Envlron. Scl. Technol. 1884, 18, 823-826
Comparison of a Chemiluminescent and a Tunable Diode Laser Absorption Technique for the Measurement of Nitrogen Oxide, Nitrogen Dioxide, and Nitric Acid James 0. Walega,*t Donald H. Stedman,+ and Richard E. Shettert
Space Physics Research Laboratory, The University of Michigan, Ann Arbor, Michigan
48109
Gervase I. Mackay, Toshlo Iguchl, and Harold I. SchM
Unisearch Associates Inc., Concord, Ontario, Canada L4K
185
rn Comparisons were made between measurements of NO, NO2, and HN03 made with an NO + O3 chemilumines-
cence (CL) system and a tunable diode laser absorption spectrometer (TDLAS) system. Good agreement was found for NO and NO2 calibrations using synthetic air mixtures in the laboratory and in the field. Good agreement was also found for NO measurements in ambient and captive downtown Los Angeles air. Interferences with the CL measurements of NO2 in ambient and captive air, primarily from PAN, were revealed by the comparison. Reasonable agreement was obtained for HN03 captive air, but the CL measurements in ambient air showed considerable variance due to fluctuating l d NO, sources. Since both techniques have response times of less than 5 min, the temporal behavior of the three gases was observed in some detail. Difficulty in passing HN03 through inlet systems made of glass was observed; in addition, while Teflon inlet systems had no difficulty in passing HN03 under normal operating conditions, there was difficulty under conditions of low humidity.
Introduction Accurate measurement techniques are required for further understanding of the atmospheric processes involved in photooxidation and acid deposition. The NO, compounds, NO, and NO2 are essential ingredients for the formation of photooxidants in urban and rural air. Three of the oxidants, HO, H202,and 03,are believed to be responsible for the conversion of S(1V) to S(V1). Nitric acid is formed mainly by reaction of HO with NO2. It is a major sink for NO, and contributes directly to acid deposition. Instrumentation for determining NO has largely been based on the chemiluminescent reaction of NO with 03. These instruments can also determine NO2, but prior chemical conversion to NO ( I , 2 )is required. Since other nitrogen-containing substances may also be converted to NO, the method is nonspecific to NOz. No previous comparisons of this method with definitive, spectroscopic methods have been reported. Measurements of HN03 have been made largely by filter techniques which require collection times of several hours. Only chemiluminescence methods using difference techniques have been reported for measuring HNO, with the response time and sensitivity required for clean tropospheric measurements (I). An earlier attempt was made to compare measurements of HN03 in ambient air at Claremont, CA, by filter and chemiluminescence techniques with a long path FTIR spectroscopic technique (3). Difficulties with the FTIR instrument and a minimum 'Present address: Department of Chemistry, University of Denver, Denver, CO 80208. Present address: National Center for Atmospheric Research, Boulder, CO 80307.
detection limit (6 ppb volume (ppbv)) which was frequently above the ambient concentrations prevented meaningful comparisons. The present paper presents the f%stcomparison between chemiluminescence and a spectroscopic method for measuring NO, NO2, and HN03 in ambient air. It also reports some of the first, rapid response measurements of the temporal behavior of HN03 in urban air. A tunable diode laser absorption spectrometer (TDLAS) fitted with a long-path absorption cell (4) is compared with a high sensitivity, chemiluminescence (CL) detector fitted with the appropriate fiiters and convertors. Both instruments have distinct advantages and limitations. The TDLAS system provides a universal method for positive, unequivocal identification of the species of interest. Because of its very high resolution, interferences from other species are highly unlikely. If interferences do happen to occur, they are readily identified by a distortion in the shape of the selected absorption line or by comparison of absorptions observed for several lines. If such interferences are observed, another line is selected. This was never necessary for the three gases being compared. However, in its present configuration the TDLAS system is limited in its detection limit to about 0.5 ppbv for these gases. The chemiluminescence instrument has a much better detection limit of 3 pptv (parts per trillion volume) but has potential interferences from the species which may chemiluminesce or be converted to a chemiluminescing species when passed through the convertors. Both instruments are capable of making real time measurements with response times of a few minutes or less. The lower cost and power requirement of the chemiluminescence instrument may make it more attractive for routine monitoring, but it does require comparison with the more definitive spectroscopic method to assess its reliability for such monitoring applications. In the work described here comparisons are made in the laboratory with synthetic air and in Los Angeles with ambient and captured urban air as part of a field study. Particular attention was paid to calibration and sampling procedures, problems these instruments share with all mervsurement instrumentation.
