Envlron. Scl. Technol. 1904, 18, 231-234
Comparison of Solvent Extraction and Thermal-Optical Carbon Analysis Methods: Application to Diesel Vehicle Exhaust Aerosol Steven M. Japar," Ann C. Szkarlat, and Robert A. Gorse, Jr.
Research Staff, Ford Motor Company, Dearborn, Mlchlgan 48121 Emily K. Heyerdahl, Richard L. Johnson, John A. Rau, and James J. Huntzlcker"
Department of Environmental Science, Oregon Graduate Center, Beaverton, Oregon 97006 Filter samples of particulate emissions from two diesel automobiles were analyzed by solvent extraction with a hot toluene/l-propanol mixture, by thermal-optical carbon analysis, and by X-ray fluorescence analysis. On the average, carbon accounted for 83% of the particulate matter, and organic carbon comprised 70% of the extractable mass. The ratio of elemental carbon as measured by the thermal-optical technique to unextractable mass was 1.05 f 0.04. For most of the filters the unextractable mass was predominantly elemental carbon. However, for the filters with the largest amounts of unextracted material the elements Fe, S, Al, Si, and Ca were present in significant amounts (0.3-5% each of the unextractable mass when expressed as oxides). Introduction
It has become apparent in recent years that carbonaceous aerosols play an important role in the chemistry and physics of the atmosphere. Carbonaceous aerosols have been implicated in the problem of climate modification (submicron elemental carbon particles can change the radiative transfer properties of the atmosphere), potential health effects (a number of particle-bound organic compounds found in the atmosphere are known mutagens), and visibility reduction (elemental carbon is an efficient light absorber, and both organic and elemental carbon aerosols scatter light). Because of this, a number of efforts have been made to develop rapid and simple analytical methods to determine the elemental, organic, and total carbon content of aerosols. These techniques include solvent extraction (1-7),thermal combustion (8-13), optical methods (14-21), and acid digestion (22-24). Comparisons of several of the analytical techniques have been reported by Cadle and Groblicki (6) and Stevens et al. (25). Cadle and Groblicki (6) note that each of the analytical techniques defines organic and elemental carbon in an operational manner; i.e., there is no accepted, standard analytical definition of organic and elemental carbon. Consequently, it is of interest to compare independent analytical methods for the determination of organic and elemental carbon. This report presents the results of such a comparison between a solvent extraction method (1) and a thermal-optical technique (8, 9) on samples of diesel vehicle exhaust aerosol. Experimental Section Diesel vehicle exhaust particulate samples were collected on a chassis dynamometer/dilution tube facility (26). The vehicles, a 1979 2.3-L Ope1 and a 1980 2.3-L Peugeot, were run,using no. 2 diesel fuel, over a series of cruises between 20 mph and 60 mph, as well as over a portion of the Federal Test Procedure (FTP), a test cycle which includes idles, cruises, and hard accelerations to speeds in excess of 50 mph. 0013-936X/84/0918-0231$01.50/0
Samples were collected on two different filters simultaneously: 47-mm Teflon-backed Teflon membrane filters (1.0 pm pore size "!&fluor", Ghia Corp.) were used to collect samples for solvent extraction, mass measurement, and X-ray fluorescence analysis while 47-mm glass fiber filters (type AE, Gelman Corp.) were used to collect samples for carbon analysis. It has been found on the basis of a large number of vehicle tests that both filters are equally efficient in the collection of submicron aerosol emitted from diesel vehicles. The Teflon filters were used for the solvent extraction and mass measurementa because they minimize artifact formation due to sorption of gaseous oxides of nitrogen and sulfur. Solvent Extractions. The organic-soluble