278 Anal. Chem. 1082, 54 , 278-282

Laser Photoacoustic Detection of Nitrogen Dioxide in the Gas-Phase Titration of Nitric Oxide with Ozone

Alan Fried" and Jimmle Hodgeson'

Center for Analytlcal Chemistty, National Bureau of Standards, Washlngfon, D. C. 20234

Gaephase tltration (OPT) studies of nitric oxide (NO) with ozone (0,) were carried out over a 1 year time span. I n these studies, NO and Os concentratlons were measured with conventional technlques of chemiluminescence and UV ab- sorption photometry, respectlveiy. Nltrogen dloxide concen- tratlons (NO,) also measured durlng the course of titration were carried out by use of a photoacoustlc detection system, a method in which NO, was measured directly without an interfering response from NO. Excellent agreement in NO and NO, concentration changes durlng titration has been demon- strated throughout thls study. The corresponding O3 mea- surements, however, were found to average 3.6% lower. AddRlonai studies, both experimental and photochemical modeling of alr stream impurities, could not account for thls dlscrepancy.

Ozone (03), nitric oxide (NO), and nitrogen dioxide (NO2) are critical components involved in the chemistry and physics of the troposphere and stratosphere and are subject to con- siderable monitoring requirements. For each of these gaseous constituents, an individual measurement standard is currently employed for calibration. In recent years several studies have been published (1-7) intercomparing measurement standards for O3 and NO via gas-phase titration (GPT) involving the bimolecular reaction

Some of these studies have shown a one-to-one agreetnent between the NO reacted and the O3 added to within the stated uncertainty, typically a few percent. Nevertheless, the results of GPT are still not devoid of controversy. Biases as large as 9% have been reported, and in some cases agreement was obtained interspersed among erratic unexplainable results. Rehme et al. (8) have very recently published a detailed study evaluating ozone calibration procedures. Four methods were studied: (1) UV photometry, (2) gas-phase titration with excess NO (GPT-NO), (3) gas-phase titration with excess O3 (GPT-03), and (4) a boric acid buffered potassium iodide technique. In this study, a total of 10 volunteers were selected from within the U.S. Environmental Protection Agency (EPA) and outside of EPA to represent technicians likely to use the four procedures. The two GPT procedures were carried out by each of the volunteers using their own GPT system and a common GPT system. Ozone concentrations assayed by GPT-NO and GPT-03 methods using the common system averaged 3.1 and 2.4% higher, respectively, than assays de- termined by a reference UV photometer. The corresponding assays by the 10 operators using their own GPT system av- eraged 7.4 and 7.9% higher, respectively, than the UV pho- tometer. One of the major conclusions of this study was that there appeared to be an equipment-dependent bias with these GPT methods. Referencing flow measurements and NO

NO(excess) + 03 = NO2 + 0 2 (1)

Present address: Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803.

cylinder assays to common standards did little to improve the results, and thuj it was concluded that errors from these sources could not account for the observed discrepancies. Preliminary investigations, however, revealed that the amount of NO2 produced in the titration agreed to within 2% of the NO consumed. Even in cases where agreement between NO and O3 has been reported, researchers have generally cast an unfavorable opinion regarding routine use of GPT. For ex- ample, Demore and Patapoff (2), after a thorough study of GPT, concluded that the method is accurate in principle but susceptible to experimental error. The present study was therefore undertaken in an effort to identify the experimental factors affecting GPT results and to improve the precision and agreement among NO, NO2, and O3 measurement standards. This technique could then be used routinely for a much needed independent determination of NO standards prepared and analyzed gravimetrically at the National Bureau of Standards (NBS). Furthermore, this information would be useful to those employing GPT in the field calibration of O3 detectors.

