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Problems in the sampling and analysis of carbon particulate
Atmospheric Entkwnenr Printed in Great Bntam
Vol. 17, No. 3. pp. 593402.
ooo‘4981~83/03059348 s03.00/0 0 1983 Pergamon Press Ltd.
1983
PROBLEMS IN THE SAMPLING AND ANALYSIS OF CARBON PARTICULATE STEVEN H. CADLE, PETER J. GROBLICKI
and PATRICIA A. MULAWA
Environmental Science Department, General Motors Research Laboratories, Warren, MI 48090-9055, U.S.A. (First received 17 May 1982 and infinalform
12 July 1982)
Abstract-Several thermal and wet chemical methods of separating organic from elemental carbon in particulate samples were examined. It is concluded that none of them represents an ideal separation procedure and that only a method-dependent operational definition of organic and elemental carbon is possible at this time. The best separation method appears to be a thermal procedure using 350°C air oxidation followed by pyrolysis in He at 950°C. There are also difficulties in sampling since dual filter techniques show that adsorption of organic compounds on various filter media accounted for at least 15 per cent of the total organic carbon collected during ambient sampling in Warren, MI. This adsorption further confuses the results and needs to be studied at other sampling sites.
INTRODUCTION Recently, increased attention has been focused on the carbonaceous component of atmospheric particulate matter because of the role elemental carbon plays in visibility reduction and atmospheric heating and because of the suspected health effects of some of the individual components of the organic carbon (Wolff and Klimisch, 1981). However, a standard procedure for the collection and analysis of this material has not been developed. Sampling has been performed with a wide variety of filter materials with relatively little attention paid to the possibility of the adsorption of gas-phase organics. The s’eparation of organic and elemental carbon has been done by several methods including thermal, digestion, extraction and optical techniques. The results obtained by these methods have been shown to correlate well, but give significantly different results (Cadle and Groblicki, 1981). Our standard analysis method has been to pyrolytically remove organic carbon by pyrolysis in He at 650°C. The volatilized carbon is catalytically oxidized to CO2 and detected with a nondispersive infrared analyzer. The remaining carbon is then removed by combustion in air and detected as COZ. While this method works very well with some samples, especially diesel particulate, most ambient samples carbonize appreciably during the pyrolysis step. Carbonization can be minimized by heating the sample in air at 350°C before pyrolysis, but it remains a problem. This was highlighted recently when the method was applied to hi-vol samples of ambient particulate matter collected near Luray, Virginia. With these samples, it was noted that the 350°C pretreatment and 650°C pyrolysis in He step caused the particulate filter deposit to blacken appreciably. In addition, the back of the filters turned from white to grey. A similar observation has been made by Dod (1981) for particulate samples collected
in California. These observations indicate that appreciable carbonization is occurring with these samples. In addition, the darkening of the entire filter could be due to the carbonization of organic compounds adsorbed throughout the filter matrix. Because of these observations, three questions were investigated. First, do organic compounds adsorb on some filter materials? Second, can the thermal analysis procedure be refined to further reduce carbonization? Third, are digestion procedures for removing organic carbon better than thermal procedures? Results of experiments designed to help answer these questions are reported below. EXPERIMENTAL Samples
Samples of atmospheric suspended particulate matter were collected during a field study near Luray, Virginia (Ferman et al., 1981).Samples of diesel particulate matter were obtained from passenger cars run on a chassis dynamometer. The test stand, which included a dilution tube, has been described previously (Cadle et al., 1980b). Wood smoke particulate matter was collected from a free-standing fireplace burning either soft or hard woods (Muhlbaier, 1981). All of these samples were collected on preheated Gelman Micro-Quartz filters. In addition, a large number of ambient particulate samples were collected on 47-mm filters mounted in doubleor triple-stacked filter holders (Nuclepore Corporation, Pleasanton, CA.) These samples were collected in Warren, MI at the General Motors Technical Center during the winter and spring of 1981. The Gelman Micro-Ouartz. Pallflex QAST’,GeTman-AE, and Gelman-A filters we;e pre-heated at 500°C for several hours to reduce the carbon blank. Selas 0.8 pm pore diameter silver membrane filters were heated for several hours at 425°C. Two lots of Selas filters were used, one lot approximately 10 years old and a current lot. Ana/ysis method Samples were analyzed using an automated carbon analyzer which has been previously described (Cadle et al., 198Oa). The analyzer operates by dropping a small filter sample into a
593
STEVENH. CADLE PETERJ. GROBLICKIand PATRICIAA. MULAWA
594
heated zone of a quartz tube under helium. The volatilized and pyrolyzed organic species are catalytically oxidized to CO, and measured with a nondispersive infrared analyzer. Air is then swept over the sample to remove the remaining carbon which is also detected as C02. These two steps will be referred to as pyrolysis in He and combustion in air. Unless otherwise stated, analysis was performed at 650°C. Analysis at 450°C required a separate heat supply to the catalyst to maintain efficiency. All samples were run in duplicate. When duplicates disagreed by more than 15 %, additional samples were run. Diyesrions
Three digestion methods were used to remove the organic carbon from filter samples of ambient particulate matter. In one method, the filters were immersed in a 1.4 M H,SO.+, 0.13 M K2S20s, 0.67” W/V AgNOS solution (Leithe, 1975). Samples were digested hot and at room temperature, with and without the Ag catalyst for various times. They were then rinsed with water and dried. In another method, the filters were immersed in a 1: 1 solution of ethanol and 4 N KOH for approximately 18 h (Rudd and Strom, 1980). They were then washed with water and dried in an oven at 100°C. The third method consisted of immersion in a 30 7; H, 0, solution and irradiation with a 200W Xe-Hg arc lamp for 0.75 h (Malaiyandi et al., 1980). These filters were also washed and dried before analysis.
