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Integrative technique for detection of atmospheric ammonia
C#-6981/80/06Ol-01
Afmospkric Environment Vol. 14, pp. 64&645. 0 Pergamon Press Ltd. 1980. Printed in Great Britain.
Y.O2.00/0
INTEGRATIVE TECHNIQUE FOR DETECTION OF ATMOSPHERIC AMMONIA W. A. Environmental
MCCLENNY and C. A.
BENNETT,
JR.
Sciences Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A. (First received 30 July 1979 and in jnal form 5 November
1979)
Abstract - Ambient gaseous ammonia (NH,) in the range 0.5-25 ppb (v/v) is collected by pulling sample air
through a tube packed with Chromosorb T*. The ammonia is thermally desorbed by raising the tube temperature to 100°C while flushing the tube with purified air. Transient response of either a chemiluminescence or an opto-acoustic detector is integrated to obtain a signal which is linearly related to the total amount of ammonia collected. The capacity of the collection tubes is such that ammonia loadings are limited to 60 ng and collection times to 40 min; for higher loadings or longer collection times, “breakthrough” occurs and sample is lost. Precision of the procedure is approx. + 3 ng as characterized by 90% confidence intervals based on five replicate measurements at each of several known loadings.
INTRODUCTION The technique for preconcentrating and detecting ammonia is the result of an EPA development pro-
gram to detect ambient NH, at concentrations as low as 0.1 ppb (v/v). Previous efforts included work on a technique using chemiluminescence (Baumgardner et al., 1979), as well as the development of open-path infrared absorption techniques (McClenny and Russwurm, 1978) that may eventually provide a means to measure NH, without sampling and the attendant problems related to sample integrity. Baumgardner et al. (1979) reviewed several techniques, including calorimetry, that have been used for determining NH,. Optical techniques such as Fourier transform spectroscopy (FTS) in the infrared spectral region (Pitts et al.. 1977) and microwave absorption (Morrison et al., 1975) have been employed. The FTSIR technique with a 900 m optical pathlength has been used to measure NH, concentrations as low as several ppb, while the microwave technique is being used for source measurements. Promising new techniques, which appear applicable to ambient measurements, include atmospheric pressure chemical ionization mass spectroscopy (APCIMS) with a projected partsper-trillion (v/v) detection sensitivity in real time (Reid et al., 1977); and collection of ammonia in an acidic solution followed by detection of ammonium ion concentrations by ion chromatography (Mulik et al., 1978) or by ion selective electrode. The present procedure for ammonia monitoring consists of collection, thermal release, detection and analysis. The collection at ambient temperatures, followed by release at an elevated temperature, increases the effective sensitivity of detector systems by a * Mention of commercial products does not constitute endorsement of the product by the U.S. Government. 641
factor of 10-20. Used with chemiluminescence or opto-acoustic detector systems, the procedure requires only ng quantities of ammonia and is effective in measuring ambient ammonia to 0.5 ppb (v/v) levels using 15 min collection periods. These characteristics make it particularly appropriate for sampling from aircraft to determine vertical concentration profiles (Hoe11 et al., 1980) or for measuring short-term variations. EXPERIMENTAL Ammonia collection is performed by drawing samples of ambient air through a Pyrex glass tube, 12.7 cm in length and 0.5 cm i.d., containing 5.0 g Chromosorb T (i.e. polytetrafluoroethylene beads of 30/60 mesh sold by Supelco, Inc.), held in place by quartz wool. Sampling flow rate is 1 Imin-’ and occurs at ambient tempelature. The collection tube is flushed with clean, dry air during desorption of the ammonia and the exit air stream is monitored for ammonia. Thermal desorption is initiated by passing electrical current through nichrome wire which is wrapped around the collection tube. Desorption occurs as the temperature of the beads is increased to 100°C. Ammonia released from the collection tubes has been determined by two entirely different techniques. The optoacoustic technique involves the use of a cylindrical absorption cell, a CO, laser and associated equipment (McClenny et al., 1978) to detect NH, directly. Laser radiation is emitted as a continuous beam which is optically chopped at a selected frequency prior to entering the opto-acoustic cell. The cell is a unique design (McClenny et al., 1979) incorporating a low frequency resonance behavior which enhances response of the system to gases which absorb the laser radiation. In operation, the laser beam is tuned to either the R(30) line (9.2 urn) of the 12C’602 laser or the R(18) line (lO.‘8pmj of’thd ‘3C160, laser. A collection tube is attached to the cell inlet and thermally desorbed. Radiation absorbed by ammonia in passing through the cell is converted to kinetic energy by collisional deactivation. The corresponding variation in air pressure during chopping is detected by a sensitive microphone, electronically amplified, and converted to a d.c. voltage during phase-sensitive detection. The d.c. voltage level is proportional to the ammonia concentration. The second detection technique is chemiluminescence.