fraction of the particulate emissions was determined by 20-h Soxhlet extractions of the Teflon filter samples in 1:l (v/v) mixtures of toluene/l-propanol. This solvent system is highly efficient for the removal of adsorbed organic material from diesel particulate material (1). To minimize loss of particulate material during the extractions, the sample filters were wrapped in a second Teflon filter. The extractable mass was determined by the difference in the two-filter weight before and after extraction. X-ray Fluorescence Analysis. Several of the Teflon filters which had been extracted were subjected to energy dispersive X-ray fluorescence analysis using an ORTECTEFA instrument. Concentrations of Fe, Zn, Pb, Al, Si, S, and Ca were determined. The results were only semiquantitative, however, because the solvent extraction procedures produced both a nonuniform deposit of particulate material on the filter and wrinkled filters. The latter effect resulted in a nonuniform distribution of distances between the X-ray source and filter and between the X-ray detector and filter. Thermal-Optical Carbon Analysis. The thermaloptical carbon analyzer (8, 9) is unique among thermal methods in that it explicitly corrects for the pyrolytic conversion of organic to elemental carbon (i.e., carbonization) which occurs during the organic analysis step in most thermal methods. In the thermal-optical approach a single glass fiber filter disk (0.25 cm2in area) was placed in a quartz boat in the cool end of the analyzer. After purging with He, the boat was inserted into the volatilization oven in which the initial temperature was 350 "C. Organic carbon that volatilized under these conditions flowed through a Mn02 bed at 1000 "C where it was oxidized to C02. (For some analyses this step was conducted in a 2% 02-98% He atmosphere. Continuous monitoring of the filter reflectance as described below indicated no oxidation of elemental carbon under these conditions.) The C 0 2was subsequently reduced to CHI and measured with a flame ionization detector. Further volatilization and measurement of the remaining organic carbon were achieved at 600 "C. Elemental carbon was analyzed by lowering the temperature to 400 "C, changing the atmosphere to 2% 02-98% He, and measuring the amount of
@ 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 4, 1984 231
Table I. Chemical Analysis of Vehicle Particulate Emissions a run 1 2 3 4 5 6 7
vehicle 2.3-L Opel
8
9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 21 28 29 30 31 32 33
2.3-L Peugeot
conditionC FTP C-50 (2-30 C-50 C-50 C-30 (3-40 FTP FTP c-20 C-40 C-50 C-30 FTP (2-50 FTP (2-30 '2-50 (2-40 C-50 (2-40 C-30 (2-50 FTP FTP C-40 C-50 FTP C-50 (2-45 C-50 C-60 FTP
total
mass, pg/cma unextr.
extract .
164.2 156.9 109.8 111.4 87.0 82.9 78.9 161.8 230.9 106.5 97.6 79.1 100.8 387.8 118.7 248.8 102.4 64.2 89.4 117.9 135.0 108.1 89.4 204.1 76.4 131.7 111.4 48.0 119.5 185.4 113.0 98.4 61.8
132.5 340.7 190.2 380.5 134.1 117.1 134.1 41.5 50.4 174.8 156.1 166.6 135.0 114.6 122.0 41.9 166.7 162.6 105.7 166.7 219.4 158.6 231.7 40.6 48.8 189.4 274.8 49.1 195.1 330.0 221.7 247.9 31.4
296.7 491.6 300.0 491.9 221.1 200.0 213.0 203.3 281.3 281.3 253.1 246.3 235.8 502.4 240.7 296.1 269.1 226.8 195.1 284.6 354.4 266.7 321.1 244.7 125.2 321.1 386.2 97.1 314.6 515.4 340.1 346.3 99.2
carbon, p g of C/cm2 total elemental organic 253 428 202 393 175 149 154 196 23 2 236 201 192 187 392 20 2 24 1 196 194 159 260 297 218 279
181 156 115 132 96 90 86 170 193 110 94 17 88 310 106 204 98 76 89 130 141 102 91
72 272 81 262 79 59 68 26 39 126 113 116 99 81 96 37 98 119 70 130 156 116 187
153 271 343
97 128 138
56 143 205
283 414 320 263 78
131 166 140 103 53
152 248 180 160 25
a Carbon concentrations are rounded to the nearest whole number. bAverage area of deposition of the filters is 12.3 c m 2 . For #28 the area is 10.2 c m 2 . FTP is a 505-second cycle including accelerations to 50 mph. C-50 is a 50 mph cruise.
