McVeety, B. D.; Hites, R. A. Atmos. Environ. 1988, 22, 511-536. Readman, J. W.; Mantoura, R. F. C.; Rhead, M. M. Fresenius 2.Anal. Chem. 1984, 319, 126-131. Webster, G. R. B.; Muldrew, D. H.; Graham, J. J.; Sarna, L. P.; Muir, D. C. G. Chemosphere 1986, 15, 1379-1386. Ruepert, C.; Grinwins, A,; Govers, H. Chemosphere 1985, 14, 279-291. Sarna, L. P.; Hodge, P. E.; Webster, G. R. B. Chemosphere 1984, 13,975-983. Shiu, W. Y.;Doucette, W.; Gobas, F. A. P. C.; Andren, A.; Mackay, D. Environ. Sei. Technol. 1988,22, 651-658.
(61) Sijm, D. T. H. M.; Wever, H.; De Vries, P. J.; Opperhuizen, A. Chemosphere 1989,19, 263-266. (62) Webster, G. R. B.; Friesen, K. J.; Sarna, L. P.; Muir, D. C. G. Chemosphere 1985, 14, 609-622. (63) Friesen, K. J.; Vilk, J.; Muir, D. C. G. Chemosphere 1990, 20, 27-32. (64) Miller, M. M.; Wasik, S. P.; Huang, G.-L.; Shiu, W.-Y.; Mackay, D. Environ. Sci. Technol. 1985,19, 522-529.
Received for review October 2, 1990. Revised manuscript received July 9,1991. Accepted July 12,1991. This study was financially supported by the Swedish Environmental Protection Agency.
Time-Averaged Measurements of Peroxyacetyl Nitrate Daniel Grosjean,' Sucha S. Parmar, and Edwin L. Williams, I 1 DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003
rn Time-integrated measurements of PAN have been
carried out by sampling ambient air on alkaline alumina cartridges followed by liquid chromatography analysis of acetate with "negative" UV detection (LC-UV). Two cartridges are operated side by side, one at ambient temperature and the other downstream of a thermal converter that decomposes PAN to products other than acetic acid. The first cartridge measures total acetate (PAN + acetic acid), the second measures acetic acid, and PAN is obtained by difference. Good agreement has been obtained between the dual alumina cartridge-LC-UV method and electron capture gas chromatography. Field results are presented and applications of the method are briefly discussed. Introduction
Measurements of peroxyacetyl nitrate (PAN) in air have involved gas chromatography with electron capture detection (1-5), infrared spectroscopy (6, 7), and luminol detection after conversion to NOz (8). Since PAN is phytotoxic (9),mutagenic (10, II), and a severe eye irritant (I), there is a need to obtain time-averaged information regarding ambient levels of PAN in order to assess, for example, population exposure, potential injury to agricultural crops, and photochemical oxidant damage to forests. While this information could be obtained by the methods mentioned above, the corresponding costs (trained personnel, expensive equipment, data reduction, etc.) are prohibitive. We describe here a simple, cost-effective method that yields time-averaged concentrations of PAN. The method involves the use of standard liquid chromatography or ion chromatography equipment of the type readily available in many analytical, environmental, and other laboratories. In addition to the applications listed above, our method can be used as an inexpensive screening method for measuring PAN at locations where no prior data are available, as well as for cost-effective measurements of PAN with good spatial resolution as part of regional air quality surveys involving numerous monitoring locations. Taking advantage of the decomposition of PAN to acetate in alkaline media (1, 12), our method involves side-by-side sampling using two alkaline cartridges. The first cartridge is operated a t ambient temperature and collects total acetate, i.e., acetic acid + PAN. The second 1864
Envlron. Scl. Technol., Vol. 25, No. 11, 1991
cartridge is operated behind a thermal converter, which decomposes PAN to products other than acetate, and therefore yields only acetic acid. Both cartridges are analyzed for acetate by liquid chromatography with ultraviolet detection, LC-UV (13, 14),and PAN is thus measured by difference. In the following sections, we describe the performance of the method and its application to measurements of 24-h-averaged ambient PAN a t two southern California locations. Experimental Methods