In the present GPT studies, O3 and NO concentrations were measured by traditional techniques of UV absorption (2,9) and chemiluminescence (IO), respectively. The NO2 mea- surements, on the other hand, were carried out with a pho- toacoustic detection system, a method in which NO2 was measured directly without an interfering response from NO. This is a significant advantage over the more traditional chemiluminescent measurements, where NO2 is measured differentially (NO, - NO * NO2) thus introducing considerable measurement uncertainky when NO is present a t high con- centrations as it is in GPT experiments.

EXPERIMENTAL SECTION Gas-Phase Titration Apparatus. The experimental setup

is schematically shown in Figure 1. A cylinder containing a NO/N2 mixture, at a nominal concentration of 50 ppm ( p d per million by volume) was used as a working standard. This gas, during titration, was directed into a glass Kjeldahl reaction chamber through a mass flow controller, Tylan Corp. Model FC 260, and a series of manual three-way ball valves. The flow rate employed, f N o , ranged from 50 to 180 cm3/min. Clean air, Matheson certified dry grade, entered the gas-phase titration system through a second flow controller at a flow rate, f ~ l ~ , typically around 5.3 L/min. Approximately 10% of the flow, fo, passed tbrough a flow controlling glass capillary into a quartz tube. Ozone concentrations ranging from 0 to approximately 14 ppm were produced in the flow stream by adjusting a sliding opaque sleeve surrounding a low-pressure mercury lamp located ap- proximately 5 cm from the quartz tube. Once in the reaction chamber, the ozonized air stream thoroughly mixed and reacted with the NO flow stream, in accordance with the bimolecular gas-phase reaction in eq 1. This flow scheme is designed to ensure locally high concentrations of reactants and thus minimize the required reaction chamber volume. The reaction chamber effluent was diluted with the remaining clean air flow and passed into a sampling manifold where the concentrations of NO, 03, and NO2 were measured by, respectively, a chemiluminescent detector (CD) built at NBS, a Dasibi Corp. UV photometer, and a photoacoustic detection system. The Dasibi photometer was used as an O3 transfer standard throughout this work and was periodically

This article not sublect to US. Copyright. Published lg82 by the American Chemlcai Society

MASS FLOW 1 0

CONTROLLER NO CHEMllUMlNESCENT DETECTOR g

Figure 1. as-phase titration apparatus. PRESSURE

FOCUSING CUACE

PHOTODlOOE FLOW MEAS

VALVE PERMEATION CAL SYSTEM

CLEAN A I L 3.WAY VRLYES SOURCE

4

PARTICLE FILTER

CLEAN AIR 3.WAY VRLYES somr - I 4

CPT SAMPLE MANIFOLO

Flgure 2. NO2 photoacoustic detection system.

calibrated by the NBS 3-m UV photometer, as described in detail by Hodgeson (7) and Wendt et al. (9). The GPT system just described was predominantly constructed of glass using stainless steel valves, flow controllers, and fittings. Ozone contacted only one metal fitting upstream of the reaction chamber. Teflon tubing, due to its porosity, was used sparingly and only in places were flexibility was required.

Photoacoustic Apparatus. The NOz photoacoustic detection system is shown in Figure 2 and will be further described in ref 11. A stainless steel photoacoustic cell (15 cm long by 0.8 cm i.d. with 0.1 cm i.d. inlet and outlet ports) continuously sampled gas through a 0.4-pm Nuclepore particle filter at pressures slightly below atmosphere, typically around 80 kPa (600 torr). As in- dicated, this cell could sample one of three different flow streams: pure diluent air; diluent air mixed with NO2 permeant from a calibration system, Kintek Corp. Model 670 modified by the incorporation of a mass flow meter; GPT sample manifold flow. Nitrogen dioxide flowing through the photoacoustic cell was ir- radiated by the 488.0-nm line from an argon-ion laser, Spectra Physics Model 164, directed along the cell axis and chopped at 250 Hz. The laser beam was weakly focused into the center of the cell using a 65-cm focal length lens in an effort to minimize large background signals produced by scattered light impinging on the cell walls and microphone. Brewster-angle windows were also employed for this purpose. Average chopped laser powers in the 488.0-nm line, measured at the cell entrance window, were typically about 120 mW throughout this work. The laser was equipped with an actively stabilized power supply. After a warm-up period of approximately 2 h, power drifts over periods as long as 8 h were less than 0.5% as measured by a photodiode shown in Figure 2.