RESULTS AND DISCUSSION Adsorption of organic carbon A large number of ambient particulate matter samples were collected on two filters run in stacked filter holders in order to estimate the amount of organic carbon adsorbed on filters during sampling. The top filter was in an open-faced filter holder. Samples were generally run for 24 h at a flow rate of 20 I min _ ‘. The exposed filter area was 14.5 cm2. This is approximately equivalent to l/2 the flow rate per unit area through a high-volume filter sample. An identical pair of filters mounted in stacked filter holders was set out during the same time period to serve as a field blank. Typical results for individual Gelman Micro-Quartz, Pallflex QAST, and Selas silver membrane filters are given in Table 1. Several points can be made from this data. First, the backup field blank filter has the same carbon loading as laboratory blanks. This iJ expected since it is protected from exposure to the atmosphere by the front filter. It is possible to obtain lower laboratory blank values for these filters immediately after heating. However, the
Table
1. Typical
results for dual sample and blank filters
Gelman Micro-quartz Organic
blanks generally average around 1.0 pg cm- ’ after storage. Second, in all cases, the field blank has a higher organic carbon loading than the backup field blank. This is not surprising since it is open to the atmosphere. However, this increase does not appear to be due to the dry deposition of particles on the filter. If dry deposition were occurring, one would expect a similar increase in both the organic and elemental carbon, since these species are frequently associated in the same particles, were present in approximately equal concentrations in the atmosphere, and are expected to have the same particle size distribution (Countess et al., 1981). Therefore, it appears that a carbon-containing species is being adsorbed by the filter. It is unlikely that the adsorbed species is CO2 since the filters had ample time to equilibrate with atmospheric CO, during storage. Most likely the adsorbed material is a mixture of various organic compounds. Third, it is apparent that the backup sample filter has gained even more organic carbon than the field blank. Since the backup sample filter showed no gain in elemental carbon it is unlikely that the increase is due to particle penetration through the first filter. Instead, it appears to be caused by the adsorption of organic species. If the value of the laboratory blank is subtracted from all samples, then the organic carbon found on the backup sample filter accounts for 12, 15 and 167;, respectively, of the organic carbon found on the Gelman Micro-Quartz, Pallflex QAST, and Selas Ag membrane sample filters shown in Table 1. If the field blank is subtracted from all samples, the organic carbon found on the backup filter accounts for 9, 11 and 87$ respectively, of the organic carbon found on the sample filter. The results just described indicate that adsorption of gaseous organic compounds can be a significant part of the collected organic carbon. To obtain more information on the extent of the problem, 60 stacked filter samples of winter- and spring-time ambient particulate were collected in Warren. The results for the average organic and elemental carbon on the front filters as well as the average organic carbon on the backup filters are given in Table 2. The organic carbon on the backup filter is expressed as a percentage of the organic carbon on the front filter. All samples are corrected for the laboratory blank. Overall, the adsorbed carbon is equivalent to an ambient concentration of approximately 1 pg m -3 carbon. This result is similar to that of
Elemental
Pallflex QAST Carbon, pg cm-’ Organic Elemental
Selas Ag Membrane Organic
Elemental
18.5 2.9
15.3 < 0.1
8.6 2.3
4.6 < 0.1
12.5 3.15
8.7 < 0.1
Field blank Backup field blank
1.4 0.8
< 0.1 < 0.1
1.5 1.2
< 0.1 < 0.1
2.35 1.41
< 0.1 < 0.1
Laboratory
0.8
< 0.1
1.2
< 0.1
1.3
< 0.1
Sample filter Backup sample filter
blank
595
Problems in the sampling and analysis of carbon particulate Table 2. Average carbon loading of ambient particulate filter samples
Filter
Average organic carbon on Number front filter of samples pgcm-’
Average elemental carbon on front filter pgcm-’
Average organic carbon on backup filter as % of organic carbon on front filter 17% 13%
Gelman Micro-Quartz Pallflex QAST Gelman A
22 13 4
22.6 10.4 15.1
19.1 7.8 12.1
Selas Ag Membrane
21
9.9
6.1
Stevens et al. (1980) who found an equivalent of 0.331.1 pgrnm3 total carbon on four glass fiber filters run behind Teflon filters. Since a 1 pg m- 3 carbon concentration is equivalent to approximately 2 ppb of a Ci gas-phase carbon compound, it is reasonable to assume that the carbon could be adsorbed from the gas phase. It is apparent from Table 2 that all the filters tested adsorbed similar quantities of carbon. This was further verified by simultaneously collecting samples on Gelman Micro-Quartz and Pallflex QAST as well as Gelman Micro-Quartz and Gelman A. The amounts of adsorbed carbon on the different backup filters were within 15 y0 of each other. Only when Pallflex QAST and Selas Ag membrane filters were run simultaneously was a difference observed. In this case, the Ag filters adsorbed much less carbon. These samples, however, were run on an old lot of Ag filters. When new Ag filters were run, they adsorbed as much carbon as the Pallflex QAST. The reason for this difference has not been determined. Gelman AE filters were also run. The carbon blanks varied between 4 and 6 pgcm-* making it impossible to determine small increases in organic carbon. The above results raise two additional questions. Is there a saturation level for adsorption of the organic carbon on the filters? And, how much adsorption occurs on the front filters? The first question was addressed by examining the data for the Micro-Quartz and Ag membrane filters. A plot of the amount of organic carbon on the front and backup sample filters corrected for the laboratory blank is given in Fig. 1. Only the data from the current production Ag filters is plotted. The correlation coefficients were 0.91 and 0.81, respectively, for the Micro-Quartz and Ag membrane filters, excluding the three data points above 25 pgcm- * for Micro-Quartz and the most deviant point for the Ag. These correlations are surprisingly good and suggest that the amount of adsorbable species increases with increasing organic particulate concentration. The slopes of the regression lines indicate that the per cent organic carbon adsorbed was 11.5 and 8.9, respectively, for the Micro-Quartz and Ag membrane filters. These results are lower than the average amount of organic on the backup filters shown in Table 2 because of the nonzero intercept of the regression line and because Table 2 contains additional data. There is no indication that the backup filters have
Range of organic carbon on backup filter 7-32 % 4-38 “/, 1417 7”
16% 13%
0
10 Organic
Fig.
20
3-29 “/,
30
I 40
I 50
Carbon on Front Filter (pg
I 60
70
cni2)
1. Relationships between organic carbon on front and backup filters.
been saturated with adsorbed organic carbon up to a level of 3 pgcm-*. At higher loadings the plot is nonlinear, but there isn’t sufficient data to conclude that the filters have lost capacity to adsorb more material. An estimate of how much adsorption occurs on the front filter can be obtained from a simple experiment. Filtered, particle-free air containing adsorbable organics is passed through two additional filters in series. The efficiency, E, of a single, clean filter for adsorbing organics from the gas phase is given by
E=l_$, 2
where A, and A, are the amounts of adsorbed organics on the second and third filters, respectively. This yields a useful, but approximate, estimate of the efficiency. The estimate is approximate because of two assumptions. Perhaps the worst assumption is that the adsorbable organics transmitted by the particulateremoving filter are typical of those in the atmosphere. It is likely that this filter would quantitatively remove the most strongly adsorbed compounds. The second assumption is that the adsorption efficiency of each filter is constant, i.e. independent of gas-phase concentration and compound type. Because of the first
596
STEVENH. CADLE, PETERJ. GROBLICKIand PATRICIAA. MULAWA
assumption,
the efficiency determined by this method should be a lower limit. To utilize this model, Warren, MI ambient particulate was collected on six sets of Micro-Quartz filters. Four of the filters showed an average collection efficiency of 34 % with a range from 27 to 46 %. One filter, which was very lightly loaded, had an apparent efficiency of 100% and one filter had an efficiency of 73 7;. On these six filters, the average organic carbon loading of the backup filter was 15% of that on the front filter. If one assumes the front filter also has a 347; efficiency for adsorbing these organic carbon compounds, then the adsorbed carbon on the front filter would amount to 23% of the total organic carbon. In the above discussion, it was assumed that the adsorbed organic carbon originated as atmospheric gaseous hydrocarbon. The fact that blanks do adsorb carbon demonstrates that this can happen, but does not eliminate the possibility that some of the organic carbon was released from the particulate on the front filter and was then adsorbed on the backup filter. If this were the case, then the carbon found on the backup filter would be indicative of sampling losses of carbon rather than adsorptive gains in carbon. It is impossible to distinguish between these two phenomena using our data. However, we consider it unlikely that 23 7; of the particulate organic carbon could desorb. This would require major shifts in equilibrium conditions during sampling. To judge the potential magnitude of organic desorption, hydrocarbon-free air (Scott Specialty Gases) was passed through two clean filters, a 47-mm section of a hi-vol filter, and then a final clean filter at a flow rate of 4 &min- 1 for 24 h. Two hi-vol filters were used with organic carbon loadings of 182 and The final filters gained 0.8 and 80.5 pg cm-‘. < 0.2 pg cm-’ organic carbon, respectively. This experiment would not produce as high a concentration of organics in the air as would be found in ambient sampling. But if it is assumed that there is little concentration dependence on the adsorption of organics then it can be concluded that the organic carbon released from the front filter is not a significant contributor to the observed adsorbed organic carbon.
