Anal. Chem. 1980, 52, 2201-2206
perature rise in the sample but also the chemical state of the elements in the sample. The PIXE technique is a valuable analytical tool for rapid multielemental determinations of complex samples. The single factor that limits the accuracy and precision of the method is sample preparation. We have shown in this work that thin sample targets of complex materials can be successfully analyzed. T h e precision as well as the accuracy of the method can be improved if inhomogeneities in the thin sample targets can be removed and sample heating minimized.
220 1
(5) Oona, H.; Kirchner, S.J.; Kresan, P. L.; Fernando, Q.; Zeitlin, H. Anal. Chem. 1979, 51, 302-303. (6) Clark, P. J.; Neal, G. F.; Allen, R. 0. Anal. Chem. 1975. 4 7 , 650-658. (7) Johansson, S. A. E.; Johansson. T. 6.Nucl. Instrum. Methods 1976, 137, 476-516. (8) Kullerud, G.; Steffen, R. M.; Simms, P. C.; Rickey, F. A. Chem. Geol. 1979, 25, 245-256. (9) Hansen, L. D.; Ryder, J. F.; Mangelson, N. F.;Hill, M. W.; Faucette. K. F.; Eatough, D. J. Anal. Chem. 1980, 52, 821-824.
RECEIVED for review July 19, 1980. Accepted September 8, 1980. This work was sponsored in part by the University of Hawaii Sea Grant College Program under Institutional Grants 04-7-158-44129 and 04-8-Mol-178 from N O M Office of Sea Grant, Department of Commerce, and by NSF Grant No. CHE 78-18576 with added support provided by the Department of Planning and Economic Development, State of Hawaii. The PIXE system is a part of the National Science Foundation Instrumentation Facility for Radiocarbon Dating and Trace Element Analysis.
LITERATURE CITED Johansson, T. B.; Akselsson, R.; Johansson, S. A. E. Nucl. Instrum. Methods 1970, 8 4 , 1411-1143. Walter, R. L.; Willis, R. D.; Gutknecht, W. F.; Joyce, J. M. Anal. Chem. 1974. 4 6 . 843-855. Cahill, T. A. "New Uses of Ion Accelerators"; Ziegler, J. F., Ed.; Plenum Press: New York. 1975; pp 1-71. Mangelson, N. F.; Hill, M. W.; Nielson, K. K.; Eatough, D. J.; Christenson, J. J.; Izatt, R. M.; Richards, D. 0. Anal. Chem. 1979, 51, 1187-1193.
Automated Carbon Analyzer for Particulate Samples Steven H. Cadle,' Peter J. Grobiicki, and David P. Stroup Environmental Science Depatfment, General Motors Research Laboratories, Warren, Michigan 48090
An automated analyzer has been developed for separation and measurement of the organic and the elemental carbon content of suspended particulate matter. The separation is accomplished by volatillzlng the organic particulate away from the elemental carbon in an oxygen-free atmosphere. Measurement Is accomplished by an infrared technique after the carbonaceous specles have been oxldlzed to carbon dloxlde over a catalyst. Automation of sample Introduction and gas switching allows 23 samples to be analyzed In 8 h of unattended operatlon. Good agreement was obtalned between determinations of carbon by this method and by other methods. This instrument is useful in analyzing filter samples from sources such as diesel vehkles and industrial smokestacks. With some samples such as ambient particulates and certain natural products, there is a tendency to overestimate elemental carbon because of charring during the volatilizatlon procedure.
