
a Basedon a 54 pprn CO exposure f 1 SD. Time weighted average from NIOSH based on a 40 h work week. As listed on material safety data sheets for inhalation.
suspected carcinogens. It should be noted, however, that the synergistic effects of some or all of these compounds have not been determined, nor have WASfor many of these compounds been determined. The results presented in this paper are currently being verified with larger scale laboratory fires and field studies. The amount and distribution of fuel burned could affect the magnitude of oxygenated organic compounds emitted. Different fire behavior in larger fires could also possibly affect the emissions.
Conclusion The emissions of condensible oxygenated organic compounds and volatile low molecular weight hydrocarbons from smoldering combustion of different components of ponderosa pine fuels have been quantified. The major condensible emissions, other than water, were acetic acid, 2-furaldehyde, vinyl acetate, acetol, and methanol. The emissions are primarily dependent on fuel chemistry and secondarily on combustion efficiency. The relative composition of three major macromolecules (cellulose,hemicellulose, and lignin) in a particular fuel influences the emissions of these compounds. These results suggest that combustion efficiency is not an effective indicator of oxygenated organic emissions unless the fuel chemistry is known. Exposure levels of wildland firefighters to these oxygenated organic compounds from fires of ponderosa pine can be estimated based on their molar emission ratios to CO. No single compound, except for vinyl acetate and 2-furaldehyde,measured in this studywas present at a toxic level.
Acknowledgments The experimental assistance of Steve Baker and financial support of the USDAForestServiceIntermountain Research Station (INT-91639-RJVA) and NASA’s Global Change Research Program (4054-GC93-0134) are gratefully acknowledged. VOL. 29, NO. 8, 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
2013
Supportiy Infomatien Available
(22) Hshieh, F. Y.; Richards, G. N. Combust. Flame 1989,76,37-47.
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Received for review November 29, 1994. Revised manuscript received April 21, 1995. Accepted April 24, 1995.@ ES9407251 @Abstractpublished in Advance ACS Abstracts, June 15, 1995.