The adsorption of organic carbon is not the only significant problem in collecting and analyzing carbonaceous particulate matter. Uncertainty is also introduced by the fact that there is no method of cleanly separating the organic from the elemental carbon. Various analytical methods provide different operating definitions of organic and elemental carbon. One of the most widely accepted definitions is that elemental carbon is any carbon which is black and organic carbon is the remainder, excluding carbonates. Thermal stripping of the organic carbon has been used since it provides one of the fastest means of separating the organic and elemental carbon and can be coupled
with very sensitive detectors. Unfortunately, optical measurements have shown that some of the organic carbon can carbonize during this kind of separation process (Huntzicker et al., 1981; Cadle and Groblicki, 1981). One thermal method for minimizing carbonization utilizes a two-step separation procedure to remove the organic from the elemental carbon. The sample is heated at 350°C in air and then pyrolyzed in He at 650” C to remove the organic carbon. The remaining elemental carbon is determined by combustion in air. Organic carbon is determined by difference utilizing total carbon analysis on a separate piece of the filter sample. Although pyrolysis in He at 650” C would be expected to remove most organic carbon compounds, it was reasoned that pyrolysis at even higher temperatures may remove some of the polymeric chars formed during carbonization and, thus, could further minimize the problem. Therefore, the effect of pyrolysis temperature on the separation of organic and elemental carbon was investigated, Multiple samples were cut from filters of ambient, diesel and woodburning particulate matter. One set of samples was analyzed for organic and elemental carbon by the pyrolysis in He-oxidation in air procedure at four temperatures; 450,650,800 and 950°C. This is referred to as the onestep procedure. A second set of samples was pretreated by heating in air at 350” C for several minutes before the pyrolysis in He-oxidation in air analysis at the same four temperatures. This procedure is referred to as the two-step separation procedure. Typical results of these analyses are shown in Figs 2-5. Total carbon, organic carbon and elemental carbon determined on separate samples at each temperature by the one step procedure are plotted. Also plotted is the elemental carbon measured at each temperature using the samples which had been pretreated at 350°C in air, i.e., the two-step separation ~ - ------
Total Carbon Organic Carbon (l-step separation procedurel Elemental Carbon (l-step separation procedure1 Elemental Carbon @step separation procedure)
1000
:0Temperature
WI
Fig. 2. Carbon analysis of an ambient particulate sample collected in Warren, Michigan.
Problems in the sampling and analysis of carbon particulate
-
Total Carbon ______ Organic Carbon {l-step separation procedure) --_ ELemental Carbon (l-step separation procadurel Elemental Carbon (2-step separation procedure)
...
o
:.
i
400
I
I
600 Temperature
1000
800 (OCl
Fig. 3. Carbon analysis of an ambient particulate sample collected near Luray, Virginia.
__---..“.
Total Carbon Organic Carbon (l-step separation procedure1 Elemental Carbon Il.-step separation procedure) Elemental Carbon (I-step separation procedure)
“O;I@----,,p------=“____r
“D
e
//
G
1’
p .5 ‘3 9
,’
..S*..,.-.. . . . . . ..I 4 . . . .~~~~tr:
/
i
/^
rt
.;..‘C ,*i
~~
: _a-
_/-
31! ____----= .* ..--
_+--
s-
I
0 400
I
I
1000
600 PC)
600 Temperstore
Fig. 4. Carbon analysis of a wo~buming
~iculate
sample.
5 _ -TotalOrganicCarbon Carbon
3 o .6$
..---..---
Elemental
Carbon
G a: __I__---~------; “N-C /’ r
0 400
I
I
600
800
Temperatuta
I 1000
PC)
Fig. 5. Carbon analysis of a diesel particulate sample.
597
procedure. The bars on individual points show the spread of duplicate analyses. Five facts are apparent from inspection of these figures. First, very little elemental carbon is removed by oxidation at 450” C. Second, there is no significant increase in total carbon when the analysis temperature is raised from 650 to 950” C. Third, the amount of organic carbon removed increases dramatically between 450 and 650°C and continues to increase up to 950” C for a11but the diesel sample. We have no explanation for the relatively high organic carbon value found at 450°C for the diesel sample. It should be noted that this sample is atypical of diesel passenger-car particulate in that it contains much more organic carbon than most samples and thus provides a stringent test for charring. The percentage increase in organic carbon between 650 and 950” C was 25, 65, 41 and - 1, respectively, for the Warren, Luray, wood-burning and diesel samples and was accompanied by a 35, 67, 8 and 10 per cent decrease, respectively, in the elemental carbon. Fourth, the elemental carbon determined after the two-step separation procedure (350” C oxidation followed by pyrolysis) also decreases with increasing temperature for the Warren, Luray and woodburning samples. Elemental carbon was not determined for the diesel sample after the two-step separation procedure since the carbon content was constant and previous results have shown that diesel samples do not carbonize significantly. Fifth and last is the fact that the difference in elemental carbon determined with and without the two-step separation disappears in the ambient samples at 95O”C, but persists in the woodsmoke samples. It is important to determine if the decrease in elemental carbon at high temperature is due to a more efficient removal of the organic carbon or is due to loss of the elemental carbon. Elemental carbon could be lost via reaction with other components of the particulate such as Hz0 liberated from the rest of the sample. A loss in elemental carbon can be determined by measuring the light absorbance of the sample. With this method it is assumed that elemental carbon is black, that heating the samples does not change the specific extinction coefficient of the elemental carbon, that changes in absorbance are not caused by loss of scattering material from the filter, and that the organic carbon does not absorb appreciably. Then, any increase in absorbance of a sample after heating can be attributed to the production of elemental carbon via carbonization of the organics, and any decrease is due to removal of elemental carbon. Absorbance measurements were made on all samples before and after pyrolysis at 650, 800 and 950°C using a modified integrating plate method (Lin et al., 1973; Groblicki et al., 1981). All but the diesel samples showed large increases in absorption even after heating at 950” C. Therefore, it is concluded that increasing the analysis temperature to 950” C does not result in the removal of any e~emen~l carbon, and that some ~rboni~tion of the organics is still occurring.