The 488.0-nm output is one of many argon-ion lines in for- tuitous coincidence with pressure-broadened lines of NO2. Ex- citation at this wavelength results in an electronic transition i-nvolving rotational-vibrational levels of the ground X 2A1 and A 2B2 excited electronic states. Photoacoustic signals resulting from collisional deactivation of these excited states were detected as a modulated pressure wave with an electret microphone, Thermoelectron Corp. Model 814C, located approximately 5 cm from the center of the cell. The signal processing electronics are shown in Figure 3. Output signals from the microphone field- effect transitor were amplified, processed by an electronic filter used in the band-pass mode, and demodulated by a lock-in am- plifier referenced to the chopping frequency. The resulting dc signals from the lock-in amplifer output were simultaneously sent into a strip chart recorder for visual display and a printing digital

ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 279

M I C R O P H O N E

CHOPPER

Figure 3. Signal processing electronics.

voltmeter for subsequent data averaging. GPT Preparatory Experiments and Calibration Proce-

dures. Gas-phase titration experiments were carried out over approximately a 1 year period. The nominal 50 ppm NO/NZ working standard was calibrated by comparison with NBS certified Standard Reference Material (SRM) cylinders and with an in- dependent set of NO/N2 gravimetric standards. A working standard concentration of 49.5 ppm was determined along with a conservative estimate of f0.5 ppm for the total uncertainty (1 standard deviation) combining both analyses. The stability of the working standard was observed by analysis on three separate occasions spanning this 1 year period, and within the limits of uncertainty no concentration changes were observed.

Accurate flow measurements are an absolute requirement in GPT studies. Flow errors would directly be reflected in NO, and in the present study, NOz concentration determinations. Con- siderable effort was therefore expended in utilizing the flow calibration facilities at NBS to make accurate flow measurements. These flow measurements, as detailed in a later section, are used in the calculations in the form of a flow dilution ratio (FDR)

(2) f N 0

f N 0 -k f A I R FDR =

where f A I R = fo + f D as indicated in Figure 1. The mass flow controllers used in achieving this flow dilution ratio were calibrated immediately after each daily experimental run by means of 3L and 1L wet test meter transfer standards, American Meter Corp. The 3L and 1L wet test meters (WTM) were in turn calibrated against a mercury sealed piston gas calibration system, using air and nitrogen, respectively, at the flow rates employed in the titration experiments. The gas calibration system, Brooks Corp. Model 1050, is rated for an absolute accuracy of better than f0.2%. Additional checks on the WTM calibrations were also carried out using an independent method for each device. The 3L WTM calibration was checked by measuring the weight of gas expended from a cylinder through the WTM as a function of indicated volumetric displacement. The 1L WTM was checked by using a calibrated bubble meter preceded by a saturator. The agreement between the independent calibrations for each WTM was excellent; agreement was found to be within 0.1% and 0.4% for the 3L and 1L WTMs, respectively. Most of the GPT ex- periments were carried out using values of (FDR)-' around 30.3 f 0.4%. The uncertainty in this flow dilution ratio (1 standard deviation) reflects the collective uncertainty in the WTM gas and water temperature, WTM timing, and WTM calibration factor. In addition to measuring the individual flows upstream of the reaction chamber, the total flow out of the sample manifold was also measured during each experiment run. This was accom- plished by using the 3L WTM at the vent port of the sample manifold with the other three sampling ports closed off. This extra measurement was typically within 0.1% of the sum of fa and f N 0 measured upstream, thus assuring against erroneous results caused by leaks.

