Analysis of Carbonaceous Material in Southern California Atmospheric Aerosols. 2 Bruce R. Appel”, Emanuel M. Hoffer, Evaldo L. Kothny, Stephen M. Wall, and Meyer Haik Air and Industrial Hygiene Laboratory, Laboratory Services Branch, California State Department of Health Services, 2151 Berkeley Way, Berkeley, Calif. 94704
Richard L. Knights Chemistry Department, University of Washington, Seattle, Wash. 98 195
Two- and fourteen-hour high-volume (hi-vol) aerosol samples were collected simultaneously a t Pasadena, Pomona, and Riverside, Calif. on four successive days. Samples were analyzed for primary (CJ, secondary (C,), and elemental carbon (C,) by selective solvent extraction-carbon analysis. High-resolution mass spectrometry was used to provide detailed analysis of organic constituents and to evaluate the selective extraction procedure. The composition of the carbonaceous material a t the three sites was found to be similar with C,/C, 2 2. The C,, as estimated by insoluble carbon, was the most abundant carbon form. Hexanedioic and pentanedioic acids were among the more abundant aerosol constituents of probable secondary origin. Elevated morning levels of C,, dicarboxylic acids, and acid nitrates as well as low morning Br/Pb ratios gave evidence of the retention of secondary organic aerosol from preceding days. Cycloalkenes appear to be the principal secondary organic aerosol precursor. Evidence was obtained of both loss of organics by volatilization and increased collection efficiency for organics with increased particle loading.
The first paper in this series ( I ) reviewed various techniques which have been applied to characterization of atmospheric carbonaceous particulate matter and described a new procedure for such characterization. The procedure described employed a combination of solvent extraction and carbon determinations. It was postulated that cyclohexane was a selective solvent for the extraction of “primary” particle phase organics (i.e., those injected into the atmosphere in the particle state), while total organics could be approximated as those solubilized by successive extraction with benzene and 1:2 v/v methanol-chloroform. Insoluble carbon was used to estimate the elemental carbon present. “Secondary” organics (Le., those formed as a result of chemical reactions in the atmosphere) were determined by subtracting primary from total organics, all expressed as carbon. Insoluble carbon also includes carbonates, if present, as well as carbon in various polymeric forms (e.g., pollen, spores, rubber particles). Samples were analyzed for carbonates but not for carbon in rubber or viable particles. Thus, the results cited for elemental carbon were upper-limit values. The present paper reports on the validation of the selective extraction approach and its application to samples collected simultaneously a t Pasadena, Pomona, and Riverside, within California’s South Coast Air Basin (SCAB). Results of analyses employing high-resolution mass spectrometeric thermal analysis (MSTA) (2, 3 ) on the same samples are also reported. In addition to analyses for carbonaceous species, the samples were analyzed for lead and bromine. The ratio of these elements can be used to assess the age of an air mass when motor vehicle exhaust is the principal source of both elements ( 4 ) . Such information was employed in interpreting diurnal and spatial variations of carbonaceous materials. Ozone was monitored a t each site to provide additional evaluation of the 98
Environmental Science & Technology
correlation between mean daytime ozone concentrations and secondary organics ( I ). The sampling and analytical design permitted assessment of both diurnal and spatial variations in primary and secondary organics and elemental carbon in the SCAB. A comparison of analyses made on short- and long-term samples provided measurement of sampling errors for carbonaceous materials employing hi-vol samplers. A more detailed account of this work is available elsewhere ( 5 ) .
