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Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air
SIMULTANEOUS NITRIC ACID, PARTICULATE NITRATE AND ACIDITY MEASUREMENTS IN AMBIENT AIR B. R. AFPEL, S. M. WALL, Y. TOKIWA and M. HAIK Air and Industrial Hygiene Laboratory, Laboratory Services Branch, California Department of Health Services, 2151 Berkeley Way, Berkeley, CA 94704, U.S.A. (First received 6 August 1979 and in~~al form 26 November 1979) Abstract - A laboratory and field study was conducted to evaluate measurement methods for atmospheric nitric acid and to elucidate the formation of artifact particulate nitrate on filter media. Nitric acid collection with nylon and NaCl-impregnated filters was compared employing Teflon pre-filters to remove atmospheric particulate matter. The two collection procedures provided 297% collection of nitric acid at the levels expected under ambient conditions. At higher levels, the efficiency of the nylon filters decreased. Nylon filters supplied by Millipore and Ghia Corporations were shown to be equally effective. Clean Teflon pre-filters permitted passage of z 98% of the HNO, to the collection filters. However, atmospheric particulate matter on the pre-filters retained HNO,. Ammonium nitrate on otherwise clean pm-filters caused positive error in HNO, measurement (and a corresponding loss of particulate nitrate) when clean air was passed through the sample. Dissociation to NH, and HNO, is a likely source of these errors. A lower limit of 22 (ppb)’ is obtained for the NH,NO, dissociation constant at 21°C. Atmospheric sampling was done in Pittsburg, California. On average, results with the two HNO, collection procedures agreed within 3% for levels in the range 0.3-1.5 ppb. Levels of HNO, and NH, were below those needed for saturation with respect to NH,NOJ formation. A glass fiber filter was shown to serve as a total inorganic nitrate sampler, collecting both particulate nitrate and HN03.
1. INTRODUCTION Previous studies (Spicer and Schumacher, 1977 ; Appel et al., 1979a) have compared atmospheric concentrations of particulate nitrate collected with relatively inert (e.g. Teflon, quartz) and glass fiber filters. The latter were deduced to form artifact particutate nitrate by collection of gaseous nitrogen oxides. From comparison of the pattern of artifact particulate nitrate on various filter types, we previously concluded that gaseous nitric acid was the dominant precursor of this artifact (Appel et al., 1979a). However, no simultaneous measurement of ambient nitric acid was attempted. The present study included a laboratory evaluation of methods for nitric acid, and ambient air sampling for this acid and aerosol constituents in Pittsburg, California, about 53 km NE of San Francisco. The location was chosen because of its proximity to stationary emission sources for both sulfuric and nitric acid. Since sampling was done in the winter, relatively few photochemica~ly produced pollutants were expected. In addition to nitrate, aerosols were analyzed for ammonium, sulfate, sulfuric acid, and strong acids. Gaseous nitric acid retained by adsorption on the sampling medium and/or particulate matter should contribute to the particulate strong acid measurement. Parallel sampling for ammonia was also done. The data are evaluated in relation to conventional particulate nitrate sampling and the chemistry of gas-toparticle conversion.
Nitric acid was sampled simultaneously at 20 1min- ’ with 47 mm, NaCI-impregnated Whatman 41 cellulose filters (Okita et al., 1976)and Duralon and Ghia Corp. nylon filters (Spicer, 1979). One-micron pore size, 47mm Teflon pm-filters (Ghia Corp., Zefluor) were used in both cases to remove particulate nitrates. Pre-filters and collection filters were rno~t~ in series in Nuclepore multiple filter holders with 3.2cm I.D., 15cm length polycarbonate inlet tubes. The efficiency of the pre-filters for collection of particulate matter is >99.9o/Qas measured with room air dust with a condensation nuclei counter (John and Reischl, 1978). Following each sampling period, ambient air, heated to a temperature which produced a relative humidity of
and sampler walls. Aqueous extracts were analyzed for ammonia (as ammonium) within 48 h by the indophenoi blue (IPB) procedure (Intersociety Committee, API-IA, 1977). The IPB procedure was shown to have a coefikent of variation of S’,,. and accuracy of98”, using (NH.&S04 aerosol. The glass fiber pre-filters wereanaiyzed for nitrate to permit evaluation of artifact particulate nitrate formation under atmospheric conditions. The resulting nitrate values were compared to the bum of particulate nitrate, as measured with t\+o, 1)trn pore size Zetluor filters sampling in parallel, and nitric acid as measured with collection filters downstream of the same Zefluor filters. Nitric acid values for this comparison were mean results for simultaneous measurement with nylon and NaCl/‘Whatman 41 filters. Particulate nitrate, ammonium sulfate, sulfuric acid and strong p~~rticu~ateacids weresampled at I.1 m3 min- ’ on 203 x 254 mm, acid-washed and heat treated, PailRex 2500 QAO quartz fiber filters (Tanner er crl., 1977). A cyclone removed particles > 3.5pm. The techniques for determination of strong acid and H,SO, are described elsewhere (Appel et iti., 1979b. 1980). Sulfa!e