During the initial phase of this work a great deal of effort was centered on determining the optimum set of conditions with which to operate the NO chemiluminescence detector. The emphasis was placed on sensitivity, linear range, and minimizing signal variations associated with changes in chamber pressure and sample and ozone flows. This detector was operated with sensitivities typically around 286 mV/ppm with long-term drifts over the course of a 4-6 h experiment less than *0.3%. The detector response over the NO concentration range of interest, 05-1.6 ppm,

280 ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982

was linear; a plot of detector response as a function of NO con- centration, prepared by known flow dilutions, resulted in residuals near &0.2% with a maximum of 0.5%.

The photoacoustic signal response was also found to be linear. A plot of the photoacoustic detector response as a function of NOz concentration over the range of interest resulted in a straight line with most residuals near f0.2%. From the slope of this plot, 0.136 pV/ppb, together with an approximate value of 0.6 pV for the signal noise, we calculated an NOz photoacoustic detection limit (SIN = 1) of approximately 5 ppb. This detection limit was based on an average laser power of 120 mW and a 40-5 integration time constant. Known NOz concentrations, [ N O Z ~ ~ ~ ~ ~ ~ ~ ~ ~ , were gen- erated in this study by varying the diluent air flow passing over a calibrated NOz permeation tube. The permeation rate of this tube was calibrated on a weekly basis according to recommended procedures (12). The diluent air flow was measured upstream of the permeation tube with a calibrated maea flow meter, Hastings Model ALL-lK, and checked periodically at a flow measurement port located at the calibration system outlet as indicated in Figure 2.

Since NOz, NO, and O3 are all highly reactive, concentration changes in the experimental apparatus were initially of some concern. Systematic losses of NO2 were checked in the Teflon transfer tubing interconnecting the GPT sample manifold to the photoacoustic detection system. This was accomplished by measuring NO2 from the output of the calibration system with and without the Teflon transfer tubing in the flow path. In a similar manner, the GPT system losses of NO and O3 were also checked. These gases were passed through the entire GPT system and measured in the sample manifold. The response from the appropriate detector was then compared with that obtained from sampling NO and O3 directly, bypassing the GPT system. In the case of 03, the generator flow was first diluted with air in an external glass mixing bulb attached to the Dasibi inlet using the same flow dilution factor as above. In all three cases no detectable systematic losses could be found.

Experimental Titration Procedures and Data Reduction. At the start and periodically throughout each experimental run, a photoacoustic signal base line was recorded by flowing diluent air without NOz permeant added into the photoacoustic cell. This signal, Sm (permeation base line signal), a r k s from three sources: (1) scattered laser light impinging on the microphone, (2) heating of the cell windows, and (3) to a very small extent turbulence from gas flowing through the cell. An identical base line was observed by sampling the effluent from the GPT sample manifold with Nz and clean air flowing. Base line signals for the Dasibi UV pho- tometer and NO detector were also recorded. Upon addition of O3 to this sample stream, a slight photoacoustic signal increase was consistently observed at the threshold of our detection limit. This signal was found to be independent of O3 concentration, and therefore may be associated with NOz generated from background levels (approximately 6 ppb) of NO. An initial NO chemilumi- nescent detector response was then recorded by turning the O3 source off and diluting the NO/N2 working standard with clean air to generate initial NO concentrations, (NO)i, ranging from 0.5 to 1.6 ppm in the sample manifold, in accordance with

(3)

A photoacoustic detector response was simultaneously recorded. This signal, which was approximately 0.7 pV greater than SPBL, was caused by 133 ppb of NOz in the NO/Nz working standard. This signal, in addition to that resulting from background levels of NO as detailed above, comprised our photoacoustic titration base line, STBL.