Experimental Materials. Reagent grade cyclohexane, benzene, and methanol and redistilled chloroform were used with nonvolatile carbon contents of 7.8,8.6,41, and 19 pg/100 mL of solvent, respectively. Extraction employed about 60 mL of solvent. Particulate Sample Collection. Gelman AE 8 X 10 in. glass fiber filters were extracted before use by refluxing for 18 h with 1:2 v/v methanol-chloroform. This lowered the total carbon blank value from 3.3 f 0.2 (before treatment) to 2.4 f 0.2 pg/cm2. The filter blank correction represented less than 7% of the total carbon in all 14-h samples and ranged from 11 to 32% in the 2-h total carbon and solvent-soluble carbon samples. Samples were collected using six high-volume samplers calibrated immediately prior to the study. The equivalence of the samplers was established by sampling side-by-side for six 24-h periods for total suspended particulate (TSP).The coefficient of variation for the six trials ranged from 3.3 to 7.1% for a range of TSP from 26 to 49 pg/m3. A t each of the three sampling sites, two samplers were used collecting one 14-h sample and seven 2-h samples, simultaneously. Immediately following collection the filter samples were refrigerated in sealed plastic bags and brought to room temperature just prior to analysis. Samples were collected on eight days in July 1975. Of these, samples taken on four successive days were chosen based upon smog intensity, surface winds data, and inversion height. Days of heavy smog intensity were of especial interest. Ozone and meteorological data for the four days selected are given in Table I and include three days of moderate smog intensity followed by a day of light smog. Solvent Extractions. Filters were cut in half diagonally. Disks totaling 8% of the sample were removed from each half for analyses not requiring solvent extraction. One filter half was extracted with cyclohexane and the other, first with benzene and then with 1:2 v/v methanol-chloroform employing 6-h Soxhlet extraction. Soxhlet extraction was terminated just prior to solvent recycling leaving ca. 5 mL of extract in the pot flask. The extracts were transferred with washings to volumetric flasks and, after diluting to known volumes, 1.0-mL aliquots were transferred to prefired ceramic crucibles resting in prefired nickel boats. Solvent was evaporated by drawing particle-free air slowly over the crucibles a t room temperature. After 2 h, any residual solvent was removed at room temperature by evacuation to ca. 1mmHg for 30 min. The latter procedure was shown to produce no measurable loss in the carbonaceous aerosol extract.
0013-936X/79/0913-0098$01 .OO/O
@
1979 American Chemical Society
Table 1. Ozone and Meteorological Data
1r5y2y”b
dale
Pasadena
m a x 0 3 , ppm a Pomona
Riverside
7/9/75 7/10/75 711 1/75 7/12/75
0.38 0.30 0.28 0.13
0.34 0.28 0.36 0.15
0.18 0.19 0.22 0.15
a
One- or two-hour average.
4.9 3.0 3.5 1.7
Two-hour average at Pasadena. Pacific Daylight Time.
time of max. bscat (PDT)
lnverslon height, m d
1100-1300 1100-1300 1100-1300 0900-1100
690 360 >910
550
At noon, El Monte.
-
Table II. Precision of Carbon Determinations (Coefficient of Variation, % ) a sample collection time, h 2 14
determination
2.6 15.5 11.8 3.3
total C cyclohexane extractable C benzene extractable C methanol-chloroform extractable C
1.9
4.7 4.2 10.3
a Mean results based on duplicate sections cut from 3 to 10 different fiiters.