Experimental Section The TbLAS system used in this work has been described by Hastie et al. ( 4 ) and the chemiluminescence (CL) instrument by Kelly et al. (I). The error limits placed on the regression coefficients shown in the subsequent analreflect the 95% confidence interval as determined by the product of the standard error and the appropriate t value (5). For large data sets ( n > 120) the error limits are approximately twice the standard error; for smaller data sets the t value increases appropriately. The value r is the standard estimate of the correlation coefficient shown to three decimal places. UM and UNI refer to the
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Envlron. Sci. Technol., VOI. 18,NO. 11, 1984 823
ppbv mixing ratios measured by the CL and the TDLAS instruments, respectively. Laboratory Comparison. In the laboratory comparison, made at The University of Michigan, calibration standards were added to a stream of cylinder air or N2 flowing through a common manifold at 5 SLPM (standard liters per minute). Both instruments sampled from the same point of this manifold through 1-2-m lengths of 6 mm 0.d. Teflon tubing. For NO the calibration of the CL instrument was based on a Matheson cylinder gas mixture containing 53.0 ppmv of NO in Nz and the TDLAS instrument on an Airco cylinder mixture containing 4.00 ppmv of NO in N2. Comparisons were made on gas streams of N2 to which various flows of these calibration gases were added. Excellent agreement was observed. Regression analysis on the eight point data set ranging from 8 to 51.5 ppbv yielded the line r= UM = (1.003 f 0.015)UNI - (0.555 f 0.438) 0.999 Calibration of the instruments for NO2 were based on permeation devices. Since these devices were found to permeate at rates different from those specified by the manufacturer, they were, in turn, calibrated by NO + O3 titration (6). A gas stream containing about 100 ppbv of NO in Nz was mixed with O3 in a 4-L flask. The decrease in the measured NO signal as observed by using the CL instrument was equated to the increase in NO2 observed by using the TDLAS. The validity of this procedure was confirmed during the field comparison portion of this study by having the TDLAS observe both the decrease in NO and increase in NOz. On the basis of a calibration at one concentration, the increase in NOz and the decrease in NO were found to be linear over a concentration range of 4-60 ppbv. This also indicated that the O3 oxidation did not go beyond the formation of NOz. On several occasions additional confirmation was obtained by monitoring, with a Dasibi O3 monitor, the concentration of O3added to the reaction vessel. Excellent agreement between the added O3 and the NO decrease was obtained. Comparisons for NO2 were made on gas streams to which small amounts of N02/N2mixtures were added by using one of two permeation tubes or a cylinder containing about 9 ppmv of NO2in N2 Analyses of the measurements of the concentrations of seven NO2 samples in the range 13-37 ppbv yielded the relationship r= UM = (1.001 f 0.133)UNI (0.121 f 3.377) 0.993
+
When comparisons were made with samples by using cylinder NOz to make up the mixtures, the CL measurements were always about 15% higher than those obtained with the TDLAS instrument. The reason for this discrepancy is not known but may have been due to some impurity in the bottled gas which interfered with the CL measurements. Attempts to compare gas mixtures containing HN03 in the laboratory were largely unsuccessful. Part of the difficulty can be attributed to the loss of this polar gas to the glass walls of the inlet system and White cell of the TDLAS system. This difficulty was subsequently overcome by using an all Telfon inlet, a Teflon liner in the White cell, and faster pumping speeds as described by Hastie et al. (4). Meaningful comparisons were then made during the field mission by using this modified system. These results are similar to the data of Bowermaster and Shaw (7), who show that nonzero humidity and Teflon surfaces are best for passing HN03. Goldan et al. (8) also 824