642
W. A. MCCLENNV and C. A. BENNETT.JR
Ammonia is converted to nitric oxide (NO) and detected by the light-producing reaction of nitric oxide and added ozone (Volltrauer, 1976). A commercial monitor (Bendix, Model 8001) with minor modifications was used for the measurement (Bendix Corporation, 1975). One of the modifications was the close coupling of the chemiluminescence reaction cell. a platinum converter operated at lOOO”C, and the collection tube. Connecting lines between the collection tube and the NH,-to-NOconverter were heated to minimize adsorption of ammonia. A second modification was to reduce the reaction chamber pressure to approximately 20 torr, thereby achieving an increase in sensitivity (Steffenson and Stedman, 1974). The response from either of the detection systems was electronically integrated using a Linear Instruments Model 252A recorder. Ammonia concentrations are determined by comparing the integrated response of the monitoring instrument with previous calibrations. For performance tests, ammonia was added to ammonia-
I
I
I
I
I
A
free, clean air in a double dilution system (Baumgardner et al., 1979) based on the permeation of ammonia through Teflon (TFE) tubing (O’Keeffe and Ortman, 1966). A temperaturedependent permeation rate was determined by gravimetric weight loss as measured on a Cahn/Ventrol Mode1 RlOO electrobalance. The permeation tube (Metronics, Inc.) was placed in a temperature-regulated cylinder which was flushed by a clean air flow rate of typically 0.5 1 min- ‘. A portion of the flow was further diluted, e.g., 0.1 1 min- ’ was combined with 1.0 I min-r ammonia-free air, to give ammonia concentrations in the l-100 ppb range. The resultant air stream was then mixed with a humidified clean air stream at equal flow rates in a Pyrex manifold.
RESULTS AND DISCUSSION
eficiency of collection The sample loading of ammonia (L,), is proportional to the sample flow rate (F,), the efficiency of collection (E,), the collection time (T), and the ammonia concentrations (C), so that L,(ng) = C(ngl-‘)F,(lmin-‘)E,T(min).
(1)
For a collection tube of practical length, the loading capacity is limited, so that L, has an upper limit, L:, at which the the efficiency of collection begins to decrease, i.e. the value of I$ is the breakthrough loading. The efficiency of collection was measured in two ways. In the first, the amount of ammonia was measured on each of two collection tubes placed in series. A concentration of 23 ppb of ammonia in air at 40% RH was sampled at a flow rate of 1.0 1 mini for 100 s. Less than 5% (within the measurement error) of the ammonia was collected on the second tube. The value of E, was also measured by continuously passing fixed concentrations of ammonia in dry, zero air through a collection tube until breakthrough occurred. Concentrations of 18, 75 and 150 ppb (v/v) NH, were used with resulting breakthrough times of 1800, 680 and 500 s, respectively, and corresponding to breakthrough loadings of 76, 120 and 176ng, respectively. Breakthrough time T* was inversely proportional to the value of the fixed concentrations raised to a power slightly greater than one. A value of T* proportional to C-’ (T*C = constant) would be expected if the breakthrough loading LF were not concentration-dependent. From these results it ap-
Fig. 1. Response ofchemiluminescence analyzer to thermally desorbed NH, (as NO). Integrated response above R, is proportional to the length of the line between integrator on and off. pears that ammonia is migrating along the collection tube during collection so that sampling time is limited even at very low concentrations. Subsequent use of the collection tubes has required the generation of numerous calibration curves ; departure from linearity between response and loading has been consistent with the values of JQ+ determined in the efficiency of collection tests. Release of ammonia The amount of NH, released (&) is proportional to the amount adsorbed and the efficiency of the release (E,), i.e. R, = E,L,. Figure 1 is a copy of a typical recorder trace showing transient chemiluminscence detector response to thermally desorbed NHB. Most of the release occurs over a time period which is short compared with the collection period. In Fig. 1 the release time can be characterized by the response full width at half maximum (FWHM) which is 1.7 min. For a typical collection time of 17min, a preconcentration factor of approx 10 can be achieved. To measure the product E,E,, a real-time concentration of ammonia was monitored directly from the manifold and a constant response vs time was recorded on a chart recorder. In this way a rectangular area, A,, of chart was associated with the product of
Integrative technique for detection of atmospheric ammonia response and a given interval of time At. A tiollection tube was then placed in a position at the gas manifold and sample air was pulled through for a time At. Subsequently, the tube was desorbed to give a response such as that shown in Fig. 1. This response was recorded as before and a second area, Al, was defined. Since the NH, monitor response is proportional to concentration, the ratio AZ/A1 gives E,E,. A test of this type using 75 ppb ammonia (100 s sampling) in humidified air (40% RH at 23°C) gave A2 values of 62.5 units and 59.4 units compared with 59.4 units for A,. Zero response from tubes which had been previously desorbed indicates that all the NH, is desorbed with one thermal desorption and is consistent with a release efIiciency of one.
integrated instrument
643
response (I,), is represented as
I, = AE,E,F,[C + C,]T + ROT + S,
(2)
Ammonia detection There are several differences in the two detection techniques. The chemihuninescence technique utilizing a high temperature platinum converter responds to a large number of N-containing compounds. This is a disadvantage if the N-containing compounds are collected by the Chromosorb T during ambient sampling. However, experiments with synthetic air mixtures of 40% and 80”/, RH containing NOz and of NO in dry air indicate that Chromosorb T does not efficiently pr~on~ntrate these molecules. Similar tests indicate that HNO, and several amines are collected. To prevent collection of HN03, an acid scrubber is used during sampling. The scrubber consists of a glass tube 0.5 cm in diameter and 5 cm in length filled with glass beads which were coated with a dilute solution of KOH (2.5 mg g- ‘). The scrubber passes ammonia but collects nitric acid at low ppb con~ntrations. Amines are collected and thermally desorbed along with NH,. In the opto-acoustic technique the presence of other trace gases which are collected and which also absorb strongly is possible although such an occurrence has not been observed. However to prevent this, a practice of replicate sampling followed by analysis using one laser line which is absorbing and one which is not has been subsequently initiated. This practice should account for any interfering broadband absorbers. Water vapor at concentrations observed in the ambient air constitutes an interference because of the occurrence of weak absorption at the CO2 laser lines most strongly absorbed by NH,. Fortunately, experiments have shown that only small amounts of H,O are retained on the collection tube.
