- Email: [email protected]
Indirect determination of nitrogen oxides by a chemiluminescence technique
Atmospheric Environment Pergamon Press 1972. Vol. 6, pp. 807-814. Printed in Great Britain.
INDIRECT DETERMINATION OF NITROGEN OXIDES BY A CHEMILUMINESCENCE TECHNIQUE R. GUICHERIT Research Institute for Public Health Engineering TNO, Delft, P.O.B. 214, The Netherlands (First received 14 April 1972 and in final form 19 May 1972)
Abstract-NO2 concentrations
in outdoor air can be determined indirectly by measuring the equilibrium ozone concentration under continuous U.V. irradiation. This concentration can be measured very accurately by a chemiluminescence technique using Bhodamine B as a light emitting compound. The reaction between ozone and Bhodamine B is highly spectic. No other components of air pollution at concentrations occurring in outdoor air interfere. The equilibrium ozone concentration on photolysis of the nitrogen dioxide present is a function of the wavelength, the light intensity and the temperature. By keeping these parameters constant, NO2 concentrations can be determined very accurately with a lower detection limit of 5 pg NO1 m-j air.
INTRODUCTION
MOST of the atmospheric NO and NOz (represented in the text by NO,) is biologically produced NO which is slowly being oxidized to NOz ; background concentrations for NO, range from 2 to 10 pg mm3 (ROBINSON and ROBBINS, 1970). The main man-made source is combustion of fossil fuels. Most of the NO, produced in this way is in the form of NO. Relatively small quantities of NO, are emitted from non-combustion, industrial chemical processes, and by the use of explosives. The most accurate and reliable method to measure NOz is the calorimetric method with Griesz-Sal&man type reagents. NO can be measured in the same way after oxidation to NOz (SALTZMAN, 1954). The disadvantages of this method are that large quantities of reagent are consumed, the stability of the reagent is poor, mostly only O-5h average NOz values can be determined due to the lack of sensitivity, and finally the reagent might be carcinogenic. Continuous coulometric determination is possible with instruments of the Masttype. However, the coulometric efficiency is very low and the reactions involved are not specific. Recently oxides of nitrogen are measured by chemiluminescent reactions. NO is determined by measuring the luminescence produced on reaction of NO and ozone in the gas-phase (FONTIJN et al., 1970). NO, is determined by a chemiluminescent reaction involving oxygen atoms and NO, in the gas-phase; a reaction similar to the one used for NO measurements (SNIJDERand WOOTEN,1969). Both methods have the drawback that rather high concentrations of ozone and oxygen atoms should continuously be generated and that the pressure in the reaction vessel should be kept at the torr level. This makes the system rather complicated, especially when the system is continuously in operation. In this paper an indirect method for the detection of oxides of nitrogen will be discussed by which ozone is produced by NO2 photolysis, and subsequently the ozone concentration is measured by making use of a chemiluminescent reaction between ozone and an organic compound (GUICHERIT,1971). 807
R. GUICHERIT
808
THEORY
NO2 can be photolysed by light of wavelengths below 400 nm. NO, F
hv
h<4OOnm
NO + 0.
(‘1
For light of wavelengths below 370 nm the quantum yield for reaction (1) is unity. For wavelengths > 370 nm dissociation becomes rapidly less, and for wavelengths > 420 nm no dissociation occurs. The generated oxygen atoms may react with NO2 to produce NO + 0,: 0+NOz+M+NO+02+M.
(2)
In air, however, M = N,/Oz reaction (2) can not occur because oxygen atoms will react in less than lob6 s* with O2 leading to the formation of ozone: 0 + 0, + M + OJ + M.
(3)
The NO formed in the initial NOz photolysis reacts very rapidly with O3 in the reaction. NO + 0, -+ NOz + OZ.
(4)
Reaction of NO2 and O3 to form nitric acid NO,+O,+NO,+O,
(5)
NO,+NO,+M+N,O,+M
(6)
N,O, + H,O --f 2 HNO,
(7)
can be neglected in this respect for at NOz concentrations occurring in outdoor air reaction (4) proceeds more than 100 times fatser than reaction (5). Summarizing we may say that only reactions (1, 3 and 4) are important and need further discussion. For reaction (1)
S1 = &,N02 = vka (NO,)
For reaction (3)
S, = k3 (0) (0,) (M)
For reaction (4)
S, = k4 (0,) (NO)
s1
= rate of oxygen atom production
= primary quantum yield (may be taken 1, if h < 370 nm) ==rate of absorption (for weak absorption In,No2= k. (NO,)) k, = number of photons absorbed per molecule NOz in unit time = rate constant. k, In a very short time the equilibrium state is achieved.