Elemental carbon particles in the atmosphere are important because of their optical absorption properties (1-3), the role they may play in atmospheric chemistry ( 4 ) ,and their contribution to the respirable fraction of the total particulate (5). However, relatively few measurements have been made of either elemental carbon emissions or elemental carbon concentrations in the atmosphere. One of the reasons for this is the absence of an analyzer designed specifically for elemental carbon determinations on filters. While many commerical carbon analyzers are available, they do not discriminate elemental carbon from the accompanying carbon compounds nor are they suited for the routine, automated analysis of the large number of filter samples generated in atmospheric field studies. Recently, Johnson and Huntzicker (6) described an instrument designed specifically for the analysis of organic and elemental carbon on filter samples of ambient particulate. The organic carbon was volatilized or pyrolyzed in an inert atmosphere, oxidized to COS,and converted to methane, which 0003-2700/80/0352-2201$01 .OO/O
was detected with a flame ionization detector. Elemental carbon was then determined after the oxidation to C 0 2 and chromatographic separation of the COPfrom 02. We have used an approach similar to Johnson and Huntzicker's to separate the organic and elemental carbon but have used nondispersive infrared (NDIR) detection of the COPto simplify the analysis. In addition, we have automated the instrument to allow for the unattended analysis of up to 23 filter samples using a modifiication of the vertical process system described by Fraisse and Semet (7). Because of a special requirement to determine very small amounts of carbon on 22-mm diameter glass disks from an impactor, the apparatus is larger and more sensitive than would normally be required if only filter samples were analyzed. This paper describes the instrument and evaluates the method.
EXPERIMENTAL SECTION Apparatus. A diagram of the carbon analyzer is shown in Figure 1. It consists of a gas flow metering and switching section, a n automatic sample dropper, a quartz pyrolysis and reaction tube mounted vertically in a furnace, a temperature controller, dual NDIR COz detectors with integrators, a recorder, and a timer. An analysis begins by loading 23 samples into the rotating sample holder of the automatic sample dropper. The small separating orifices in the quartz tube allow air to pass through the oxidizing catalyst while maintaining an oxygen-free helium atmosphere in the pyrolysis section. After the system is purged (45 min) of the oxygen which is introduced during sample loading, the turntable rotates one position and drops a sample down the heated quartz tube. The organic compounds which are volatilized and pyrolyzed are carried by the He flow through the orifice. They are oxidized to COz over the catalyst and measured by the NDIR COz analyzer. This step gives a measure of the carbon contained in organic compounds, which is referred to as apparent organic carbon. The automatic switching valve then passes the air stream over the sample, producing COz from the carbon remaining on the filter. This carbon, which will be referred to as apparent elemental carbon, is a measure of the elemental carbon present in the sample plus whatever carbon may be formed by charring 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
F l
I
A I
w Flow
Controller
Figure 1. Diagram of the carbon analyzer.
of organic compounds during the first stage of the analysis. The entire cycle is repeated automatically, for up to 23 samples. When all samples have been dropped, the timer shuts down the system. The residue is removed from the sample compartment before reloading the sample dropper. Details of the sample dropper are shown in Figure 2. It consists of a Teflon plate resting on an aluminum housing. Holes cut through the Teflon plate serve as sample holders. A Ledex stepping motor (Dayton, OH) is used to bring successive samples over the inlet to the quartz tube. A helium inlet is provided to purge air from the sample dropper. Care must be exercised to ensure that good seals are maintained at the O-rings on the quartz tube and stepping motor shaft as well as at the gasket for the aluminum cover plate; any oxygen leak will cause elemental carbon to be removed during the pyrolysis stage of the analysis. The number of samples which can be accommodated depends on the size of Teflon plate and the sample. In this work, a 14-cmdiameter Teflon plate with either 12 or 24 holes was used. Three, l-mmdiameter orifices separate the quartz reaction tube into two compartments, each with a side arm. The lower compartment is filled with Coleman Cuprox platinized copper oxide catalyst (Oak Brook, IL). Air is supplied through the lower side arm to the catalyst during the pyrolysis step. During the oxidation step the air passes through the upper side arm to remove the elemental carbon from the sample. It is also important to keep the opening between the compartments as small as possible to prevent the back-diffusion of oxygen into the sample compartment. It was found that the three 1-mm holes were adequate for this purpose. A quartz cup in the upper compartment is used to catch the samples. This facilitated removal of the residues and prevented clogging of the passages between the two compartments. The reaction tube is mounted in a Lindberg furnace (Watertown, WI) which is operated isothermally at 650 "C. The flow rates were 150 mL/min helium through the sample dropper, 53 mL/min helium through the upper side arm, and 31 mL/min air through the lower side arm. A Carle automatic valve (Fullerton, CA) was used to switch the helium and air flows between the side arms. Calibrations with CHI or COPcould be done by using the Carle gas sampling valve (0.5 mL) or by syringe injections through the septum. An Horiba PIR-2ooO NDIR (Irvine, CA) with a range of 0.5-100 ppm COzand a Beckman Model 315A NDIR (Fullerton, CA) with a range of 12-1200ppm COz were connected in series. Two NDIRS were used to ensure no loss of data during unattended operation. A Spectrum Model-1021 electronic filter (Newark, DE) was used with the Horiba detector. Both detectors were connected to Spectra Physics System I integrators (Santa Clara, CA) and a Gould Model-110 dual-pen strip-chart recorder (Cleveland, OH). Timing signals from one of the integrators were used to control
TOP VIEW
Aluminum Plate
Bonom
Aluminum Cover Plate
Neoprene Gasket
i
L "~w;;'
SIDE VIEW
Figure 2. Automated sample dropper.