where the constant A converts ng to chart area, C0 denotes an impurity NH3 contribution in the carrier gas, T’ denotes integration time and S denotes the chart area equivalent of either a blank value due to residual NH, on the tube (positive offset) or an NH, demand by the tube (negative offset). The ~libration equation is generally modified from Equation (2) so that I, is plotted as a function of NH3 loading. Assuming C, =fC whenfis a constant, the first term in Equation (2) becomes AE,( 1 + f) ~2~A nonzero value offincreases the slope of the I, vs L, curve. Water used to humidify a sample air stream is a potential source of impurity NH,. Nonzero values of S give a nonzero intercept as does the term RoT’ if the bias level of the integrating recorder is incorrectly set. Since T’ depends on the bias level, the term RoT’ underestimates the true intercept if R, < 0. A cafibration run consisted of the measurement of five replicates at each of three loadings. Assuming a normal distribution of integrated signal responses at constant loadings, 90% confidence levels were calculated using the five integrated responses for each loading. Table 1 tabulates the 90% confidence levels and the results of a linear regression analysis for three calibration runs. Calibration run No. 2 is a repeat of calibration run No. 1 using a different tube. Both calibration No. 1 and No. 2 were run using a dry sample air stream containing 14ppbNH,. In calibration No. 3, the NH, concentration of the synthetic mixture was changed to 3.6 ppb NH, in dry air. The slightly different values of the slope and offset are not statistically significant. Note that the 90”/0confidence levels suggest a measurement precision of approx & 3 ng. The addition of H,O to the sample increases the slope of the calibration line presumably due to NH, released from the water used for humidi~cation. A contribution to the intercept value due to the term S would be evident if the tubes were not initially conditioned. A newly-packed tube has an ammonia demand and is conditioned by sampling directly from vapors accumulated over ammonium hydroxide and then thermaIly desorbing at 100°C. Tubes which are desorbed for several hours also exhibit an NH, demand of approx. 5ng on first loading. However, on subsequent loadings the integrated response is proportional to loading.
Response factors - linearity and reproduc~biZity A number of tests were run to determine the linearity and reproducibility of the chemiluminescence monitor as a function of tube loading. To perform these tests, the NH, delivery system was stabilized at a given concentration and the collection time was varied. The use of the integrating recorder requires the setting of a bias level to offset any standing response level on the chemilu~nes~n~ monitor. The difference between these two levels (R,), is initially adjusted to zero. The
Ambient measurements Ambient ammonia measurements have been made in three Iocations on the east coast. Values detected have ranged from 0.2 to 5.8 ppb as shown in Table 2. Measurements on Cedar Island, NC and near the EPA facility at the Research Triangle Park, NC were performed in 1978 with both the chemiluminescence monitor and the opto-acoustic monitor. Both monitors were used with a TefIon prefilter to eliminate particulate interference, e.g. from (NH&S04. The
644 Table
W. A. M&LENNY and C. A. BENNETT, JR. 1. Representative
Run
Tube
1
1
2
2
3
1
Loading*,
calibration
ng
runs
Cont.,
25.0 37.5 50.0 25.0 37.5 50.0 20.8 31.2 41.7
for NH,,
results of linear determinations
Average of five replicate responses, units of area
ppb
14 14 14 14 14 14 3.6 3.6 3.6
regression
Slope, units of area ng-
21.2 44.9 60.1 29.2 44.0 63.0 31.1 43.5 63.7
analysis
and 90% confidence
level (Cl)
Standard error of slope
Intercept units of area
Standard error of intercept
SOS/”CL, units of area
1.31
0.06
- 5.1
2.5
1.35
0. to
- 5.3
3.8
1.56
0.2 1
- 2.6
6.8
2.7 3.0 3.3 2.6 2.6 3.1 7.7 2.5 3.8
’
_~ * Accuracy
of known
loadings
estimated
as + 5”“.
sampling train for the chemiluminescence monitor also contained a nitric acid scrubber. To prevent Hz0 from condensing on the filter/scrubber yhen sampling at high relative humidities, the prefilter and scrubber are slightly heated; otherwise loss of NH, by dissolution into collected H,O is observed. Measurements shown in Table 2 indicate ammonia
Table 2. Ambient
Date
Site
Technique
17 July 17 July 14 July 4 Aug. 4 Aug. 4 Aug. 4 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 7 Aug. 7 Aug. 8 Aug. 10 Aug. 10 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 14 Sept. 14 Sept.
