Acknowledgment
We thank Dr. Don Schell for providing details of the CO214C analytical procedures.
L i t e r a t u r e Cited (1) Peakall, D. B. Res. Reu. 1972,44, 1-38. (2) Fisher, N. S.; Carpenter, E. J.; Remsen, C. C.; Wurster, C. F. Microb. Ecol. 1974,i, 39-50. (3) Walsh, F.; Mitchell, R. Nature (London) 1974,249, 673-4. (4) Crosby, D. G.; Moilanen, K. W. Bull. Enuiron. Contam. Toxicol. 1973,9, 372-7. (5) Sayler, G. S.; Thomas, R.; Colwell, R. R. Estuarine Coastal Mar. Sci. 1978,6, 553-67. (6) Sundstrom, G.; Hutzinger, 0.; Safe, S. Chemosphere 1976, 267-98. (7) Koeman, J. H.; DeBrauw, M. C. T. N.; DeVos, R. H. Nature (London) 1969,221, 1126-8. (8) Baxter, R. A.; Gilbert, P. E.; Lidgett, R. A.; Mainprize, J. H.; Vodden, H. A. Sci. Total Enuiron. 1975,4, 53-61. 19) Furukawa. K.: Tonomura. K.: Kamibavashi. A. A n d Enuiron. Microbiol. 1978; 35, 223-7. (10) Clark, R. R.; Chian, E. S. K.; Griffin, R. A. Appl. Enuiron. Microbiol. 1979,37, 680-5. (11) Nisbet, I. C. T.; Sarofim, A. F. Enuiron. Health Perspect. 1972, 1, 21-38. I
.
(12) Harvey, G. R.; Steinhauer, W. G.; Miklas, H. P. Nature (London) 1974,252, 387-8. (13) Sayler, G. S.; Shon, M.; Colwell, R. R. Microb. Ecol. 1977, 3, 241-55. (14) Augood, D. R.; Hey, D. H.; Williams, G. H. J. Chem. SOC.1953, 44-50. (15) Reichardt, P. B.; Schuttner, S. E. J. Labelled Compd. Radiopharm. 1976,12, 243-6. (16) Bowden, B. W. Appl. Enuiron. Microbiol. 1977,33, 1229-35. (17) Cole, M. A.; Reichardt, P. B.; Button, D. K. Bull. Enuiron. Contam. Toxicol 1979,23, 44-50. (18) [Jrey, J. C.; Kricher, J. C.; Boylan, J. M. Bull. Enuiron. Contam. Toxicol. 1976,16, 81-5. (19) Sodergren, A. Oikos 1968,19, 126-38. (20) Catelani, D.; Sorlini, C.; Treccani, V. Experientia 1971, 27, 1173-4. (21) Ahmed, M.; Focht, D. D. Can. J. Microbiol. 1973,19, 47-52. (22) Furukawa, K.; Matsumura, F. J . Agric. Food Chem 1976,24, -251 _ _ -6I.
(23) Wong, P. T. S.; Kaiser, K. L. E. Bull. Enuiron. Contam. Toxicol. 1975,13, 249-56. (24) Schulte, E.; Acker, L. Naturwissenschaften 1974,61, 79-80.
Received for review November 13, 1979. Accepted October 6,1980. This work was supported by the National Science Foundation (OCE 76-84266).
