respectively. Record the analyzer's NO and NOx responses for each concentration. Plot the analyzer responses versus the respective calculated NO and NOx concentrations and draw or calculate the NO and NOx calibration curves. For subsequent calibrations where linearity can be assumed, these curves may be checked with a two-point calibration consisting of a zero air point and NO and NOx concentrations of approximately 80% of the URL.

1.5.9 Preparation of NO2 calibration curve.

1.5.9.1 Assuming the NO2 zero has been properly adjusted while sampling zero air in step 1.5.7, adjust Fo and FD as determined in section 1.4.2. Adjust FNo to generate an NO concentration near 90% of the URL of the NO range. Sample this NO concentration until the NO and NOx responses have stabilized. Using the NO calibration curve obtained in section 1.5.8, measure and record the NO concentration as [NO]orig. Using the NOx calibration curve obtained in section 1.5.8, measure and record the NOX concentration as [NOX]orig.

1.5.9.2 Adjust the O3 generator to generate sufficient 03 to produce a decrease in the NO concentration equivalent to approximately 80% of the URL of the NO2 range. The decrease must not exceed 90% of the NO concentration determined in step 1.5.9.1. After the analyzer responses have stabilized, record the resultant NO and NOx concentrations as [NO]rem and [NOX]rem.

1.5.9.3 Calculate the resulting NO2 concentration from:

recheck the zero and span adjustments by repeating steps 1.5.7 and 1.5.9.3. Record the NO2 concentration and the corresponding analyzer NO2 and NOx responses.

1.5.9.4 Maintaining the same FNO, Fo, and FD as in section 1.5.9.1, adjust the ozone generator to obtain several other concentrations of NO2 over the NO2 range (at least five evenly spaced points across the remaining scale are suggested). Calculate each NO2 concentration using equation (13) and record the corresponding analyzer NO2 and NOx responses. Plot the analyzer's NO2 responses versus the corresponding calculated NO2 concentrations and draw or calculate the NO2 calibration curve.

1.5.10 Determination of converter efficiency.

1.5.10.1 For each NO2 concentration generated during the preparation of the NO2 calibration curve (see section 1.5.9) calculate the concentration of NO2 converted from:

where:

[NO2]CONV = concentration of NO2 converted, ppm

[NOx]orig = original NOx concentration prior to addition of O3, ppm

[NOX]rem = NOx concentration remaining after addition of O3, ppm

NOTE: Supplemental information on calibration and other procedures in this method are given in reference 13.

Plot [NO2]CONV (y-axis) versus [NO2]OUT (Xaxis) and draw or calculate the converter efficiency curve. The slope of the curve times 100 is the average converter efficiency, Ec The average converter efficiency must be greater than 96%; if it is less than 96%, replace or service the converter.

2. Alternative B-NO2 permeation device.
Major equipment required:
Stable O3 generator.

Chemiluminescence NO/NOx/NO2 analyzer with strip chart recorder(s).

NO concentration standard.
NO2 concentration standard.

2.1 Principle. Atmospheres containing accurately known concentrations of nitrogen dioxide are generated by means of a permeation device. (10) The permeation device emits NO2 at a known constant rate provided the temperature of the device is held constant (10.1 °C) and the device has been accurately calibrated at the temperature of use. The NO2 emitted from the device is diluted with zero air to produce NO2 concentrations suitable for calibration of the NO2 channel of the NO/NOx/NO2 analyzer. An NO concentration standard is used for calibration of the NO and NOx channels of the analyzer.

2.2 Apparatus. A typical system suitable for generating the required NO and NO2 concentrations is shown in Figure 2. All connections between components downstream from the permeation device should be of glass, Teflon®, or other non-reactive material.

2.2.1 Air flow controllers. Devices capable of maintaining constant air flows within ±2% of the required flowrate.

2.2.2 NO flow controller. A device capable of maintaining constant NO flows within ±2% of the required flowrate. Component parts in contact with the NO must be of a non-reactive material.

2.2.3 Air flowmeters. Calibrated flowmeters capable of measuring and monitoring air flowrates with an accuracy of ±2% of the measured flowrate.

2.2.4 NO flowmeter. A calibrated flowmeter capable of measuring and monitoring NO flowrates with an accuracy of ±2% of the measured flowrate. (Rotameters have been

reported to operate unreliably when measuring low NO flows and are not recommended.)

2.2.5 Pressure regulator for standard NO cylinder. This regulator must have a non-reactive diaphragm and internal parts and a suitable delivery pressure.

2.2.6 Drier. Scrubber to remove moisture from the permeation device air system. The use of the drier is optional with NO2 permeation devices not sensitive to moisture. (Refer to the supplier's instructions for use of the permeation device.)

2.2.7 Constant temperature chamber. Chamber capable of housing the NO2 permeation device and maintaining its temperature to within 10.1 °C.

2.2.8 Temperature measuring device. Device capable of measuring and monitoring the temperature of the NO2 permeation device with an accuracy of ±0.05 °C.