‘p I,,,,
hv NOz + 02 f By
NO + OS.
(8)
this process light is being absorbed and subsequently converted into heat. For the steady state of dissociation and generation of NOz, S1 equals 5’4. 9% (NO,) = k, (0,) (NO).
* All rate constants are taken from New York 1961.
LEIGHTONP. A., Photochemistry
(9 of Air Pollution, Academic Press,
809
Indirect Determination of Nitrogen Oxides by a Chemiluminescence Technique
From equation (8) it follows that (NO) + (NO,) = NO, = constant.
(10)
Starting with pure NOa it further follows from stoichiometric considerations that (11)
(0,) = (NW From equations (9, 10 and 11) we can caMate
the ozone concentration
T = temperature A = wavelength of light I = intensity of light. In FIG. 1 the ozone ~n~ntration is given as a function of the NOZ ~on~nt~tion for different values of krr/k4. These curves are calculated from equation (12). $30
800
700
=90
600
=50
om 500 yE 460 9 300
= IO
200
ko z-1
IO0
0'
FIG. 1. Ozone
100 200 300 400
500 600 700 600 900 1000
concentration as a function of the NO2 concentration for different values of k./k., in PP~~I.
The graphs show that for the concentration range of interest in outdoor air, i.e. NOZ concentrations ranging from O-1000 pg rnm3, the greater the J&,/k*values are, the more the function approaches a linear one. From the graphs it also foffows that fluctuations of the ka/k4 value lead to far lower fluctuations in the photolysis yield. Since ozone concentratior+ of 1 pg rn-$ can very well be determined by the chemiluminescent method employed (GUICHERIT, 1971) the dete&on limit for NOZ by this indirect method, depending on the kJk4 value by photolysis, will also be very low. In our case the detection limit was better than 5 cl&NOZ mW3 air. A.&6/l1-B
810
R. GUICHBRIT EXPERIMENTAL
The flowscheme and the principal component parts of the detection system are shown in FIG. 2.
I
NO, NO,
I
IO
1. 2. 3. 4. 5. 6. 7.
9
IO
FIG. 2. Detection system for NO,. 8. High voltage supply Air sample 9. Chemiiu~~nt compound NOz removal 10. Photomultiplier OS removal; NO-NO2 oxidation 11. Flow meter Photolysis unit (h 3600 A) 12. Air pump Timing unit 13. Recorder Fourway valve Clean air tilter
In the photolysis unit a Philips HPW 125 W blac~ght lamp is used, the relative spectral distribution of which is given in FIG. 3. Since the lamp does not emit wavelengths below 300 nm, no ozone is formed by photolysis of air oxygen, From the relative spectral distribution of the lamp, given in FIG. 3 it also follows that no wavelengths above 300 nm are emitted at which O3 absorbs, so photolysis of OS can also be neglected. The photomultip~er tubes Philips XP 1006 have a very IO
6
f&i 31310 334.2
36k.5 360.6 464. ,?I
w
2.75
0.06
0.16
Fro. 3. Relative spectral energy distribution
0.03 0.03
407-S
0.03
of the Philips HPW black light
lamp.