Artifact Sulfate and Nitrate Formation at Two Sites in the South Coast Air Basin. A Collaborative Study between the South Coast Air Quality Management District and the California Air Resources Board Samuel Witz" South Coast Air Quality Management District, 9150 Flair Drive, El Monte, California 91731
Jerome G. Wendt Haagen-Smit Laboratory, California Air Resources Board, 9528 Telstar Avenue, El Monte, California 91731
In a field study involving a comparison of Whatman EPM 1000 glass-fiber filters with quartz (Gelman Micro-Quartz, Pallflex Tissuquartz) and Teflon-coated glass-fiber (Pallflex or E R T Teflon-coated) filters, artifact sulfate and nitrate averaged 2.0 pg/m3 (standard deviation, f 2 . 0 pg/m3; range, 0-10.9 pg/m3) and 8.3 pg/m3 (standard deviation, f6.0 pglm3; range, 0-25.8 pg/m3), respectively, for 24-h hi-vol samples collected a t Azusa and Anaheim from September to midNovember 1978. The validity of Coutant's chemical model for predicting artifact sulfate formation under a given set of conditions (ambient SO2 levels, relative humidity, temperature, filter alkalinity, and sampling rate) was confirmed in the present study. However, filter alkalinity, as determined by an aqueous extraction and titration procedure (ASTM D202), does not provide an adequate guideline to the magnitude of artifact nitrate that might be expected. Secondary reactions, involving nitration or nitrosation of filter components, are probably responsible for the high values of artifact nitrate observed.
Introduction
Atmospheric sulfate and nitrate particulate matter have been of interest to air pollution chemists and modelers for many years and are currently receiving considerable attention in the open iiterature. The particulate materials which are formed from SO, and NO, source emissions in poorly understood atmospheric processes have been linked to atmospheric haze, visibility reduction, and acid rainfall with its 0013-936X/81/0915-0079$01.00/0 @ 1981 American Chemical Society
many side effects and, in the case of sulfates, to significant adverse human health effects. The California Air Resources Board underscored the importance of atmospheric particulate sulfates in 1976 with the adoption of an ambient air quality standard. Laboratory studies reported by Coutant ( l ) Spicer , (2),and Appel(3) have suggested that significant errors may exist in the generally accepted sampling methodology for particulate sulfates and nitrates. The errors result from artifacts produced by the adsorption of acidic sulfur and nitrogen-containing gases (e.g., SO2, NO2, HN03, etc.) with subsequent conversion on the collection filters to sulfates and nitrates. Artifact sulfate and nitrate as high as 8 and 11.5 pg/m3, respectively, were reported in these earlier studies (California's Ambient Air Quality Standard for particulate sulfate is 25 pg/m3, 24-h average). The purpose of this study, conducted jointly by the Air Resources Board (ARB) and the South Coast Air Quality Management District (SCAQMD), was to evaluate the magnitude of artifact sulfate and nitrate formation under actual field sampling conditions. Field sampling was conducted a t two locations in the South Coast Air Basin by using filters of varying alkalinity. The two sampling sites, Azusa and Anaheim, were selected for their past history of high sulfate, nitrate, sulfur dioxide, nitrogen dioxide, and water vapor concentrations. Filters included in this study were Whatman glass fiber (EPM lOOO),Gelman Micro-Quartz, Pallflex Tissuquartz (2500 QAO), Pallflex Teflon-coated glass fiber (purchased from Environmental Research and Technology Inc., Westlake, CA, under their designation TX40H120(ERT)), and acetic Volume 16, Number 1, January 1981 79
acid-washed Gelman AE (supplied by the California Air Resources Board). In this study, Whatman glass-fiber filters (EPM lOOO), used by the EPA, ARB, and all California air pollution control districts to sample particulates in 1978,were compared to the other filters known to exhibit low artifact sulfate and nitrate formation. The amount of artifact sulfate reported for Pallflex Teflon-coated glass (used in the Sulfate Regional Experiment) and pH neutral or acid quartz filters has ranged from 0.42 to 0.82 pg/cm2 ( 4 , 5 ) .If these represent saturation values, then, for a 24-h hi-vol(8 X 10 in. or 406-cm2 collection surface) operating at 45 cfm, this would correspond to an artifact sulfate concentration of only 0.09-0.18 pg/m3 and hence can be considered insignificant. With respect to artifact nitrate formation, a value of 1.1pg/cm2 has been reported for Pallflex Teflon-coated glass (SURE) filters ( 4 ) . Both of these values were obtained by the backup-filter method which is claimed to seriously overestimate the amount of artifact nitrate in a sample, presumably because an acidic aerosol tends to concentrate on the prefilter and suppress artifact formation ( 4 ) . One further objective of this investigation was the verification of Coutant’s chemical model, which is potentially useful in predicting the magnitude of artifact sulfate formation ( I ) . In Coutant’s model, artifact sulfate formation depends on the SO2 concentration, relative humidity, temperature, and filter alkalinity; thus