0 0 0 1
Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi OA Chemi OA Chemi OA Chemi OA OA
Time 1500 1600 1600 1400 1400 1600 1600 2400 2400 1400 1400 1600 1600 1730 1600 2000 2400 1400 1630 1730 1730 1808 1808 1938 1938 1600 1800 .___
Code for sites
1 1 1
1 1 1
1 I I 2 3 3 4 1 5 6 6 6 6 6 6 0 0
values which were determined within minutes of the sample collection, just as in the calibration procedure. Prior to sampling, the particulate filter and HNO, scrubber were conditioned with ambient air for 30 min. Examples of NH, demand developed during sampling on the particulate filter have been noted and require further study.
measurement
of ammonia*
Sample time, Flow min rate, I min -- 1 IO
20 15 30 30 60 60 30 30 30 30 30 30 30 30 30 20 32 30 30 30 30 30 30 30 16 16
1.0 1.0 1.0 0.88 0.88 0.88 0.88 I.0 1.0 1.0 1.0 1.0 1.0 1.o 0.9 0.9 1.0
1.o 0.9 1.0 1.0
1.o 1.0 1.o 1.0
1.o 1.o
Loading,
ng
(* 3ng)
wb
41.9 82.4 43.3 21.0 25.0 25.6 20.0 20.4 21.0 32.0 32.0 16.3 19.5 18.4 64.0 46.0 19.4 16.5 5.6 6.2 8.7 10.3 4.7 6.2 5.6 33.0 27.1
5.8 5.7 4.0 1.1 1.3 0.7 0.5 0.7 0.9 1.5 1.5 0.8 0.9 0.9 3.4 2.4 1.4 0.7 0.3 0.3 0.4 0.5 0.2 0.3 0.3 2.8 2.3
Additional information
replicate replicate replicate replicate replicate
amines
present
rain rain rain rain rain rain rain
: Site O-Ambient,
Research Triangle Park, NC l-Cedar Island, NC 10 ft height 2Xedar Island, NC 6 ft over grass 3-Cedar Island, NC 2 in. over marsh grass at edgewater +-Cedar Island, NC in marsh, 2 in. above ground 5 -Cedar Island, NC 2 ft height over land &Cedar Island, NC, covered entrance while raining. * Readings taken on 11 August during rain represent a lower limit to the actual ambient NH3 concentration, dissolution of NH, in Hz0 on the unheated surface of the particulate filter probably occurred.
since losses by
Integrative technique for detection of atmospheric ammonia CONCLUSIONS
Ammonia in the concentration range OS-25 ppb can be measured by preconcentration on Chromosorb T followed by thermal desorption and detection. Independent analytical techniques of chemiluminescence and opto-acoustics give comparable NH, concentrations at the same location under similar conditions, Further development of the technique is needed to document the effect of increasing the period of time between sampIing and analysis and to identify other potential interferents. The ability to store tubes prior to analysis would allow a procedure for remote sample collection and return to a central analysis system to be developed. Alternatives to the chemiluminescence and opto-acoustic detection techniques may be available that give more specificity to NH,, e.g. thermally desorbed NH3 could be collected as ammonium ions in an acidic solution and analyzed by ion exchange chromatography (Mulik et al,, 1978). REFERENCE
Baumgardner R. E., McClenny W. A. and Stevens R. K. (1979) Optimized Chemiluminescence System for Measuring Atmdspheric Ammonia, EPA Technology Series, Publication EPA 600/2-79-028. Environmental Protection Agency, Research ‘Triangle Park, NC. 27711. Bendix Corporation (1975) Instruction Manual for Model 8101-B, NO, NOz, NO, Analyzer, Process Instruments Division, Lewisburg, WV. Hoe11 J. M., Harward C. N. and McClenny W. A. (1980) Comparison of Remote Infrared Heterodyne Radiometer and Point Measurements of Ambient Ammonia. To be presented at 1980 CLEOS Conf. in San Diego, CA.
~.a 14/6-a
645
McClenny W:A. and Russwurm G. M. (1978) Laser-based, long path monitoring of ambient gases - analysis of two systems. Atmospheric Enuironme$ 12, 1443. McCienny W. A., Russwurm G. M. and Paur R. J. (1978) New Optical Techniques for Detecting Gaseous Air Pollutants.
Paper ThB3-2, Topical Meeting on Atmospheric Spectroscopy, 30 August - 1 September, Keystone, CO. McCfenny W. A., Bennett C. A., Jr., Russwurm G. M. and Richmond R. (1979) Optoacoustic Cell Design for Trace Gas Analysis using a Heimoholtz Resonator. Proc. Topical meeting of Pfioto~oustic Spectroscopy, 1-3 August, WBilP l-4. Morrison R. L., Maddux A. and Hrubesh L. (1975) A Portable Microwave Spectrometer Analyzer for Chemical Contaminants in Air - A Feasibility Study. Lawrence Livermore Laboratory, Report UCRL-51945, University of California, Livermore, CA 94550. Mulik J. D., E.stes E. and Sawicki E. (1978) Ion chromatographic analysis of ammonium ion in ambient aerosols. In Ion Chromatographic Analysis of Environmental Pollutants (Edited by E. Sawicki, J. D. Mulik and E. Wittgenstein), Ann Arbor Sciences, Ann Arbor, MI. O’Keeffe A. E. and Ortman G. C. (1966) Primary standards for trace gas analysis. Anulyt. Chem. 38, 760-764. Pitts J. N., Jr., Finlayson B. F. and Winer A. M. (1977) Optical systems unravel smog chemistry. En&. Sci. ~echno~. 11, 568-573.
Reid N. M., French J. B., Buckley J. A., Lane D. A., Lovett A. M. and Rosenblatt G. (1977) Real-time Analysis of Gaseous Atmospheric Pollutants to the PPT Level using a Mobile API Mass Spectrometer System. Application Note No. 677-P, Sciex, Inc., 55 Glenameron Road, Suite 202, Thornhill, Ontario, Canada. Steffenson D. M. and Stedman D. H. (1974) Optimization of the operating parameters of chemiluminescence nitric oxide detectors. Anulyt. Chem. 46,1704-1709. Volltrauer H. N, (1976) Instruction Manual for Ultra Sensitive NO/NO, Monitor. AeroChem Research Laboratories, Inc., Princeton, NJ.