2.2.9 Valves. A valve may be used as shown in Figure 2 to divert the NO2 from the permeation device when zero air or NO is required at the manifold. A second valve may be used to divert the NO flow when zero air or NO2 is required at the manifold.

The valves should be constructed of glass, Teflon®, or other nonreactive material.

2.2.10 Mixing chamber. A chamber constructed of glass, Teflon®, or other nonreactive material and designed to provide thorough mixing of pollutant gas streams and diluent air.

2.2.11 Output manifold. The output manifold should be constructed of glass, Teflon®, or other non-reactive material and should be of sufficient diameter to insure an insignificant pressure drop at the analyzer connection. The system must have a vent designed to insure atmospheric pressure at the manifold and to prevent ambient air from entering the manifold.

2.3 Reagents.

2.3.1 Calibration standards. Calibration standards are required for both NO and NO2. The reference standard for the calibration may be either an NO or NO2 standard, and must be traceable to a National Bureau of Standards (NBS) NO in N2 Standard Reference Material (SRM 1683 or SRM 1684), and NBS NO2 Standard Reference Material (SRM 1629), or an NBS/EPA-approved commercially available Certified Reference Material (CRM). CRM's are described in Reference 14, and a list of CRM sources is available from the address shown for Reference 14. Reference 15 gives recommended procedures for certifying an NO gas cylinder against an NO

0:

SRM or CRM and for certifying an NO2 permeation device against an NO2 SRM. Reference 13 contains procedures for certifying an NO gas cylinder against an NO2 SRM and for certifying an NO2 permeation device against an NO SRM or CRM. A procedure for determining the amount of NO2 impurity in an NO cylinder is also contained in Reference 13. The NO or NO2 standard selected as the reference standard must be used to certify the other standard to ensure consistency between the two standards.

2.3.1.1 NO2 Concentration standard. A permeation device suitable for generating NO2 concentrations at the required flow-rates over the required concentration range. If the permeation device is used as the reference standard, it must be traceable to an SRM or CRM as specified in 2.3.1. If an NO cylinder is used as the reference standard, the NO2 permeation device must be certified against the NO standard according to the procedure given in Reference 13. The use of the permeation device should be in strict accordance with the instructions supplied with the de2 vice. Additional information regarding the use of permeation devices is given by Scaringelli et al. (11) and Rook et al. (12).

2.3.1.2 NO Concentration standard. Gas cylinder containing 50 to 100 ppm NO in N2 with less than 1 ppm NO2. If this cylinder is used as the reference standard, the cylinder must traceable to an SRM or CRM as specified in 2.3.1. If an NO2 permeation device is used as the reference standard, the NO cylinder must be certified against the NO2 standard according to the procedure given in Reference 13. The cylinder should be recertified on a regular basis as determined by the local quality control program.

2.3.3 Zero air. Air, free of contaminants which might react with NO or NO2 or cause a detectable response on the NO/NO/NO2 analyzer. When using permeation devices that are sensitive to moisture, the zero air passing across the permeation device must be dry to avoid surface reactions on the device. (Refer to the supplier's instructions for use of the permeation device.) A procedure for generating zero air is given in reference 13. 2.4 Procedure.

2.4.1 Assemble the calibration apparatus such as the typical one shown in Figure 2.

2.4.2 Insure that all flowmeters are calibrated under the conditions of use against a reliable standard such as a soap bubble meter or wet-test meter. All volumetric flowrates should be corrected to 25 °C and 760 mm Hg. A discussion on the calibration of flowmeters is given in reference 13.

2.4.3 Install the permeation device in the constant temperature chamber. Provide a small fixed air flow (200-400 scm3/min) across the device. The permeation device should always have a continuous air flow across it to prevent large buildup of NO2 in the system and

a consequent restabilization period.

Record the flowrate as FP. Allow the device to stabilize at the calibration temperature for at least 24 hours. The temperature must be adjusted and controlled to within ±0.1 °C or less of the calibration temperature as monitored with the temperature measuring device.

2.4.4 Precautions must be taken to remove O2 and other contaminants from the NO pressure regulator and delivery system prior to the start of calibration to avoid any conversion of the standard NO to NO2. Failure to do so can cause significant errors in calibration. This problem may be minimized by

(1) Carefully evacuating the regulator, when possible, after the regulator has been connected to the cylinder and before opening the cylinder valve;

(2) Thoroughly flushing the regulator and delivery system with NO after opening the cylinder valve;

(3) Not removing the regulator from the cylinder between calibrations unless absolutely necessary. Further discussion of these procedures is given in reference 13.

2.4.5 Select the operating range of the NO/ NOX NO2 analyzer to be calibrated. In order to obtain maximum precision and accuracy for NO2 calibration, all three channels of the analyzer should be set to the same range. If operation of the NO and NOx channels on higher ranges is desired, subsequent recalibration of the NO and NOx channels on the higher ranges is recommended.

