Environ. Sci. Techno/.1995, 29, 2695-2702
Method for Determining Black Cahon in Residues ofVegetation Fires T. A . J . K U H L B U S C H * Max Planck Institute for Chemistry, Airchemistry Department, P.O. Box 3060, 55020 Mainz, Germany
W e present here a simple versatile method for the direct determination of black carbon in residues of vegetation fires. Differentiation of black carbon from organic carbon is based on solubility, vapor pressure, and oxidative stability. Charring, the formation of black carbon due to the analytical method, is significantly reduced, achieved by a solvent extraction prior t o a thermal treatment. It is shown that the final step for the differentiation, the thermal treatment, can be looked at as a fractionation of carbon compounds by their molar H/C ratio. The molar H/C ratio determined for black carbon in ash was 0.19 f 0.05 but has to be corrected for some hydrogen bonded to elements other than carbon, most likely silicate. Thus, black carbon appears to have high aromaticity and a l o w organic carbon content. Black carbon comprised 16.0 f 2.0% of the total carbon in the ash residues. Due to a favorable intercomparison to another method determining black carbon in aerosols and to the possibility to adjust the solvent extraction to different matrices, w e believe that the method presented here is satisfactory and can be used to derive information on the global cycling of black carbon.
Introduction The term black carbon (BC) is used here to describe a certain, relatively inert and ubiquitous form of carbon comprising a range of materials from highly polyaromatic to elemental or graphitic carbon. BC is produced by any incomplete combustion process and contained in the smoke or residues on the ground. BC can be found in the atmosphere, soils, ice, and sediments (1-3). Its atmospheric appearance is well investigated because of its optical and physicochemical properties. BC is the main component in aerosols that strongly absorb solar radiation. Black carbon particles may not act as good cloud condensation nuclei (CCN) on their own due to the hydrophobic nature of BC, but in smoke it is often combined with other chemicals of CCN nature, thus directly and indirectly influencing the solar radiation budget. BC formation is always accompanied by the formation of polyaromatic * Present address: National Research Council, c/o U.S. Environmental Protection Agency, NERL, 960 College Station Road, Athens, GA 30605.
0013-936X/95/0929-2695$09.00/0
@ 1995 American Chemical Society
hydrocarbons (PAHs),some of which are carcinogenic, e.g., benzo[alpyrene. Chang etal. (4) point out the possibility that atmospheric BC acts as a catalyst forvarious reactions. These properties and effects have led to investigations of BC emissions by vegetation fires and industrial combustion processes as well as quantitative determinations of atmospheric BC. Another property of black carbon is its chemical and microbial inertness, which leads to residence times in soils, sediments, and ice of several million years. Especially because of this inertness, BC and charcoal (term used for the total residue of vegetation fires containing organic carbon, inorganic carbon, and black carbon) are used by several scientists as tracers in sediments to investigate the fire history of different regions (2, 5, 6'). The potential importance of the inertness of black carbon on the global carbon cycle was first pointed out by Seiler and Crutzen ( 7). Since the carbon, being thermally altered by vegetation fires, was first part of the relatively fast bioatmospheric carbon cycle and thereafter sequestered from this fast cycle reservoir to the slow geological carbon cycle, black carbon production represents a sink for the fast cycle and thus for atmospheric COz. To estimate this sink and to derive information on the cycle of black carbon, meaning production, transport, and final deposition, we need to know the amount of black carbon that is emitted to the atmosphere and that is deposited in the residues of the fires. Onlytwo attempts to estimate the quantities of thermally altered carbon in the residues of vegetation fires have so far been conducted (8-10). Feamside et al. (9) sampled all blackened material (charcoal)after a deforestation fire, dried it, and weighed it. Since the blackened material will still contain some degradable organic matter, not all of its carbon can be considered to be inert and thus as a sink for the bioatmospheric carbon cycle. Comery et al. (10) analyzed the residues of forest fires for organic and total carbon. The total carbon content was measured by burning the sample at 1650"C in oxygen and following quantification of the produced carbon dioxide. Organic carbon was determined by chemical oxidation with Cr2072-and following quantificationof the excess Cr2072-by titration with standard FeS04 solution (11). Black carbon was then calculated by subtraction of organic carbon from total carbon, leading to an overestimation of black carbon formation by neglecting the carbonate carbon that is certainly present in residues of forest fires. For the above reasons, a method for determining black carbon, two organic carbon fractions,and carbonate carbon in the residues of fires was developed and is presented here. The comparability of this method to two thermal methods quantifying BC in aerosols will be shown. The method will be used in the near future to study dependencies of the formation of black carbon, to estimate the annual amount of black carbon produced by vegetation fires, and to derive information on the global cycle of black carbon (12, 13).