598
STEVENH. CADLE, PETERJ. GROBLICKIand PATRICIAA. MIJLAWA
The reason for the improved removal of organic carbon at elevated temperatures is not clear. One possible explanation is that more rapid heating results in less carbonization. This would be consistent with studies of the charring of natural materials. For example, Brunner and Roberts (1980) have found that increasing the temperature programming rate from ll”Cmin-’ to 70” C min- l, when heating cellulose to 700°C decreases the char yield from 20 to 12 per cent. Cadle and Groblicki (1981) reported decreased carbonization of ambient particulate samples collected in Denver, Colorado when the heating rate was increased. Therefore, ambient particulate samples from Warren and Luray as well as diesel and fireplace samples were temperature programmed in He to 650” C at rates of 5, 25, 125 and 400” C min- 1 to determine heating rate effects. The samples were analyzed by pyrolysisoxidation at 650” C for elemental carbon. No significant effect of programming rate on the amount of elemental carbon was found for any of the samples. Therefore, it is concluded that the increased removal of organic carbon at elevated temperature is not a heating rate effect for these samples. An alternate explanation is that the increase is caused by the thermal decomposition of carbonoxygen surface complexes and polymeric chars. It is well documented that various surface carbonoxygen complexes exist on almost all types of carbon (Puri, 1970). The amount of oxygen present varies due to several factors, but is frequently 10 % of the carbonaceous material on a mass basis. In general, the oxygen can be pyrolytically removed as H,O, CO and CO2 at temperatures between 300 and looo” C. If it is assumed that all of the oxygen is removed as CO, then decomposition of these surface complexes could account for the observed 8-10 % decrease in elemental carbon with increasing pyrolysis temperature for diesel and woodburning samples. However, it is unlikely that it could account for the much larger decrease in elemental carbon, i.e. 35 and 67 per cent, respectively found with the Warren and Luray samples. The possibility that the increase in organic carbon with increasing temperature was due to the decomposition of carbonates can be ruled out by the fact that no significant increase in total carbon occurred with increasing temperature. Therefore, it appears that these changes must be due in part to the thermal decomposition of polymeric chars. The actual contribution of CO and CO, evolved from surface functional groups to our operationally defined organic- and elemental-carbon results is not known since neither the oxygen content of the elemental carbon nor the decomposition kinetics have been determined for our samples. It is not even clear whether this carbon should be classified as organic or elemental carbon. If the definition of elemental carbon as black carbon is accepted, then these surface complexes should be included in the organic carbon category since they would not be expected to absorb visible light. However, since they are bonded to the elemental carbon, they would not normally be consid-
ered part of the organic fraction. With most ambient samples, heating at 950” C appears necessary for removal of chars. Therefore, the CO and CO, evolved from surface functional groups will probably be classified as organic carbon. In view of the experiments reported here, revised thermal procedures are recommended for the analysis of particulate matter on quartz or silver filters. When the nature of the particulate matter is unknown, the two-step procedure using oxidation-in-air at 350” C followed by pyrolysis and oxidation at 950” C is the best thermal method. The 350” C air-oxidation step is particularly needed for ambient or source samples containing woodsmoke. If woodsmoke is known to be a small constituent of the carbonaceous aerosol, increased speed and accuracy can be obtained using a single-step pyrolysis at 950” C. In the special case of diesel particulate, a single-step pyrolysis at 800” C will minimize loss of surface functional groups, while producing rapid combustion and good peak shapes. Digestion methods
Since our data suggest that all thermal methods are subject to some error due to carbonization, alternate wet digestion procedures for the removal of organics were investigated. Included were a 0.75 h digestion in u.v.-irradiated H,O,, an 18 h ethanolic-KOH digestion, and an 18 h digestion with room-temperature, silver-catalyzed acidic persulfate solution. These solutions are known to be effective oxidizers of organic compounds. Samples were analyzed by the 650” C pyrolysis-oxidation procedure after digestion. Results for seven filter samples of ambient particulate matter are given in Table 3. Samples l-6 were collected in Warren, Michigan on Micro-Quartz filters. Sample 7 was collected in Denver, Colorado on a Spectrograde glass filter. Elemental carbon was determined by two methods. In the first, it was assumed that the total carbon remaining on the filter after digestion was elemental carbon. If this were a valid assumption, then organic and elemental carbon could be determined by commercial total-carbon analyzers. In the second, it was assumed that some organic carbon remained on the filter. This carbon was removed by pyrolysis at 650°C. Elemental carbon was then determined by combustion. Also included in Table 3 are elemental carbon results determined after the three thermal separation procedures; 650” C pyrolysis, 350” C oxidation-650” C pyrolysis and 950” C pyrolysis. Inspection of Table 3 shows that the two-step (350” C/6500 C) thermal-separation procedure gives lower elemental carbon values than the single 650” C procedure. Elemental carbon after 950” C pyrolysis is even lower, consistent with the previous discussion. Total carbon determinations after ethanolic-KOH and u.v.-H,O, digestion give results similar to those obtained by the thermal methods. However, when elemental carbon is determined after both the digestion and 650°C pyrolysis, the results are consistently lower than those obtained after the 950” C pyrolysis.
Problems in the sampling and analysis of carbon particulate
599
There are two possible explanations for this result. First, the organic carbon may not be completely removed during the digestion; second, the elemental carbon may be partially oxidized during digestion to give surface carbon-oxygen complexes which decompose to CO and CO2 upon pyrolysis. It has been shown that treatment of various carbons by oxidizing agents such as sulfate, nitric acid and hydrogen peroxide does increase the amount of surface carbonoxygen complexes that evolve CO and CO, upon heating (Mahajan et al., 1980). Visual evidence that these digestions attacked the elemental carbon comes from the observation that all digested filters were visually lighter than the original sample. It was not possible to quantify this observation by light absorbance measurement of the filters because they were too heavily loaded with light-absorbing particulate matter. However, it was concluded that the elemental carbon determined after digestion and 650” C pyrolysis is probably too low due to oxidation of the black carbon. In all but one sample, the persulfate digestion removed most of the carbon and, thus, was obviously too strong an oxidizing agent. Alternative persulfate procedures utilizing hot-acidic persulfate solutions with and without the silver catalyst were tried for varying digestion times. These appeared to be at least as strong as the procedure used in Table 3 and resulted in the removal of almost all the elemental carbon. Therefore, persulfate digestion does not appear to be a viable technique. There is no apparent advantage of these digestion methods over the simpler thermal methods.
CONCLUSIONS AND RECOMMENDATIONS
The adsorption of organic carbon on glass and quartz fiber filters and silver membrane filters may be a major problem in the determination of organic particulate matter. With some samples, the amount of carbon adsorbed by the backup filter was 30 % of that on the front filter. The actual amount of adsorbed carbon on the front filter may be even greater if adsorption efficiency is taken into consideration. The use of field blanks for organic carbon samples will lower the error, but will not eliminate it. Since adsorption is apt to vary with sampling location and time, it is recommended that double stack filter samples be collected to assess the magnitude of organic carbon adsorption. Irrespective of sampling problems, it remains difficult to unambiguously determine the organic or elemental carbon content of ambient particulate matter. Thermal methods have the advantage of being rapid, but can cause carbonization of the organic carbon, even at 950 “C. The optimum thermal method varies with different samples. When the nature of the sample is unknown, a two-step separation procedure employing heating at 350°C in air followed by a
STEVEN H. CADLB PETER J. GROBLICKI
600
950°C pyrolysis should be used. The digestion methods investigated remove some of the elemental carbon
methods.
and
have
no advantage
over
the thermal
The lack of a clear separation
organic and elemental carbon limits the methods to Drovidint? oDerational definitions . materials. Y
1
between analysis of these
Acknowledgement-We gratefully acknowledge the efforts of C. C. Ang which helped make this project possible.
REFERENCES Brunner P. H. and Roberts P. V. (1980) The significance of heating rate on char yield and char properties in the pyrolysis of cellulose. Carbon 18, 217-224.. Cadle S. H. and Groblicki P. J. (1981) An evaluation of methods for the determination of organic and elemental carbon in particulate samples. In Particulate Carbon: Atmospheric Life Cycle (Edited bv Wolff G. T. and Klimisch R. L.)“,89-109. Plenum Press, New York. Cadle S. H., Groblicki P. J. and Stroup D. P. (198Oa) An automated carbon analyzer for particulate samples. Analyt. Chem. 52, 2201-2206. Cadle S. H., Nebel G. J. and Williams R. L. (1980b) Measurements of unregulated emissions from General Motors’ light-duty vehicles. S.A.E. Trans. 87,2381-2401. Countess R. J., Cadle S. H., Groblicki P. J. and Wolff G. T. (1981) Chemical analysis of size segregated samples of Denver’s ambient particulate. J. Air Pollut. Control Ass. 31,247-252.