Environ. Scl. Technoi., Vol. 18, No. 11, 1984
show that Teflon is somewhat better than glass, but their results on humidity appear to be contradictory to those of Bowermaster and Shaw (7). Goldan et al. do not comment on this discrepancy. Field Measurements. The field study was conducted on the roof of the Ethyl Percy Andrus Gerontology Center of the University of Southern California. The sampled gas, either ambient, captive, or synthetic air, or N2 was drawn through a 12.5 mm 0.d. Teflon manifold. During ambient and captive air measurements the total air flow through the common manifold was about 10 SLPM of which 2 SLPM entered the TDLAS instrument and 3.5 SLPM entered the CL instrument through separate 3-m lengths of 6 mm 0.d. Teflon tubing. The captive air was obtained by filling the 40-m3 Teflon bag sometime between 08:OO and 0930 PDT; the filling took about 15 min during which time the bag was covered with black plastic to prevent photochemical reactions. The cover was then removed to expose the captive air to sunlight. Samples were taken through the manifold which extended about 40 cm inside the bag. Measurements were made before and after the bag was uncovered. Ambient air was sampled from the manifold located about 2 m above the roof of the building. Synthetic samples were produced by adding calibration standards to a stream of cylinder air or N2 flowing through the manifold. NO. Comparison of one set of 10 synthetic air mixtures ranging in NO concentrations from 4 to 235 ppbv yielded the relationship r= UM = (1.013 f 0.007)UNI (0.291 f 0.770) 0.999
+
Similar agreement was obtained for comparison of six other synthetic mixture sets made with five different cylinder calibration sources throughout the field program. The excellent agreement found, both in the laboratory and in the field comparisons for synthetic mixtures, was also found for ambient and captive air measurements. Although there was considerable fluctuation in the ambient air concentrations with time, 116 samples of ambient air ranging in NO concentrations from 2 to 133 ppbv gave the regression line UM = (0.950 f 0.051)UNI (1.529 f 2.285) r = 0.961
+
After the cover was removed, the concentration of NO in the captive air decreased monotonically with time without showing the fluctuations observed in ambient air. A data set of 273 measurements of captive air ranging in NO concentrations between 2 and 106 ppbv gave the regression line r= UM = (0.905 f O.OO9)UNI (0.340 f 0.234) 0.996
+
The small (1%) uncertainty in the slope of the captive air measurements provides an indication of the long-term (9 h) stability of the two instruments. The somewhat larger error (5%)in the ambient air measurements undoubtedly arises from the larger fluctuations of NO in ambient air within the 1-min time responses of the instruments. The slope of the ambient air regression line agrees with those of the captive and synthetic air slopes within the combined uncertainties. The slope of the captive and synthetic air regression lines differ marginally outside their combined uncertainties. NOz. Comparisons of synthetic air and N2to which NO2 was added either from permeation tubes or from the NO O3 titration showed good agreement. Comparison of NOz concentrations in ambient and captive air is com-
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PACIFIC DRYLIGHT TIDE ( 10/30/81 1 F@we1. Nitrogen Dioxkle (NO,) captive air measurement comparison. UM data offset +25 ppbv.