Afmospkric Environment Vol. 14, pp. 64&645. 0 Pergamon Press Ltd. 1980. Printed in Great Britain.
Y.O2.00/0
INTEGRATIVE TECHNIQUE FOR DETECTION OF ATMOSPHERIC AMMONIA W. A. Environmental
MCCLENNY and C. A.
BENNETT,
JR.
Sciences Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A. (First received 30 July 1979 and in jnal form 5 November
1979)
Abstract - Ambient gaseous ammonia (NH,) in the range 0.5-25 ppb (v/v) is collected by pulling sample air
through a tube packed with Chromosorb T*. The ammonia is thermally desorbed by raising the tube temperature to 100°C while flushing the tube with purified air. Transient response of either a chemiluminescence or an opto-acoustic detector is integrated to obtain a signal which is linearly related to the total amount of ammonia collected. The capacity of the collection tubes is such that ammonia loadings are limited to 60 ng and collection times to 40 min; for higher loadings or longer collection times, “breakthrough” occurs and sample is lost. Precision of the procedure is approx. + 3 ng as characterized by 90% confidence intervals based on five replicate measurements at each of several known loadings.
INTRODUCTION The technique for preconcentrating and detecting ammonia is the result of an EPA development pro-
gram to detect ambient NH, at concentrations as low as 0.1 ppb (v/v). Previous efforts included work on a technique using chemiluminescence (Baumgardner et al., 1979), as well as the development of open-path infrared absorption techniques (McClenny and Russwurm, 1978) that may eventually provide a means to measure NH, without sampling and the attendant problems related to sample integrity. Baumgardner et al. (1979) reviewed several techniques, including calorimetry, that have been used for determining NH,. Optical techniques such as Fourier transform spectroscopy (FTS) in the infrared spectral region (Pitts et al.. 1977) and microwave absorption (Morrison et al., 1975) have been employed. The FTSIR technique with a 900 m optical pathlength has been used to measure NH, concentrations as low as several ppb, while the microwave technique is being used for source measurements. Promising new techniques, which appear applicable to ambient measurements, include atmospheric pressure chemical ionization mass spectroscopy (APCIMS) with a projected partsper-trillion (v/v) detection sensitivity in real time (Reid et al., 1977); and collection of ammonia in an acidic solution followed by detection of ammonium ion concentrations by ion chromatography (Mulik et al., 1978) or by ion selective electrode. The present procedure for ammonia monitoring consists of collection, thermal release, detection and analysis. The collection at ambient temperatures, followed by release at an elevated temperature, increases the effective sensitivity of detector systems by a * Mention of commercial products does not constitute endorsement of the product by the U.S. Government. 641
factor of 10-20. Used with chemiluminescence or opto-acoustic detector systems, the procedure requires only ng quantities of ammonia and is effective in measuring ambient ammonia to 0.5 ppb (v/v) levels using 15 min collection periods. These characteristics make it particularly appropriate for sampling from aircraft to determine vertical concentration profiles (Hoe11 et al., 1980) or for measuring short-term variations. EXPERIMENTAL Ammonia collection is performed by drawing samples of ambient air through a Pyrex glass tube, 12.7 cm in length and 0.5 cm i.d., containing 5.0 g Chromosorb T (i.e. polytetrafluoroethylene beads of 30/60 mesh sold by Supelco, Inc.), held in place by quartz wool. Sampling flow rate is 1 Imin-’ and occurs at ambient tempelature. The collection tube is flushed with clean, dry air during desorption of the ammonia and the exit air stream is monitored for ammonia. Thermal desorption is initiated by passing electrical current through nichrome wire which is wrapped around the collection tube. Desorption occurs as the temperature of the beads is increased to 100°C. Ammonia released from the collection tubes has been determined by two entirely different techniques. The optoacoustic technique involves the use of a cylindrical absorption cell, a CO, laser and associated equipment (McClenny et al., 1978) to detect NH, directly. Laser radiation is emitted as a continuous beam which is optically chopped at a selected frequency prior to entering the opto-acoustic cell. The cell is a unique design (McClenny et al., 1979) incorporating a low frequency resonance behavior which enhances response of the system to gases which absorb the laser radiation. In operation, the laser beam is tuned to either the R(30) line (9.2 urn) of the 12C’602 laser or the R(18) line (lO.‘8pmj of’thd ‘3C160, laser. A collection tube is attached to the cell inlet and thermally desorbed. Radiation absorbed by ammonia in passing through the cell is converted to kinetic energy by collisional deactivation. The corresponding variation in air pressure during chopping is detected by a sensitive microphone, electronically amplified, and converted to a d.c. voltage during phase-sensitive detection. The d.c. voltage level is proportional to the ammonia concentration. The second detection technique is chemiluminescence.