where F is the air volume per unit area (m3/crn2) in a 24-h sample, A is the filter alkalinity (pequiv/cm2),T is the absolute temperature (K), P is the partial pressure of SO2 (ppb), RH is the relative humidity (expressed as a decimal fraction), and z is the blank sulfate content of the filter (pequiv/cm2). The atmospheric parameters necessary to evalute Coutant’s model were monitored a t hourly intervals at each of the two locations during the sampling period. Experimental Section Sampling. Twenty-four-hour hi-vol samples were taken over a period of 3 months from September through midNovember in 1978. Some of the highest sulfate levels generally occur at this time of the year. Samples were taken from 9 a.m. to 9 a.m. at Anaheim and 10 a.m. to 10 a.m. at Azusa. In any one day, two or three filter types might be run in parallel for comparison. Filters were rotated among the hi-vols on a daily basis to minimize sampler bias. Hi-vol samplers from General Metal Works Inc. (Model GMW-2000H) were utilized. The samplers were operated a t a nominal flow rate of 45 cfm. A total of 119 sample pairs were collected for analyses. Sulfate Analysis. One quadrant of each suspended particulate sample collected during the study was extracted in boiling distilled deionized water for a period of 1h. The filter extract of 50 mL was then filtered through a prewashed Whatman No. 42 filter paper into a volumetric cylinder or flask. Each extracted filter quadrant, with the exception of the Gelman Micro-Quartz samples, was washed with three 10-mL portions of boiling distilled deionized water and filtered into volumetric containers. The filter extracts were diluted to 100 mL after cooling to room temperature. The extraction procedure was modified for the Gelman Micro-Quartz samples because of poor sulfate recoveries. The latter resulted from pulping of the filter on boiling, making complete recovery of the extract on subsequent filtration difficult. For the Gelman Micro-Quartz samples the three rinses were increased to 50-mL portions of-boiling distilled deionized water, and the final sample volume was increased to 250 mL. 80
Environmental Science & Technology
The samples were analyzed for their sulfate content by using the AIHL Method 61 procedure (based on turbidimetry) (6).
Nitrate Analysis. The filter extracts collected for sulfate analysis were analyzed for their nitrate content by using the automated cadmium-reduction method (7). Filter Alkalinity and pH. Alkalinities and pHs of the filters were determined by ASTM D-202 procedures (8). The method consists of a hot-water extraction of the filter followed by a pH measurement or an alkalinity-acidity titration of the extract solution. Acid-Washed Filters. Acid-washed filters used in the study were prepared from Gelman AE grade glass-fiber filters. The filters were soaked in 10% acetic acid for 24 h, washed with deionized water to a neutral overflow pH, and oven-dried at 82 “C. Results Artifact Sulfate. The sulfate values obtained on comparison of Whatman EPM 1000 glass-fiber filters with a number of other filters ranged from 0 to 41.3 pg/m3. A Student’s t test of the sulfate values for each set of filters (i.e., Whatman and reference filter) indicated that the differences were significant at the 95% confidence level. The artifact sulfate represents the difference in sulfate values for the Whatman and each of the other filters. The average values for the artifact sulfate were derived in one of two ways: either (a) as the arithmetic mean of the differences in sulfate between the Whatman and reference filter or (b) as the intercept in a linear regression plot of each of the sulfate values in a particular set. Results summarized in Table I indicate that the “average” value for artifact sulfate is of the order of 1-3 pg/m3. Artifact Nitrate. A similar treatment of the nitrate data summarized in Table I1 indicates the following: (a) With nitrate values ranging from 0.4 to 57.5 pg/m3, the artifact nitrate (defined as the difference between the Whatman nitrate value and that of the other test filter) ranged from 0.1 to 29 pg/m3. A Student’s t test indicated that the differences observed Table 1. Filter Comparison of Artifact Sulfate Formation no. 01 samples
Whatman EPM 1000 compared lo
28
Pallflex Teflon-coated glass Gelman quartz Pallflex quartz acid-washed
22 37 32
artifact sulfate, pg/m3 mean max regression difference a difference intercept
2.6
7.9
1.5
2.1 1.6 1.2
3.7 10.9 2.9
1.7 1.1 1.9
a Arithmetic mean of differences in sulfate values between the Whatrnan and reference filter. Intercept of a linear regression plot of the sulfate values for the Whatman (y axis) and reference filters ( x axis).