Time-Averaging PAN Sampler. The PAN sampler includes two sampling units housed in a single compact carry-on case, 22 X 10 X 8 in. Each sampling unit includes a '/,-in.-diarneter Teflon tubing inlet, an alkaline-coated alumina cartridge, a calibrated flowmeter (range 0-1.0 L/min.), a differential pressure gage (Minihelic 11, range 0-50 psig, Dwyer Instruments) to measure pressure drop through the cartridge, a small sampling pump, and the corresponding air flow and electrical connections. Both sampling units are connected to the same programmable timer. One sampling unit also includes a thermal converter placed upstream of the sampling cartridge. This thermal converter consists of 2 f t of heating tape (resistance 140 a, Barnstead/Thermolyne) coiled around a l-ft section of '/,-in.-diameter Teflon tubing and wrapped in insulation material. The temperature of the heating tape is controlled by a dimmer switch and is recorded with a digital thermometer (Universal Enterprises, Model DT 2). A 0.25-A fuse, added as a safety feature, goes off (should the dimmer fail) when the converter temperature exceeds 250 O C . The converter temperature obtained varies linearly with current through the heating tape (range tested 90-180 "C). Alumina Cartridge Preparation. Batches of 30-50 basic alumina cartridges (Sep-Pak, Waters-Millipore) are first cleaned with 5 mL of HPLC-grade water, rinsed with 5 mL of HPLC-grade methanol, coated with 3 mL of 4 X N KOH, dried in a vacuum desiccator containing KOH-coated filters to minimize cartridge contamination, sealed with Teflon tape, wrapped in aluminum foil, stored refrigerated in the dark, and shipped to and from field sampling locations in plastic bags containing alkaline-coated 47-mm fiiters that act as passive samplers to minimize cartridge contamination. Liquid Chromatography Analysis. After sampling, the cartridges are eluted with 3 mL of deionized water containing 40 pL of chloroform added as a biocide, and
0013-936X/91/0925-1864$02.50/0
0 1991 American Chemical Society
150-pL aliquots of the eluate are analyzed for acetate by LC-UV. The detection limit has been improved over that obtained in previous work (13,14) by using an anion-exchange column and “negative” UV detection; Le., the analyte absorbs much less than the eluent and elutes as a “negative” peak. The LC-UV components included a SSI Model 300 pump, a Perkin-Elmer LC75 UV-visible detector (190-600 nm), a Valco injection valve, and a Hitachi D-2000 recorder-integrator. The analytical column was a 4.1 X 250 mm Hamilton PRP-X-100 anion-exchange column (styrene-divinylbenzene copolymer with trimethylammonium groups), the eluent was 1 mM KHP in deionized water containing 5% by volume HPLC-grade acetonitrile (pH = 4.5), the eluent flow rate was 1.0 mL/min, the column pressure was 2000 psig, and the detection wavelength was 250 nm. All samples and standards were injected into the sampling loop with a syringe equipped with a 0.2 pm pore size Teflon filter. Under these conditions, acetate is well-resolved from other weak anions such as formate and nitrite, which are potential interferents when ambient air is sampled. Quantitative analysis involved calibrations with external standards, i.e., dilute solutions of acetic acid or sodium acetate. Calibration curves, Le., absorbance (peak height, mm) vs concentration, were constructed with acetate concentrations in the range 0.07-4.0 pg and yielded linear plots with near-zero intercepts, correlation coefficients greater than 0.99, and standard errors on the slopes of 1.7-6.0% (15, 16). Laboratory Tests with PAN. Tests were carried out with PAN alone in purified air and with PAN prepared in situ as a product of NOx-hydrocarbon photooxidation. PAN was synthesized in the liquid phase (17) and was stored refrigerated in the dark as a solution in n-dodecane. Low levels of PAN in purified humid air (RH = 55 f 10%) were obtained in a 1-m3Teflon-lined Plexiglass container by passing pure air through small impingers containing aliquots of the PAN solution in n-dodecane. In situ preparation of PAN involved sunlight irradiation, in a 3.5-m3chamber made of 200A Teflon film,0.25 ppm nitric oxide, and 1 ppm of the reactive olefin 2-methyl-2-butene in purified air (18). The concentration of PAN was measured in all tests with an electron capture gas chromatograph calibrated by use of a chemiluminescence NO, analyzer and two certified NOz permeation tubes (5). Since acetic acid is produced along with PAN in the photooxidation of 2-methyl-2-butene (mainly by the reaction of 2-methyl-2-butene with ozone; see ref 18),alkaline cartridge samples in tests involving PAN prepared in situ were collected downstream of an impinger containing 5 mL of deionized water, which removes acetic acid but not PAN. Field Tests. Field measurements were carried out at two southern California locations, Perris (90 km eastsoutheast of Los Angeles) and Palm Springs (150 km east of Los Angeles) from June 1989 to June 1990. Additional details regarding the sampling protocol and other air quality measurements performed at these locations can be found elsewhere (15).