Dod R. L. (1981) In Particulate Carbon: Atmospheric Lge Cycle (Edited bv Wolff G. T. and Klimisch R. L.). II 86. Pienum Press, New York. Ferman M. A., Wolff G. T. and Kelly N. A. (1981) The nature and sources of haze in the Shenandoah Valley/Blue
and
PATRICIA A. MULAWA
Ridge Mountains area. J. Air Pollut. Control Ass. 31, 1074-1082. Groblicki P. J., Wolff G. T. and Countess R. J. (1981) Visibility reducing species in the Denver ‘Brown Cloud’ Part 1. Relationships between extinction and chemical comoosition. Atmosoheric Environment 15. 2473-2484. Huntzlfcker J. J., Johnson R. L., Shah J. J. and Gary R. A. (1981) Analysis of organic and elemental carbon in ambient aerosols bv a thermal-ontical method. In Particulate Carbon: Atmospheric Li& Cvcle (Edited by Wolff G. T. and Klimisch k. L.), 79-8i. Plenum Press, New York. Leithe W. (1973) The Analysis ofOraanic Pollutants in Water and Waste Water, 48-51. Ann Arbor Science, Ann Arbor, MI. Lin C. I., Baker M. and Charlson R. J. (1973) Absorption coefficient of atmospheric aerosol: a method for measurement. Appl. Optics 12, 1356-1363. Mahajan 0. P., Moren-Castilla C. and Walker P. L., Jr. (1980) Surface-treated activated carbon for removal of phenol from water. Sep. Sci. Technol. 15, 1733-1752. Malaiyandi M., Sadar M. H., Lee P. and G’Grady R. (1980) Removal of oraanics in water usine hvdroeen neroxide in presence of ultraviolet light. Ware; Res. 12, 1’131-1135. Muhlbaier J. L. (1981) Characterization of emissions from woodburning fireplaces. In Residental Solid Fuels: Environmental Imoacts and Solutions (Edited by Cooper _ J. A. and Malek D.) 164187. Puri B. R. (1970) Surface comolexes on carbons. In Chemistry and Physics of Carbon. Vol. 6, New York. Marcel Dekker, 191-282. Rudd C. J. and Strom K. A. (1980) Accumulation of diesel exhaust particles in guinea pig lung: measurement by an optical technique. General Motors Research Report Bl172. Stevens R. K., Dzubay T. G., Shar R. W., Jr., McClenny W. A., Lewis C. W. and Wilson W. E. (1980) Characterization of the aerosol in the Great Smokey Mountains. Enoir. Sci. Technol. 14, 1491-1498. Wolff G. T. and Klimisch R. L. (Eds) (1981) Particulate carbon: Atmospheric Itye cycle. Plenum Press, New York.
Vol. 17, No. 3. pp. 593402.
ooo‘4981~83/03059348 s03.00/0 0 1983 Pergamon Press Ltd.
1983
PROBLEMS IN THE SAMPLING AND ANALYSIS OF CARBON PARTICULATE STEVEN H. CADLE, PETER J. GROBLICKI
and PATRICIA A. MULAWA
Environmental Science Department, General Motors Research Laboratories, Warren, MI 48090-9055, U.S.A. (First received 17 May 1982 and infinalform
12 July 1982)
Abstract-Several thermal and wet chemical methods of separating organic from elemental carbon in particulate samples were examined. It is concluded that none of them represents an ideal separation procedure and that only a method-dependent operational definition of organic and elemental carbon is possible at this time. The best separation method appears to be a thermal procedure using 350°C air oxidation followed by pyrolysis in He at 950°C. There are also difficulties in sampling since dual filter techniques show that adsorption of organic compounds on various filter media accounted for at least 15 per cent of the total organic carbon collected during ambient sampling in Warren, MI. This adsorption further confuses the results and needs to be studied at other sampling sites.
INTRODUCTION Recently, increased attention has been focused on the carbonaceous component of atmospheric particulate matter because of the role elemental carbon plays in visibility reduction and atmospheric heating and because of the suspected health effects of some of the individual components of the organic carbon (Wolff and Klimisch, 1981). However, a standard procedure for the collection and analysis of this material has not been developed. Sampling has been performed with a wide variety of filter materials with relatively little attention paid to the possibility of the adsorption of gas-phase organics. The s’eparation of organic and elemental carbon has been done by several methods including thermal, digestion, extraction and optical techniques. The results obtained by these methods have been shown to correlate well, but give significantly different results (Cadle and Groblicki, 1981). Our standard analysis method has been to pyrolytically remove organic carbon by pyrolysis in He at 650°C. The volatilized carbon is catalytically oxidized to CO2 and detected with a nondispersive infrared analyzer. The remaining carbon is then removed by combustion in air and detected as COZ. While this method works very well with some samples, especially diesel particulate, most ambient samples carbonize appreciably during the pyrolysis step. Carbonization can be minimized by heating the sample in air at 350°C before pyrolysis, but it remains a problem. This was highlighted recently when the method was applied to hi-vol samples of ambient particulate matter collected near Luray, Virginia. With these samples, it was noted that the 350°C pretreatment and 650°C pyrolysis in He step caused the particulate filter deposit to blacken appreciably. In addition, the back of the filters turned from white to grey. A similar observation has been made by Dod (1981) for particulate samples collected
in California. These observations indicate that appreciable carbonization is occurring with these samples. In addition, the darkening of the entire filter could be due to the carbonization of organic compounds adsorbed throughout the filter matrix. Because of these observations, three questions were investigated. First, do organic compounds adsorb on some filter materials? Second, can the thermal analysis procedure be refined to further reduce carbonization? Third, are digestion procedures for removing organic carbon better than thermal procedures? Results of experiments designed to help answer these questions are reported below. EXPERIMENTAL Samples
Samples of atmospheric suspended particulate matter were collected during a field study near Luray, Virginia (Ferman et al., 1981).Samples of diesel particulate matter were obtained from passenger cars run on a chassis dynamometer. The test stand, which included a dilution tube, has been described previously (Cadle et al., 1980b). Wood smoke particulate matter was collected from a free-standing fireplace burning either soft or hard woods (Muhlbaier, 1981). All of these samples were collected on preheated Gelman Micro-Quartz filters. In addition, a large number of ambient particulate samples were collected on 47-mm filters mounted in doubleor triple-stacked filter holders (Nuclepore Corporation, Pleasanton, CA.) These samples were collected in Warren, MI at the General Motors Technical Center during the winter and spring of 1981. The Gelman Micro-Ouartz. Pallflex QAST’,GeTman-AE, and Gelman-A filters we;e pre-heated at 500°C for several hours to reduce the carbon blank. Selas 0.8 pm pore diameter silver membrane filters were heated for several hours at 425°C. Two lots of Selas filters were used, one lot approximately 10 years old and a current lot. Ana/ysis method Samples were analyzed using an automated carbon analyzer which has been previously described (Cadle et al., 198Oa). The analyzer operates by dropping a small filter sample into a
593
STEVENH. CADLE PETERJ. GROBLICKIand PATRICIAA. MULAWA
594
heated zone of a quartz tube under helium. The volatilized and pyrolyzed organic species are catalytically oxidized to CO, and measured with a nondispersive infrared analyzer. Air is then swept over the sample to remove the remaining carbon which is also detected as C02. These two steps will be referred to as pyrolysis in He and combustion in air. Unless otherwise stated, analysis was performed at 650°C. Analysis at 450°C required a separate heat supply to the catalyst to maintain efficiency. All samples were run in duplicate. When duplicates disagreed by more than 15 %, additional samples were run. Diyesrions
Three digestion methods were used to remove the organic carbon from filter samples of ambient particulate matter. In one method, the filters were immersed in a 1.4 M H,SO.+, 0.13 M K2S20s, 0.67” W/V AgNOS solution (Leithe, 1975). Samples were digested hot and at room temperature, with and without the Ag catalyst for various times. They were then rinsed with water and dried. In another method, the filters were immersed in a 1: 1 solution of ethanol and 4 N KOH for approximately 18 h (Rudd and Strom, 1980). They were then washed with water and dried in an oven at 100°C. The third method consisted of immersion in a 30 7; H, 0, solution and irradiation with a 200W Xe-Hg arc lamp for 0.75 h (Malaiyandi et al., 1980). These filters were also washed and dried before analysis.