Table 111. Mean Extraction Efficiency of Solvents for Carbonaceous Material in Atmospheric Samples of total C in extract
Oh
solvent
cyclohexane benzene benzene plus MeOH-CHCI3 _________
17 25 57
Carbon Analysis. Following solvent removal, t h e carbonaceous aerosol extracts were combusted to CO? and the latter determined by gas chromatography using the technique detailed by Mueller et al. (6).The total carbon in the sample was obtained from two 1-in. disks cut from opposite sides of the filter prior to ex1 raction. Accuracy was established by interlaboratory comparison of an atmospheric particulate sample supplied by Euratom, Ispra, Italy. The carbon value obtained was 100%o f t h e interlab mean. Carbonate carbon was determined on two 1-in. disks cut from opposite sides of each 14-h filter by acidification of the samples and COS analysis (6).The procedure was modified by heating the sample to 50 “C to improve recovery of carbonate carbon. Recovery of Na2C03 from filter disks was 86 f 19%. The precision for the determinations varied with filter sampling time and the solvents used, since both influenced
the amount of carbon extracted. The precision of results, after correction for filter blank, is summarized in Table 11. Ozone Determination. Ozone was monitored continuously a t Pasadena and Pomona with Dasibi analyzers calibrated against a UV photometer. The R E M chemiluminescent analyzer employed at Riverside was calibrated by the 2% neutral buffered KI method. The resulting 03 data were corrected t o be comparable to those obtained with the UV calibration method by multiplying all Riverside 0 3 data by 0.78 ( 7 ) . b,,,, Determination. T h e light scattering coefficient was monitored a t Pasadena with calibration as described elsewhere ( I ) . High-Resolution Mass Spectrometry Thermal Analysis (MSTA).Filter samples and solvent extracts, obtained on the two successive days with highest ozone levels, were analyzed by MSTA. The procedure has been described elsewhere (2, 3). Bromine and Lead. One-inch filter disk samples were analyzed for Br and P b by wavelength dispersive X-ray fluorescence analysis. Bromine loss was shown t o be negligible under analytical conditions.
Results arid Discussion Validation of the‘selective Solvent Extraction Technique. The extraction efficiencies for atmospheric particulate carbon of the solvents used are compared in Table 111. Cyclohexane extracted significantly less carbon compared with t h e other solvent systems. T o evaluate t h e hypothesis that cyclohexane is selective for the extraction of primary organics, results of the MSTA of cyclohexane extracts of 14-h filter samples were compared with MSTA for direct analysis of the same samples without extraction (Table IV). MSTA provides information on individual compounds and, in some cases, on classes of compounds. Alkanes plus alkenes and alkylbenzenes were used as model primary organics while the dicarboxylic acids and difunctional nitrates plus nitrites were used as model secondary organics. Comparing the MSTA of cyclohexane extracts and filter samples, cyclohexane enhanced the recovery of alkanes plus alkenes relative to direct filter anal-
___--_____
Table IV. Comparison of Cyclohexane Soluble Organics and Direct Analysis of Filter Samples by MSTA ( W m 3 )a
+
episode
July July July July July July
9 9 9 10
10 10
site
alkanes alkenes CgHi2
filter
alkylbenzenes CgHi2 filter
hexanedioic acid C6H12 filler
pentanedioic acid Cd12 filter
Pasadena 5.8 3.1 0.14 0.38 1.0 1.4 1.6 0.16 Pomona 4.5 3.0 0.37 0.43 0.1 1 1.6 0.05 1.4 0.047 Riverside 2.8 1.3 0.34 0.15 0.47 0.88 0.15 0.84 Pasadena 4.4 2.3 0.67 0.44 1.o 1.8 1.3 Pomona 4.7 2.6 0.10 0.34 0.29 1.1 1.4 0.022 Riverside 1.9 1.o 0.32 0.24 0.18 1.o 0.96 mean ratio total secondary organicsc/total primary organicsd = 1.1 (filters);0.13 (cyclohexane extract)
0.39 0.22 0.17 0.17 0.090 0.032
+
‘’The compounds determined and their identifying mass fragments are: total alkanes plus alkenes (C~HI, CSHg), alkylbenzenes (C7H7 f C~HI, -t- CsHll iC9H12), hexanedioic acid (C5H802),pentanedioic acid ( C ~ H ~ Oand S ) , organic nitrates plus nitrites (C,H,NO, where x = 5, 6,7; y = 7,9, l l ; z = 4, 5). Where no value IS shown, analysis is below limit of detection. The sum of acid nitrates and nitrites and aldehyde nitrates and nitrites. Hexanedioic acid i- pentanedioic acid i- organic nitrates and nitrites. Alkanes alkenes alkylbenzenes.
+
+
--_
Volume 13, Number 1, January 1979
99
y = 0.18+0.4x
p = 0.81
1 .n
0 1.0
0
2 0
IaLrent r x r r . r t t o n .