the automatic mode of the system. The Spectra-Physics integrator provides up to six "time function control signals". These signals occur a t user-selected times after the start of integration and consist of a 6-s digital logic (TTL) low. These signals were interfaced to the switching valve electric operator (Carle Model-4200) and the stepping motor (Ledex Models 216 612-032 or 216 624-032) by use of the circuit shown in Figure 3. This interface also provided manual controls and lights to indicate the mode of operation. A typical run sequence is shown in Figure 4. At time TI(0.1 min) a sample
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2203
or
Chassis
Chassis
+5 - 14 AND Gate
Figure 3. Control circuit for automation of the carbon analyzer.
9
Valve
Helium 13 . Oxidatlan Ends -
Inlet
I
Purge Bsg8ns
7 4 . Integrate Peak Ares
T 5 . R e ~ t mI n i s g r a m
0-Ring Figure 4. Typical run sequence for analyzing filter samples.
was dropped into the tube. After 1.6 min, T2,the air was switched over the sample. The air was switched back at time T3(3.6 min). The data were printed out at T4(3.7 min). The system was then allowed to purge for 11min, up to Ts (15 min), before the sequence was repeated, This timing schedule was varied somewhat with different filter media and sample types to optimize the response. If carbonate is present in the samples, they should be acidified before analysis to release the carbonate as C 0 2 . To determine the amount of carbonate, the acidification can be carried out in a cell such as the one shown in Figure 5 , which is spliced into the flow system. The cell is flushed with helium and sealed before the 1-cm2filter is treated with 0.1 mL of 25% phosphoric acid and allowed to stand for 30 min. The valves are opened and the C 0 2 is swept to the NDIR analyzers. Reagents. Spectrograde benzene was obtained from Burdick and Jackson. Absolute ethanol was obtained from U.S. Industrial Chemicals. Reagent grade phosphoric acid, calcium carbonate, and potassium carbonate were obtained from Baker. Helium (99.999%), carbon dioxide (99.9%),and methane (99%) were from Matheson Gas Products. Hydrocarbon-free air was from Scott Research Laboratories. The polynuclear aromatic hydrocarbons were from Eastman Chemicals. All other hydrocarbons were obtained from Supelco.
--ti+
Septum
-
Outlet
€I--
"
20 Trap
Figure 5. Cell used to determine carbonate.
Standards and Samples. Hydrocarbon standards and samples were prepared by two methods. Liquids were spotted in known volumes on quartz fiber filters and air-dried. The solids were either placed directly on the filter and weighed or were dissolved in a suitable solvent and then spotted on the filter. Carbonate standards were prepared by weight on glass fiber filters. High-volume and low-volume filter samples of particulate from ambient air, as well as Nter samples of diesel particulate collected from an emissions test facility were used in the validation of the method. Sections of the filter used for analysis were cut with a cork borer. Filter areas of 1.05 and 0.314 cm2,respectively, were used in the carbon analyzer for lightly loaded and heavily loaded
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Table I. Carbon Content of Various Filter Media
Table 11. Recovery of High-Molecular-WeightCompounds
amt of C, pg/cm2 organic elemental brand
C
Not Stored in CO, Gelman Micro-Quartz 1.8 Gelman Spectrogradea 8.0 Dexiglas (acid treated) 2.0 Selas Silver 0.97 5.8 Reeve Angel Glassa
a
After Storage in CO, Gelman Micro-Quartz 3.0 Gelman SpectrogradeQ 11.9 Dexiglas (acid treated) 1.9 Selas Silver 0.8 Reeve Angel Glassu 11.9 Not preheated.