Results and Discussion Sampling and Analytical Performance. The analytical detection limit (signal/noise ratio 5) was 10 ng for a 150-pL injection, or 200 ng/cartridge. For ambient air typically collected at 0.5 L/min the analytical detection limit corresponds to 0.12 ppb PAN in a 24-h sample, (sample volume 0.72 m3), 0.06 ppb PAN in a 48-h sample, and so on. In practice, detection may also be limited by the variability (30) in cartridge acetate background con-
Table I. Removal of P A N by Alkaline-Coated Alumina Cartridges
matrix airn P M P P M P
cartridge coating
PAN (measd by EC-GC),ppb upstream downstream cartridge removal of of cartridge cartridge efficiencyb
97 f 2 0.1 N KOH 0.1 N KOH 40 f 2 265 f 10 noneC 110 f 5 noneC 4 x 10-3 N K O H 115 f 5 4 x 10-3 N KOH 80 f 2
0.987
(4) (3) (1)
(2) (2) (2)
“P, PAN in purified air; M, PAN from irradiation of 2-methyl2-butene-NO mixture in purified air. Copollutants present along with PAN in this mixture include NOz, ozone, formaldehyde, acetaldehyde, acetone, other carbonyls, nitric acid, acetic acid, and formic acid (18). *Number of cartridges tested in parentheses. Alumina cartridge is alkaline, pH = 8-8.5. Verified for 24 h; all other tests were of 2-6-h duration unless otherwise indicated. eVerified for 17 h. ’Verified for 10 h, sampling flow rate of 0.7 L/min; all other tests involved air flow rates through the cartridge of 0.5-0.6 L/min.
tent, which varies from batch to batch (range 0.5-3.3 pg/cartridge for 13 batches). The background variability (3a) within each batch was 8-12% (average 10%) or 50-320 nglcartridge and corresponded to ambient air detection limits of 0.03-0.19 ppb (24-h sample, volume of air sampled 0.72 m3). Calibration curves constructed from independently prepared standards yielded slopes that were within *5% of each other. Replicate analyses of standard solutions yielded relative standard deviations (RSD) of 1.7-6.0% with a mean value of 3.7%. Replicate analyses of field samples yielded RSD = 0-25 % , with more scatter as expected at low (near-detection) acetate content. The average relative standard deviation for 51 sets of replicates was 3.4% (15,16). Analyte recovery, calculated from the measured acetate content of two consecutive elutions of field samples, averaged 90.0 f 8.2% for 43 field samples (15, 16). Results for three sets of colocated samplers, a measure of overall precision (sampling + analytical), yielded an average RSD = 11.0%. Cartridge Collection Efficiency for Acetic Acid, PAN, and Total Acetate. The collection efficiency of alkaline-coated cartridges for acetic acid has been established (13). The collection efficiency for PAN was measured in the laboratory by taking electron capture gas chromatography readings upstream and downstream of an alumina cartridge. The results are summarized in Table I and indicate essentially complete removal of PAN. The collection efficiency for total acetate (acetic acid + PAN as acetate) was measured in field samples involving the collection of ambient air on two cartridges placed in series and the determination of the acetate content of the upstream and downstream cartridges. The collection efficiencies thus measured for a set of nine field samples averaged 82.2 f 13.7% (15, 16). Comparison of Cartridge-LC-UV and EC-GC Methods. Summarized in Table I1 are the results of a comparison of the two methods employed to measure PAN, EC-GC and alumina cartridge sampling followed by LC-UV. The LC-UV data are for samples collected over 4-24 h at flow rates of 0.3-0.7 L/min, and the EC-GC data are averages of instantaneous readings taken every 30-60 min. The results indicate good agreement between the two methods, with cartridge-LC-UV/EC-GC ratios of 0.92-1.17 (average ratio 1.04). Environ. Sci. Technol., Vol. 25, No. 11, 1991
1885
Table 11. Comparison of PAN Measurements Made Using Two Methods, EC-GC and Alkaline Cartridge-LC-UV
matrix ai?
P P M P P P M
a1u m ina cartridge coating*
PAN, ppb cartridgeEC-GC LC-UVc
0.1 N KOH 0.1 N K O H none none
ratio, cartridge LC-UV/ EC-GC
74 f 2 80 f 5 (2) 97 f 2 114 f 10 (4) 42 f 6 (3)(d) 40 f 2 80f 2 74 f 7 (2) 265 f 10 255 f 10 (2) 110 f 5 105 I 10 (2) 115 f 5 135 f 10 (2) av
1.08 1.17 1.05 0.92 0.96 0.94 1.17 1.04
"See footnote a in Table I. b 4 X N KOH unless otherwise indicated. 'The number of cartridge samples is given in parentheses. dAfter correction for acetic acid present in matrix air.