COz evolved at 400,500, and 600 "C. To correct for the pyrolytic conversion of organic to elemental carbon the filter reflectance was continuously monitored with a He-Ne laser (633 nm). The amount of elemental carbon combustion necesbary to return the filter reflectance to its original value (i.e., before pyrolytic production of elemental carbon occurred) was taken to be the correction. The three-step elemental carbon combustion process permitted adequate resolution of the point when the filter reflectance returned to its original value. For the 56 measurements (including replicates) the average correction amounted to 3 f 3% (la) of the total carbon on the filter. This is considerably less than usually observed for ambient air filters (9). To assess the accuracy of the carbon analysis procedure for total carbon, known amounts of sucrose were deposited on filter disks and analyzed. The average ratio of measured to expected carbon was 1.01 f 0.04 (95% confidence interval).
Results and Discussion The results are presented in Table I in terms of mass (pg) or carbon (pg of C) loading per centimeter squared of filter. Replicate carbon analysis (two to five per sample) was performed on 13 filters. From these results the following analytical precisions ( f l standard deviation, n = 38) were determined by analysis of variance: organic carbon (OC), f16%; elemental carbon (EC), f5.4%; total carbon (TC = OC + EC), f4.0%; OC/TC and EC/TC, f0.027. The large uncertainty in organic carbon was caused by the three filters (numbers 9, 14, and 16) with the highest concentrations of elemental carbon (193,310, 232 Envlron. Sci. Technol., Vol. 18, No. 4, 1984
and 204 pg of C/cm2, respectively). For these filters small fractional uncertainties in elemental carbon due to uncertainties in the speciation between organic and elemental carbon resulted in large fractional uncertainties for organic carbon, which was the minor species for all three filters. When these filters were removed from the analysis of variance, the uncertainty for organic carbon dropped to f7.1% while the uncertainties for elemental carbon, total carbon, OC/TC, and EC/TC remained essentially unchanged. It is apparent from Table I that the nature of the particulate emissions is dependent on the vehicle operating conditions. For the Opel the extractable fraction of the total mass was 23 f 4% for FTP's and 63 f 2% for cruises where the uncertainties correspond to one standard error of the mean. For the Peugeot FTP runs the extractable mass was 42 f 4% of the total and for cruises 66 f 2%. These can be compared with the on-road vehicle results of Szkarlat and Japar (27)in the 1981 Allegheny Tunnel experiment in which extractable mass was 24% of the total mass associated with diesel vehicles (primarily trucks). For the FTP runs organic carbon comprised 19 f 2% of total carbon for the Opel and 34 & 2% for the Peugeot and for the cruise runs 53 f 2% for the Opel and 58 f 2% for the Peugeot. For all runs carbon (i.e., organic plus elemental) constituted 83 f 2% of total mass. This is in good agreement with the results of Pierson and Brachaczek (28) for the 1977 Tuscarora Tunnel experiment (84% carbon) and with the results of Szkarlat and Japar (27) for the 1981 Allegheny Tunnel experiment (76%). Elemental Carbon/Unextractable Mass Relationships. In comparing the results of the carbon analyses and
Table 11. Statistical Analysis of Elemental Carbon (EC)/Unextractable (UEM) Dataa EC = u(UEM) t b
(EC/UEM)
data set
1.03 i 0.06 1.03 i 0.04 1.05 i 0.04 1.05 i 0.04 1.05 i 0.04 1.05 i. 0.04
(1)all filters ( 2 ) delete 1 4 ( 3 ) delete 9, 14, and 16 ( 4 ) delete 9, 14, 16, and 30 ( 5 ) delete 9, 14, 16, 30, 1, and 8 ( 6 ) delete 9, 14, 16, 30, 1, 8, and 2
r
b
U
32i 9 i 9 i 0.4 5 i
0.74 i. 0.04 0.78 i 0.05 0.96 i 0.07 1.05 i 0.08 1.00 i 0.10 1.04 i 0.12
5 8 8 i
0.96 0.94 0.93 0.93 0.90 0.88
9
10
1i 12
a Uncertainties in (EC/UEM) correspond t o 95% confidence intervals for the mean and in the regression coefficients t o one standard error. r is the linear correlation coefficient. 400
Table 111. Elemental Concentrations (pg/cm') on Extracted Filtersa