642
W. A. MCCLENNV and C. A. BENNETT.JR
Ammonia is converted to nitric oxide (NO) and detected by the light-producing reaction of nitric oxide and added ozone (Volltrauer, 1976). A commercial monitor (Bendix, Model 8001) with minor modifications was used for the measurement (Bendix Corporation, 1975). One of the modifications was the close coupling of the chemiluminescence reaction cell. a platinum converter operated at lOOO”C, and the collection tube. Connecting lines between the collection tube and the NH,-to-NOconverter were heated to minimize adsorption of ammonia. A second modification was to reduce the reaction chamber pressure to approximately 20 torr, thereby achieving an increase in sensitivity (Steffenson and Stedman, 1974). The response from either of the detection systems was electronically integrated using a Linear Instruments Model 252A recorder. Ammonia concentrations are determined by comparing the integrated response of the monitoring instrument with previous calibrations. For performance tests, ammonia was added to ammonia-
I
I
I
I
I
A
free, clean air in a double dilution system (Baumgardner et al., 1979) based on the permeation of ammonia through Teflon (TFE) tubing (O’Keeffe and Ortman, 1966). A temperaturedependent permeation rate was determined by gravimetric weight loss as measured on a Cahn/Ventrol Mode1 RlOO electrobalance. The permeation tube (Metronics, Inc.) was placed in a temperature-regulated cylinder which was flushed by a clean air flow rate of typically 0.5 1 min- ‘. A portion of the flow was further diluted, e.g., 0.1 1 min- ’ was combined with 1.0 I min-r ammonia-free air, to give ammonia concentrations in the l-100 ppb range. The resultant air stream was then mixed with a humidified clean air stream at equal flow rates in a Pyrex manifold.
RESULTS AND DISCUSSION
eficiency of collection The sample loading of ammonia (L,), is proportional to the sample flow rate (F,), the efficiency of collection (E,), the collection time (T), and the ammonia concentrations (C), so that L,(ng) = C(ngl-‘)F,(lmin-‘)E,T(min).
(1)
For a collection tube of practical length, the loading capacity is limited, so that L, has an upper limit, L:, at which the the efficiency of collection begins to decrease, i.e. the value of I$ is the breakthrough loading. The efficiency of collection was measured in two ways. In the first, the amount of ammonia was measured on each of two collection tubes placed in series. A concentration of 23 ppb of ammonia in air at 40% RH was sampled at a flow rate of 1.0 1 mini for 100 s. Less than 5% (within the measurement error) of the ammonia was collected on the second tube. The value of E, was also measured by continuously passing fixed concentrations of ammonia in dry, zero air through a collection tube until breakthrough occurred. Concentrations of 18, 75 and 150 ppb (v/v) NH, were used with resulting breakthrough times of 1800, 680 and 500 s, respectively, and corresponding to breakthrough loadings of 76, 120 and 176ng, respectively. Breakthrough time T* was inversely proportional to the value of the fixed concentrations raised to a power slightly greater than one. A value of T* proportional to C-’ (T*C = constant) would be expected if the breakthrough loading LF were not concentration-dependent. From these results it ap-
Fig. 1. Response ofchemiluminescence analyzer to thermally desorbed NH, (as NO). Integrated response above R, is proportional to the length of the line between integrator on and off. pears that ammonia is migrating along the collection tube during collection so that sampling time is limited even at very low concentrations. Subsequent use of the collection tubes has required the generation of numerous calibration curves ; departure from linearity between response and loading has been consistent with the values of JQ+ determined in the efficiency of collection tests. Release of ammonia The amount of NH, released (&) is proportional to the amount adsorbed and the efficiency of the release (E,), i.e. R, = E,L,. Figure 1 is a copy of a typical recorder trace showing transient chemiluminscence detector response to thermally desorbed NHB. Most of the release occurs over a time period which is short compared with the collection period. In Fig. 1 the release time can be characterized by the response full width at half maximum (FWHM) which is 1.7 min. For a typical collection time of 17min, a preconcentration factor of approx 10 can be achieved. To measure the product E,E,, a real-time concentration of ammonia was monitored directly from the manifold and a constant response vs time was recorded on a chart recorder. In this way a rectangular area, A,, of chart was associated with the product of
Integrative technique for detection of atmospheric ammonia response and a given interval of time At. A tiollection tube was then placed in a position at the gas manifold and sample air was pulled through for a time At. Subsequently, the tube was desorbed to give a response such as that shown in Fig. 1. This response was recorded as before and a second area, Al, was defined. Since the NH, monitor response is proportional to concentration, the ratio AZ/A1 gives E,E,. A test of this type using 75 ppb ammonia (100 s sampling) in humidified air (40% RH at 23°C) gave A2 values of 62.5 units and 59.4 units compared with 59.4 units for A,. Zero response from tubes which had been previously desorbed indicates that all the NH, is desorbed with one thermal desorption and is consistent with a release efIiciency of one.
integrated instrument
643
response (I,), is represented as
I, = AE,E,F,[C + C,]T + ROT + S,
(2)
Ammonia detection There are several differences in the two detection techniques. The chemihuninescence technique utilizing a high temperature platinum converter responds to a large number of N-containing compounds. This is a disadvantage if the N-containing compounds are collected by the Chromosorb T during ambient sampling. However, experiments with synthetic air mixtures of 40% and 80”/, RH containing NOz and of NO in dry air indicate that Chromosorb T does not efficiently pr~on~ntrate these molecules. Similar tests indicate that HNO, and several amines are collected. To prevent collection of HN03, an acid scrubber is used during sampling. The scrubber consists of a glass tube 0.5 cm in diameter and 5 cm in length filled with glass beads which were coated with a dilute solution of KOH (2.5 mg g- ‘). The scrubber passes ammonia but collects nitric acid at low ppb con~ntrations. Amines are collected and thermally desorbed along with NH,. In the opto-acoustic technique the presence of other trace gases which are collected and which also absorb strongly is possible although such an occurrence has not been observed. However to prevent this, a practice of replicate sampling followed by analysis using one laser line which is absorbing and one which is not has been subsequently initiated. This practice should account for any interfering broadband absorbers. Water vapor at concentrations observed in the ambient air constitutes an interference because of the occurrence of weak absorption at the CO2 laser lines most strongly absorbed by NH,. Fortunately, experiments have shown that only small amounts of H,O are retained on the collection tube.