was determined with the AIHL microsulfate method (Hoffer rt (I/., 1979). Ozone and NO, were monitored with a Bendix Model 8002 chemiluminescence analyzer and a Teco Model 14 chemiluminescence analyzer. respectively. The latter was also used to measure HNO, plus NO, in laboratory-~ene~ted mixtures in air. Nitric acid measurement methods were evaluated using techniques and apparatus previously described (hppel rr al, 1979al. The efficiencv of NaCI-imoreenated filters for HNO, colleciion wasdetertkned by sampling with two such filtersin series. The percent of the total collected which was retained on the initial filter was used as a measure of the filter’s efficiency. Efficiency for nylon filters was determined by comparison with NaGimpregnated tiltera sampling m parallel. It was also obtained by addition of ;I nylon filter to the inlet of the NO, monitor, sampling about 70 ppb HNO, containing a small percentage of N02. Since the response of the instrument to HN03 and NO2 was equivalent, and since the NO2 still penetrated the filter, the percentage decrease in response provided a lower limit value of collection efficiency for HNO,. Using 47 mm filters, sampling rates were 20 I min _ ’ except with the NO, monitor which sampled at llmin‘. Ammonium nitrate aerosol was generated with a nebulizer to produce particles 10.3 inn in optical diameter. A manifold permitted three filters to be loaded simultaneously with NH,NO, particles. Ammonium nitrate loadings on the three filters were equal within ?“,,, _I
3. RESUI.‘I’S
AND
DISCUSSION
The filters intended for use in nitric acid sampling were evaluated for retention of NOZ at 90T0 relative humidity. The NO, concentration used, 940 pg m- 3, was close to the upper limit expected in ambient air. Collection of NO, has been reported for NaClimpregnated cellulose fiiters (Okita et ul., 1976). Results with NO, are compared to those obtained with clean air run immediately before (Table 1). Nylon and NaCl-impregnated filters were evaluated both with and without Zefluor pre-filters. The observed NO, retention, expressed as jig mm3 NO;, was < 0.39;. The addition of 70 pg m- ’ ammonia to 940 /dg me3 NO, caused a decrease in NOz retention paralleling results of prior work (Appel et al., 1979a). This decrease may be due to repressed desorption of contaminant nitric acid from the system walls. Thepercentageretention of
NO,
on
these
filter
systems
is concluded
to be
negligible.
The efficiency of the Duralon nylon and NaCl on Whatman 41 filters for nitric acid collection is shown in Fig. I. Values for NaCl-impregnated filters were 97.9 F 1.3’?,,. With nylon filters, efficiencies varied greatly with the amount of HNO, sampled. For ambient levels of HNO,, which can reach 25 big m -’ (Miller and Spicer, 1975), < 250 frg would be collected in an 8h period at 20 1min ‘. Thus approximately equal and complete collection is predicted for both filter types in ambient air. Nitric acid collection efficiencies for Ghia nylon and Duralon nylon filters were compared at two flo% rates and relative humidity values (Table 2). The results show excellent agreement between filter types over the range from about 700 to 34OOpg HNO, collected. For accurate determination of ambient nitric acid, it is essential that the pre-filters retain negligible amounts of the acid. Using clean Teflon (Zefluor) filters. and 200--300 ,q rn- 3 HNO, at 50 or 807; RH, retention of HNO, was <20, when the samplers were heated continuously at 4o’C or for 3 min at 50°C following sampling. However, the presence of atmospheric particulate matter cotlected without size segregation in Berkeley, caused increased retention of the pre-filter (Fig. 2). Accordingly, short term sampling is desirable to minimize error. Nitric acid determination is also subject to positive error ifit is liberated from nitrate salts collected on the pre-filter. Liberation of HNO, by reaction of nitrate salts with H,S04 has been demonstrated (Harker er trl., 1977). In addition, loss of nitrate from the pre-filter may occur by dissociation of NH*NO, to HNO, and NH, (Stelson rr ai., 1979; Doylert al., 1979). The latter process was evaluated in the present study by passing HNO,and NH,-free air through a Teflon filter bearing NH,NO, aerosol. The loss of HNO, was measured by the decrease in nitrate on the Teflon filter and the increase in nitrate on an NaCl-impregnated Whatman 41 filter immediately downstream (Table 3). For trials at SO and 80:‘,, relative humidjty, 45 & I I’iJ of the nitrate was lost from the loaded filter or trapped by the after filter with no consistent trend with relative humidity. The presence of atmospheric particulate matter as well as NH, and HNO, in the ambient air should decrease this source of error. Accordingly the present results are considered upper limit values to the sampling error for nitric acid and particulate nitrate from volatilization. These data also permit estimation of the equilibrium constant for NH,NO, dissociation : NH,NO,
irt NH, + HNO,
K = (NH,)WNOJL
Based on a loss of 45 _+ 1It’,, K 2 tZ(ppb)‘. Since the air stream may not be saturated on passing through the loaded filters, this result is a lower limit value. This result compares to the value cu 10(ppb)2 at 20°C for pure NH,NO, based on extrapolated dissociation pressure values (Steison et al., 1979).