RESULTS AND DISCUSSION Adsorption of organic carbon A large number of ambient particulate matter samples were collected on two filters run in stacked filter holders in order to estimate the amount of organic carbon adsorbed on filters during sampling. The top filter was in an open-faced filter holder. Samples were generally run for 24 h at a flow rate of 20 I min _ ‘. The exposed filter area was 14.5 cm2. This is approximately equivalent to l/2 the flow rate per unit area through a high-volume filter sample. An identical pair of filters mounted in stacked filter holders was set out during the same time period to serve as a field blank. Typical results for individual Gelman Micro-Quartz, Pallflex QAST, and Selas silver membrane filters are given in Table 1. Several points can be made from this data. First, the backup field blank filter has the same carbon loading as laboratory blanks. This iJ expected since it is protected from exposure to the atmosphere by the front filter. It is possible to obtain lower laboratory blank values for these filters immediately after heating. However, the
Table
1. Typical
results for dual sample and blank filters
Gelman Micro-quartz Organic
blanks generally average around 1.0 pg cm- ’ after storage. Second, in all cases, the field blank has a higher organic carbon loading than the backup field blank. This is not surprising since it is open to the atmosphere. However, this increase does not appear to be due to the dry deposition of particles on the filter. If dry deposition were occurring, one would expect a similar increase in both the organic and elemental carbon, since these species are frequently associated in the same particles, were present in approximately equal concentrations in the atmosphere, and are expected to have the same particle size distribution (Countess et al., 1981). Therefore, it appears that a carbon-containing species is being adsorbed by the filter. It is unlikely that the adsorbed species is CO2 since the filters had ample time to equilibrate with atmospheric CO, during storage. Most likely the adsorbed material is a mixture of various organic compounds. Third, it is apparent that the backup sample filter has gained even more organic carbon than the field blank. Since the backup sample filter showed no gain in elemental carbon it is unlikely that the increase is due to particle penetration through the first filter. Instead, it appears to be caused by the adsorption of organic species. If the value of the laboratory blank is subtracted from all samples, then the organic carbon found on the backup sample filter accounts for 12, 15 and 167;, respectively, of the organic carbon found on the Gelman Micro-Quartz, Pallflex QAST, and Selas Ag membrane sample filters shown in Table 1. If the field blank is subtracted from all samples, the organic carbon found on the backup filter accounts for 9, 11 and 87$ respectively, of the organic carbon found on the sample filter. The results just described indicate that adsorption of gaseous organic compounds can be a significant part of the collected organic carbon. To obtain more information on the extent of the problem, 60 stacked filter samples of winter- and spring-time ambient particulate were collected in Warren. The results for the average organic and elemental carbon on the front filters as well as the average organic carbon on the backup filters are given in Table 2. The organic carbon on the backup filter is expressed as a percentage of the organic carbon on the front filter. All samples are corrected for the laboratory blank. Overall, the adsorbed carbon is equivalent to an ambient concentration of approximately 1 pg m -3 carbon. This result is similar to that of
Elemental
Pallflex QAST Carbon, pg cm-’ Organic Elemental
Selas Ag Membrane Organic
Elemental
18.5 2.9
15.3 < 0.1
8.6 2.3
4.6 < 0.1
12.5 3.15
8.7 < 0.1
Field blank Backup field blank
1.4 0.8
< 0.1 < 0.1
1.5 1.2
< 0.1 < 0.1
2.35 1.41
< 0.1 < 0.1
Laboratory
0.8
< 0.1
1.2
< 0.1
1.3
< 0.1
Sample filter Backup sample filter
blank
595
Problems in the sampling and analysis of carbon particulate Table 2. Average carbon loading of ambient particulate filter samples
Filter
Average organic carbon on Number front filter of samples pgcm-’
Average elemental carbon on front filter pgcm-’
Average organic carbon on backup filter as % of organic carbon on front filter 17% 13%
Gelman Micro-Quartz Pallflex QAST Gelman A
22 13 4
22.6 10.4 15.1
19.1 7.8 12.1
Selas Ag Membrane
21
9.9
6.1
Stevens et al. (1980) who found an equivalent of 0.331.1 pgrnm3 total carbon on four glass fiber filters run behind Teflon filters. Since a 1 pg m- 3 carbon concentration is equivalent to approximately 2 ppb of a Ci gas-phase carbon compound, it is reasonable to assume that the carbon could be adsorbed from the gas phase. It is apparent from Table 2 that all the filters tested adsorbed similar quantities of carbon. This was further verified by simultaneously collecting samples on Gelman Micro-Quartz and Pallflex QAST as well as Gelman Micro-Quartz and Gelman A. The amounts of adsorbed carbon on the different backup filters were within 15 y0 of each other. Only when Pallflex QAST and Selas Ag membrane filters were run simultaneously was a difference observed. In this case, the Ag filters adsorbed much less carbon. These samples, however, were run on an old lot of Ag filters. When new Ag filters were run, they adsorbed as much carbon as the Pallflex QAST. The reason for this difference has not been determined. Gelman AE filters were also run. The carbon blanks varied between 4 and 6 pgcm-* making it impossible to determine small increases in organic carbon. The above results raise two additional questions. Is there a saturation level for adsorption of the organic carbon on the filters? And, how much adsorption occurs on the front filters? The first question was addressed by examining the data for the Micro-Quartz and Ag membrane filters. A plot of the amount of organic carbon on the front and backup sample filters corrected for the laboratory blank is given in Fig. 1. Only the data from the current production Ag filters is plotted. The correlation coefficients were 0.91 and 0.81, respectively, for the Micro-Quartz and Ag membrane filters, excluding the three data points above 25 pgcm- * for Micro-Quartz and the most deviant point for the Ag. These correlations are surprisingly good and suggest that the amount of adsorbable species increases with increasing organic particulate concentration. The slopes of the regression lines indicate that the per cent organic carbon adsorbed was 11.5 and 8.9, respectively, for the Micro-Quartz and Ag membrane filters. These results are lower than the average amount of organic on the backup filters shown in Table 2 because of the nonzero intercept of the regression line and because Table 2 contains additional data. There is no indication that the backup filters have
Range of organic carbon on backup filter 7-32 % 4-38 “/, 1417 7”
16% 13%
0
10 Organic
Fig.
20
3-29 “/,
30
I 40
I 50
Carbon on Front Filter (pg
I 60
70
cni2)