where the constant A converts ng to chart area, C0 denotes an impurity NH3 contribution in the carrier gas, T’ denotes integration time and S denotes the chart area equivalent of either a blank value due to residual NH, on the tube (positive offset) or an NH, demand by the tube (negative offset). The ~libration equation is generally modified from Equation (2) so that I, is plotted as a function of NH3 loading. Assuming C, =fC whenfis a constant, the first term in Equation (2) becomes AE,( 1 + f) ~2~A nonzero value offincreases the slope of the I, vs L, curve. Water used to humidify a sample air stream is a potential source of impurity NH,. Nonzero values of S give a nonzero intercept as does the term RoT’ if the bias level of the integrating recorder is incorrectly set. Since T’ depends on the bias level, the term RoT’ underestimates the true intercept if R, < 0. A cafibration run consisted of the measurement of five replicates at each of three loadings. Assuming a normal distribution of integrated signal responses at constant loadings, 90% confidence levels were calculated using the five integrated responses for each loading. Table 1 tabulates the 90% confidence levels and the results of a linear regression analysis for three calibration runs. Calibration run No. 2 is a repeat of calibration run No. 1 using a different tube. Both calibration No. 1 and No. 2 were run using a dry sample air stream containing 14ppbNH,. In calibration No. 3, the NH, concentration of the synthetic mixture was changed to 3.6 ppb NH, in dry air. The slightly different values of the slope and offset are not statistically significant. Note that the 90”/0confidence levels suggest a measurement precision of approx & 3 ng. The addition of H,O to the sample increases the slope of the calibration line presumably due to NH, released from the water used for humidi~cation. A contribution to the intercept value due to the term S would be evident if the tubes were not initially conditioned. A newly-packed tube has an ammonia demand and is conditioned by sampling directly from vapors accumulated over ammonium hydroxide and then thermaIly desorbing at 100°C. Tubes which are desorbed for several hours also exhibit an NH, demand of approx. 5ng on first loading. However, on subsequent loadings the integrated response is proportional to loading.
Response factors - linearity and reproduc~biZity A number of tests were run to determine the linearity and reproducibility of the chemiluminescence monitor as a function of tube loading. To perform these tests, the NH, delivery system was stabilized at a given concentration and the collection time was varied. The use of the integrating recorder requires the setting of a bias level to offset any standing response level on the chemilu~nes~n~ monitor. The difference between these two levels (R,), is initially adjusted to zero. The
Ambient measurements Ambient ammonia measurements have been made in three Iocations on the east coast. Values detected have ranged from 0.2 to 5.8 ppb as shown in Table 2. Measurements on Cedar Island, NC and near the EPA facility at the Research Triangle Park, NC were performed in 1978 with both the chemiluminescence monitor and the opto-acoustic monitor. Both monitors were used with a TefIon prefilter to eliminate particulate interference, e.g. from (NH&S04. The
644 Table
W. A. M&LENNY and C. A. BENNETT, JR. 1. Representative
Run
Tube
1
1
2
2
3
1
Loading*,
calibration
ng
runs
Cont.,
25.0 37.5 50.0 25.0 37.5 50.0 20.8 31.2 41.7
for NH,,
results of linear determinations
Average of five replicate responses, units of area
ppb
14 14 14 14 14 14 3.6 3.6 3.6
regression
Slope, units of area ng-
21.2 44.9 60.1 29.2 44.0 63.0 31.1 43.5 63.7
analysis
and 90% confidence
level (Cl)
Standard error of slope
Intercept units of area
Standard error of intercept
SOS/”CL, units of area
1.31
0.06
- 5.1
2.5
1.35
0. to
- 5.3
3.8
1.56
0.2 1
- 2.6
6.8
2.7 3.0 3.3 2.6 2.6 3.1 7.7 2.5 3.8
’
_~ * Accuracy
of known
loadings
estimated
as + 5”“.
sampling train for the chemiluminescence monitor also contained a nitric acid scrubber. To prevent Hz0 from condensing on the filter/scrubber yhen sampling at high relative humidities, the prefilter and scrubber are slightly heated; otherwise loss of NH, by dissolution into collected H,O is observed. Measurements shown in Table 2 indicate ammonia
Table 2. Ambient
Date
Site
Technique
17 July 17 July 14 July 4 Aug. 4 Aug. 4 Aug. 4 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 5 Aug. 7 Aug. 7 Aug. 8 Aug. 10 Aug. 10 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 11 Aug. 14 Sept. 14 Sept.
0 0 0 1
Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi Chemi OA Chemi OA Chemi OA Chemi OA OA
Time 1500 1600 1600 1400 1400 1600 1600 2400 2400 1400 1400 1600 1600 1730 1600 2000 2400 1400 1630 1730 1730 1808 1808 1938 1938 1600 1800 .___
Code for sites
1 1 1
1 1 1
1 I I 2 3 3 4 1 5 6 6 6 6 6 6 0 0
values which were determined within minutes of the sample collection, just as in the calibration procedure. Prior to sampling, the particulate filter and HNO, scrubber were conditioned with ambient air for 30 min. Examples of NH, demand developed during sampling on the particulate filter have been noted and require further study.