551
Nitric acid, particulate nitrate and acidity measurements Table 1. NO1 retention on clean filters (pg mm3 as NO;)* 940pgm-3NOz 70 fig mm3 NH,
Filter
Clean air?
940 pg mm3 NO,
Duralon nylon$ NaCl/Whatman 41 Ghia nylon Zefluor pre-filter plus Duralon after-filter Zefluor pre-filter plus NaCIW41 after-filter
2.6kO.2 8.5 kO.3
6.1 kO.1 X6*1 5.3 k 0.6 2.5s
2.6kO.l 2.9 + 0.4 1.9kO.2 1.og
1.65
0.55
* RH = SO%,flow rate = 20 I min-‘, T = 2O”C, 6-h exposures. Where errors are shown, results are mean values k 1 u for two filters exposed simultaneously. No filter heating was employed. t The exposure system was cleaned with water and detergent and purged for 72 h with dry, purified air before this trial. f Millipore Corp. These filters are no longer commercially available. 5 Total for both filters.
24-
20
0
Filter
heated
0
Filter
not heated
It 0
-
0 16 -
0
0
Fig. 1. HN03 collection efficiency of nylon and NaCIimpregnated Whatman 41 filters @O-80% RH).
pg particulate/47m
filter
Fig. 2. Retention of HNO, on atmospheric particulate matter (300-500 pg mm3 HNO, at 50-80% RH).
Table 2. Comparison of Duralon and Ghia* nylon filters for HN03 collection HNO, Sampled (as /cg NO; )
Ghia nylon/Duralon
6WJ 136g$,§ 1699t,$ 339% 1) Mean: * Ghia Corp., Pleasanton, CA. 7 lOlmin_‘. $ 50% RH. 5 201min-‘. 1180% RH.
1.13 0.96 1.00 0.99 l.O&O.l
Table 3. Volatilization ofnitric acid from ammonium nitrate aerosol*,t Relative humidity (%)
Loss of nitrate (%)
Gain of nitrate on NaCI/W41 after filter (%)
50 80
40*2 50*3
57*5 33+3
l Initial loading 185+7 pg of 50.3 pm NH4N0, on 47 mm lpm Zefluor filters. t Ammonia- and nitric acid-free air passed through filters at 20 I min-’ at 21°C. Total volume sampled 7.0 m3. Results are means for two trials, expressed as a percent of the initial loading on the Zefluor filter.
552
H. R. Arn,~,
3.2 Results from Calforniu
S. M. WAI
I Y. TOKIWhand M. HAIK
umbirnt uir sumpliny in Pittsburq,
3.2.1 Nitric acid sampler results. Nitric acid results with the two sampling techniques are compared in Table 4. Also shown are mean results _t 1 G for nitrate determinations on the two Zefluor pre-filters. The precision for parallel sampling for particulate nitrate, as measured by the median coefficient of variation, was 5.97,. Eight-hour average concentrations of nitric acid ranged from 0.3 to 1.5 ppb (0.8 to 3.9 )lg m 3 expressed as nitrate). The ratio of means (NaCI on Whatman 41), nylon, was 0.97. Eight-hour average nitric acid values, representing mean results for two samplers, are compared to diurnal variations for NO, NO, and ozone (Fig. 3). There is no obvious correlation between HNO, and 0, such as was reported at other locations (Miller and Spicer, 1975). The levels of both ozone and nitric acid are relatively low, however. Figure 4 includes a comparison of diurnal variations of nitric acid, NH, and particulate constituents NH:, NO;, SOi- and total strong acid, H ‘, as collected with quartz filters. To facilitate mass balance calculation, results are expressed in pequivalents m 3. Diurnal variations of NH,, NHf , SO: and NO, are clearly correlated, consistent with the significance of
Table 4. Nitric acid sampler
results,
Teflon pre-filter NO; *
NH, in aerosol formation. However, concentrations may vary proportionately due to meteorological effects. Ammonium is present in about 507, excess relative to the sum ofNO; and SO:-. This suggests that other anions (e.g. (~‘I- ) are present at substantial concentrations. Prior studies of the volatility of atmospheric particulate NH,f are consistent with the presence of relatively volatile NH,CI (Appel et (I/., 1976). The collection of nitric acid by adsorption on either particulate matter or the filter medium could contribute to the aerosol acidity. Comparison of diurnal variations for HNO, and aerosol H+ shows only moderate correlation, however. Analysis of these filter samples for H,SOI (Appel rr (II., 1980) showed no values above the limit of detection (0.002 to 0.008 pequiv rn-- ‘). Since < lo?,, of the observed H + was due to Fe3 ’ , which titrates as a dibasic strong acid in water, the observed strong acid may be largely a combination of NH,HSO, and adsorbed nitric acid. 3.22 Efict ofNH, and HNO, lerels UII prrrflculrrte nitrate concentrations. To evaluate the influence of the equilibrium between NH,NO, and gaseous f_1NO, and NH,, short term NH, samples were averaged to permit calculation ofconcentration products, (HNO,) (NH,), using 8-h average values. Resulting values
Pittsburg,
HNO,
California
(pg rn--’ as NO;)
on NaCl/W4lt
HNO,
on Duralon
Date
Time (PST)
215-6179
07 15 15 23 23 07
13.6kO.8 4.4k2.1 1.7kO.3
2.2 3.8 0.9
3.3 3.3 0.9
2& 7179
07 15 15-23 23-07
0.4+0.05 2.5: 1.6kO.3
1.7 1.0 0.7
1.5 1.0 0.8
2/l--8/79
07 ~15 15-23 23 -07
4.9 f 0.2 10.4 + 0.4 18.9:
2.6 3.9 3.5
2.6 4.0 4
* Mean values f 1 B for two Zefluor Teflon pre-filters t HNO, on NaGimpregnated Whatman 41 filters. $ Single value. 9 Sample lost.