Table II. Filter Comparison of Artifact Nitrate Formation no. of
samples
28 22 37 32
Whatman EPM 1000 compared to
Pallflex Teflon-coated glass Gelman quartz Pallflex quartz acid-washed
artifact nitrate, pg/m3 mean max regression difference a difference intercept
9.5
23.4
8.5
4.5 9.6 3.7
15.4 25.8 29.0
4.9 8.6 0.5
a Arithmetic mean of differences in nitrate values between the Whatman and reference filter. Intercept of a linear regression plot of the nitrate values for the Whatrnan (yaxis) and reference filters ( x axis).
N
% E L FREO DFFERENCES
1
b
2&
//I
I I I I I I 1 I I \ I I I I I I I I I x,
DIFFERENCE BETWEEN WHATMAN AND REFERENCE FILTER,
!JG/M’
Figure 1. Distribution of differences between Whatman EPM 1000 and Gelman Micro-Quartz, Pallflex tissuquartz, and ERT Teflon-coated glass-fiber filters.
between the Whatman and each of the other filters were statistically significant at the 95% confidence level. (b) “Average” values of the artifact nitrate for the different sets of filters ranged from -1 to 10 pg/m3.
Discussion Artifact Sulfate. In the present study, individual values for artifact sulfate on the Whatman filter ranged from 0 to 10.9 pg/m3 (or 0-37 pg/cm2) depending on the filter used for reference. When the artifact sulfate data from the comparison of the Whatman and the weakly basic and acid filters (i.e., Gelman Quartz, Pallflex Quartz, and Pallflex Teflon-coated glass) are combined, a mean value of 2.0 pg/m3 (standard deviation = 1.97 pg/m3) is obtained. As shown in Figure 1,a
plot of the combined data yields a normal distribution. The artifact sulfate data from the acid-washed filters were excluded from this overall average since the experimentally determined pH of the treated filters indicated that they were still quite alkaline (Le., pH 9.82). These values for artifact sulfate are consistent with the maxima of 3.4-8 pg/m3 reported by other investigators for 24-h hi-vol sampling on alkaline glass-fiber filters. Thus, in a comparison of MSA 1106 BH glass-fiber filter (pH 9.4) with Fluoropore, a Teflon membrane filter, a high of 4.8 pg/m3 artifact sulfate was observed ( 3 ) .Maxima of 7-8 pg/m3 were reported in a study in which SO2 at 0.15 ppm was added to the ambient air of one of a pair of equivalent samples. Field studies involving comparisons between hi-vol alkaline glassfiber (Gelman A) and lo-vol dichotomous fluorocarbon filters produced artifact sulfate values ranging from 0.2 to 4.8 pg/m3 (with means of 2-2.5 pg/m3 (9)).Recent studies by Pierson et al. ( 4 ) ,involving the use of tandem filters in other than 24-h sampling periods, have yielded artifact sulfate values of 29 pg/cm2 (406-cm2 collection surface, 12-h hi-vol, 1068-m3 sampling volume) and 33.7 pg/cm2 (98-h hi-vol, 5571 m3) on alkaline backup filters of Pallflex Tissuquartz 2500 QAOS (pH 9.7) and Gelman A (pH 9.9), respectively. The above values appear comparable to the maximum value of 36.7 pg/cm2 (406 cm2, 24-h hi-vol, 1367 m3), observed on