IndirwtDeterminationof NitrogenOxidesby a Chemilumin-a
Technique
811
low dark current, a high gain, and a spectral response of the b&alkali D-type. AS a chemiluminescent compound Rhodamine B Merck No. 7599 on Eastman chromatogram sheets (6061 Silicagel) was used. The analysis of NO and NO2 requires two separate analyses (FIG.2). In one analysis sample air is analysed for NO, and in the other sample air is analysed for total NO,. The NOz concentration is the difference between the total NO, and the value for NO. In order to carry out analysis for NO; NOz is removed from the air by means of a triethanolamine on pumice scrubber (BEREZKINand GORSHWOV, 1965). To carry out analyses for NO, and NO; the sample air is passed through an oxidizing scrubber to convert NO into NOZ. Oxidation of NO into NO2 is effectuated by a MnOz/ KHS04. scrubber described by HARTKAMP (1970). This scrubber also completely removes ozone from the air sample. Removal of ozone from the air and oxidation of NO into NOz is essential since ozone concentrations which are being measured after photolysis of NOZ (see equation 12) are derived for zero ozone and NO concentration of the air sample to be analysed. By means of a Cway valve, an intermittent sampling procedure is applied. In one cycle NO, after oxidation into NOz and photolysis of the N02, is introduced into the detection unit for 5 s, followed by the introduction of clean air for 25 s. In the next cycle NO + NOz, after oxidation of NO into NOz and photolysis of the NOa, is introduced into the detection unit for the same fixed time (5 s) followed by the introduction of clean air for 25 s. Each air sample, which is being analysed is also irradiated for 55 s. In this way also a check on the stability of the “zero-point” of the detection system, which corresponds more or less with the dark current of the photo multiplier tubes is obtained. It is noteworthy that by introducing outdoor air by means of a 5-way valve directly into the detection unit, the ozone concen~ation of the air can also be determined. The system is then a real multi-component detection unit by which, with one instrument the NO, NOz and O3 concentration of outdoor air can, one after another, directly be determined. ?7ae NO2 source for calibration purposes
The system for the production of NO, for calibration purposes is shown in FIG. 4. Permeation tubes (O’KEEWE and ORTMANN,1966) were used for the preparation of known NO2 concentrations. For dilution, purified air was used. NO is produced by photolysis of the NO2 in nitrogen. By using nitrogen as the carrier gas the competition reactions (2 and 3) cannot occur during photolysis. Rapid dilution of the produced NO by purified air renders the reaction of NO with air oxygen to reform NOZ exceedingly sluggish at this level. In this way almost 95 per cent of the NOZ is converted into NO and hardly any NO* is found in the airstream at the sampling site. Linearity and agreement with other measurements
The linearity has been investigated by stepwise variation of the NO, concentration at the sampling site. For the system described the solid line of FIG. 5 was found. The graph shows that for concentrations of &500 pg NOZ mW3 linearity is better than 5 per cent. Linearity for con~n~ations over 500 pg mm3 can be obtained by better focusing the long wave U.V. light of the black light lamps, or by using another type
812
R. GUICHER~T
FIG.4. System for the production 1. 2. 3. 4. 5. 6.
Nitrogen Constant temperature Permeation tubes Air Filter section Gas meter
water bath
of NO, 7. Photolysis unit 8. Mixing vessel 9. Flow meter 10. Filter 11. Sampling site
of lamp which gives greater k,,/k, values. Still another possibility is to dilute the air to be analysed with clean air free of nitrogen oxides. From a great many experiments it followed that the reproducibility was very good. Deviations of more than 5 per cent have not been observed. Continuous measurements of rapid changes of the NO, concentration in outdoor air show excellent quantitative agreement between results obtained with the chemihtminescent method and with a modified Griesz-Saltzman reagent after HUYGEN (1970) (FIG. 6). The system proved to be stable in outdoor air sampling procedures for at least 2 months. During that time no recalibration is required. Because the photolysis yield, the sensitivity of the light detection and amplification system, as well as the luminescence efficiency of Rhodamine B are temperature dependent, the temperature of the detection units as a whole should be controlled within I-2’C. INTERFERENCES
As the detection of ozone by chemiluminescent measurement on reaction with Rhodamine B is highly specific no interference occurs for other components of air pollution, at least not at concentrations usually encountered in outdoor air. This means that interference of the NO, detection by ozone production by photolysis can only be expected in the case of (a) Photolysis of other components other than NO2 occurring in outdoor air to yield ozone, at wavelengths of 360 nm. (b) Losses of ozone due to reaction with materials of the detection system and/or with other components occurring in outdoor air which have not been removed by the filter sections. (4 Losses of ozone due to reaction with NO formed by photolysis of NO2 between the photolysis unit and the reaction chamber. (a) LEIGHTON(1961) has calctdated the rate of absorption for the most significant absorbing molecules occurring in polluted outdoor air and has found that for the wavelengths we used for photolysis, NO, is the only component of importance
(4 FIG. 6. Comparative NO, measurements in outdoor air for 3 concentration ranges by the chemiluminescent method and a continuous calorimetric method using a modified GrieszSaltzman type reagent. Calorimetric method : upper graphs. Chemiluminescent method : lower graphs. (a) Concentration range O-100 pg NO, mS3, (b) Concentration range O-400 pg NO, mm3, (c) Concentration range O-800 pg NO, rnm3. (Fucing p. 812)
Indirect Determination of Nitrogen Oxides by a Chemilumineacence Technique
813
i
.e :: 50 ._ E 0) E ii 0 w
100 200
300 400
SO0 603 700 800 900 lo00 1100
p9 me3
FIG. 5. Light emission as a function of the NO+oncentration.