1 0
secm&ryl*'~ry
Figure 1. Correlation between organic analysis by selective solvent extraction and MSTA
ysis. Alkylbenzenes recovered in cyclohexane were about equal to the level on the filters. On the average, cyclohexane extracted about 40% of the hexanedioic acid and a small fraction of the pentanedioic acid and difunctional nitrates and nitrites. The ratio, total model secondary .organic indicatorshotal model primary organics, for the filters was about 10 times higher than in the cyclohexane extracts. T h e enhanced recovery of alkanes plus alkenes in cyclohexane accounts for, on the average, a value of 1.7 for the above ratio. The remainder reflects the relatively low solubility of secondary organics in cyclohexane. Additional indications of the selectivity of cyclohexane are seen in Figure 1,which plots the ratio of secondary to primary organics by solvent extraction against the ratio of total model secondary organics to total model primary organics by MSTA of 14-h samples. A relatively high correlation coefficient ( p = 0.81) is observed with a small intercept. A slope less than one is consistent with the omission of significant contributors to the total secondary organics from the set of compounds used as model secondary organics in MSTA (e.g., isomers of heptanedioic acid). We conclude t h a t while cyclohexane is not perfectly selective for primary organic aerosol constituents, it provides a useful upper limit to the primary organics. Direct comparison of the selectivity of the solvents by mass spectrometric analysis of solvent extracts was not possible since secondary organics were shown to react with solvent components using methanol-chloroform under extraction conditions (e.g., methyl ester formation by carboxylic acids). As a consequence of the reactivity of aerosol components with ~ _ _ _ _ _ _ _ _ _
polar solvents, the determination of secondary organic carbon is subject to positive error. In the present study this error is estimated to be 110%. It may be noted t h a t previous evaluations of extraction efficiency for organic aerosols, for example ref 8, employing, in part, reactive polar solvents would also be subject to such errors. Depending on the reactants and the use of mass or carbon analyses, errors may be positive or negative. Composition of 14-h Samples. Table V lists the results obtained by the solvent extraction-carbon analysis technique for 14-h samples. Insoluble carbon, as a measure of elemental carbon, was the largest C constituent, averaging somewhat more than 40% of the total C. Secondary organic carbon was 2 to 3 times more abundant than primary. In Riverside, which can receive pollutants formed during transport from locations to the west, there were somewhat more secondary organics in two of the four trials. On three of the four sampling days, the abundance of primary organic carbon was slightly less a t Riverside although the differences are relatively small. I n all cases, the proportion of secondary organic carbon was somewhat lower a t Pomona compared t o the other sites. MSTA results for 14-h filter samples for the 2 days of highest ozone levels were given in Table IV. These results are expressed relative to the concentration of total alkanes plus alkenes in the same sample in Table VI. Dicarboxylic acids are seen to be about as abundant as the model primary organics. The relative concentration of secondary organics a t Riverside was higher than a t the other sites on both days. Similar to the solvent extraction-carbon analysis results, the proportion of secondary organics was significantly