During titration the responses of all three detectors were si- multaneously recorded with each incremental addition of 0% The NOz concentration at each titration point was carefully determined by sequentially measuring the photoacoustic response from the permeation calibration system, Sp, and the titration, ST, in a repetitive fashion. The total NOz concentration, ANOz, formed with each O3 addition was calculated from

(Noli = FDR X (NO)oylinder

The corresponding concentration of NO reacted, ANO, was

calculated from the initial and final chemiluminescent detector response using the relation

(5)

where Ri = measured response with (NO)i, Rf = measured response after each O3 addition, and R B ~ = measured base line response with only clean air flowing. The titration was always carried out using a large excess of NO. This fact, together with a sufficiently long reaction chamber residence time, guaranteed complete re- action as evidenced by the lack of residual O3 in the sampling manifold. The total concentration of O3 added was measured at each titration point by replacing the NO/N2 mixture with pure Nz at the same flow rate. By use of the same flow dilution ratio, the O3 concentration measured was the same as that added during titration. The Dasibi UV photometric data thus obtained were reduced to the corresponding O3 concentrations, PO3, by appli- cation of the Beer-Lambert law using an absorption coefficient of 308.5 cm-l atm-' and the appropriate Dasibi span factor, temperature, pressure, path length, and calibration parameters as detailed by Wendt et al. (9).

RESULTS AND DISCUSSION The results of all GPT runs from this study are summarized

in Table I. These-results are grouped according to the O3 concentration added during titration, which is indicated by the O3 generator sleeve setting given in the fifth column. Because of differences in mercury lamp intensity, these set- tings only indicate nominal O3 concentrations. Therefore, only concentrations shown in the same row should be compared. Results obtained from the same experimental run are desig- nated by identical letters in the first column. The uncer- tainties shown with each concentration represent 1 standard deviation and were determined by an error propagation analysis on the appropriate concentration expression. The total uncertainty derived from this analysis is comprised of terms indicating the precision of repeated measurements and terms indicating estimates of variance for which repeated measurements could not be performed. For NOz, these numbers were derived from the total uncertainty in the permeation rate, permeation oven temperature and pressure, diluent flow, and, most important, the photoacoustic mea- surement imprecision. This last contribution was found to decrease with increasing signal strength. Photoacoustic measurements of NO2 were therefore only carried out at high O3 generator concentrations where the total NOz uncertainty could be kept around 1 % or better. The corresponding figures for NO were calculated based upon the uncertainty in the determination of the NO cylinder concentration, the flow dilution ratio, the long-term chemiluminescent detector drift, and, to a lesser extent, the measurement imprecision. For 03, the total uncertainty was based upon the corresponding un- certainty of the UV photometric cell temperature, pressure, and path length along with uncertainties in the absorption coefficient and measurement of absorption. At concentrations of 0.5 ppm and greater, the predominate term was found to be the uncertainty in the absorption coefficient, approximately f1.5%.

The results for ANO, and AN0 shown in Table I are in excellent agreement. This agreement is shown more clearly in Figbe 4a, in which the mean value of the percent difference between corresponding AN02 and AN0 concentrations, 6, is displayed along with associated error bars at each O3 generator sleeve setting. This figure shows no clear-cut trend in the NO2, NO comparisons with increasing O3 concentrations. The corresponding 03, NO comparisons on the other hand, pres- ently reveal an entirely different situation. The data in Table I and in Figure 4b, in which the corresponding differences between A 0 3 and AN0 are displayed, clearly show a definite bias; the A 0 3 values are lower than the corresponding AN0 and AN02 values, in concurrence with biases found in most

Table I. Summary of Results Grouped by 0, Generator Sleeve Setting

ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 281

&Or I I I I I I I I I :

runa a b

d e f a b

e f a b

d e f a b

e f a b a

d g g m m e d g h h i

C

C

C

C

C

j j j k k C e h i i k 1 1 m n

g

i k 1 d

e i 1 m m

C

C

AN02 , ppb

4722 5 4 6 5 t 5 4 7 0 r 6 4792 6

6 8 9 r 12 7 0 2 r 11 7 1 3 r 8 717 r 12 7 0 9 r 12 7 1 5 t 11 7002 12 7 0 4 t 10