octacosane C,, octacosane c,, tetratetracontane C, benzo[a]pyrene hexadecanoic acid hexacosanoic acid hexadecanoic acid/ carbon black pentanedioic acid/ carbon black heptanedioic acid/ carbon black
C
0.36 0.10
0.26 0.40 0.10
0.64 0.10
0.28 0.56 0.10
filters. Triplicate analyses were performed on the smaller samples to ensure that inhomogeneous filter loadings did not introduce errors. RESULTS AND DISCUSSION Calibration, Precision, a n d Detection Limits. The instrument was calibrated daily with syringe injections of 0.5-100 p L of COz. Peak area was always proportional to micrograms of carbon injected for both analyzers. This calibration varied 2-570 from day-to-day. Catalyst efficiency was also checked daily by injecting equal volumes of COz and CHI. Methane was chosen for this purpose since it is the most difficult hydrocarbon to catalytically oxidize to COz. Catalyst efficiency was consistently 99% or greater. Precision was determined by running a large number of duplicate and triplicate filter samples. Most results were within 2% of each other which is considered excellent since this includes possible errors due to filter inhomogeneity. T h e limit of detection under the operating conditions employed was 0.05 pg of organic or elemental carbon. Although a lower limit of detection could easily be obtained by operating on a more sensitive scale of the Horiba NDIR, this was not necessary for any of the samples we have analyzed. Generally, the lower limit of detection of the organic carbon is determined by the filter blank which is due to a combination of adsorbed hydrocarbons and COz. This blank can be minimized by preheating the filters at 500 "C for several hours. For ambient work, 47-mm Micro-Quartz filters were preferred. All filters could be successfully analyzed at 650 "C. Recovery of carbon was quantitative, and the fused beads from glass filters did not appear to contain any particulate matter. Glass cover slips (22-mm) used as impactor plates contained 1 pg of organic carbon and negligible elemental carbon. Blanks from other materials are also presented in Table I. The effect of COz on filters was determined by analyzing blanks stored in a COz atmosphere. The results, shown in Table I, indicate a small increase in blank for some filters. Validation. The method was validated by determining the recovery of known quantities of pure hydrocarbons spotted on both clean and carbon-black-loaded filters as well as by comparing filter sample results with those obtained from a Perkin-Elmer Model-240 elemental analyzer and a DuPont thermogravimetric analyzer. Recovery data for several compounds are given in Table 11. The 93-111% recoveries are good evidence that the pyrolysis and catalytic oxidation of the organics are well-behaved for pure compounds. Comparison of total carbon determined by a Perkin-Elmer elemental analyzer with the sum of the organic and elemental carbon is shown in Table I11 for seven samples. The average difference between the methods was 5% with the greatest
total mass % loading, p g recovery
compound
20 30 20 20 19
104 108
22
27
106 96
23
94
25
94
93 101 111
Table 111. Comparisons of Total Carbon total carbon. sam-
Pie
description
1
ambient particulate ambient particulate ambient particulate ambient particulate diesel particulate diesel particulate diesel particulate
2
3 4
5 6 7
UB
elemental carbon analyzer analyzer 155 179 194 291 313 314 489
168 180 193 309 3 04 3 54 433
- rc p loo
! I
80
y 60
g 5
40
?
-e
g 20
< #
0 0
20
40
80
80
loo
%Volatile Material (TGA)
Comparison between percent apparent organic carbon and percent volatile material for diesel particulate samples.