Analytical Section In his seminal book Black Carbon in the Environment, Goldberg (14) pointed out that there is no clear definition of black carbon. It often is defined by scientists on the
VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2896
Eslbonmenon
m**".l(urm
drllrmid
r l
FIGURE 1. Flow diagram showing the determination of the different carbonfractions: TC.total carbon inthe residue;TCl. carbon content alter the solvent extraction: OC1. organic carbon removed by the solvent extraction; IC, inorganic carbon, carbonates; BC, carbon content after solvent extraction and thermal treatment. All values calculated in % dm; similar lor hydrogen.
basis of their techniques of isolation and measurement. Despite thehighlyvariable nature ofblackcarbon, we may describe it as acombustion-derivedcarbon fractionofblack color defined by the ratio of the carbon to the main impurities, hydrogen and oxygen. Therefore. we developed asimpleandversatile method forthe quantification ofblack carbon and its H/C ratio. The method involves two pretreatment steps to remove all inorganic and organic carbon. Black carbon and its corresponding H/C ratio are directly quantified after these two steps by an elemental analysis conducted for carbon and hydrogen. A schematic flow diagram of this method is shown in Figure 1. Acronyms used are as follows: BC, black carbon; BH, hydrogen associated with the black carbon; OC1 and OH1, organic carbon and hydrogen removed by solvent extraction, first pretreatment step: OC2 and OH2,organiccabonand hydrogenremoved by thermal treatment, second pretreatment step;IC, inorganic carbon, carbonates: CE, carbon exposed to fire: TC + TH, total carbon and hydrogen content in the residue; TC1 THl, total carbon and hydrogen content in the sample after the solvent extraction; mass removed, mass removed by the extraction; W dm, W dry matter of the initial residue. Solvent Ewtraction. For the first pretreatment step, a weighed, dried, and pulverized fraction (ca. 1 g; average particle diameter 40 pm, maximum diameter 60 pm) of a residue sample, following named subsample, is placed in a cenuifuge tube (made out of glass to avoid any contamination) andtreatedwith 10-20mLofdifferent solvents in the order: 2x NaOH (1M), l x 70 mass W HN03, 5 x 1 M NaOH, 1x 1mass % HCI, and 2x twice-distilled water. The subsample is placed for 10-20 min in an ultrasonic bath to ensure efficiency of reactions after each solvent has been added. Thereafter, the subsample is centrifuged, and the supernatant is discarded. Following treatment with the last solvent, the subsample is dried in an oven (105 "C, 24 h) to determine the remaining dry weight of the subsample. The subsample is then pulverized and homogenized again to an average particle sue of 40 pm for the elemental analysis and the thermal treatment. This first solvent extraction removes all IC and as much organic carbon as possible. By conducting an elemental analysis, we obtain the carbon and hydrogen content of the subsample after the extraction VC1+ THl). Thus, we are able to quantify the removed carbon (OC1 IC) by subtracting TC1 from TC and similarly for hydrogen. The solvents were chosen for the following reasons: By adding the acids HCI and HN03, carbonates are removed and evolved as COZ. Carbonates are resistant to the second