801 r 9 8 0 9 r 7 8 1 9 r 8 813 t 10 8252 8 8 2 8 t 8 804 r 10

916 r 7 9 1 9 r 10 924 r 10

1 0 1 4 t 9 1 0 2 5 t 10 1ooo t 10 1014 r 10

A NO, PPb 8 3 r 3 84 t 3 852 3 91 r 3 8 3 % 3 8 5 t 3

1 8 1 r 3 1 8 2 % 3 1 8 8 % 3 185 t 3 1 8 8 r 3 2782 4 287 r 4 292 r 4 301 r 4 294 r 4 293 r, 4 3921: 5 4032 5 4 0 5 r 5 4 0 3 r 5 4 0 2 r 5 447 t 6 461 r 6 502 r 6 5 1 9 r 7 527 i: 7 471 t 6 473 r 6 474 r 6 477 r 6 627 t 7 757 r 8 694 t 8 6 9 0 r 10 6 9 7 r 10 7 0 6 r 8 711 t 11 7 1 2 t 1 2 7 0 8 t 11 7 0 2 r 8 707 t 8 8 4 0 r 10 8 4 5 t 10 8022 11 8 1 4 r 9 823 t 9 8142 9 8 2 0 t 9 821 r 9 8 0 2 r 9 764 t 9 8 4 5 t 10 948 r 11 9 2 0 t 10 917 r 11 9195 10

1065 r 11 1035 r 11 10461 11 1 0 1 2 t 11 1014 t 11 988t 11

1005 r 11

0 3

AO, , sleeve ppb setting 81 t 2 81 t 2 86k 2 8 8 r 2 8 2 r 2 842 2

171 t 4 174 r 4 1 8 6 t 4 180t 4 1 8 2 r 4 267 2 5 277 t 5 291 r 6 293 r 5 284. 5 2 8 5 r 5 3 7 0 r 7 3 8 8 r 7 397 t 6 391 t 5 3 9 2 t 6 431 i 8 4 4 0 r 8 4902 8 504 r 7 509r 7 4 4 9 r 5

474 r 6 607 r 7 7 3 2 r 9 6 5 3 t 8

6 4 6 t 8 6 7 5 r 7 7 0 0 r 8

670.1 8 822r 10 8 2 0 t 10 7532 9

7 7 9 t 9 7 7 5 r 9 7 9 9 r 10

781 t 9 7 3 6 r 9 801 r 10 8992 11 8792 10 8 7 6 r 11 903 r 11

1035 t 12 990k 11

1011 i 12 9722 11

1000 r 12 9 7 2 t 1 2

1.0

2.0

3.0

4.0

4.5

5.0

6.0 7.0

8.0

8.5 9.0

10.0

a Identical letters indicate data taken the same day.

0 2 4 6 0 10

O3 Generator Sleeve Setting

Flgure 4. (a) Display of the mean value of 6 and associated error bars for the AN02, AN0 comparisons at each O3 generator sleeve setting. (b) Corresponding display of the mean value of 6 and associated error bars for the A03, AN0 comparisons at each 0, generator sleeve setting.

previous studies. The average percent difference in these comparisons, however, also shows no clear-cut O3 concentra- tion dependence.

The results for the corresponding values of ANOz, ANO, and A 0 3 were fit using a linear regression. Since the uncer- tainties in all three determinations are comparable in mag- nitude, a weighted regression was employed using an iterative technique described by York (13); the residuals in both the ordinate and abscissa were simultaneously minimized, in contrast to ordinary least-squares data treatment. All 26 ANOz (Yi) and AN0 (Xi) data pairs were fit by using weighting factors equal to the reciprocal of the relative uncertainties squared. This resulted in the expression AN02 (ppb) =

[1.016 f 0.006lANO (ppb) - [ lo f 5 ppb] (6) Substitution of observed AN0 values into this relationship yields ANOz values 0.6 f 1.4% higher. The four data pairs a t sleeve setting 10, three of which show approximately a 1% higher AN02 concentration, heavily influence the regression fit because of relatively large weighting factors. More inde- pendent experimental runs in this concentration range are therefore needed. If all four data pairs at this O3 concentration are excluded from the fit, slightly lower values of slope and negative intercept are obtained AN02 (ppb) =