Figure 6.
difference being 11%. This was accepted as reasonable agreement between the methods. More extensive comparisons were made between TGA data on diesel particulate filters and the organic and elemental carbon. Williams and Begeman (8) have divided weight loss in TGA runs into two parts. The weight lost during inertatmosphere pyrolysis to 550 "C will be referred to as the volatile material while the weight lost during the subsequent oxidation will be referred to as TGA oxidizable material. Diesel particulate was chosen for these comparisons because it consists almost entirely of carbonaceous material and should be similar to carbonaceous material from combustion sources found in atmospheric samples. Typically, 80% of this material is carbon with the remainder being oxygen, hydrogen, nitrogen, and sulfur. Although elemental analysis was not performed on these samples, it was expected that TGA results would be higher than the carbon results, due to the weight of these other elements. A comparison of the percent of apparent organic carbon to the percent of volatile material is shown in Figure 6 along with a least-squares fit of the data and the equivalence line. The TGA volatiles averaged 11% greater than the apparent organic carbon. A comparison of the percent of ap-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2205
Table IV. Materials Tested for Charring pure compounds
complex mixtures
%
R'0968
B ."C
0
m.104
20
40
60
BO
100
% Oxidizable Materiel (TGA)
Figure 7. Comparison between percent apparent elemental carbon and percent oxidizable material for diesel particulate samples. Equivalence Llr
R = 0 988 m = 0 926
" 0
20
40
60
80
loo
% Extractable Material
Figure 8. Comparison between percent apparent organic carbon and percent extractable material for diesel particulate samples.
parent elemental carbon to the percent of TGA oxidizable material is shown in Figure 7. Apparent elemental carbon averaged 9% higher than the TGA oxidizable material, suggesting a systematic error was present in one of the analysis sets. Considering the difference in the parameter measured by each of these methods, the excellent correlations and the agreement to about 10% both indicate that the carbon analyzer is performing the proper organic-elemental separation. The weight of extractable material from diesel particulate is commonly used as a measure of the organic fraction (8). Therefore, a comparison was made between the percent of apparent organic carbon and the percent of extractable material on the same diesel particulate samples. Benzene/ethanol (80:20) was used as the extraction solvent since previous experience has shown this solvent removes the most mass. The results in Figure 8 show excellent agreement. This may, in part, be fortuitous due to opposing errors. T h e extracted material will include some sulfate as well as hydrogen, oxygen, and nitrogen and thus should represent a larger percentage than that detected by the carbon analyzer. Conversely, no extraction will remove all the organics; thus the extraction will remove less carbon than the pyrolysis step of the carbon analyzer. The results suggest that these two factors offset each other for these diesel particulate samples. Interferences. Huntzicker (9) has recently reported that some natural products such as wood, or soot from wood burning, will char during the pyrolysis step of the analysis, thereby decreasing the amount of organic carbon detected and increasing the quantity of elemental carbon. This is not surprising, since the pyrolysis of carbonaceous material such as hardwoods, softwoods, nut shells, fruit pits, coal, vegetable wastes, and papermill residues (10) forms the basis of the charcoal industry.
%
material
charring
material
octacosane tetratetracontane hexadecene decosene-1 benzaldehyde methyl hexacosanoate methyl tetracosanoate methyl hexadecanoate benzoic acid hexadecanoic acid hexacosanoic acid sucrose lactose pyrene benzo[a]pyrene phenanthrene benzo[e]pyrene perylene coronene pyranthrone isoviolanthrone hexabenzocoronene
0 0
Silicone grease Vaseline 30W motor oil tire tread
4 0 0 0 0
0 0 0 2 0 2 57
0
0 2 0
rubber
Ashland A 240 Pitch
44
natural products
0 0 0 0 6 8
charring
%
material
charring
cellulose sawdust
0 26
grass
23
mushroom tree bark pine sap pollen (marigold) dried leaf
23 41 2
16
33
69 34