sampling
in parallel
except as noted.
Nitric acid, particulate nitrate and acidity measurements
553
Gelman A filters. During daylight hours, nitrogen dioxide concentrations were typically between 20 and 40 ppb (40 and 80 pg mm3). The collection of NOz on Gelman A does not appear to be significant, in accord with prior studies (Appel et al., 1979a). 3.3 Conclusions
(1) Nitric acid at ambient concentrations can be measured with > 97% efficiency using either nylon or NaCl-impregnated filters. (2) NOz at ambient levels shows no interference in HNO, sampling at 90% RH, but atmospheric particulate matter on Teflon pre-filters retains HNO,. This implies a need for short term sampling to minimize errors. Loss of NH,NO, from the pre-filter by volatilization can cause a large positive error in HNO, measurements. (3) For low HNO, levels, a glass fiber filter serves as a total nitrate sampler in ambient air sampling, quantitatively retaining all nitric acid and particulate nitrate. Atmospheric nitrogen dioxide did not contribute significantly to artifact particulate nitrate formation on a glass fiber filter. (4) The concentrations of NH, and HNO, at Pittsburg, California were insufficient to cause saturation with respect to NH,NO, aerosol formation. Fig. 4. Diurnal variations for HNO,, NH, and aerosol constituents at Pittsburg, California. ranged from 0.65to 27(ppb)‘, with a median value of 4.3(ppb)‘. Thus the NH, and HNO, levels were generally below the values needed for saturation with respect to NH,NO, formation. This is consistent with the lack of negative correlation between the concentrations of these gases. 3.2.3 Artifact particulate nitrateformation on a glass fiberJilter. Table 5 compares particulate nitrate results from sampling with Gelman A glass fiber filters to the total nitrate observed with Teflon pre-filters and HNO, collectors. On average, the results agree within 3%. Thus, under these conditions, the results are consistent with the complete collection ofnitric acid on
Table 5. Glass fiber as a total nitrate sampler (pgrnm3 as NO;) Teflon pre-filter NO; 13.6 4.4 1.7 0.4 2.5 1.6 4.9 10.4 18.9
Mean HNO, 2.8 3.5 0.9 1.6 1.0 0.8 2.6 3.9 3.5
Ratio of means, Gelman
Glass fiber (Gelman A) NO,
Particulate NO; + HNO, 16.3 7.9 2.3 2.0 3.6 2.4 7.5 14.3 22.4 A NO;/Total
18.2 6.9 2.0 2.3 4.4 2.2 7.7 13.8 19.2 NO;
= 0.97
Acknowledgements The authors express their appreciation to D. Levaggi and the staff of the Bay Area Air Quality Management District for making available the Pittsburg monitoring site for portions of this work. L. Raftery and J. Benzing provided very capable assistance. The financial support of the California Air Resources Board Research Division is gratefully acknowledged. The statements and conclusions in this report are those of the authors and not necessarily those of the California Air Resources Board. The 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 of such product. REFERENCES
Appel B. R., Wesolowski J. J., Hoffer E., Twiss S., Wall S., Chang S. G. and Novakov T. (1976) An intermethod comparison of X-ray photoelectron spectroscopic (ESCA) analysis of atmospheric particulate matter. Int. J. Enuir. Analyt. Chem. 4, 169-181. Appel B. R., Wall S. M., Tokiwa Y. and Haik M. (1979a) Interference effects in sampling particulate nitrate in ambient air. Atmospheric Enoironment 11, 873-876. Appel B. R., Tokiwa Y., Wall S. M., Haik M., Kothny E. L. and Wesolowski J. J. (1979b) Determination of sulfuric acid, total particle-phase acidity and nitric acid in ambient air. Final report to the California Air Resources Board under Contract A6-209-30. Appel B. R., Haik M., Kothny E. L., Wall S. M. and Tokiwa Y. (1980) Evaluation of techniques for sulfuric acid and particulate strong acidity measurements in ambient air. Atmospheric Environment 14, 559-563. Doyle G. J., Tuazon E. C., Graham R. A. Mischke T. M., Winer A. M. and Pitts J. N., Jr. (1979) Simultaneous concentrations of ammonia and nitric acid in a polluted atmosphere and their equilibrium relationship to particulate ammonium nitrate. En&-. Sci. Tech&. 13, 1416-1419. Harker A. B., Richards L. W. and Clark W. E. (1977) The