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step, the thermal treatment, and thus would be measured together with the black carbon if not removed by the first pretreatment step. Nitric acid like the other solvents is onlyusedat room temperatureandfor 10-20min, because Cadle and Groblicki (15) clearly showed that bot nitric acid attacks black carbon. By using these solvents at room temperature, we do not attack or produce black carbon. Additionally, the pH value of the last supernatant should be below pH 6 to ensure that no carbonates are formed again. Niuicacidwas chosen as the first acid. This ensured that the nitrogen oxides were removed by the following solvents since otherwise it would lead to charring during the drying process. By changing the pH (1-13) twice, we avoid the isoelectric points of substances like proteins, nucleic acids, and amino acids and dissolve as much organic carbon as possible. Inorganic solvents were used to avoid any contamination and to be able to directlydetermine the H/C ratio for OC1 and OC2. This removal method for organiccarbonreducescharring(formationofblackcarbon) by the followingthermaltreatmentandenablesthe samples to be placed for the second step directly into an oven at 340 OC without causing further burning. Thermal Treatment and Black Carbon Determination. To remove the residual organic carbon (OC2) 0.01-0.1 g ofthepretreatedmaterialisweighedinaquartz glass sample boat and then placed in an oven at 340 i 3 "C in a pure oxygen flow (500 mL/min) for 2 h. The carbon that is resistant to the solvent emactionand the thermaltreatment is defined as black carbon and quantified by an elemental analysis for carbon and hydrogen directly after the thermal treatment. OC2 and OH2 respectively are calculated by subtraction ofBC or BH fromTCl orTH1. The abbreviation BH black hydrogen is used here showing that the hydrogen is associated to black carbon. The thermal treatment was chosen as the last step for the differentiation between organic carbon and black carbon because by using oxygen as the carrier gas the separationofthe carbon fractions is basedontheir stability versus temperature and oxidation. These stabilities especially versus oxidation can be looked at as an indicator of the resistance of the remaining carbon to chemical and microbial breakdown. Secondly, thermal evolution methods are commonly used for aerosols. This ensures a comparability of data sets of black carbon, which will be shown later. The samples are directly placed at the pretreatment temperature of 340 "C since rapid heating of the sample leads to fast volatilization of organic compounds and thus suppressionofchaning (16). CadleandGroblicki (15) stated that "carbonization in helium can be minimized by rapid heating" and that "air oxidationinstead ofbelium pyrolysis prevents most of the carbonization". Therefore, pure oxygen is preferred as the carrier gas. The pretreatment temperature of 340 "C was chosen based on investigations by Delumyea et al. ( 1 7) and Ohta and Okita (18),showing no significant loss or production of BC at that temperature. Delumyea et al. ( 1 7) determined the temperature dependence of the light reflection of soot produced by butane flames. They did not see any change of light reflection below340OCinair. OhtaandOkita (18) tooksamplefilters of ambient air particulates and heated them for 30 min. They did not see any change of color at 300 "C; the color became fainter at 330 "C and turned to gray at 360 "C. For this reason and because of similar experiences by Cachier
to the thermal pretreatment
data recording
I ____.$._._......mass flow controller (1) I................ !:I:::::
._____ j
temperature control r catalvst oven
IR-oven (3)
catalyst oven (4)
Bin1
IR-spektrometer (5) FIGURE 2. Experimental setup for the elemental analyses and the thermograms. Carbon and hydrogen are determined by oxidation to COz and HtO and detection by NDIR detectors.