below that a t the other sites on July 10. Diurnal Variations of Aerosol Constituents. The diurnal variations of carbonaceous materials on 2 successive days, obtained by the solvent extraction-carbon analysis and MSTA techniques (on filter samples), are shown in Figures 2 through 7. Figure 2 shows diurnal variations for aerosol constituents a t Pasadena on July 9, the first day of a relatively polluted episode. Ozone peaked a t midday, simultaneous with secondary, primary organic, and elemental carbon using the solvent extraction technique. At its peak, total carbon was about 55 kg/m3. Similarly, midday maxima were also found for model primary and secondary organic indicators by MSTA as well as for lead. The midday minimum in the ratio Br/Pb is consistent with the influx of a relatively aged air mass. Figure 3 shows comparable results for the following day a t Pasadena. Midday maxima are again observed for 0 3 , primary, secondary, and elemental carbon, but the peak in secondary carbon precedes the 0 3 peak and the early morning secondary carbon is relatively more abundant. The early morning en-
~
Table V. Composition of 14-h Carbonaceous Samples by Solvent Extraction-Carbon Analysis ( % of Total Carbon) episode
July 9
July 10
July 11
July 12
100
soluble C
primary C
secondary C
17.5 18.5 15.3 13.7 16.8 15.3 19.9 20.8 15.4 20.4 16.5 14.1
40.7 35.6 43.2 44.1 31.7 43.2 47.8 37.8 42.2 35.4 33.5 38.6
41.8 45.9 41.5 42.2
Riverside
58.2 54.1 58.5 57.8 48.5 58.5 67.7 58.6 57.6 55.8 50.1 42.8
mean:
56.5
17.0
39.5
43.5
site
Pasadena Pomona Riverside Pasadena Pomona Riverside Pasadena Pomona Riverside Pasadena Pomona
Environmental Science & Technology
elemental C
51.5 41.5 32.4 41.4 42.4 44.2 50.0 47.2
Table VI. Analysis of 14-h Filter Samples by MSTA a total alkanes
org nitrites
+
episode
July 9 July 9 July 9 July 10 July 10
July a
10
site
alkenes
alkylbenzenes
hexanedioic acid
Pasadena Pomona Riverside Pasadena Pomona Riverside
1.o 1.o 1.o 1.o 1.o 1.o
0.12 0.14 0.12 0.19 0.13 0.24
0.45 0.53 0.68 0.78 0.42 1 .o
pentanedioic acid
0.52
+
X (secondary)
nitrates
0.13 0.07 0.13 0.07 0.03 0.03
0.47 0.65
0.57 0.54 1.o
1.1 1.1 1.5 1.4 1.o
2.0
The sum of hexanedioic and pentanedioic acids and organic nitrates
Results relative to the concentration of total alkanes plus alkenes in the same sample.
pius nitrites.
:::m Tod Pnmary (MSTA)
T o d Carbon
o'6c
Alkylbenzener
3.0 2.0
30 20
8.0 r
T o u l Secondary (MSTA)
Alkanes +Alkenes 2.0
Penunedioic Acid
1.0
0.5
RENO&
TlIb4I4
0.5
7
9
11
13
15
17 19 21
0.10 0.15
7
9
Time (PDT)
11 13
15
17
19
7
21
9
Time (FQT)
11
13
15
17
19
21
Time (FQT)
Figure 2. Diurnal variations for aerosol constituents, Pasadena, July 9, 1975 (pglm3)
::fLdL
~:~&
T o d Grbon
20 30
0.2
32.0 0
Ucmentd G r b o n
Totd Sccondan lMSTAl
n
201
+ Alk'ener
3.0
Rimnry
Grbon 3.0 2.0 1.0
-- --
--
Pcntanedioic Acid
Pb
4nr
RON02+RONO
Secondary 0.12 0.02
7
9
13 15 17 Time (PDT)
11
19
21
7
9
11 13
15
Time (FQT)
17
19
21
7
9
11 13
15
17
19
21
Time (PDT)
Figure 3. Diurnal variations for aerosol constituents, Pasadena, July 10, 1975 (pglm3)
hancement, relative to t h e preceding day, in secondary organics by MSTA is clearly evident. T h e concentration of P b showed early morning, midday, and evening maxima. T h e midday Br/Pb ratio again suggests a relatively aged aerosol. T h e ratio observed in the early morning was about t h e same as t h e preceding day. Results for the same two days at Pomona are given in Figures 4 and 5 . On July 9 t h e peak in secondary organics by solvent extraction followed that for 03,while primary organic carbon showed two weak maxima and elemental carbon peaked earlier in the day. MSTA showed a similar pattern, two