plicated by the fact that other nitrogenous compounds such as PAN and methyl nitrate are not differentiated from NO2 by the CL instrument. Simultaneous measurements of NO2in ambient air were made on Oct 24, 26, and 31,1981. Although there were differences in the behavior of ambient NO2on these 3 days, some general trends were observed. First, the maximum mixing ratios occurred about noon; second, the maximum mixing ratios were in the 100 ppbv range; third, the CL measurements were significantly higher than the TDLAS measurements throughout most of the day. Because of the interferences encountered with the chemiluminescence technique, a comparison curve with a slope of unity was not obtained. The difference between the averages of the data sets obtained by the two techniques showed that, on average, 18% of the signal obtained by the CL instrument was due to PAN or other, convertible compounds. Captive air measurements of NO2 showed similar temporal behavior, but with much less variability. Figure 1 shows the measurements obtained by the two techniques on Oct 30, 1 of 3 days on which captive air comparisons were made over the entire day. The two sets of measurements agree with one another until local noon but show increasing differences during the afternoon. This behavior is consistent with the buildup of NO2 in the morning with increasing generation of secondary pollutants such as PAN in the afternoon. Similar behavior was observed on seven other occasions when comparisons were made over part of the day. The maximum NO2concentrations occurred between 12:OO and 1400 local time and varied between 50 and 600 ppbv. The mixing ratios measured by the two techniques generally agreed within 10% in the morning but showed increasing deviations as the day progressed. Differences as large as 80% were observed at the end of some of the experiments, with an average difference of 15% over the entire set of measurements. "0,. Comparison of the HNO, mixing ratios in captive air were made on 6 days. Similar temporal behavior was observed on each of these days. Figure 2 shows the data obtained on Nov 2. The mixing ratios increased until about noon and then remained essentially constant until the captive air in the bag decreased to about 20% of its initial volume, after which the mixing ratios decreased. The CL data shown after 12:30 are 10-min averages taken from chart records due to failure of the data acquisition system. For the six sets of measurements, results from the two instruments were in reasonable agreement in the morning, but the CL instrument gave significantly higher mixing ratios than the TDLAS instrument in the late
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11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 PACIFIC DAYLIGHT TINE ( 11/02/81 1 Flgwe 2. Nitric acid ("0,) captive air measurement comparison. UM data offset 4-5 ppbv. After 12:30 PDT UM data were reduced from chart records as 10-min averages.
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Flgure 3. Nitric acid ("4) captive air measurement comparison linear regression. Data on the line would represent perfect agreement between the two instruments.
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Figure 4. Nitric acid ("0,)
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afternoon for reasons which are presently not understood. Both instruments gave erratic results when attempts were made to measure samples in which HNO, was added to dry synthetic air. This was traced to the low humidity of the bottled gases which apparently caused sampling problems when the walls of the instruments are dry. Such problems were never encountered in measurements of air with normal humidities. Figure 3 shows the comparison plot of the 581 measurements obtained with captive air for which the HNO, mixing ratios ranged from 1 to 25 ppbv. Regression analyses of this data give the relationship UM = (0.947 f 0.032)UNI - (0.603 f 0.307) r= 0.924 Environ. Scl. Technoi., Voi. 18, No. 11, 1984
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Figure 4 shows the comparison of the ambient measurements made by the two instruments on Nov l. The CL measurements show a great deal of scatter, demonstrating the difficulties inherent in the chemiluminescent technique for HNO, measurements in highly polluted air having many local sources. The technique depends on determining the small difference, due to HN03 (5-10 ppbv), between the larger signals obtained from the sum of all the nitrogenous species (NO, NO2,"OB, PAN, etc.). Fluctuations of the concentrations of any of these species on a time scale comparable to that required to make the sequential conversions and subtractions result in large uncertainties in the resultant "OB determinations. These fluctuation are inevitable close to NO, sources in an urban area. Earlier measurements of well-mixed ambient air taken downwind of Los Angeles by Shetter et al. (9) showed much smaller fluctuations, similar to those obtained with the captive air measurements shown in Figure 2. In clean air in which rapid NO, fluctuations are entirely absent such as on Niwot Ridge in the Mountains of Colorado, we have shown (1) that stable pptv concentrations of nitric acid could be determined. Our results are similar to the filter measurements of Huebert et al. at the same site (IO);however, the filter measurements of HNO, have a better detection limit and noise immunity since they measure the HNOBabsorbed by nylon rather than the NO, difference with and without nylon.