Since wavelengths below 300 nm are not used, photolysis of air oxygen to yield ozone need not be considered. (b) Due to the highly reactive nature of ozone, only glass or Teflon should be used as materials for ozone sampling. In this way losses of ozone due to reaction with materials of the detection ‘system can be minimized. LEIGHTON(1961) has also demonst~ted that next to reaction with NO only reaction of ozone with reactive olefins are fast enough to be of importance as a source of ozone losses, due to reaction with other components of air pollution. Our experiments with mixtures of CzH4, I-&H8 and C,H, at concentration levels of up to 1 ppm each, showed ozone losses of less than 3-4 per cent for NO2 concentrations of 200 pg mm3 at a photolysis time of 55 s. The olefin concentrations mentioned are higher than usually encountered in outdoor air. Reaction with certain internal olefins might lead to greater losses. The concentration of these olefins in outdoor air are, however, usually in the lower part per billion range, while the most reactive ones are also partly or totally removed from the air stream by the scrubbers used. Reaction with NO need not be considered because NO has been oxidized to NO2 before photolysis. (c) It can be calculated that the residence time of the air in the ducts between the photolysis unit and the detection unit is about O-3 s. The rate of reaction between NO and O3 is too slow to lead to appreciable losses of ozone in this short time. For example at the 10 pphm level the rate of reaction between NO and OJ to form NO2 is about 1700 pphm hm2. This wouid result in an ozone loss of about 1 per cent. Concluding we may state that this method appears to be very useful and reliable in monitoring ambient air NO, concentrations over very long periods of time. Acknowledgement-The author is indebted to PhilipsIndustries, The Netherlands, for providing the equipment required to develop and test the method, and also to Ir. C. HUYGEN of the Research Institute for Public Health Engineering TN0 for critically reading the manuscript and calculating the graphs of FIG. 2.
814
R. GUICHERIT REFERENCES
BERBZKINV. G. and GORSHUNOV0. L. (1965) Zzv. Akad. Nauk, SSSR Ser. Khim 11,2069-2070. FONTYNA., SWELL A. J. and RONCO R. J. (1970) Homogenesus chemiluminescent measurement of NO with ozone. Anal. Chem. 42,575-579. GOODYR. M. and WALSHAWC. D. (1953) The origin of atmospheric nitrous oxide. Q. J. Met. Sot. * 496-500. G~I~HERITR. (1971) Ozone analysis by chemiluminescence measurement. Zeitschr. Anal. Chem. 256, 177-182. HARTKAMPH. (1970) Untersuchungen tiber die Oxydation und die Messung von Stickstoffmonoxid in kleinen Konzentrationen. Schriftreihe der Landesanstalt fiir Immissions- und Bodennutzungsschutz des Landes Nordrhein-Westfalen. Essen. Heft 18,55-74. HUY~EN C. (1970) The reaction of nitrogen dioxide with Griesz type reagents. Anal. Chem. 42, 407-409. O’K~EFFEA. E. and ORTMANNG. C. (1966) Primary standards for trace gas analysis. Ad Chem. 38, 760-763. ROBINSONE. and ROBBINSR. C. (1970) Gaseous atmospheric pollutants from urban and natural sources. J. Air. Pollut. Contr. Ass. 20, 303-306. SALTZMAN B. E. (1954) Calorimetric microdetermination of NOz in the atmosphere. Anal. Chem. 26, 1949-1955. SNIJDER A. D. and WOOTBNG. W. (1969) Feasibility study for the development of a multifunctional emission detector for NO, CO and CO1. Mosanto Research Corp. Dayton, Ohio. Final Rep. under NAPCA Contr. NO. CPA 22-69-8.