N
14
15
16
30
9.7 0.50 1.0 1.6 3.0 3.4 2.8
0.45 0.23 1.2 0.10 0.38 0.86 0.35
9.0 0.34 0.87 1.6 3.3 2.4 1.8
0.14 0.20 0.10 0.09 0.26 0.52 0.15
total 22 38 total as oxidesb unextractedmass- 7 8 i elemental carbon 17
3.6 6.2 13 i 6
19 34 45 t 11
I
1
I
I
I
I
I
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I
-
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Fe Zn Pb A1 Si S Ca
I
\
filter element
I
I
1.5 2.9 19t 9
z
-
0
0.14 0.10 0.04 0.00 0.21 0.23 0.16 0.88 2.8 (-27 i 8)
300-
i
31
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2u _J
-
w
100 -
a Iz I W
i W I
200 -
:
200
I00
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300
I 400
UNEXTRACTABLE MASS p g / c m '
The uncertainties in (unextracted mass - elemental carbon) were taken t o be the 5.4% uncertainty in elemenAssumed to be Fe,O,, ZnO, PbO, A1,0,, tal carbon. SiO,, SO,2-, and CaO.
Flgure 1. Relation between elemental carbon (thermal-optical method) and unextractable mass. The solid line corresponds to the regression relation of data set 3 In Table I1 which excludes filters 14, 16, and 9, the three points with the largest unextractablemass listed in descending order. The dashed line is an extrapolation of the solid line.
the extractions it can be assumed that the unextractable mass on the filter represents an upper limit to the mass of the elemental carbon determined by combustion. Deviations from a 1:l relationship between the two values would be expected if a significant fraction of the emissions were unextractable organic or inorganic material, i.e., (elemental carbon)/(residual mass) < 1. To investigate the relationship between elemental carbon and unextractable mass, the data were subjected to statistical analysis. As shown in Table I1 the average ratio of elemental carbon to unextractable mass was 1.03 f 0.06 (f95% confidence interval) for all filters. Least-squares regression analysis, however, gave a slope of only 0.74. Further investigation revealed that the deviation from unit slope was strongly influenced by the three filters with the largest unextractable mass concentrations (filters 9,14 and 16). When these three points were removed, the regression slope (data set 3) approached unity, and the average ratio of elemental carbon to unextractable mass increased slightly to 1.05 f 0.04 (195% confidence interval). Further removal of the high concentration runs produced only small changes in the regression coefficients and the average
ratio. The elemental carbon/unextractable mass results are plotted in Figure 1 in which the solid line represents the regression results of data set 3 in Table 11. To investigate the large differences between unextractable mass and elemental carbon for filters 9, 14, and 16, a selected group of filters (14, 15, 16, 30, and 31) was analyzed for elemental content by X-ray fluorescence analysis. The results of the analysis are given in Table 111. Filters 14 and 16 are clearly distinguished from filters 15, 30, and 31 by the concentrations of Fe, Al, Si, S, and Ca. When the elemental concentrations are expressed as oxides, the totals for filters 14 and 16 account for a significant share of the difference between elemental carbon and unextractable mass for these two filters. The sources of the major elements were not determined although it is likely that S originated from the fuel, Fe, Si, and A1 from the exhaust train during the accelerations in the FTP cycle, and Ca from the motor oil (28). In experiments prior to the one reported here, aluminum-coated particulate traps fabricated from cordierite (an aluminosilicate mineral) had been used in the exhaust trains, and it is probable that the A1 and Si observed in this experiment resulted from debris
a
Table IV. Statistical Analysis of Organic Carbon (OC)/Extractable Mass (EM) Dataa OC =u(EM) t b data set (1)all filters
(OCIEM)
0.70 i 0.05 ( 2 ) delete 14 0.70 i 0.05 ( 3 ) delete 9, 14, and 16 0.70 i 0.05 ( 4 ) delete 9, 14, 16, and 30 0.70 i 0.05 ( 5 ) delete 9, 14, 16, 30, 1, and 8 0.71 2 0.05 a Uncertainties in (OC/EM) correspond t o 95% confidence intervals standard error. r is the linear correlation coefficient.