Whatman EPM 1000 glass-fiber filter (pH 9.9) in our study. It should be noted that the studies by Pierson et al. demonstrated that the backup or tandem filter method appears to overestimate the amount of artifact sulfate and nitrate in a sample, owing to the influence of the acidic aerosol in suppressing artifact formation on the prefilter. When neutral filters were used in tandem collections, the H+ concentration was always found to be higher on the prefilter. Coutant’s Chemical Model for Artifact Sulfate. On the basis of Coutant’s model (which takes into account filter alkalinity, ambient SO2 levels as well as sampling rate, tem-
Table 111. Comparison of Observed and Calculated Artifact Sulfate and Nitrate sample date (1978)
slte
9-13
Azusa
9-19 9-10
Azusa Anaheim
9-19
Anaheim
filters compared
artttact sulfate, pg/m3
artlfact nitrate, pg/m3
obsd a
calcd (Coutant *)
obsd *
calcd
What-Gel Q What-Gel Q
3.7 3.1
3.0
2.2
6.4
4.3
2.0
2.9
6.4
2.8
8.2 8.2
What-Gel Q
3.1
3.2
1.1
6.4
3.2 6.1
1.6 6.1
(2.8)
>
Gel Q (7.29)
>
Pal Q (36-acid) Pal Q (4.35)
What > acid-washed > Gel Q > Teflon-ctd > Pal Q What > acid-washed > Gel Q > Teflon-ctd > Pal Q What > acid-washed > Gel Q > Pal Q > Teflon-ctd What > acid-washed > Gel Q > Pal Q > Teflon-ctd
TSP, pg/m3 1150
>
Teflon-ctd
What > acid-washed > Pal Q > Gel Q > Teflon-ctd What > Pal Q > Teflon-ctd > Gel Q > acid-washed
Table V. Relation between Total Suspended Particulates and Artifact Nitrate and Artifact Sulfate Formation
>150
>
(22)
(75)
filter pH
acid-washed
57
74
43
26
37 63
Represents percentage of total number of cases.
In light of t h e s e considerations, it is not surprising that the r a n k i n g of the observed a r t i f a c t s u l f a t e and n i t r a t e or comb i n e d artifact sulfate and n i t r a t e (calculated as total number of equivalents of SO2 or N02/HN03) does n o t always coincide with the experimentally d e t e r m i n e d filter alkalinities or pH (see Table IV). One other f a c t o r w h i c h could upset the correlation between filter alkalinity and artifact sulfate formation is prior adsorption of HN03 or NO2 on the filter surface. T h i s is e x p e c t e d to occur m o r e f r e q u e n t l y at Azusa and A n a h e i m w h e r e the r a t i o i n a m b i e n t N02/S02 concentrations w a s relatively h i g h (average 21/1). The more acidic e n v i r o n m e n t produced b y the prior adsorption of HN03 or NO2 would tend to i n h i b i t the oxidation of SO2 b y 0 2 or O3 since the rate is k n o w n to be pH d e p e n d e n t (oxidation rate decreases w i t h increasing acidity (18)). In a recent observation b y other investigators, decreased levels of artifact n i t r a t e and sulfate were observed at elevated TSP levels, as a result of “poisoning” of the active sites on the filter b y the collected aerosol (19).This observation was not b o r n e out b y o u r data (see Table V). L i t e r a t u r e Cited (1) Coutant, R. W. Enuiron. Sci. Technol. 1977,11, 873.