INDIRECT DETERMINATION OF NITROGEN OXIDES BY A CHEMILUMINESCENCE TECHNIQUE R. GUICHERIT Research Institute for Public Health Engineering TNO, Delft, P.O.B. 214, The Netherlands (First received 14 April 1972 and in final form 19 May 1972)
Abstract-NO2 concentrations
in outdoor air can be determined indirectly by measuring the equilibrium ozone concentration under continuous U.V. irradiation. This concentration can be measured very accurately by a chemiluminescence technique using Bhodamine B as a light emitting compound. The reaction between ozone and Bhodamine B is highly spectic. No other components of air pollution at concentrations occurring in outdoor air interfere. The equilibrium ozone concentration on photolysis of the nitrogen dioxide present is a function of the wavelength, the light intensity and the temperature. By keeping these parameters constant, NO2 concentrations can be determined very accurately with a lower detection limit of 5 pg NO1 m-j air.
INTRODUCTION
MOST of the atmospheric NO and NOz (represented in the text by NO,) is biologically produced NO which is slowly being oxidized to NOz ; background concentrations for NO, range from 2 to 10 pg mm3 (ROBINSON and ROBBINS, 1970). The main man-made source is combustion of fossil fuels. Most of the NO, produced in this way is in the form of NO. Relatively small quantities of NO, are emitted from non-combustion, industrial chemical processes, and by the use of explosives. The most accurate and reliable method to measure NOz is the calorimetric method with Griesz-Sal&man type reagents. NO can be measured in the same way after oxidation to NOz (SALTZMAN, 1954). The disadvantages of this method are that large quantities of reagent are consumed, the stability of the reagent is poor, mostly only O-5h average NOz values can be determined due to the lack of sensitivity, and finally the reagent might be carcinogenic. Continuous coulometric determination is possible with instruments of the Masttype. However, the coulometric efficiency is very low and the reactions involved are not specific. Recently oxides of nitrogen are measured by chemiluminescent reactions. NO is determined by measuring the luminescence produced on reaction of NO and ozone in the gas-phase (FONTIJN et al., 1970). NO, is determined by a chemiluminescent reaction involving oxygen atoms and NO, in the gas-phase; a reaction similar to the one used for NO measurements (SNIJDERand WOOTEN,1969). Both methods have the drawback that rather high concentrations of ozone and oxygen atoms should continuously be generated and that the pressure in the reaction vessel should be kept at the torr level. This makes the system rather complicated, especially when the system is continuously in operation. In this paper an indirect method for the detection of oxides of nitrogen will be discussed by which ozone is produced by NO2 photolysis, and subsequently the ozone concentration is measured by making use of a chemiluminescent reaction between ozone and an organic compound (GUICHERIT,1971). 807
R. GUICHERIT
808
THEORY
NO2 can be photolysed by light of wavelengths below 400 nm. NO, F
hv
h<4OOnm
NO + 0.
(‘1
For light of wavelengths below 370 nm the quantum yield for reaction (1) is unity. For wavelengths > 370 nm dissociation becomes rapidly less, and for wavelengths > 420 nm no dissociation occurs. The generated oxygen atoms may react with NO2 to produce NO + 0,: 0+NOz+M+NO+02+M.
(2)
In air, however, M = N,/Oz reaction (2) can not occur because oxygen atoms will react in less than lob6 s* with O2 leading to the formation of ozone: 0 + 0, + M + OJ + M.
(3)
The NO formed in the initial NOz photolysis reacts very rapidly with O3 in the reaction. NO + 0, -+ NOz + OZ.
(4)
Reaction of NO2 and O3 to form nitric acid NO,+O,+NO,+O,
(5)
NO,+NO,+M+N,O,+M
(6)
N,O, + H,O --f 2 HNO,
(7)
can be neglected in this respect for at NOz concentrations occurring in outdoor air reaction (4) proceeds more than 100 times fatser than reaction (5). Summarizing we may say that only reactions (1, 3 and 4) are important and need further discussion. For reaction (1)
S1 = &,N02 = vka (NO,)
For reaction (3)
S, = k3 (0) (0,) (M)
For reaction (4)
S, = k4 (0,) (NO)
s1
= rate of oxygen atom production
= primary quantum yield (may be taken 1, if h < 370 nm) ==rate of absorption (for weak absorption In,No2= k. (NO,)) k, = number of photons absorbed per molecule NOz in unit time = rate constant. k, In a very short time the equilibrium state is achieved.