554
B. R. AIJP~I, S. M. WAI.I. Y. TOKIWA and M. HAIK
effect of atmospheric SO, photochemistry upon observed nitrate concentrations in aerosols. Atmo,spheric Enuironmem II, 87 91. Hoffer E. M., Kothny E. L. and Appel B. R. (1979) Simple method for microgram amounts of sulfate in atmospheric particulates. Atmospheric Environment 13, 3033306. Intersociety Committee, APHA, Method 42604-Ol-72T, Tentative Method of Analysis for Ammonia in the Atmosphere (Indophenol Method). In Methods of‘ Air Sampling and .4na!)sis (Edited by M. Katz) 2nd Edn. 1977, Am. Pub. Health Ass. John W. and Reischl G. (1978) Measurements of the filtration efficiencies of selected filter types. Atmospheric Environment 12, 2015. 2019. Lazrus A. (1979) Nitric acid measurements in western USA. In Current Methods to Measure Atmospheric Nitric Acid urld Nitrate Artifacts (Edited by R. K. Stevens) EPA Report 600/2-79-051. Miller D. F. and Spicer C. W. (1975) Measurement of nitric acid in smog. J. Air Pollut. Control Ass. 25, 940-942. Okita I., Morimoto S., Izawa S. and Konno S. (1976) The measurement of gaseous and particulate nitrates in the atmosphere. Atmospheric Enkmmmt 10, 1085-1089.
Richards L. W., Johnson K. R. and Shepard I,. 5. 1197X) Sulfate aerosol study. Final Report for Coordinating Research Council, Contract No. CAPA- 3-76 (1 76). Spicer C. W. and Schumacher P. (1977) Interference in sampling atmospheric particulate nitrate. .4tmr~spheric~ Environment 11, 873. X76. Spicer C. W. (1979) Measurement of gaseous HNO, by electrochemistry and chemiluminescence. In C’urrcm Muthods to Meusurr Atmospheric Nitric Acid und Nitrau Arfjfacb (Edited by R. K. Stevens) EPA Report 600/2-79051. Stelson A. W., Friedlander S. K. and Seinfeld J. 11. (iY7Y) A note on the equilibrium relationship between ammonia and nitric acid and particulate ammonium nitrate. Atmospheric Environment 13, 369 371. Tanner R. L., Cederwall R., Garber R., Leahy D., Marlow W.. Meyers R., Phillips M. and Newman L. (1977) Separation and analysis of aerosol sulfate species at ambient concentrations. Atmosphrric Enrironment 11, 955 966. Technicon Industrial Systems (1973) Nitrate and nitrite m water and wastewater. Method No. 100-70W. Technicon Instruments Corp.. Tarrytown. NY.
1. INTRODUCTION Previous studies (Spicer and Schumacher, 1977 ; Appel et al., 1979a) have compared atmospheric concentrations of particulate nitrate collected with relatively inert (e.g. Teflon, quartz) and glass fiber filters. The latter were deduced to form artifact particutate nitrate by collection of gaseous nitrogen oxides. From comparison of the pattern of artifact particulate nitrate on various filter types, we previously concluded that gaseous nitric acid was the dominant precursor of this artifact (Appel et al., 1979a). However, no simultaneous measurement of ambient nitric acid was attempted. The present study included a laboratory evaluation of methods for nitric acid, and ambient air sampling for this acid and aerosol constituents in Pittsburg, California, about 53 km NE of San Francisco. The location was chosen because of its proximity to stationary emission sources for both sulfuric and nitric acid. Since sampling was done in the winter, relatively few photochemica~ly produced pollutants were expected. In addition to nitrate, aerosols were analyzed for ammonium, sulfate, sulfuric acid, and strong acids. Gaseous nitric acid retained by adsorption on the sampling medium and/or particulate matter should contribute to the particulate strong acid measurement. Parallel sampling for ammonia was also done. The data are evaluated in relation to conventional particulate nitrate sampling and the chemistry of gas-toparticle conversion.