[1.006 f 0.007lANO (ppb) - [4 f 5 ppb] (7) The ANOz concentrations obtained from both expressions, however, only differ by 0.4%, and in both cases are in agreement with the observed AN0 values within experimental error. The corresponding linear regression fit of the 56 AO& Y J , ANO(Xi) data pairs shown in Table I yields A 0 3 (ppb) = [0.964 * 0.005]ANO (ppb) - [3 f 4 ppb]

(8) In an attempt to identify the source of this bias, other

reactions have been considered which cause the oxidation of NO to NO2 without using up 0% The most obvious

2 N 0 + 02 - 2N02

only accounts for approximately 0.2%, as both measured and calculated in this study. An alternative set of reactions also considered involves the photochemistry of impurities in the “clean air” source. Periodic chromatographic measurements of our air cylinders revealed impurities such as NzO, CH4, CO,

282 ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982

and C02 at ambient concentrations. In addition, water-vapor concentrations predominately in the range between 10 and 40 ppm have also been measured. Occasional measurements, however, revealed values as high as a few hundred parts per million. The chemistry of these impurities centers around the production of the OH radical in the O3 generator which subsequently generates CH302 and H02 radicals, both of which are known to react very rapidly with NO to produce NOz. Fourty-seven reactions involving these impurities were studied by computer modeling our GPT experiments using typical flow and concentration conditions. Details of this model will be presented by the authors in a future publication. The com- puter routine used in this analysis was developed by Brown a t NBS (14). At the relative humidities measured in these experiments, the results from the modeling indicate a max- imum increase in AN0 relative to A 0 3 on the order of a few tenths of 1%, much too low to account for the observed discrepancy of 3.6%. Relative humidities as high as 50% would be required to generate a 2% discrepancy in these species. We are, however, continuing to study the air stream source for, as of yet, undetected impurities which may effect the oxidation of NO to NOz. We have also found that those species known to be present in the air source, as well as those reactive and stable end products predicted by the model, are not major interferences in the UV absorption measurements of O3 at 253.7 nm.

In addition to the chemistry of impurities just described, we were also concerned about trace impurities in the air stream reacting directly with 03, which, because of the split flow stream arrangement detailed in Figure 1, could potentially yield lower Os concentrations during measurement than ac- tually present in the gas-phase titration. A series of GPT experiments were therefore carried out using a different flow scheme. The O3 sowce, reaction, and mixing chambers shown in Figure 1 were replaced by two larger glass vessels, a 1L reactor followed by a 2L mixing chamber. Ozone was intro- duced into the system by passing the ozonized air stream from the NBS 3-m UV photometer directly into the reaction chamber. Not only did we eliminate the split flow stream in this manner but it also enabled us to make simultaneous O3 and NO measurements, thus eliminating a possible source of error due to lamp fluctuations in the O3 source during the time between titration and O3 measurement. This arrangement also enabled us to eliminate the necessity of the Dasibi transfer standard along with questions concerning the use of a different air source in the calibration and GPT phases. The results of these experiments using high-purity air with water vapor concentrations less than 3 ppm and total hydrocarbon con- centrations as CHI less than 0.1 ppm are very similar to previous GPT runs; the AN0 concentrations as measured by chemiluminescence and calibrated by gravimetrically prepared standards ranged from 3 to 5% higher than the corresponding A 0 3 concentrations as measured by UV photometry.