‘p I,,,,
hv NOz + 02 f By
NO + OS.
(8)
this process light is being absorbed and subsequently converted into heat. For the steady state of dissociation and generation of NOz, S1 equals 5’4. 9% (NO,) = k, (0,) (NO).
* All rate constants are taken from New York 1961.
LEIGHTONP. A., Photochemistry
(9 of Air Pollution, Academic Press,
809
Indirect Determination of Nitrogen Oxides by a Chemiluminescence Technique
From equation (8) it follows that (NO) + (NO,) = NO, = constant.
(10)
Starting with pure NOa it further follows from stoichiometric considerations that (11)
(0,) = (NW From equations (9, 10 and 11) we can caMate
the ozone concentration
T = temperature A = wavelength of light I = intensity of light. In FIG. 1 the ozone ~n~ntration is given as a function of the NOZ ~on~nt~tion for different values of krr/k4. These curves are calculated from equation (12). $30
800
700
=90
600
=50
om 500 yE 460 9 300
= IO
200
ko z-1
IO0
0'
FIG. 1. Ozone
100 200 300 400
500 600 700 600 900 1000
concentration as a function of the NO2 concentration for different values of k./k., in PP~~I.
The graphs show that for the concentration range of interest in outdoor air, i.e. NOZ concentrations ranging from O-1000 pg rnm3, the greater the J&,/k*values are, the more the function approaches a linear one. From the graphs it also foffows that fluctuations of the ka/k4 value lead to far lower fluctuations in the photolysis yield. Since ozone concentratior+ of 1 pg rn-$ can very well be determined by the chemiluminescent method employed (GUICHERIT, 1971) the dete&on limit for NOZ by this indirect method, depending on the kJk4 value by photolysis, will also be very low. In our case the detection limit was better than 5 cl&NOZ mW3 air. A.&6/l1-B
810
R. GUICHBRIT EXPERIMENTAL
The flowscheme and the principal component parts of the detection system are shown in FIG. 2.
I
NO, NO,
I
IO
1. 2. 3. 4. 5. 6. 7.
9
IO
FIG. 2. Detection system for NO,. 8. High voltage supply Air sample 9. Chemiiu~~nt compound NOz removal 10. Photomultiplier OS removal; NO-NO2 oxidation 11. Flow meter Photolysis unit (h 3600 A) 12. Air pump Timing unit 13. Recorder Fourway valve Clean air tilter
In the photolysis unit a Philips HPW 125 W blac~ght lamp is used, the relative spectral distribution of which is given in FIG. 3. Since the lamp does not emit wavelengths below 300 nm, no ozone is formed by photolysis of air oxygen, From the relative spectral distribution of the lamp, given in FIG. 3 it also follows that no wavelengths above 300 nm are emitted at which O3 absorbs, so photolysis of OS can also be neglected. The photomultip~er tubes Philips XP 1006 have a very IO
6
f&i 31310 334.2
36k.5 360.6 464. ,?I
w
2.75
0.06
0.16
Fro. 3. Relative spectral energy distribution
0.03 0.03
407-S
0.03
of the Philips HPW black light
lamp.