Nitric acid was sampled simultaneously at 20 1min- ’ with 47 mm, NaCI-impregnated Whatman 41 cellulose filters (Okita et al., 1976)and Duralon and Ghia Corp. nylon filters (Spicer, 1979). One-micron pore size, 47mm Teflon pm-filters (Ghia Corp., Zefluor) were used in both cases to remove particulate nitrates. Pre-filters and collection filters were rno~t~ in series in Nuclepore multiple filter holders with 3.2cm I.D., 15cm length polycarbonate inlet tubes. The efficiency of the pre-filters for collection of particulate matter is >99.9o/Qas measured with room air dust with a condensation nuclei counter (John and Reischl, 1978). Following each sampling period, ambient air, heated to a temperature which produced a relative humidity of
and sampler walls. Aqueous extracts were analyzed for ammonia (as ammonium) within 48 h by the indophenoi blue (IPB) procedure (Intersociety Committee, API-IA, 1977). The IPB procedure was shown to have a coefikent of variation of S’,,. and accuracy of98”, using (NH.&S04 aerosol. The glass fiber pre-filters wereanaiyzed for nitrate to permit evaluation of artifact particulate nitrate formation under atmospheric conditions. The resulting nitrate values were compared to the bum of particulate nitrate, as measured with t\+o, 1)trn pore size Zetluor filters sampling in parallel, and nitric acid as measured with collection filters downstream of the same Zefluor filters. Nitric acid values for this comparison were mean results for simultaneous measurement with nylon and NaCl/‘Whatman 41 filters. Particulate nitrate, ammonium sulfate, sulfuric acid and strong p~~rticu~ateacids weresampled at I.1 m3 min- ’ on 203 x 254 mm, acid-washed and heat treated, PailRex 2500 QAO quartz fiber filters (Tanner er crl., 1977). A cyclone removed particles > 3.5pm. The techniques for determination of strong acid and H,SO, are described elsewhere (Appel et iti., 1979b. 1980). Sulfa!e was determined with the AIHL microsulfate method (Hoffer rt (I/., 1979). Ozone and NO, were monitored with a Bendix Model 8002 chemiluminescence analyzer and a Teco Model 14 chemiluminescence analyzer. respectively. The latter was also used to measure HNO, plus NO, in laboratory-~ene~ted mixtures in air. Nitric acid measurement methods were evaluated using techniques and apparatus previously described (hppel rr al, 1979al. The efficiencv of NaCI-imoreenated filters for HNO, colleciion wasdetertkned by sampling with two such filtersin series. The percent of the total collected which was retained on the initial filter was used as a measure of the filter’s efficiency. Efficiency for nylon filters was determined by comparison with NaGimpregnated tiltera sampling m parallel. It was also obtained by addition of ;I nylon filter to the inlet of the NO, monitor, sampling about 70 ppb HNO, containing a small percentage of N02. Since the response of the instrument to HN03 and NO2 was equivalent, and since the NO2 still penetrated the filter, the percentage decrease in response provided a lower limit value of collection efficiency for HNO,. Using 47 mm filters, sampling rates were 20 I min _ ’ except with the NO, monitor which sampled at llmin‘. Ammonium nitrate aerosol was generated with a nebulizer to produce particles 10.3 inn in optical diameter. A manifold permitted three filters to be loaded simultaneously with NH,NO, particles. Ammonium nitrate loadings on the three filters were equal within ?“,,, _I
3. RESUI.‘I’S
AND
DISCUSSION
The filters intended for use in nitric acid sampling were evaluated for retention of NOZ at 90T0 relative humidity. The NO, concentration used, 940 pg m- 3, was close to the upper limit expected in ambient air. Collection of NO, has been reported for NaClimpregnated cellulose fiiters (Okita et ul., 1976). Results with NO, are compared to those obtained with clean air run immediately before (Table 1). Nylon and NaCl-impregnated filters were evaluated both with and without Zefluor pre-filters. The observed NO, retention, expressed as jig mm3 NO;, was < 0.39;. The addition of 70 pg m- ’ ammonia to 940 /dg me3 NO, caused a decrease in NOz retention paralleling results of prior work (Appel et al., 1979a). This decrease may be due to repressed desorption of contaminant nitric acid from the system walls. Thepercentageretention of
NO,
on
these
filter
systems
is concluded
to be
negligible.
The efficiency of the Duralon nylon and NaCl on Whatman 41 filters for nitric acid collection is shown in Fig. I. Values for NaCl-impregnated filters were 97.9 F 1.3’?,,. With nylon filters, efficiencies varied greatly with the amount of HNO, sampled. For ambient levels of HNO,, which can reach 25 big m -’ (Miller and Spicer, 1975), < 250 frg would be collected in an 8h period at 20 1min ‘. Thus approximately equal and complete collection is predicted for both filter types in ambient air. Nitric acid collection efficiencies for Ghia nylon and Duralon nylon filters were compared at two flo% rates and relative humidity values (Table 2). The results show excellent agreement between filter types over the range from about 700 to 34OOpg HNO, collected. For accurate determination of ambient nitric acid, it is essential that the pre-filters retain negligible amounts of the acid. Using clean Teflon (Zefluor) filters. and 200--300 ,q rn- 3 HNO, at 50 or 807; RH, retention of HNO, was <20, when the samplers were heated continuously at 4o’C or for 3 min at 50°C following sampling. However, the presence of atmospheric particulate matter cotlected without size segregation in Berkeley, caused increased retention of the pre-filter (Fig. 2). Accordingly, short term sampling is desirable to minimize error. Nitric acid determination is also subject to positive error ifit is liberated from nitrate salts collected on the pre-filter. Liberation of HNO, by reaction of nitrate salts with H,S04 has been demonstrated (Harker er trl., 1977). In addition, loss of nitrate from the pre-filter may occur by dissociation of NH*NO, to HNO, and NH, (Stelson rr ai., 1979; Doylert al., 1979). The latter process was evaluated in the present study by passing HNO,and NH,-free air through a Teflon filter bearing NH,NO, aerosol. The loss of HNO, was measured by the decrease in nitrate on the Teflon filter and the increase in nitrate on an NaCl-impregnated Whatman 41 filter immediately downstream (Table 3). For trials at SO and 80:‘,, relative humidjty, 45 & I I’iJ of the nitrate was lost from the loaded filter or trapped by the after filter with no consistent trend with relative humidity. The presence of atmospheric particulate matter as well as NH, and HNO, in the ambient air should decrease this source of error. Accordingly the present results are considered upper limit values to the sampling error for nitric acid and particulate nitrate from volatilization. These data also permit estimation of the equilibrium constant for NH,NO, dissociation : NH,NO,
irt NH, + HNO,
K = (NH,)WNOJL
Based on a loss of 45 _+ 1It’,, K 2 tZ(ppb)‘. Since the air stream may not be saturated on passing through the loaded filters, this result is a lower limit value. This result compares to the value cu 10(ppb)2 at 20°C for pure NH,NO, based on extrapolated dissociation pressure values (Steison et al., 1979).