When charcoal is produced from wood, approximately 60% of the carbon goes into the charcoal which has a composition corresponding to CI6Hl0O2. Another industrial process involving pyrolysis is the production of carbon fibers. These fibers are produced from a variety of materials including coal tar and petroleum pitch. Typically, 60% of the carbon in these pitches is aromatic, suggesting large quantities of PNAs. For the special case of mesophase pitch, 7 5 8 5 % of the carbon content can be converted to carbon fiber (11). Carbonization is a complication in the analysis which could become important when large quantities of natural or pitchlike products are present. For example, pollen, plant debris, insects, and bacteria as well as soot from wood or vegetation burning could, in some instances, be a significant fraction of the carbon on ambient hi-vol particulate samples. T o investigate the possibility of charring, we analyzed a number of pure compounds, complex mixtures, and some natural products. Since some of the compounds were volatile and the carbon content of the natural products was not known, the results are presented in Table IV as percent charring, assuming no elemental carbon in the sample. In the special case of tread rubber, the carbon-black content (36% of the carbon) was obtained independently. No excess elemental carbon due to charring was detected in the tire tread rubber. Of the pure compounds studied, only a few extremely large polynuclear aromatic hydrocarbons and the two sugars charred significantly. Most pure compounds were recovered completely in the organic carbon fraction. The mixture studies listed in the latter part of Table I1 also showed that these materials were recovered in the organic carbon fraction when desorbing from a soot loaded filter. Even the complex mixtures such as motor oil and Vaseline had negligible charring. Only the highly aromatic, high-molecular-weightAshland pitch charred. On the other hand, a few large aromatic compounds and natural products do char and produce an excess of elemental carbon. Because of this charring possibility, the two fractions separated by the analyzer are called apparent organic carbon and apparent elemental carbon. The confidence with which these fractions may be assigned to organic and elemental carbon requires other data on the probability of
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ANALYTICAL CHEMISTRY, VOL. 52, N O . 13, NOVEMBER 1980
Table V.
Effect of Temperature on Carbon Recovery oven temp, " C
ambient particulate o n Micro-Quartzc organic elemental
550a 29.7 15.1 600 29.0 25.4 650 30.7 24.9 700 30.4 26.1 750 31.6 25.4 800 31.6 24.8 a Not enough heat to remove elemental carbon. runs. uelsamde.
ambient particulate on Spectrograde GlassC
organic
elemental
diesel particulate on DexiglasbaC organic elemental
54.3 43.5 47.1 239.3 60.1 55.0 48.7 251.8 62.5 52.2 51.6 255.7 64.0 43.2 53.1 255.9 72.7 36.2 ND ND 71.1 35.5 51.4 258.3 Temperature increased to 800 " C to remove elemental carbon on all
charring occurring in the various types of samples. Effect of Pyrolysis T e m p e r a t u r e . Analysis a t 650 "C was found to remove all black material from the analyzed filter samples and thus was the standard operating temperature. However, it was found that some carbonaceous materials such as fly ash and diesel soot burn more efficiently, giving a sharper peak, when analyzed a t 800 "C. Therefore, equal portions of the same filter were analyzed a t different temperatures. Results of these determinations are given in Table V. All temperatures in this report are the measured furnace temperature, which was found to be approximately 30 "C higher than the actual temperature of the gas stream. Raising the temperature from 650 to 800 "C had little effect on either the Micro-Quartz or Dexiglas samples but did increase the amount of organic carbon obtained from the Spectrograde glass hi-vol sample. In addition, the total quantity of carbon decreased on the latter filter with increasing temperature which is attributed to losses of elemental carbon in the fused filter material a t the higher temperatures. Analysis of Carbonate. The cell shown in Figure 5 was used to analyze 10 carbonate standards and several ambient hi-vol samples. T h e average recovery of the carbonate standards was 104%, but the standard deviation was 34%. Clearly, improvements need to be made in the accuracy of the carbonate analyses. However, the accuracy was sufficient to demonstrate that the carbonate averaged less than 1% of the carbon in the ambient particulate samples. D I S C U S SIO N The method presented and evaluated here appears suitable for its intended purpose-the rapid automatic analysis of large numbers of atmospheric samples for organic and elemental carbon content. T h e total carbon determination has