weak maxima for total primary and a single afternoon maximum for secondary organics. The lead concentration peaked in early morning, consistent with the early morning traffic peak, low wind speed, and the expected low mixing height. T h e Br/Pb ratio was fairly constant throughout the day which contrasts greatly with results for the following day. On July 10, both primary and secondary organic particulate levels were greatly elevated in t h e early morning. While the morning P b level was high, the B r P b ratio suggests relatively aged aerosol was being sampled. These results are similar to those in Pasadena. The high Br/Pb ratio Volume
13, Number 1, January 1979
101
0.8
r
Alkvlbenzenes
T o d Carbon
40
35
Alkanes + Alkcnn 10 Primary Carbon
LdTtm 7
9
11 13 15 17 19 Time (pm)
Pcntlnedioic Acid
I H ]
4.01
21
7
11 13 15 17 19 Time (m)
9
21
7
9
11 13 15 Time ( P M )
17
19
21
Figure 4. Diurnal variations for aerosol constituents, Pomona, July 9, 1975 (pg/m3)
40 r
Total Carbon
Alkylbcnzenes
20
6.0
10
5.0
9.0r
SI-
I I
4.0
:L Primary Carbon
RONOpRONO 0.70 0.50 0.10
0.30 7
I I 13 15 17 Tunc ( P M )
9
Time (PDT)
19
21
1
0.10
I 7
9
11 13 15 Time ( P M )
17
19 21
Figure 5. Diurnal variations for aerosol constituents, Pomona, July 10, 1975 (pg/m3) 50 r
T o d Carbon
% d d l 0.4 0.2 0
I
Total Primary (MSTA)
8.0
Alkylbcnrenn
W e m e n d carbon
4.0
3.0
.."
8.0
r
Total Secondary (MSTA)
Primary Carbon Penunedioic
sccond.ry carbon
n
10
7
9
11 13 15 Time
17 19
(PM)
ai
7
9
11 13 15 17 Time (PDT)
I] 19 21
7
9
11 13 15 Time ( P M )
17
19 21
Figure 6. Diurnal variations for aerosol constituents, Riverside, July 9, 1975 (pg/m3)
observed during the evening, 0.7, reflects either nonautomotive sources of Br or analytical error in this sample. Figures 6 and 7 display the diurnal variations a t Riverside 102
Environmental Science & Technology
on these days. For July 9, ozone exhibited a broad maximum with similar diurnal variations for secondary and primary organics, both by solvent extraction and MSTA. A high
Table VII. Comparison of Observed and Calculated 14-h Values a calcd/obsd 14-h analyses b , c BtC
site
CT
Ltl.,
July 9
Pasadena
1.11 (0.04)
0.73 (0.13)
July 9
Pomona
July 9
Riverside
1.21 (0.04) 1.18 (0.04)
July 10
Pasadena
July 10
Pomona
July 10
Riverside
episode
July 11
MCC"
C.
0.52 (0.07)
1.60 (0.18)
1.11 (0.15)
0.75 (0.13) 0.63 (0.11)
0.60 (0.08) 0.53 (0.07)
1.98 (0.22) 1.52 (0.17)
1.15 (0.13) 1.29 (0.16)
1.12 (0.04)
1.02 (0.18)
0.59 (0.08)
1.59 (0.18)
1.08 (0.15)
1.10 (0.04)
0.79 (0.15)
0.67 (0.09)
1.85 (0.21)
0.92 (0.11)
1.26
0.66
(0.05)
(0.13)
0.68 (0.09)
1.65 (0.18)
1.21 (0.15)
1.28
0.64 (0.13)
0.54 (0.07)
1.45 (0.16)
1.78 (0.29)
1.35 (0.05) 1.16 (0.04) 1.34 (0.06) 1.09 (0.04) 1.34 (0.05)
0.77 (0.14) 0.60 (0.12) 0.54 (0.12) 0.67 (0.13) 1.12
0.87 (0.11) 0.59 (0.08) 0.65 (0.09) 0.59 (0.08) 0.58
1.72 (0.19) 1.58 (0.18) 1.68 (0.19)
1.34 (0.19) 1.15 (0.16) 1.46 (0.19)
1.80 (0.20) 1.94
0.93 (0.12) 1.36
(0.15)
(0.08)
(0.22)
(0.15)
1.21
0.74
0.62
1.70
1.23
Pasadena
(0.05) July 11
Pomona
July 11
Riverside
July 12
Pasadena
July 12
Pomona
July 12
Riverside
mean ratio
a "Observed" value from 14-h sample. "Calculated" value is a 14-h mean calculated from seven successive 2-h samples collected simultaneously with the 14-h sample. * CT,total carbon; CEC, cyclohexane soluble C; BEC, benzene soluble C; MCC, methanol-chloroform soluble C; C, elemental C estimated by insoluble carbon, CT - (BEC -1MCC). One u value shown below each ratio. Following extraction for BEC.