551
Nitric acid, particulate nitrate and acidity measurements Table 1. NO1 retention on clean filters (pg mm3 as NO;)* 940pgm-3NOz 70 fig mm3 NH,
Filter
Clean air?
940 pg mm3 NO,
Duralon nylon$ NaCl/Whatman 41 Ghia nylon Zefluor pre-filter plus Duralon after-filter Zefluor pre-filter plus NaCIW41 after-filter
2.6kO.2 8.5 kO.3
6.1 kO.1 X6*1 5.3 k 0.6 2.5s
2.6kO.l 2.9 + 0.4 1.9kO.2 1.og
1.65
0.55
* RH = SO%,flow rate = 20 I min-‘, T = 2O”C, 6-h exposures. Where errors are shown, results are mean values k 1 u for two filters exposed simultaneously. No filter heating was employed. t The exposure system was cleaned with water and detergent and purged for 72 h with dry, purified air before this trial. f Millipore Corp. These filters are no longer commercially available. 5 Total for both filters.
24-
20
0
Filter
heated
0
Filter
not heated
It 0
-
0 16 -
0
0
Fig. 1. HN03 collection efficiency of nylon and NaCIimpregnated Whatman 41 filters @O-80% RH).
pg particulate/47m
filter
Fig. 2. Retention of HNO, on atmospheric particulate matter (300-500 pg mm3 HNO, at 50-80% RH).
Table 2. Comparison of Duralon and Ghia* nylon filters for HN03 collection HNO, Sampled (as /cg NO; )
Ghia nylon/Duralon
6WJ 136g$,§ 1699t,$ 339% 1) Mean: * Ghia Corp., Pleasanton, CA. 7 lOlmin_‘. $ 50% RH. 5 201min-‘. 1180% RH.
1.13 0.96 1.00 0.99 l.O&O.l
Table 3. Volatilization ofnitric acid from ammonium nitrate aerosol*,t Relative humidity (%)
Loss of nitrate (%)
Gain of nitrate on NaCI/W41 after filter (%)
50 80
40*2 50*3
57*5 33+3
l Initial loading 185+7 pg of 50.3 pm NH4N0, on 47 mm lpm Zefluor filters. t Ammonia- and nitric acid-free air passed through filters at 20 I min-’ at 21°C. Total volume sampled 7.0 m3. Results are means for two trials, expressed as a percent of the initial loading on the Zefluor filter.