etal. (19,201we used 340 "C for the thermal pretreatment. Cachier et al. (21) used the same thermal treatment as presented here to determine BC in aerosols. We believe that our results of BC in residues are lower because of less charring (because of the additional solvent extraction, see Table 2) but of comparable magnitude. Aproblernexistswhenusingonlythe thermaltreatment for untreated residue samples, especially partially burned residues. Some of these residue samples do reburn under these conditions. Therefore, the solvent extraction pretreatment is important in order to avoid reburn and to reduce charring during the thermal treatment as far as possible. An extra resistance wire tube furnace oven is used for the thermal pretreatment ofthe samples so that five samples could be treated at the same time. Thus, the total analyses time for asamplewasreduced to about 1.5 h. Byrecording athermal profile at thebeginning andthe end ofthethermal treatment, we obtain the actual treatment temperature for each sample. Experimental SetupandThermograms. Figure2 shows the experimental setup of the apparatus for the elemental analysis. It basically consists of a mass flow controller [l], a sample inlet [ Z ] ,an infrared burning oven 131,an oven for the catalysts [41, the NDIR detectors for COZand HzO [51, andacomputer to recordandintegratethedetectorsignals. For the elemental analyses, a sample is weighed and placed in the sample inlet and then pushed into the burning oven. At 950 "C in the burning oven and with the constant flow of oxygen as the carrier gas, all carbon and hydrogen ofthe sample are oxidized to COz and HzO and recorded. The signal is then integrated, and the corresponding mass of carbon and hydrogen is computed. To ensure that all carbon and hydrogen pass the detectors fuUy oxidized, an oven with catalysts is placed between the burning oven and the detectors. Cobalt(I1,III)oxide (Cos04)and copper oxide (CuO) are used as oxidizing catalysts at 720-740 and 650-700 "C, respectively. Silverwoolisplacedatbothends of the oven at 500-600 "C to clean the carrier gas from
halides and oxides of sulfur that might otherwise influence the detection of COz and HzO. The infrared burning oven and the IR detectors were chosen for several reasons. First, infrared ovens have the advantage that they heat up and cool down rapidly. Thus, it was possible to redry all samples prior to an elemental analysis at 100 "C so that adsorbed water did not influence the results for hydrogen. Second,byusinga programmable IR oven, we were able to heat up samples at given rates. Thus, in combination with the IR detectors continously detectingthe mixing ratios, we could record thermograms. Thermograms show the emitted carbon and hydrogen of a sample as a function of increasing temperature. A temperature sensor is placed directly above the sample in the carrier gas flow to determine the actual temperature at the sample. We verified the molar H/C ratios that were determined (see Figure 6 ) by running thermograms of NaHC03. This compound dissociates at about 300 "C to COz, HzO, andNazCOs,givingatheoreticalmolarH/C ratio of 2. We determined molar HIC ratios of 1.85 f 0.20 in the temperature range of220-400 "C (83data points) showing good agreement to the theoreticalvalue. The thermograms presented here were recorded in the temperature range from 100 to 1000 "C with a temperature program of 100 " C h i n and a carrier gas mixture of 17 vol % Oz in NZ,
Results and Discussion Reproduciblllty and Charring. Table 1 gives an overview of the reproducibility obtained with the method presented here. The reproducibility for TC and TH was checked by measuring four residue samples 11-16 times, yielding relative standard deviations of 2% and 8% for TC and TH, respectively. We divided samples of different types of vegetation into several subsamples to study the reproducibilityfor the removed mass, TCl, TH1, BC, and BH. After the treatment of the subsamples, we compared the results for (a) mass removed, (b) TCl and TH1, and (c) BC and BH between subsamples from the same original sample. The results of five samples are shown as examples in Table 1. VOL. 29. NO. IO. 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY m 2697
TABLE 1
Reproducibility of Removed Mass, TC1, TH1, BC, and BH TCa
TH'
no.