IndirwtDeterminationof NitrogenOxidesby a Chemilumin-a
Technique
811
low dark current, a high gain, and a spectral response of the b&alkali D-type. AS a chemiluminescent compound Rhodamine B Merck No. 7599 on Eastman chromatogram sheets (6061 Silicagel) was used. The analysis of NO and NO2 requires two separate analyses (FIG.2). In one analysis sample air is analysed for NO, and in the other sample air is analysed for total NO,. The NOz concentration is the difference between the total NO, and the value for NO. In order to carry out analysis for NO; NOz is removed from the air by means of a triethanolamine on pumice scrubber (BEREZKINand GORSHWOV, 1965). To carry out analyses for NO, and NO; the sample air is passed through an oxidizing scrubber to convert NO into NOZ. Oxidation of NO into NO2 is effectuated by a MnOz/ KHS04. scrubber described by HARTKAMP (1970). This scrubber also completely removes ozone from the air sample. Removal of ozone from the air and oxidation of NO into NOz is essential since ozone concentrations which are being measured after photolysis of NOZ (see equation 12) are derived for zero ozone and NO concentration of the air sample to be analysed. By means of a Cway valve, an intermittent sampling procedure is applied. In one cycle NO, after oxidation into NOz and photolysis of the N02, is introduced into the detection unit for 5 s, followed by the introduction of clean air for 25 s. In the next cycle NO + NOz, after oxidation of NO into NOz and photolysis of the NOa, is introduced into the detection unit for the same fixed time (5 s) followed by the introduction of clean air for 25 s. Each air sample, which is being analysed is also irradiated for 55 s. In this way also a check on the stability of the “zero-point” of the detection system, which corresponds more or less with the dark current of the photo multiplier tubes is obtained. It is noteworthy that by introducing outdoor air by means of a 5-way valve directly into the detection unit, the ozone concen~ation of the air can also be determined. The system is then a real multi-component detection unit by which, with one instrument the NO, NOz and O3 concentration of outdoor air can, one after another, directly be determined. ?7ae NO2 source for calibration purposes
The system for the production of NO, for calibration purposes is shown in FIG. 4. Permeation tubes (O’KEEWE and ORTMANN,1966) were used for the preparation of known NO2 concentrations. For dilution, purified air was used. NO is produced by photolysis of the NO2 in nitrogen. By using nitrogen as the carrier gas the competition reactions (2 and 3) cannot occur during photolysis. Rapid dilution of the produced NO by purified air renders the reaction of NO with air oxygen to reform NOZ exceedingly sluggish at this level. In this way almost 95 per cent of the NOZ is converted into NO and hardly any NO* is found in the airstream at the sampling site. Linearity and agreement with other measurements
The linearity has been investigated by stepwise variation of the NO, concentration at the sampling site. For the system described the solid line of FIG. 5 was found. The graph shows that for concentrations of &500 pg NOZ mW3 linearity is better than 5 per cent. Linearity for con~n~ations over 500 pg mm3 can be obtained by better focusing the long wave U.V. light of the black light lamps, or by using another type
812
R. GUICHER~T
FIG.4. System for the production 1. 2. 3. 4. 5. 6.
Nitrogen Constant temperature Permeation tubes Air Filter section Gas meter
water bath
of NO, 7. Photolysis unit 8. Mixing vessel 9. Flow meter 10. Filter 11. Sampling site
of lamp which gives greater k,,/k, values. Still another possibility is to dilute the air to be analysed with clean air free of nitrogen oxides. From a great many experiments it followed that the reproducibility was very good. Deviations of more than 5 per cent have not been observed. Continuous measurements of rapid changes of the NO, concentration in outdoor air show excellent quantitative agreement between results obtained with the chemihtminescent method and with a modified Griesz-Saltzman reagent after HUYGEN (1970) (FIG. 6). The system proved to be stable in outdoor air sampling procedures for at least 2 months. During that time no recalibration is required. Because the photolysis yield, the sensitivity of the light detection and amplification system, as well as the luminescence efficiency of Rhodamine B are temperature dependent, the temperature of the detection units as a whole should be controlled within I-2’C. INTERFERENCES
As the detection of ozone by chemiluminescent measurement on reaction with Rhodamine B is highly specific no interference occurs for other components of air pollution, at least not at concentrations usually encountered in outdoor air. This means that interference of the NO, detection by ozone production by photolysis can only be expected in the case of (a) Photolysis of other components other than NO2 occurring in outdoor air to yield ozone, at wavelengths of 360 nm. (b) Losses of ozone due to reaction with materials of the detection system and/or with other components occurring in outdoor air which have not been removed by the filter sections. (4 Losses of ozone due to reaction with NO formed by photolysis of NO2 between the photolysis unit and the reaction chamber. (a) LEIGHTON(1961) has calctdated the rate of absorption for the most significant absorbing molecules occurring in polluted outdoor air and has found that for the wavelengths we used for photolysis, NO, is the only component of importance
(4 FIG. 6. Comparative NO, measurements in outdoor air for 3 concentration ranges by the chemiluminescent method and a continuous calorimetric method using a modified GrieszSaltzman type reagent. Calorimetric method : upper graphs. Chemiluminescent method : lower graphs. (a) Concentration range O-100 pg NO, mS3, (b) Concentration range O-400 pg NO, mm3, (c) Concentration range O-800 pg NO, rnm3. (Fucing p. 812)
Indirect Determination of Nitrogen Oxides by a Chemilumineacence Technique
813
i
.e :: 50 ._ E 0) E ii 0 w
100 200
300 400
SO0 603 700 800 900 lo00 1100
p9 me3
FIG. 5. Light emission as a function of the NO+oncentration.