a0
T o 4 Grbon
L
Alkylbcnzcner
20
.20
3.0 2.0 1.0
Totd Secondary (MSTA)
r 2.0
2
Penunedioic Acid
1 2.0 1.5 1.0 Secondary Carbon
RONOzcRONO
5 7
9
11 13
Time
15
17
19 21
7
9
(m)
11
13
15
T i e (FVT)
17 19
21
7
9
11
13
15
17
19 21
Time (PWT)
Figure 7. Diurnal variations for aerosol constituents, Riverside, July 10, 1975 (pglm3)
morning P b level coincided with a high B r E b ratio indicating the sampling of relatively fresh combustion aerosol. As in Pasadena and Pomona, this behavior contrasted with that on the following day. On July 10, primary, secondary, and elemental carbon showed early morning maxima. The peak in secondary carbon preceded the 0 3 maxima. T h e P b concentration was elevated in the morning but the Br/Pb ratio indicates a relatively aged aerosol. T h e diurnal variations of alkylbenzenes and total alkanes plus alkenes may be compared to those for P b a t the three sites. Similarity in diurnal variations would be consistent with
automobile exhaust as the principal source of these carbonaceous materials. The general lack of similarity indicates the importance of alternative sources of these organics. Sampling Errors. Table VI1 compares the 14-h sample results t o those calculated from the corresponding seven, successive, 2-h samples for total carbon (CJ, elemental carbon (C&, cyclohexane (CEC), benzene (BEC), and methanolchloroform (MCC) soluble carbon. T h e calculated 14-h average values for Ct consistently exceed those observed a t all sites. Of the fractions contributing to the total carbon, MCC (mean ratio 1.7) and C, (ratio 1.2) showed similar ratios. Volume 13, Number 1, January 1979
103
Ratios > 1.0 for carbonaceous materials may reflect the loss of more volatile constituents not strongly adsorbed on other materials during the prolonged (14-h) sampling, consistent with Della Fiorentina’s observations (9,101. This hypothesis can serve to rationalize the high ratios for MCC. However, a ratio > 1.0 for C, cannot be explained by volatilization and may be indicative of other sources of error. Thus, only ratios > 1.2, as observed for C, (e.g., 1.7 for MCC), are here considered to be indicative of losses of organics due to volatilization. Ratios < 1.0 may reflect both sampling and analytical errors. If gas-phase organics are adsorbed on previously collected, nonvolatile materials (e.g., soot) the efficiency of such collection of gas-phase organics should increase with increased particulate loading. This would result in greater levels of carbon from this source in 14-h samples than calculated from the 2-h samples. Since the atmospheric concentration of hydrocarbons in the gas phase appears to be substantially greater than that of polar organic materials, the collection of gas phase organics by adsorption on particulate matter would be expected to enhance the BEC and CEC fractions. Possible sources of analytical errors leading to ratios < 1.0 were considered as part of quality assurance studies reported elsewhere ( 5 ) .The results suggest these to be of minor importance.