been verified by recovery studies on pure compounds and by comparison with samples analyzed by standard carbon analyzer techniques. Comparison of the organic/elemental fractionation with the standard techniques of TGA and extraction suggests that the organic/elemental ratio should be accurate to within 10%. Although the precision of carbonate determiation was poor, it did show that carbonate was negligible in our ambient samples. Similar results have been reported by Mueller et al. (12) for the Los Angeles aerosol; however, it should not be assumed that carbonate is always negligible. T h e only problem with the method is the possibility that highly aromatic organic compounds or natural products will char during analysis, thus increasing the amount of elemental carbon. Miller et al. (13) have measured the ratio of aromatic hydrogens to aliphatic hydrogens in organic material extracted from atmospheric particulate matter. At most, the ratio was 0.15. If all the aromatic carbons were associated with one hydrogen, and if there were two hydrogens per aliphatic carbon, then about 25% of the carbon would be aromatic. If this aromatic carbon charred in the same proportion as the Ashland pitch, then about 10% of the organic carbon would be counted as elemental carbon. This is a "worst case" es-
timate, since the highest observed aromatic fraction and the highest charring fraction were both used. The probable error due to the presence of plant and animal debris is impossible to estimate. However, these materials whould be expected to be in the large particle sizes. Thus, the influence of the charring of natural materials can be minimized by separating aerosol into fine (C3 pm) and coarse (>3 pm) modes. Generally, the fine mode will contain the combustion-related carbon while the coarse mode will contain the natural materials. Evidence that charring will not cause a large fraction of the organic carbon to be counted as elemental carbon comes from a series of analyses performed on total ambient particulate samples from a rural site. In these samples, the average elemental carbon was 16% of the total carbon. Of course, the charring phenomena may be highly influenced by local sources, so these results cannot be safely generalized. T o further resolve this question, we are currently conducting experiments to estimate the extent of charring in urban size-segregated filter samples. ACKNOWLEDGMENT We wish to thank Joel Ager for his help in performing many of the carbon analyses, Carolina Ang and Gene Fincham for their help in setting up the method, and Ralph Thomas for designing and fabricating the electronic interface. The efforts of Warren Florance in running elemental analyses, William Lee in running TGA analyses, and David Pribich in analyzing the TGA data are also greatly appreciated. LITERATURE CITED (1) Hansen, A. D. A; Rosen, H.; Dod, R. L.; Benner, W. H.; Novakov, T. LBL-8696; Atmospheric Aerosol Research Annual Report 1977; Lawrence Berkeley Laboratory: Berkeley, CA, 1978; p 42. (2) Rosen, H.; Hansen, A. D. A.; Gundel, L.: Novakov, T. Appl. Opt. 1978, 17, 3859. (3) Novakov, T., Ed., LBL-9037; Proceedings of the Carbonaceous Particles in the Atmosphere Conference, March 1978; Lawrence Berkeley Laboratory: Berkeley, CA, June 1979. (4) Chang, S. G.; Brodzinsky, R.; Tmssi, R.; Markowitz, S. S.; Novakov. T. LBL-9037; Proceedings of the Carbanaceow Particles in the Atmosphere Conference, March 1978; Lawrence Berkeley Laboratory: Berkeley. CA, June 1979; p 122. (5) Countess, R. J.; Wolff, G. T.; Cadle, S. H., J . Air follut. Contr. Assoc., in press. (6) Johnson, R. L.: Huntzicker, J. J. LBL-9037: Proceedings of Carbonaceous Particles in the Atmosphere Conference, March 1978; Lawrence Berkeley Laboratory: Berkeley, CA, June 1979. (7) Fraisse, D.; Semet, R. Microchem. J . 1978, 23, 197. (8) Williams, R. L.; Begeman. C. R. Presented at 17th Annual Purdue Air Quality Conference May 1979; General Motors Research Publication GMR-2970, Warren, MI. (9) Huntzicker, J. J. Oregon Graduate Center, private communication. (10) Moscowitz, C. M. U . S . Environ. Prof. Agency, OM.Res. Dev. [Rep] E f A 60012-78-0042, 1978. (. 11). Volk. H. F. In "Kirk-Othmer Encyclopedia of Chemical Technology", 3rd ed.;Wiiey: New York, 1978; Vol. 4 . (12) Mueller, P. K.; Mosley, R. W.; Pierce, L. B. J. Colloid Interface Sci. 1972, 39. .. 235. (13) Miller, D. F.; Schwartz, W. E.; Gemma, J. L.; Levy, A. "Haze Formation: Its Nature and Origin"; Battelle Columbus Laboratories: Columbus. OH, March 1975.
.
~~~
RECEIVED for review June 19,1980. Accepted August 22,1980.