CONCLUSIONS At the present time we cannot offer an unequivocal ex-

planation for the bias found between the UV photometric measurements of O3 and corresponding NO and NO2 mea- surements performed by chemiluminescence and photoa- coustic detection, respectively. Some important accom- plishments from the present study, however, are worth noting. Excellent agreement in NO and NOz concentration changes in GPT has been demonstrated using two different mea- surement principles, thus indicating that NO is quanitatively converted to NOz. Nitrogen dioxide concentrations produced

in GPT were measured by a direct technique, thus minimizing experimental measurement uncertainty. The extreme care given to measurements of flow and NO and NO2 concentra- tions eliminate obvious contributions to the observed dis- crepancy. Gas-phase titration experiments were carried out in all cases with flow uncertainties about *0.4%. The si- multaneous measurements of NO and O3 in the later GPT studies further eliminate potential sources of systematic error. We have investigated the role of the photochemistry of air stream impurities flowing through ’the O3 generator and re- action chamber via a limited set of chemical reactions and as of yet have found that this by itself could not explain the large bias in these experiments. In future studies, we will continue to investigate air stream impurities by modeling, gas chro- matography, and long path Fourier transform infrared spec- trometry. This last technique will also be used in conjunction with UV photometry to measure O3 concentrations in the titrations.

ACKNOWLEDGMENT The authors are deeply indebted to Arnold Bass and Jim

Norris at NBS for calibrations of the Dasibi transfer standard and John Travis and Ron Shideler, also on the staff of NBS, for their gracious loan of equipment and technical advice. Acknowledgment is also given to Sally Howe and Robert Brown at NBS for providing us with the computer programs to carry out, respectively, the York iterative regression and the numerical modeling routine for solving systems of chemical rate equations. The authors also wish to acknowledge the invaluable assistance of Donald Stedman of the University of Michigan and the machining expertise of Eric Mott, presently at the Solar Energy Research Institute, in the design and construction of the photoacoustic cell.

(7)

LITERATURE CITED Demore, W. B.; Romanovsky, J. C.; Feldstein, M.; Hamming, W. J.; Mueller, P. K. “Comparlson of Oxidant Callbration Procedures”, final report of the Ad Hoc Oxidant Measurement Commlttee of the Callfor- nia Alr Resources Board, Feb 3, 1975. Demore, W. B.; Patapoff, M. Envlron. Scl. Techno/. 1976, 10 (9), 897. Rehme, K. A.; Martin, B. E.; Hodgeson, J. A. “Tentatlve Method for the Callbratlon of Nitrlc Oxide, Nitrogen Dioxlde and Ozone Analyzers by Gas Phase Titratlon”; EPA Publication No. EPA-R2-73-246; U.S. Envi- ronmental Protection Agency, Research Triangle Park, NC, March 1974. Stedman, D. H.; Daby, E. E.; Stuhi, F.; Nikl, H. J . Alr Po//ut. Control Assoc. 1972, 260. Paur, R. J. Proceedings of the Elgth International Materials Research Symposlum; NBS, Galhersburg, MD, 1970. Hodgeson, J. A.; Hughes, E.; Schmidt, W.; Bass, A. “Methodology for Standardizatlon of Atmospheric Ozone Measurements”, presented at the International Conference on Photochemical Oxldant Pollutlon and Its Control, Raleigh, NC, Sept 1976. Hodgeson, J. A. “A Survey of Calibratlon Techniques for Atmospheric Ozone Monkors”; Natlonal Bureau of Standards Publication No. NBSIR 76-1191, Dec 1976. Rehme, K. A,; Puzak, J. C.; Beard, M. E.; Smlth, F.; Paur, R. J. “Evaluation of Ozone Calibratlon Procedures”, EPA Pubilcation No. EPA-600/S4-80-050; U.S. Envlronmental Protectlon Agency, Research Trlangle Park, NC, Feb 1981. Wendt. J.: Kowalski. J.: Bass, A.; Ellis, C.; Patapoff, M. NBS Spec. Publ. ( U S . ) 1976, No. 529. Fontlln, A.; Sabadell. A. J.; Ronco, R. J. Anal. Chem. 1970, 42, 575. Fried, A. “A Study of Measurement Interference In the Optoacoustic Detection of NOp by Argon-Ion Laser Excitation”, to be submltted for publication.

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RECEIVED for review September 2,1981. Accepted November 2, 1981.