a
b
r
0.75 i 0.03 -7.5 i 6.4 0.97 0.75 i: 0.03 -7.6 2 6.6 0.97 0.76 i 0.04 -10.4 i 7.5 0.97 0.76 i 0.04 -9.6 * 7.9 0.96 0.75 i 0.04 -7.3 i 8.8 0.96 for the mean and in the regression coefficient t o one
Environ. Sci. Technol., Vol. 18, No. 4, 1984
233
Registry No. C, 7440-44-0; toluene, 108-88-3; 1-propanol, 9 71-23-8. “25 t 1 Literature Cited 400
i
300
i
‘ I :loor zoo
a u n
(3
0
I
I
0;
100
1
I
200
I
I
300
I
I 400
EXTRACTABLE MASS p g / c m 2
Flgure 2. Relation between organic carbon (thermal-optical method) and extractable mass. The line corresponds to the regresslon relation of data set 1 in Table I V .
from the particulate traps which remained in the exhaust trains after the traps had been removed. The slight excess of elemental carbon relative to unextracted mass (with filters 9,14, and 16 excluded) could be due either to a systematic error in the carbon analysis speciation or to wash-off of elemental carbon particles from the Teflon filters during solvent extraction. Although the solvent extraction procedure was designed to minimize such wash-off, such a possibility cannot be excluded. The results of Johnson (29) suggested that wash-off of elemental carbon particles from fibrous filters occurred during extraction with a mixture of polar and nonpolar solvents. Organic Carbon/Extractable Mass Relationships. Statistical analysis of the organic carbon/extractable mass data was &o performed, and the results are given in Table IV and plotted in Figure 2. In contrast to the elemental carbon/unextractable mass relationship the organic carbon/extractable mass regressions are invariant with respect to the selective removal of various runs and, in particular, the removal of filters 9, 14, and 16. The slope of the regression line was 0.75, and the average ratio of organic carbon to extractable mass was 0.70 f 0.05 (k95% confidence interval). Such a ratio is not unreasonable in view of other vehicle emission studies (27) and is due to the presence of both oxygenated organic compounds and (possibly) inorganic sulfates in the extracted mass.
Conclusion Elemental carbon concentrations as determined by the thermal-optical carbon analysis procedure (2, 3) were compared with the mass of material left on the filter after extraction with a hot toluene/l-propanol mixture. As a working hypothesis this residual mass was taken to be an upper limit to the concentration of elemental carbon on the filter. With the exception of the three most heavily loaded samples which were obtained under FTP conditions, the average ratio of elemental carbon to unextracted mass was 1.05 f 0.04 (i95% confidence interval), and the slope of the regression line was 1.05. Thus, for most of the filters the unextractable mass was predominantly elemental carbon.
Acknowledgments We thank Richard DeCesar of the Oregon Graduate Center for performing the X-ray fluorescence analysis.