1. Relationships between organic carbon on front and backup filters.
been saturated with adsorbed organic carbon up to a level of 3 pgcm-*. At higher loadings the plot is nonlinear, but there isn’t sufficient data to conclude that the filters have lost capacity to adsorb more material. An estimate of how much adsorption occurs on the front filter can be obtained from a simple experiment. Filtered, particle-free air containing adsorbable organics is passed through two additional filters in series. The efficiency, E, of a single, clean filter for adsorbing organics from the gas phase is given by
E=l_$, 2
where A, and A, are the amounts of adsorbed organics on the second and third filters, respectively. This yields a useful, but approximate, estimate of the efficiency. The estimate is approximate because of two assumptions. Perhaps the worst assumption is that the adsorbable organics transmitted by the particulateremoving filter are typical of those in the atmosphere. It is likely that this filter would quantitatively remove the most strongly adsorbed compounds. The second assumption is that the adsorption efficiency of each filter is constant, i.e. independent of gas-phase concentration and compound type. Because of the first
596
STEVENH. CADLE, PETERJ. GROBLICKIand PATRICIAA. MULAWA
assumption,
the efficiency determined by this method should be a lower limit. To utilize this model, Warren, MI ambient particulate was collected on six sets of Micro-Quartz filters. Four of the filters showed an average collection efficiency of 34 % with a range from 27 to 46 %. One filter, which was very lightly loaded, had an apparent efficiency of 100% and one filter had an efficiency of 73 7;. On these six filters, the average organic carbon loading of the backup filter was 15% of that on the front filter. If one assumes the front filter also has a 347; efficiency for adsorbing these organic carbon compounds, then the adsorbed carbon on the front filter would amount to 23% of the total organic carbon. In the above discussion, it was assumed that the adsorbed organic carbon originated as atmospheric gaseous hydrocarbon. The fact that blanks do adsorb carbon demonstrates that this can happen, but does not eliminate the possibility that some of the organic carbon was released from the particulate on the front filter and was then adsorbed on the backup filter. If this were the case, then the carbon found on the backup filter would be indicative of sampling losses of carbon rather than adsorptive gains in carbon. It is impossible to distinguish between these two phenomena using our data. However, we consider it unlikely that 23 7; of the particulate organic carbon could desorb. This would require major shifts in equilibrium conditions during sampling. To judge the potential magnitude of organic desorption, hydrocarbon-free air (Scott Specialty Gases) was passed through two clean filters, a 47-mm section of a hi-vol filter, and then a final clean filter at a flow rate of 4 &min- 1 for 24 h. Two hi-vol filters were used with organic carbon loadings of 182 and The final filters gained 0.8 and 80.5 pg cm-‘. < 0.2 pg cm-’ organic carbon, respectively. This experiment would not produce as high a concentration of organics in the air as would be found in ambient sampling. But if it is assumed that there is little concentration dependence on the adsorption of organics then it can be concluded that the organic carbon released from the front filter is not a significant contributor to the observed adsorbed organic carbon.
The adsorption of organic carbon is not the only significant problem in collecting and analyzing carbonaceous particulate matter. Uncertainty is also introduced by the fact that there is no method of cleanly separating the organic from the elemental carbon. Various analytical methods provide different operating definitions of organic and elemental carbon. One of the most widely accepted definitions is that elemental carbon is any carbon which is black and organic carbon is the remainder, excluding carbonates. Thermal stripping of the organic carbon has been used since it provides one of the fastest means of separating the organic and elemental carbon and can be coupled
with very sensitive detectors. Unfortunately, optical measurements have shown that some of the organic carbon can carbonize during this kind of separation process (Huntzicker et al., 1981; Cadle and Groblicki, 1981). One thermal method for minimizing carbonization utilizes a two-step separation procedure to remove the organic from the elemental carbon. The sample is heated at 350°C in air and then pyrolyzed in He at 650” C to remove the organic carbon. The remaining elemental carbon is determined by combustion in air. Organic carbon is determined by difference utilizing total carbon analysis on a separate piece of the filter sample. Although pyrolysis in He at 650” C would be expected to remove most organic carbon compounds, it was reasoned that pyrolysis at even higher temperatures may remove some of the polymeric chars formed during carbonization and, thus, could further minimize the problem. Therefore, the effect of pyrolysis temperature on the separation of organic and elemental carbon was investigated, Multiple samples were cut from filters of ambient, diesel and woodburning particulate matter. One set of samples was analyzed for organic and elemental carbon by the pyrolysis in He-oxidation in air procedure at four temperatures; 450,650,800 and 950°C. This is referred to as the onestep procedure. A second set of samples was pretreated by heating in air at 350” C for several minutes before the pyrolysis in He-oxidation in air analysis at the same four temperatures. This procedure is referred to as the two-step separation procedure. Typical results of these analyses are shown in Figs 2-5. Total carbon, organic carbon and elemental carbon determined on separate samples at each temperature by the one step procedure are plotted. Also plotted is the elemental carbon measured at each temperature using the samples which had been pretreated at 350°C in air, i.e., the two-step separation ~ - ------
Total Carbon Organic Carbon (l-step separation procedurel Elemental Carbon (l-step separation procedure1 Elemental Carbon @step separation procedure)
1000
:0Temperature
WI
Fig. 2. Carbon analysis of an ambient particulate sample collected in Warren, Michigan.
Problems in the sampling and analysis of carbon particulate
-
Total Carbon ______ Organic Carbon {l-step separation procedure) --_ ELemental Carbon (l-step separation procadurel Elemental Carbon (2-step separation procedure)
...
o
:.
i
400
I
I
600 Temperature
1000
800 (OCl
Fig. 3. Carbon analysis of an ambient particulate sample collected near Luray, Virginia.
__---..“.
Total Carbon Organic Carbon (l-step separation procedure1 Elemental Carbon Il.-step separation procedure) Elemental Carbon (I-step separation procedure)
“O;I@----,,p------=“____r
“D
e
//
G
1’
p .5 ‘3 9
,’
..S*..,.-.. . . . . . ..I 4 . . . .~~~~tr:
/
i
/^
rt
.;..‘C ,*i
~~
: _a-
_/-
31! ____----= .* ..--
_+--
s-
I
0 400
I
I
1000
600 PC)
600 Temperstore
Fig. 4. Carbon analysis of a wo~buming
~iculate
sample.
5 _ -TotalOrganicCarbon Carbon
3 o .6$
..---..---
Elemental
Carbon
G a: __I__---~------; “N-C /’ r
0 400
I
I
600
800
Temperatuta
I 1000
PC)
Fig. 5. Carbon analysis of a diesel particulate sample.
597
procedure. The bars on individual points show the spread of duplicate analyses. Five facts are apparent from inspection of these figures. First, very little elemental carbon is removed by oxidation at 450” C. Second, there is no significant increase in total carbon when the analysis temperature is raised from 650 to 950” C. Third, the amount of organic carbon removed increases dramatically between 450 and 650°C and continues to increase up to 950” C for a11but the diesel sample. We have no explanation for the relatively high organic carbon value found at 450°C for the diesel sample. It should be noted that this sample is atypical of diesel passenger-car particulate in that it contains much more organic carbon than most samples and thus provides a stringent test for charring. The percentage increase in organic carbon between 650 and 950” C was 25, 65, 41 and - 1, respectively, for the Warren, Luray, wood-burning and diesel samples and was accompanied by a 35, 67, 8 and 10 per cent decrease, respectively, in the elemental carbon. Fourth, the elemental carbon determined after the two-step separation procedure (350” C oxidation followed by pyrolysis) also decreases with increasing temperature for the Warren, Luray and woodburning samples. Elemental carbon was not determined for the diesel sample after the two-step separation procedure since the carbon content was constant and previous results have shown that diesel samples do not carbonize significantly. Fifth and last is the fact that the difference in elemental carbon determined with and without the two-step separation disappears in the ambient samples at 95O”C, but persists in the woodsmoke samples. It is important to determine if the decrease in elemental carbon at high temperature is due to a more efficient removal of the organic carbon or is due to loss of the elemental carbon. Elemental carbon could be lost via reaction with other components of the particulate such as Hz0 liberated from the rest of the sample. A loss in elemental carbon can be determined by measuring the light absorbance of the sample. With this method it is assumed that elemental carbon is black, that heating the samples does not change the specific extinction coefficient of the elemental carbon, that changes in absorbance are not caused by loss of scattering material from the filter, and that the organic carbon does not absorb appreciably. Then, any increase in absorbance of a sample after heating can be attributed to the production of elemental carbon via carbonization of the organics, and any decrease is due to removal of elemental carbon. Absorbance measurements were made on all samples before and after pyrolysis at 650, 800 and 950°C using a modified integrating plate method (Lin et al., 1973; Groblicki et al., 1981). All but the diesel samples showed large increases in absorption even after heating at 950” C. Therefore, it is concluded that increasing the analysis temperature to 950” C does not result in the removal of any e~emen~l carbon, and that some ~rboni~tion of the organics is still occurring.