measurement
of ammonia*
Sample time, Flow min rate, I min -- 1 IO
20 15 30 30 60 60 30 30 30 30 30 30 30 30 30 20 32 30 30 30 30 30 30 30 16 16
1.0 1.0 1.0 0.88 0.88 0.88 0.88 I.0 1.0 1.0 1.0 1.0 1.0 1.o 0.9 0.9 1.0
1.o 0.9 1.0 1.0
1.o 1.0 1.o 1.0
1.o 1.o
Loading,
ng
(* 3ng)
wb
41.9 82.4 43.3 21.0 25.0 25.6 20.0 20.4 21.0 32.0 32.0 16.3 19.5 18.4 64.0 46.0 19.4 16.5 5.6 6.2 8.7 10.3 4.7 6.2 5.6 33.0 27.1
5.8 5.7 4.0 1.1 1.3 0.7 0.5 0.7 0.9 1.5 1.5 0.8 0.9 0.9 3.4 2.4 1.4 0.7 0.3 0.3 0.4 0.5 0.2 0.3 0.3 2.8 2.3
Additional information
replicate replicate replicate replicate replicate
amines
present
rain rain rain rain rain rain rain
: Site O-Ambient,
Research Triangle Park, NC l-Cedar Island, NC 10 ft height 2Xedar Island, NC 6 ft over grass 3-Cedar Island, NC 2 in. over marsh grass at edgewater +-Cedar Island, NC in marsh, 2 in. above ground 5 -Cedar Island, NC 2 ft height over land &Cedar Island, NC, covered entrance while raining. * Readings taken on 11 August during rain represent a lower limit to the actual ambient NH3 concentration, dissolution of NH, in Hz0 on the unheated surface of the particulate filter probably occurred.
since losses by
Integrative technique for detection of atmospheric ammonia CONCLUSIONS
Ammonia in the concentration range OS-25 ppb can be measured by preconcentration on Chromosorb T followed by thermal desorption and detection. Independent analytical techniques of chemiluminescence and opto-acoustics give comparable NH, concentrations at the same location under similar conditions, Further development of the technique is needed to document the effect of increasing the period of time between sampIing and analysis and to identify other potential interferents. The ability to store tubes prior to analysis would allow a procedure for remote sample collection and return to a central analysis system to be developed. Alternatives to the chemiluminescence and opto-acoustic detection techniques may be available that give more specificity to NH,, e.g. thermally desorbed NH3 could be collected as ammonium ions in an acidic solution and analyzed by ion exchange chromatography (Mulik et al,, 1978). REFERENCE
Baumgardner R. E., McClenny W. A. and Stevens R. K. (1979) Optimized Chemiluminescence System for Measuring Atmdspheric Ammonia, EPA Technology Series, Publication EPA 600/2-79-028. Environmental Protection Agency, Research ‘Triangle Park, NC. 27711. Bendix Corporation (1975) Instruction Manual for Model 8101-B, NO, NOz, NO, Analyzer, Process Instruments Division, Lewisburg, WV. Hoe11 J. M., Harward C. N. and McClenny W. A. (1980) Comparison of Remote Infrared Heterodyne Radiometer and Point Measurements of Ambient Ammonia. To be presented at 1980 CLEOS Conf. in San Diego, CA.
~.a 14/6-a
645
McClenny W:A. and Russwurm G. M. (1978) Laser-based, long path monitoring of ambient gases - analysis of two systems. Atmospheric Enuironme$ 12, 1443. McCienny W. A., Russwurm G. M. and Paur R. J. (1978) New Optical Techniques for Detecting Gaseous Air Pollutants.
Paper ThB3-2, Topical Meeting on Atmospheric Spectroscopy, 30 August - 1 September, Keystone, CO. McCfenny W. A., Bennett C. A., Jr., Russwurm G. M. and Richmond R. (1979) Optoacoustic Cell Design for Trace Gas Analysis using a Heimoholtz Resonator. Proc. Topical meeting of Pfioto~oustic Spectroscopy, 1-3 August, WBilP l-4. Morrison R. L., Maddux A. and Hrubesh L. (1975) A Portable Microwave Spectrometer Analyzer for Chemical Contaminants in Air - A Feasibility Study. Lawrence Livermore Laboratory, Report UCRL-51945, University of California, Livermore, CA 94550. Mulik J. D., E.stes E. and Sawicki E. (1978) Ion chromatographic analysis of ammonium ion in ambient aerosols. In Ion Chromatographic Analysis of Environmental Pollutants (Edited by E. Sawicki, J. D. Mulik and E. Wittgenstein), Ann Arbor Sciences, Ann Arbor, MI. O’Keeffe A. E. and Ortman G. C. (1966) Primary standards for trace gas analysis. Anulyt. Chem. 38, 760-764. Pitts J. N., Jr., Finlayson B. F. and Winer A. M. (1977) Optical systems unravel smog chemistry. En&. Sci. ~echno~. 11, 568-573.
Reid N. M., French J. B., Buckley J. A., Lane D. A., Lovett A. M. and Rosenblatt G. (1977) Real-time Analysis of Gaseous Atmospheric Pollutants to the PPT Level using a Mobile API Mass Spectrometer System. Application Note No. 677-P, Sciex, Inc., 55 Glenameron Road, Suite 202, Thornhill, Ontario, Canada. Steffenson D. M. and Stedman D. H. (1974) Optimization of the operating parameters of chemiluminescence nitric oxide detectors. Anulyt. Chem. 46,1704-1709. Volltrauer H. N, (1976) Instruction Manual for Ultra Sensitive NO/NO, Monitor. AeroChem Research Laboratories, Inc., Princeton, NJ.