552
H. R. Arn,~,
3.2 Results from Calforniu
S. M. WAI
I Y. TOKIWhand M. HAIK
umbirnt uir sumpliny in Pittsburq,
3.2.1 Nitric acid sampler results. Nitric acid results with the two sampling techniques are compared in Table 4. Also shown are mean results _t 1 G for nitrate determinations on the two Zefluor pre-filters. The precision for parallel sampling for particulate nitrate, as measured by the median coefficient of variation, was 5.97,. Eight-hour average concentrations of nitric acid ranged from 0.3 to 1.5 ppb (0.8 to 3.9 )lg m 3 expressed as nitrate). The ratio of means (NaCI on Whatman 41), nylon, was 0.97. Eight-hour average nitric acid values, representing mean results for two samplers, are compared to diurnal variations for NO, NO, and ozone (Fig. 3). There is no obvious correlation between HNO, and 0, such as was reported at other locations (Miller and Spicer, 1975). The levels of both ozone and nitric acid are relatively low, however. Figure 4 includes a comparison of diurnal variations of nitric acid, NH, and particulate constituents NH:, NO;, SOi- and total strong acid, H ‘, as collected with quartz filters. To facilitate mass balance calculation, results are expressed in pequivalents m 3. Diurnal variations of NH,, NHf , SO: and NO, are clearly correlated, consistent with the significance of
Table 4. Nitric acid sampler
results,
Teflon pre-filter NO; *
NH, in aerosol formation. However, concentrations may vary proportionately due to meteorological effects. Ammonium is present in about 507, excess relative to the sum ofNO; and SO:-. This suggests that other anions (e.g. (~‘I- ) are present at substantial concentrations. Prior studies of the volatility of atmospheric particulate NH,f are consistent with the presence of relatively volatile NH,CI (Appel et (I/., 1976). The collection of nitric acid by adsorption on either particulate matter or the filter medium could contribute to the aerosol acidity. Comparison of diurnal variations for HNO, and aerosol H+ shows only moderate correlation, however. Analysis of these filter samples for H,SOI (Appel rr (II., 1980) showed no values above the limit of detection (0.002 to 0.008 pequiv rn-- ‘). Since < lo?,, of the observed H + was due to Fe3 ’ , which titrates as a dibasic strong acid in water, the observed strong acid may be largely a combination of NH,HSO, and adsorbed nitric acid. 3.22 Efict ofNH, and HNO, lerels UII prrrflculrrte nitrate concentrations. To evaluate the influence of the equilibrium between NH,NO, and gaseous f_1NO, and NH,, short term NH, samples were averaged to permit calculation ofconcentration products, (HNO,) (NH,), using 8-h average values. Resulting values
Pittsburg,
HNO,
California
(pg rn--’ as NO;)
on NaCl/W4lt
HNO,
on Duralon
Date
Time (PST)
215-6179
07 15 15 23 23 07
13.6kO.8 4.4k2.1 1.7kO.3
2.2 3.8 0.9
3.3 3.3 0.9
2& 7179
07 15 15-23 23-07
0.4+0.05 2.5: 1.6kO.3
1.7 1.0 0.7
1.5 1.0 0.8
2/l--8/79
07 ~15 15-23 23 -07
4.9 f 0.2 10.4 + 0.4 18.9:
2.6 3.9 3.5
2.6 4.0 4
* Mean values f 1 B for two Zefluor Teflon pre-filters t HNO, on NaGimpregnated Whatman 41 filters. $ Single value. 9 Sample lost.
sampling
in parallel
except as noted.
Nitric acid, particulate nitrate and acidity measurements
553
Gelman A filters. During daylight hours, nitrogen dioxide concentrations were typically between 20 and 40 ppb (40 and 80 pg mm3). The collection of NOz on Gelman A does not appear to be significant, in accord with prior studies (Appel et al., 1979a). 3.3 Conclusions
(1) Nitric acid at ambient concentrations can be measured with > 97% efficiency using either nylon or NaCl-impregnated filters. (2) NOz at ambient levels shows no interference in HNO, sampling at 90% RH, but atmospheric particulate matter on Teflon pre-filters retains HNO,. This implies a need for short term sampling to minimize errors. Loss of NH,NO, from the pre-filter by volatilization can cause a large positive error in HNO, measurements. (3) For low HNO, levels, a glass fiber filter serves as a total nitrate sampler in ambient air sampling, quantitatively retaining all nitric acid and particulate nitrate. Atmospheric nitrogen dioxide did not contribute significantly to artifact particulate nitrate formation on a glass fiber filter. (4) The concentrations of NH, and HNO, at Pittsburg, California were insufficient to cause saturation with respect to NH,NO, aerosol formation. Fig. 4. Diurnal variations for HNO,, NH, and aerosol constituents at Pittsburg, California. ranged from 0.65to 27(ppb)‘, with a median value of 4.3(ppb)‘. Thus the NH, and HNO, levels were generally below the values needed for saturation with respect to NH,NO, formation. This is consistent with the lack of negative correlation between the concentrations of these gases. 3.2.3 Artifact particulate nitrateformation on a glass fiberJilter. Table 5 compares particulate nitrate results from sampling with Gelman A glass fiber filters to the total nitrate observed with Teflon pre-filters and HNO, collectors. On average, the results agree within 3%. Thus, under these conditions, the results are consistent with the complete collection ofnitric acid on
Table 5. Glass fiber as a total nitrate sampler (pgrnm3 as NO;) Teflon pre-filter NO; 13.6 4.4 1.7 0.4 2.5 1.6 4.9 10.4 18.9
Mean HNO, 2.8 3.5 0.9 1.6 1.0 0.8 2.6 3.9 3.5
Ratio of means, Gelman
Glass fiber (Gelman A) NO,
Particulate NO; + HNO, 16.3 7.9 2.3 2.0 3.6 2.4 7.5 14.3 22.4 A NO;/Total
18.2 6.9 2.0 2.3 4.4 2.2 7.7 13.8 19.2 NO;
= 0.97
Acknowledgements The authors express their appreciation to D. Levaggi and the staff of the Bay Area Air Quality Management District for making available the Pittsburg monitoring site for portions of this work. L. Raftery and J. Benzing provided very capable assistance. The financial support of the California Air Resources Board Research Division is gratefully acknowledged. The statements and conclusions in this report are those of the authors and not necessarily those of the California Air Resources Board. The 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 of such product. REFERENCES
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