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(1) Schuetzle, D.; Lee, F. S.-C. In “Informational Report of the Measurement and Characteristics of Diesel Exhaust Emissions”;Perez, J. M.; Hill, F. J.; Schuetzle, D.; Williams, R. L., Eds.; Coordinating Research Council, 1980, Report No. 516. (2) Pierson, W. R.; Russell, P. A. Atmos. Environ. 1979, 13, 1623-1628. (3) Grosjean, D. Anal. Chem. 1975,47, 797-805. (4) Appel, B. R.; Colodny, P.; Wesolowski, J. J. Environ. Sci. Technol. 1976,10, 359-363. (5) Appel, B. R.; Hoffer, E. M.; Kothny, E. L.; Wall, S. M.; Haik, M.; Knights, R. L. Enuiron. Sci. Technol. 1979,13,98-104. (6) Cadle, S. H.; Groblicki, P. J. In “Particulate Carbon: Atmospheric Life Cycle”; Wolff, G. T.; Klimisch, R. L., Eds.; Plenum Press: New York, 1982; pp 89-109. (7) Daisey, J. M.; Leyko, M. A.; Kleinman, M. T.; Hoffman, E. Ann. N.Y. Acad. Sci. 1979,322, 125-141. (8) Johnson, R. L.; Shah, J. J.; Cary, R. A.; Huntzicker, J. J. In “Atmospheric Aerosol: SourcefAir Quality Relationships”; Macias, E. S.; Hopke, P. K., Eds.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. NO. 167, pp 223-233. (9) Huntzicker, J. J.; Johnson, R. L.; Shah, J. J.; Cary, R. A. In “Particulate Carbon: Atmospheric Life Cycle”; Wolff, G. T.; Klimisch, R. L., Eds.; Plenum Press: New York, 1982. (10) Mueller, P. K.; Fung, K. K.; Heisler, S. L.; Grosjean, D.; Hidy, G. M. In “Particulate Carbon: Atmospheric Life Cycle”; Wolff, G. T.; Klimisch, R. L., Eds.; Plenum Press: New York, 1982, pp 343-370. (11) Cadle, S. H.; Groblicki, P. J.; Stroup, D. P. Anal. Chem. 1980,52,2201-2206. (12) Tanner, R. L.; Gaffney, J. S.; Phillips, M. F. Anal. Chem. 1982,54, 1627-1630. (13) Ellis, E. C.; Novakov, T. Sci. Total Enuiron. 1982, 23, 227-238. (14) Lin, G I . ; Baker, M.; Charlson, R. J. Appl. Opt. 1973,12, 1356-1363. (15) Delumyea, R. G.; Chu, L.-C.; Macias, E. S. Atmos. Environ. 1980,14, 647-652. (16) Japar, S. M.; Killinger, D. K. Chem. Phys. Lett. 1979,66, 207-209. (17) Japar, S. M.; Szkarlat, A. C. Trans. SOC.Automot. Eng. 1981,90,3624-3631. (18) Roessler, D. M.; Faxvog, F. R. J. Opt. SOC.Am. 1979,69, 1699-1702. (19) Pleil, J. D.; Russworm, G. M.; McClenny, W. A. Appl. Opt. 1982,21, 133-135. (20) Rosen, H.; Hansen, A. D. A,; Dod, R. L.; Novakov, T. Science (Washington, D.C.) 1980,208, 741-744. (21) Heintzenberg, J. Atmos. Environ. 1982, 16, 2461-2469. (22) McCarthy, R.; Moore, C. E. Anal. Chem. 1952,24,411-412. (23) Kukreja, V. P.; Bove, J. L. Environ. Sci. Technol. 1976,10, 187-189. (24) Pimenta, J. A.; Wood, G. R. Environ. Sci. Technol. 1980, 14, 556-561. (25) Stevens, R. K.; McClenny, W. A.; Dzubay, T. G.; Mason, M. A.; Courtney, W. J. In “Particulate Carbon: Atmospheric Life Cycle”; Wolff, G. T.; Klimisch, R. L., Eds.; Plenum Press: New York, 1982; pp 111-129. (26) McKee, D. E.; Ferris, F. C.; Goeboro, R. E. Society of Automotive Engineers, 1978, Paper No. 780592. (27) Szkarlat, A. C.; Japar, S. M. J. Air Pollut. Control Assoc. 1983,33, 592-597. (28) Pierson, W. R.; Brachaczek, W. W. Aerosol Sci. Technol. 1983,2, 1-40. (29) Johnson, R. L. M.S. Thesis, Oregon Graduate Center, 1981.
Received for review December 20, 1982. Revised manuscript received October 4, 1983. Accepted October 18, 1983.