(YOdm)
(YOdm)
pine needle
a b
ash
c
29.60 29.60 29.60
1.82 1.82 1.82
hay ash
a b
19.93 19.93
savanna grass ash
a b
savanna grass partially charred savanna grass uncharred
sample
TC1 (K dm1
TH1 (70dm)
BC
BH
(% dm]
(% dm)
70.59 71.25 71.72
13.50 f 0.02 13.40 f 0.02 13.23 f 0.02
0.698 f 0.034 0.612 f 0.008 0.574 f 0.006
4.10 f 0.64 3.91 f 0.27 4.22 f 0.06
0.042 f 0.008 0.053 i 0.005 0.056 f 0.004
1.20 1.20
45.87 46.45
9.84 f 0.58 9.61 f 0.59
0.272 f 0.010 0.274 f 0.023
3.49 f 0.35 3.48 f 0.16
0.047 f 0.018 0.052 f 0.013
14.26 14.26
1.07 1.07
32.27 32.72
8.15 f 0.13 8.59 f 0.12
0.251 f 0.010 0.263 f 0.012
2.44 f 0.25 2.34 f 0.06
0.050 f 0.002 0.052 i 0.020
a b
31.99 31.99
3.37 3.37
58.89 56.64
5.20 f 0.46 5.65 f 0.29
0.472 f 0.031 0.501 f 0.013
0.64 0.82 f 0.02
0.031 0.023 i 0.009
a b
44.70 44.70
5.59 5.59
66.80 65.07 4.8 (26)
13.14 f 0.09 13.90 f 0.40 3.7 (7)
1.872 f 0.019 1.909 f 0.042 3.6 (7)
0.22 f 0.04 0.20 f 0.05 11.1 (26)
reproducibilityb
mess
removed (YO)
0.005 f 0.001 0.008 f 0.001 15.5 (26)
Analyses by the 0rg.-Chemischen lnstitut der Universitat Mainz. Reproducibility: Standard deviation between the subsamples; number of samples being divided in at least two subsamples in parentheses. Abbreviations are defined in the Analytical Section.
The mean relative reproducibility between the subsamples and thus the overall relative reproducibility for TC1, TH1, and mass removed were 3.7% [71, 3.6% [71, and 4.8% [261, respectively, and 11.1%[26] for BC and 15.5% [26] for BH (numbers of samples being divided in at least two subsamples in brackets). The precision and reproducibility of the results for both pretreatment steps were satisfactory. The average relative variability in BC and BH from one subsample was 7.0% and 7.9%,respectively (averagerelative standard deviation of 89 subsamples analyzed several times). The absolute carbon values were cross checked with those obtained by independent analyses conducted at Johannes Gutenberg-Universitat Mainz, OrganischChemisches Institut (W.Dindorf, personal communication) and TU Berlin, Fachgebiet Luftreinhaltung (E. Ulrich, personal communication) and found to be within the standard deviation. The lower detection limits during these measurements were 10 pg for C and 1 pg for H due to calibration limits. Tests to increase the detection limits are currently on the way. The carbon removed by the extraction step (TC TC1) for all samples analyzed was in the range of 23-83% of total carbon. The highest amount of carbon was removed from partly burned (83%) and unburned (70%) samples (TC - TC1, Table 1). This indicates a good removal efficiency for natural organic matter by the developed solvent extraction step. To prove that charring is suppressed by our analytical method and that the black carbon we measure is really derived by the combustion process, we conducted measurements with vegetation sampled prior to the fire, humic acids, and cellulose. These samples were treated in exactly the same way as residue samples, and the charred carbon was determined after the pretreatment steps. The results are presented in Table 2 and compared to results obtained by two other thermal methods. Four vegetation samples showed charring of less than 0.5% of the initial carbon content. The same blank test conducted with cellulose gave charring of 2%. The solvent extraction removed about 99.3% of the carbon contained in a mixture of humic acids. Thus, the amount of charred carbon for humic acids is given to be less than 0.7%. Comparing these results to values obtained with the method developed by Cadle et al. (22)and values for the thermal treatment described above
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TABLE 2
Charring Results of Three Methods Determining Black Carbon carbon method. a
b
C
compound
(% dm)
humic acids cellulose savanna grass (Ven.) a savanna grass Wen.) b savanna grass (African 91) savanna grass (African 92) cellulose lactose grass dried leaf humic acids glucose dried leafs (particle size)
29.2 45.9 44.7 44.7 45.3 43.5 45.9 40.0