Since wavelengths below 300 nm are not used, photolysis of air oxygen to yield ozone need not be considered. (b) Due to the highly reactive nature of ozone, only glass or Teflon should be used as materials for ozone sampling. In this way losses of ozone due to reaction with materials of the detection ‘system can be minimized. LEIGHTON(1961) has also demonst~ted that next to reaction with NO only reaction of ozone with reactive olefins are fast enough to be of importance as a source of ozone losses, due to reaction with other components of air pollution. Our experiments with mixtures of CzH4, I-&H8 and C,H, at concentration levels of up to 1 ppm each, showed ozone losses of less than 3-4 per cent for NO2 concentrations of 200 pg mm3 at a photolysis time of 55 s. The olefin concentrations mentioned are higher than usually encountered in outdoor air. Reaction with certain internal olefins might lead to greater losses. The concentration of these olefins in outdoor air are, however, usually in the lower part per billion range, while the most reactive ones are also partly or totally removed from the air stream by the scrubbers used. Reaction with NO need not be considered because NO has been oxidized to NO2 before photolysis. (c) It can be calculated that the residence time of the air in the ducts between the photolysis unit and the detection unit is about O-3 s. The rate of reaction between NO and O3 is too slow to lead to appreciable losses of ozone in this short time. For example at the 10 pphm level the rate of reaction between NO and OJ to form NO2 is about 1700 pphm hm2. This wouid result in an ozone loss of about 1 per cent. Concluding we may state that this method appears to be very useful and reliable in monitoring ambient air NO, concentrations over very long periods of time. Acknowledgement-The author is indebted to PhilipsIndustries, The Netherlands, for providing the equipment required to develop and test the method, and also to Ir. C. HUYGEN of the Research Institute for Public Health Engineering TN0 for critically reading the manuscript and calculating the graphs of FIG. 2.
814
R. GUICHERIT REFERENCES
BERBZKINV. G. and GORSHUNOV0. L. (1965) Zzv. Akad. Nauk, SSSR Ser. Khim 11,2069-2070. FONTYNA., SWELL A. J. and RONCO R. J. (1970) Homogenesus chemiluminescent measurement of NO with ozone. Anal. Chem. 42,575-579. GOODYR. M. and WALSHAWC. D. (1953) The origin of atmospheric nitrous oxide. Q. J. Met. Sot. * 496-500. G~I~HERITR. (1971) Ozone analysis by chemiluminescence measurement. Zeitschr. Anal. Chem. 256, 177-182. HARTKAMPH. (1970) Untersuchungen tiber die Oxydation und die Messung von Stickstoffmonoxid in kleinen Konzentrationen. Schriftreihe der Landesanstalt fiir Immissions- und Bodennutzungsschutz des Landes Nordrhein-Westfalen. Essen. Heft 18,55-74. HUY~EN C. (1970) The reaction of nitrogen dioxide with Griesz type reagents. Anal. Chem. 42, 407-409. O’K~EFFEA. E. and ORTMANNG. C. (1966) Primary standards for trace gas analysis. Ad Chem. 38, 760-763. ROBINSONE. and ROBBINSR. C. (1970) Gaseous atmospheric pollutants from urban and natural sources. J. Air. Pollut. Contr. Ass. 20, 303-306. SALTZMAN B. E. (1954) Calorimetric microdetermination of NOz in the atmosphere. Anal. Chem. 26, 1949-1955. SNIJDER A. D. and WOOTBNG. W. (1969) Feasibility study for the development of a multifunctional emission detector for NO, CO and CO1. Mosanto Research Corp. Dayton, Ohio. Final Rep. under NAPCA Contr. NO. CPA 22-69-8.