598
STEVENH. CADLE, PETERJ. GROBLICKIand PATRICIAA. MIJLAWA
The reason for the improved removal of organic carbon at elevated temperatures is not clear. One possible explanation is that more rapid heating results in less carbonization. This would be consistent with studies of the charring of natural materials. For example, Brunner and Roberts (1980) have found that increasing the temperature programming rate from ll”Cmin-’ to 70” C min- l, when heating cellulose to 700°C decreases the char yield from 20 to 12 per cent. Cadle and Groblicki (1981) reported decreased carbonization of ambient particulate samples collected in Denver, Colorado when the heating rate was increased. Therefore, ambient particulate samples from Warren and Luray as well as diesel and fireplace samples were temperature programmed in He to 650” C at rates of 5, 25, 125 and 400” C min- 1 to determine heating rate effects. The samples were analyzed by pyrolysisoxidation at 650” C for elemental carbon. No significant effect of programming rate on the amount of elemental carbon was found for any of the samples. Therefore, it is concluded that the increased removal of organic carbon at elevated temperature is not a heating rate effect for these samples. An alternate explanation is that the increase is caused by the thermal decomposition of carbonoxygen surface complexes and polymeric chars. It is well documented that various surface carbonoxygen complexes exist on almost all types of carbon (Puri, 1970). The amount of oxygen present varies due to several factors, but is frequently 10 % of the carbonaceous material on a mass basis. In general, the oxygen can be pyrolytically removed as H,O, CO and CO2 at temperatures between 300 and looo” C. If it is assumed that all of the oxygen is removed as CO, then decomposition of these surface complexes could account for the observed 8-10 % decrease in elemental carbon with increasing pyrolysis temperature for diesel and woodburning samples. However, it is unlikely that it could account for the much larger decrease in elemental carbon, i.e. 35 and 67 per cent, respectively found with the Warren and Luray samples. The possibility that the increase in organic carbon with increasing temperature was due to the decomposition of carbonates can be ruled out by the fact that no significant increase in total carbon occurred with increasing temperature. Therefore, it appears that these changes must be due in part to the thermal decomposition of polymeric chars. The actual contribution of CO and CO, evolved from surface functional groups to our operationally defined organic- and elemental-carbon results is not known since neither the oxygen content of the elemental carbon nor the decomposition kinetics have been determined for our samples. It is not even clear whether this carbon should be classified as organic or elemental carbon. If the definition of elemental carbon as black carbon is accepted, then these surface complexes should be included in the organic carbon category since they would not be expected to absorb visible light. However, since they are bonded to the elemental carbon, they would not normally be consid-
ered part of the organic fraction. With most ambient samples, heating at 950” C appears necessary for removal of chars. Therefore, the CO and CO, evolved from surface functional groups will probably be classified as organic carbon. In view of the experiments reported here, revised thermal procedures are recommended for the analysis of particulate matter on quartz or silver filters. When the nature of the particulate matter is unknown, the two-step procedure using oxidation-in-air at 350” C followed by pyrolysis and oxidation at 950” C is the best thermal method. The 350” C air-oxidation step is particularly needed for ambient or source samples containing woodsmoke. If woodsmoke is known to be a small constituent of the carbonaceous aerosol, increased speed and accuracy can be obtained using a single-step pyrolysis at 950” C. In the special case of diesel particulate, a single-step pyrolysis at 800” C will minimize loss of surface functional groups, while producing rapid combustion and good peak shapes. Digestion methods
Since our data suggest that all thermal methods are subject to some error due to carbonization, alternate wet digestion procedures for the removal of organics were investigated. Included were a 0.75 h digestion in u.v.-irradiated H,O,, an 18 h ethanolic-KOH digestion, and an 18 h digestion with room-temperature, silver-catalyzed acidic persulfate solution. These solutions are known to be effective oxidizers of organic compounds. Samples were analyzed by the 650” C pyrolysis-oxidation procedure after digestion. Results for seven filter samples of ambient particulate matter are given in Table 3. Samples l-6 were collected in Warren, Michigan on Micro-Quartz filters. Sample 7 was collected in Denver, Colorado on a Spectrograde glass filter. Elemental carbon was determined by two methods. In the first, it was assumed that the total carbon remaining on the filter after digestion was elemental carbon. If this were a valid assumption, then organic and elemental carbon could be determined by commercial total-carbon analyzers. In the second, it was assumed that some organic carbon remained on the filter. This carbon was removed by pyrolysis at 650°C. Elemental carbon was then determined by combustion. Also included in Table 3 are elemental carbon results determined after the three thermal separation procedures; 650” C pyrolysis, 350” C oxidation-650” C pyrolysis and 950” C pyrolysis. Inspection of Table 3 shows that the two-step (350” C/6500 C) thermal-separation procedure gives lower elemental carbon values than the single 650” C procedure. Elemental carbon after 950” C pyrolysis is even lower, consistent with the previous discussion. Total carbon determinations after ethanolic-KOH and u.v.-H,O, digestion give results similar to those obtained by the thermal methods. However, when elemental carbon is determined after both the digestion and 650°C pyrolysis, the results are consistently lower than those obtained after the 950” C pyrolysis.
Problems in the sampling and analysis of carbon particulate
599
There are two possible explanations for this result. First, the organic carbon may not be completely removed during the digestion; second, the elemental carbon may be partially oxidized during digestion to give surface carbon-oxygen complexes which decompose to CO and CO2 upon pyrolysis. It has been shown that treatment of various carbons by oxidizing agents such as sulfate, nitric acid and hydrogen peroxide does increase the amount of surface carbonoxygen complexes that evolve CO and CO, upon heating (Mahajan et al., 1980). Visual evidence that these digestions attacked the elemental carbon comes from the observation that all digested filters were visually lighter than the original sample. It was not possible to quantify this observation by light absorbance measurement of the filters because they were too heavily loaded with light-absorbing particulate matter. However, it was concluded that the elemental carbon determined after digestion and 650” C pyrolysis is probably too low due to oxidation of the black carbon. In all but one sample, the persulfate digestion removed most of the carbon and, thus, was obviously too strong an oxidizing agent. Alternative persulfate procedures utilizing hot-acidic persulfate solutions with and without the silver catalyst were tried for varying digestion times. These appeared to be at least as strong as the procedure used in Table 3 and resulted in the removal of almost all the elemental carbon. Therefore, persulfate digestion does not appear to be a viable technique. There is no apparent advantage of these digestion methods over the simpler thermal methods.
CONCLUSIONS AND RECOMMENDATIONS
The adsorption of organic carbon on glass and quartz fiber filters and silver membrane filters may be a major problem in the determination of organic particulate matter. With some samples, the amount of carbon adsorbed by the backup filter was 30 % of that on the front filter. The actual amount of adsorbed carbon on the front filter may be even greater if adsorption efficiency is taken into consideration. The use of field blanks for organic carbon samples will lower the error, but will not eliminate it. Since adsorption is apt to vary with sampling location and time, it is recommended that double stack filter samples be collected to assess the magnitude of organic carbon adsorption. Irrespective of sampling problems, it remains difficult to unambiguously determine the organic or elemental carbon content of ambient particulate matter. Thermal methods have the advantage of being rapid, but can cause carbonization of the organic carbon, even at 950 “C. The optimum thermal method varies with different samples. When the nature of the sample is unknown, a two-step separation procedure employing heating at 350°C in air followed by a
STEVEN H. CADLB PETER J. GROBLICKI
600
950°C pyrolysis should be used. The digestion methods investigated remove some of the elemental carbon
methods.
and
have
no advantage
over
the thermal
The lack of a clear separation
organic and elemental carbon limits the methods to Drovidint? oDerational definitions . materials. Y
1
between analysis of these
Acknowledgement-We gratefully acknowledge the efforts of C. C. Ang which helped make this project possible.
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