(2) Spicer, C. W.; Schumacher, P. M.; Kouyoumjian, J. D. W. “Sampling and Analytical Methodology for Atmospheric Nitrates”; Final Report, Battelle Columbus Laboratories, U.S. Environmental Protection Agency, Contract No. 68-02-2213,J a n 1978. (3) Appel, B. R.; Tokiwa, Y.; Wall, S. M.; Hoffer, E. M.; Haik, M.; Wesolowski, J. J. “Effect of Environmental Variables and Sampling Media on the Collection of Atmospheric Sulfate and Nitrate”; Final
Report, Air and Industrial Hygiene Laboratory, California Air Resources Board Contract No. ARB 5-1032, J a n 1978. (4) Pierson, W. R.; Brachaczek, W. W.; Korniski, T. J., Truex, T. J.; Butler, J. W. J . Air Pollut. Control Assoc. 1980,30, 30. (5) Pierson, W. R.; Hammerle R. H.; Brachaczek, W. W. Anal. Chem. 1976,48, 1808. (6) Air and Industrial Hygiene Laboratory, Department of Health, Berkeley, CA, AIHL Method 61, “Determination of Sulfate in HiVol Particulate Samules: Turbidimetric Barium Sulfate Method”. Revised July 1976. (7) US.Environmental Protection Aeencv. “Methods for Chemical Analysis of Water and Wastes”, EFA-660-4-79-020, March 1979. (8) ASTM D202-77 In Annu. Book of ASTM Stand. 1977,27, 8. (9) Stevens, R. K.; Dzubay, T. G.; Mage, D. T.; Burton, R.; Russwurm, G.; Tew, E. “Comparison of Hi-Vol and Dichotomous Sampler Results on Nitrates and Sulfates”, presented to the Division of Environmental Chemistry, 176th National Meeting of the American Chemical Society, Miami Beach, FL, Paper No. 98, Sept 10-15, 1978. (10) Summers, J. C. Enuiron. Sci. Technol. 1979,13, 321. (11) Peri, J. B. J . Phys. Chem. 1965,69, 211. (12) Appel, B. R.; Tokiwa, Y.; Haik, M. Berkeley, CA, May 1980, Air and Industrial Hygiene Laboratory (AIHL) Report CA/DOH/ AIHL/R-213. (13) Forrest, J.; Tanner, R. L.; Spandaw, D.; D’Ottavio, T.; Newman, L. Washington, D.C., 1979, in U.S. Environmental Protection Agency Report 60012-79-051. (14) Harker, A. B.; Richard, L. W.; Clark, W. E. Atmos. Enuiron. 1977,11, 87. (15) Okita, T.; Okita, S. “Measurement of Nitrogen Compounds in the Atmosphere and in Cloud Water in Relation t o Their Transformation in the Atmosphere”, presented to the Division of Environmental Chemistry, 175th National Meeting of the American Chemical Society, Anaheim, CA, Paper No. 140, March 12-17, 1978. (16) Miller, D. F.; Spicer, C. W. J.Air Pollut. Control Assoc. 1975, 25,940. (17) Hubert, B. J.; Lazarus, A. L. “Global Measurements of Nitric Acid Vapor and Particulate Nitrate”, presented to the Division of Environmental Chemistry, 175th National Meeting of the American Chemical Society, Anaheim, CA Paper No. 141, March 12-17, 1978. (18) McKay, H. A. C. Atmos. Enuiron. 1971,5, 7. (19) Pitts, J. N.; Grosjean D. “Detailed Characterization of Gaseous and Size-Resolved Particulate Pollutants a t a South Coast Air Basin Smog Receptor Site: Levels and Modes of Formation of Sulfate, Nitrate and Organic Particulates and Their Implications for Control Strategies”; Final Report, Statewide Air Pollution Research Center, CARB Contracts No. ARB 5-384 and No. A6171-30, Dec 1978. Received for review March 7,1980. Accepted August 26,1980.
Volume 15, Number 1, January 1981 83