PAN Decomposition. The decomposition of PAN involves two pathways, Le., the equilibrium R1 and the unimolecular decomposition to methyl nitrate, reaction R2. CHSC(O)OON02
CH3CO3 + NO2
CH3C(O)OON02 e CH3ON02 + C02
(Rl) (R2)
The unimolecular decomposition pathway R2 is slow (19) and under the conditions of our study PAN decomposes according to R1, yielding NO2 and the CH3COBradical. The data in Table I11 indicate that essentially complete decomposition is achieved at T = 120 "C at residence times in the converter that correspond to sampling flow rates of 0.3-1.0 L/min. Measurements of acetate were made in cartridges collected side by side, one without thermal converter and the other downstream of the thermal converter. Acetate concentrations downstream of the thermal converter and of a water impinger agreed well with the
corresponding PAN concentrations measured by EC-GC. When PAN prepared from 2-methyl-2-butene was sampled, acetate concentrations downstream of the thermal converter but without a water impinger were higher than the corresponding PAN concentrations by a few ppb, as expected since some acetic acid is indeed produced in the 2-methyl-2-butene-ozone reaction (18). Thus, the results indicate that no acetic acid is produced when the products of PAN decomposition, which include the CH3C03radical, are sampled on alkaline alumina cartridges. Comparison of PAN Measurements by EC-GC and Using the Time-Averaged PAN Sampler. Side-by-side measurements of PAN were carried out by EC-GC and with the PAN sampler. The results summarized in Table IV indicate good agreement under the conditions tested. The results of the three types of laboratory validation tests are summarized in Figure 1 and indicate good agreement between the two methods. Linear regression of the data yielded a near-unity slope, (0.958 f 0.037), a small positive intercept (4.53 f 3.79), and a correlation coefficient of 0.989. Application to Field Measurements. The PAN sampler was employed to measure PAN a t two southern California locations often impacted by photochemical air pollution transported from the urban Los Angeles area. All samples were collected for 24 h every sixth day over a 1-year period, thus encompassing a wide range of air quality and meteorological conditions. Individual results can be found elsewhere (15). The highest 24-h-averaged values were 9.1 ppb in Perris (second highest 8.0 ppb) and 7.6 ppb in Palm Springs (second highest 7.3 ppb). Most 24-h-averaged values were in the range 1-3 ppb, with late summer high values and late winter low values a t both locations (15). The levels measured in Palm Springs (PAN has not been measured in Perris prior to this study) are consistent with those of 3-15 ppb (6-8 h averages) we measured in 1987 a t the same location (5).
Table 111. Summary of Thermal Converter Performance Studies converter temp, OC
flow rate, L/min
100
0.3 0.3 0.3 0.3 0.3 0.3
200 200 200 200 200 123 200 200
0.3
1.0 1.0
PAN (EC-GC)," ppb upstream downstream of converter of converter 66 66 103 91 100
107 50 102 104
11
3.5 13 7 6 11
98 95
16 12 4 8
water impinger
6 11
" All experiments were carried out with PAN prepared from 2-methyl-2-butene. Table IV. Comparison of EC-GC and Thermal Converter Sampler PAN Measurements
PAN (EC-GC), PPb
sampler side without thermal converter (total acetate)
102"
100
104"
107 86 152
66" 155* 906
111
acetate (LC-UV),' ppb sampler side with thermal converter (acetic acid) 6 11
7 12 9
PAN (by difference)
Ratio, PAN sampler/EC-GC
94 95 79 140 102 av
0.92 0.91 1.19 0.90 1.13 1.01
"Converter temperature 200 OC, flow rate 1.0 L/min. bConverter temperature 125 f 5 "C, flow rate 0.6 L/min. "PAN from sunlightirradiated NO-2-methyl-2-butene mixture in all tests. 1868 Environ. Sci. Technol., Vol. 25, No. 11, 1991
320
280
Tarallo prepared the draft and final versions of the manuscript. Registry No. PAN, 2278-22-0.