Conclusions A comparison between a chemiluminescence instrument and a tunable diode laser absorption spectrometer system showed that both had sufficient sensitivity to measure NO, NO2, and HNOBin ambient and captive Los Angeles air, with response times less than 5 min. Good agreement was found between the instruments for measurements of NO in ambient and captive air. Agreement was also found for synthetic air to which known quantities of NO were added. Since the TDLAS system provides unequivocal identification, these agreements justify the widespread use of chemiluminescence instruments for measuring NO in ambient air. Although agreement was found for NO2 measurements in synthetic air to which known concentrations of NO2 were added, discrepancies were found in comparisons of measurements made in ambient and in captive air. The chemiluminescence instrument gave higher readings, and the discrepancy increased throughout the day. Differences appear to be due to the inability of the CL instrument to distinguish NOz from other secondary nitrogenous materials such as PAN which build up during the day. The NO, measurements from these instruments must be interpreted to include these other substances. Reasonable agreement between the techniques was obtained for HNO, measurements in captive air but not for
826
Envlron. Sci. Technol., Vol. 18, No. 11, 1984
Los Angeles ambient air for which the CL method gave erratic results. The measurement of HNO, by the CL instrument involves an indirect technique requiring subtraction of two large signals obtained with different convertors. The fluctuations in the larger signals observed for NO and NO, in Los Angeles ambient air in the time scale taken for the CL technique is undoubtedly responsible for the erratic HNO, determinations. The CL method for HNO, may therefore be limited to measurements in well-mixed air with no large local NO, sources. The tunable diode laser absorption spectrometer system can provide a valuable standard against which other techniques can be compared and calibrated. Comparison experiments with filter and tungstic acid denuder techniques will be reported in a subsequent paper.
Acknowledgments We thank K. G . Walega, T. A. Bahls, R. Fukuda, and B. Walunas for their assistance in the Michigan component of this program. We also thank Environmental Research and Technology of California who were responsible for the field arrangements. Registry No. PAN, 2278-22-0; NO, 10102-43-9;NOz, 1010244-0; "OB,
7697-37-2;methyl nitrate, 598-58-3.
Literature Cited Kelly, T. J.; Stedman, D. H.; Ritter, J. A.; Harvey, R. T. J . Geophys. Res. 1980,85, 7417. Kley, D.; McFarland, M. Atmos. Technol. 1980, 12, 63. Spicer, C. W.; Holmes, J. E., Jr.; Bishop, T. A.; Arnold, L. H.; Stevens, R. K.; Forrest, J.; Huebert, B.; Kelly, T. J.; Kok, G.; Lazrus, A.; Paw, R.; Shaw, R.; Stedman, D. H.; Tanner, R.; Tew, E.; Tuazon, E. Atmos. Environ. 1982, 16, 1497-1500. Hastie, D. R.; Mackay, G. I.; Iguchi, T.; Ridley, B. A.; Schiff, H. I. Environ. Sci. Technol. 1983, 8, 352A-3646. Bennet, G. A.; Franklin, N. L. "Statistical Analyses in Chemistry and Chemical Industry"; Wiley: New York, 1954. Stedman, D. H. J . Air Pollut. Control Assoc. 1976,26,62. Bowermaster, J.; Shaw, R. W., Jr. J . Air Pollut. Control Assoc. 1981, 31, 787. Goldan, P. D.; Kuster, W. C.; Albritton, D. L.; Fehsenfeld, F. C.; Connell, P. S.; Norton, R. B.; Huebert, B. J. Atmos. Environ. 1983, 17, 1355. Shetter, R. E.; Stedman, D. H.; West, D. H. J . Air Pollut. Control Assoc. 1983, 33, 3. Huebert, B. J.; Norton, R. B. Bollinger, M. J.; Parish, D. D.; Murphy, P. C.; Albritton, D. L.; Fehsenfeld, F. C. Symp. Compos. Nonurban Troposphere, 2nd 1982, 163. Received for review July 29,1983. Revised manuscript received March 7,1984. Accepted May 22,1984. This work was performed with the financial support of The Coordinating Research Council of Atlanta, Georgia (CAPA 17/19-80). The field site was provided by the Ethyl Percy Andrus Gerontology Center of the University of Southern California at Los Angeles, Dr. J . E. Birren, Director.