Summary and Conclusions The solvent extraction-carbon analysis approach for estimating primary and secondary organics is a useful technique and correlates reasonably well with results by mass spectrometry. For four days in July 1975 in California’s South Coast Air Basin, the carbonaceous fractions in order of abundance were elemental C > secondary organic C > primary organic C. The extent to which polymeric forms of carbon (e.g., in spores, pollen, tire dust) contributed to the estimate for elemental carbon remains unclear. Field sampling results lead to the additional conclusions: The period July 9-10, 1975, represented a stagnation episode during which aerosols were retained in the SCAB from one day to the next. The indicators of such aerosol retention are elevated concentrations of secondary organics preceding the diurnal ozone peak and low Br/Pb ratios for early morning aerosol samples. Because of the possible retention of aerosols from one day to the next, high correlations between ozone concentration and secondary organic materials may not be generally observed. Similarity in diurnal variations between primary and secondary organic aerosols a t fixed locations suggests pollutant transport and/or dilution as dominant factors, rather than degree of conversion of precursors, in determining con-
104
Environmental Science & Technology
centrations of secondary organics. Sampling for particulate organics is subject to negative errors, probably by volatilization following collection, and to positive errors probably by adsorption of gas-phase organics onto previously collected, relatively nonvolatile materials (e.g., soot). Negative errors appear to involve loss of polar organics, and positive errors, an increase in hydrocarbon content. Sampling errors are similar in magnitude at the three sites used in the present study. Thus, conclusions about spatial variations are probably valid. Further work is needed to assess the influence of such errors on diurnal variations of carbonaceous materials as obtained from short-term samples. Sampling procedures offering improvement over the conventional hi-vol are needed to obtain measures of ambient carbonaceous particulate matter which are less subject to error. Acknowledgments The authors wish to express their appreciation to A. Alcocer,
S. Twiss, D. Grosjean, S. K. Friedlander, S. Heisler, S. Marsh, and the staff of the Southern California Air Pollution Control District for assistance in various phases of this study. Literature Cited (1) Appel, B. R., Colodny, P., Wesolowski,J. J., Enuiron. Sci. Tech-
nol., 10, 359 (1976). (2) Schuetzle, D., Cronn, D., Crittenden, A. L., Charlson, R. J., ibid., 9, 838 (19751. (3) Cronn, D. R., Charlson, R. J., Knights, R. L., Crittenden, A. L., Appel, B. R., Atmos. Enuiron., 11,929 (1977). (4) Robbins, J. A., Snitz, F. L., Enuiron. Sci. Technol., 6, 164 (1972). ( 5 ) Appel, B. R., Hoffer, E. M., Haik, M., Wall, S. M., Kothny, E. L., Knights, R. L., Wesolowski, J. J., “Characterization of Organic Particulate Matter”, Final Report to California Air Resources Board, Contract No. ARB 5-682, NTIS Report PB 279 209/1 WP, 1977. (6) Mueller, P. K., Mosley, R. W., Pierce, L. B., J . Colloid Interface Sei., 39, 235 (1972). (7) Grosjean, D., Statewide Air Pollution Research Center, Riverside, Calif.. Drivate communication. 1976. (8) Grosjean, D., Anal. Chem., 47,797 (1975). (9) Della Fiorentina. H.. Dewiest, F., DeGraeve, J., Atmos. Enuiron., 9, 517 (19751. (10) Rondia, D., Dewiest, F., Della Fiorentina, H., paper presented at the International Conference on Environmental Sensing and Assessment, Paper No. 24-5, Las Vegas, Nev., Sept 1975. Receiued for reuieu: May 30,1978. Accepted September 11, 1978. This uvrk was carried out under the sponsorship of the California Air Resources Board Research Section. The statements and conclusions in this report are those of the authors and not necessarily those of the California Air Resources Board. T h e mention of commercial products, their sources, or use in connection with material reported herein is not to be construed as either an actual or implied endorsement o f such products.