4
0
40
80
120
160
PAN, ppb, EC
200
240
280
320
- GC
Figure 1. Comparison of PAN measurement methods: EC-GC and dual alumina cartridge-LC-UV with thermal converter. Open squares, direct camMge measurements of PAN as total acetate; solid triangles,
acetate measurements downstream of converter; solid squares, acetate measurements by difference (total acetate - acetic acid). Acknowledgments
We thank Eric Grosjean for assistance in field operations, liquid chromatography analyses, and data reduction, and the Technical Services Division staff of the South Coast Air Quality Management District for their cooperation in the field measurements. Denise Yanez and Tina
Literature Cited (1) Stephens, E. R. Adv. Environ. Sci. Technol. 1969, I, 119- 146. (2) Darley, E. F.; Kettner, K, A,; Stephens, E. R. Anal. Chem. 1963,35, 589-591. (3) Singh, H. B.; Salas, J. L. Atmos. Environ. 1989,23,231-238. (4) Grosjean, D. Environ. Sci. Technol. 1983, 17, 13-19. (5) Williams, E. L., Q Grosjean, D. Atmos. Environ. 1990,24A, 2369-2377. (6) Tuazon, E. C.; Winer, A. M.; Pitts, J. N., Jr. Environ. Sci. Technol. 1981,15, 1232-1237. (7) Hanst, P. L.; Wong, N. W.; Bragin, J. Atmos. Enuiron. 1982, 16, 969. (8) Blanchard, B.; Shepson, P. B.; So, K. W.; Schiff, H. I.; Bottenheim, J. W.; Gallant, A. J.; Drummond, J. W.; Wong, P. Atmos. Environ. 1990, 24A, 2839-2846. (9) Taylor, 0. C. J. Air Pollut. Control Assoc. 1969,19,347-351. (10) Peak, M. J.; Belser, W. L. Atmos. Environ. 1969,3,385-397. (11) Kleindienst, T. E.; Shepson, P. B.; Edney, E. 0.;Claxton, L. D. Mutat. Res. 1985, 157, 123-128. (12) Grosjean, D.; Harrison, J. Environ. Sci. Technol. 1985,19, 749-752. (13) Grosjean, D.; Van Neste, A.; Parmar, S. S. J. Liq. Chromatogr. 1989,12,3007-3017. (14) Grosjean, D. Atmos. Enoiron. 1990,24A, 2695-2698. (15) Williams, E. L., 11; Grosjean, D. Inland Areas Air Quality Study: a one-year survey of ambient levels of aldehydes, nitric acid and peroxyacetyl nitrate in Palm Springs and Perris, June 1989-June 1990. Final report t o South Coast Air Quality Management District, DGA, Inc., Ventura, CA, November 1990. (16) Grosjean, D.; Williams, E. L. Acidic pollutants in the Sierra Nevada: formic acid and acetic acid. Final report to the University of Nevada Desert Research Institute, DGA, Inc., Ventura, CA, November 1990. (17) Gaffney, J. S.; Fajer, R.; Senum, G. I. Atmos. Environ. 1984, 18, 215-218. (18) Grosjean, D. Environ. Sci. Technol. 1990,24, 1428-1432. (19) Roberts, J. M.; Fajer, R. W. Environ. Sci. Technol. 1989, 23, 945-951.
Received for review December 27, 1990. Revised manuscript received June 17, 1991. Accepted June 28, 1991.
Comparability of Wet-Only Precipitation Chemistry Measurements from the United States’ National Atmospheric Deposition Program (NADP) to Those of the Canadian Network for Sampling Acid Precipitation (CANSAP) Davld S. Blgelow
Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523
A 3-yea.r)six-site, direct comparison between NADP and CANSAP deposition monitoring programs reveals a positive bias in reported CANSAP values over those reported by NADP. The bias is significant at most locations for most measured analytes. Reasons given for the bias include an inadequate seal on the mechanical lid of the CANSAP collector. Poor siting of the sampling equipment is further shown to exacerbate the problem. Introduction
Beginning in 1981, the National Oceanic and Atmospheric Administration (NOAA) sponsored a 3-year, direct 0013-936X/91/0925-1867$02.50/0
comparison of atmospheric deposition monitoring protocols used by the National Atmospheric Deposition Program (NADP) and the Canadian Network for Sampling Acid Precipitation [CANSAP (1-4)].At the time of the comparison, these two deposition monitoring programs represented the primary long-term deposition monitoring initiatives in both the United States and Canada. The purpose of the comparison was to determine if data from these two networks could be used as a single data set to characterize atmospheric deposition in North America. As a single data set, the networks would become a 200+ site monitoring program for North America and would improve the understanding and interpretation of the transboundary
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 11, 1991 1867