NOTE: Some analyzer designs may require identical ranges for NO, NOx, and NO2 during operation of the analyzer.

2.4.6 Connect the recorder output cable(s) of the NO/NOx/NO2 analyzer to the input terminals of the strip chart recorder(s). All adjustments to the analyzer should be performed based on the appropriate strip chart readings. References to analyzer responses in the procedures given below refer to recorder

responses.

2.4.7 Switch the valve to vent the flow from the permeation device and adjust the diluent air flowrate, FD, to provide zero air at the output manifold. The total air flow must exceed the total demand of the analyzer(s) connected to the output manifold to insure that no ambient air is pulled into the manifold vent. Allow the analyzer to sample zero air until stable NO, NOx, and NO2 responses are obtained. After the responses have stabilized, adjust the analyzer zero control(s).

NOTE: Some analyzers may have separate zero controls for NO, NOx, and NO2. Other analyzers may have separate zero controls only for NO and NOx, while still others may have only one zero common control to all three channels.

Offsetting the analyzer zero adjustments to +5% of scale is recommended to facilitate observing negative zero drift. Record the stable zero air responses as ZNO, ZNOX, and ZNO2.

URL = nominal upper range limit of the NO channel, ppm

NOTE: Some analyzers may have separate span controls for NO, NOx, and NO2. Other analyzers may have separate span controls only for NO and NOx, while still others may have only one span control common to all three channels. When only one span control is available, the span adjustment is made on the NO channel of the analyzer.

If substantial adjustment of the NO span control is necessary, it may be necessary to recheck the zero and span adjustments by repeating steps 2.4.7 and 2.4.8.1. Record the NO concentration and the analyzer's NO re

2.4.8.3 Generate several additional concentrations (at least five evenly spaced points across the remaining scale are suggested to verify linearity) by decreasing FNO or increasing FD. For each concentration generated, calculate the exact NO and NOx concentrations using equations (16) and (18) respectively. Record the analyzer's NO and NOx responses for each concentration. Plot the analyzer responses versus the respective calculated NO and NOx concentrations and draw or calculate the NO and NOx calibration curves. For subsequent calibrations where linearity can be assumed, these curves may be checked with a two-point calibration consisting of a zero point and NO and NOx concentrations of approximately 80 percent of the URL.

2.4.9 Preparation of NO2 calibration curve.

2.4.9.1 Remove the NO flow. Assuming the NO2 zero has been properly adjusted while sampling zero air in step 2.4.7, switch the valve to provide NO2 at the output manifold.

2.4.9.2 Adjust FD to generate an NO2 concentration of approximately 80 percent of the URL of the NO2 range. The total air flow must exceed the demand of the analyzer(s) under calibration. The actual concentration of NO2 is calculated from:

[NO2]OUT = diluted NO2 concentration at the output manifold, ppm

R = permeation rate, μg/min

K = 0.532 μl NO2/μg NO2 (at 25 °C and 760 mm Hg)

FD diluent air flowrate, scm3/min

NOTE: If the analyzer has only one or two span controls, the span adjustments are made on the NO channel or NO and NOx channels and no further adjustment is made here for NO2.

If substantial adjustment of the NO2 span control is necessary it may be necessary to recheck the zero and span adjustments by repeating steps 2.4.7 and 2.4.9.2. Record the NO2 concentration and the analyzer's NO2 response. Using the NOx calibration curve obtained in step 2.4.8, measure and record the NOx concentration as [NOX]M.

2.4.9.3 Adjust FD to obtain several other concentrations of NO2 over the NO2 range (at least five evenly spaced points across the remaining scale are suggested). Calculate each NO2 concentration using equation (20) and record the corresponding analyzer NO2 and NOx responses. Plot the analyzer's NO2 responses versus the corresponding calculated NO2 concentrations and draw or calculate the NO2 calibration curve.

2.4.10 Determination of converter efficiency. 2.4.10.1 Plot [NOx]м (y-axis) versus [NO2]OUT (x-axis) and draw or calculate the converter efficiency curve. The slope of the curve times 100 is the average converter efficiency, Ec. The average converter efficiency must be greater than 96 percent; if it is less than 96 percent, replace or service the converter.

NOTE: Supplemental information on calibration and other procedures in this method are given in reference 13.

3. Frequency of calibration. The frequency of calibration, as well as the number of points necessary to establish the calibration curve and the frequency of other performance checks, will vary from one analyzer to another. The user's quality control program should provide guidelines for initial establishment of these variables and for subsequent alteration as operational experience is accumulated. Manufacturers of analyzers should include in their instruction/operation manuals information and guidance as to these variables and on other matters of operation, calibration, and quality control.

REFERENCES

1. A. Fontijn, A. J. Sabadell, and R. J. Ronco, "Homogeneous Chemiluminescent Measurement of Nitric Oxide with Ozone," Anal. Chem., 42, 575 (1970).

2. D. H. Stedman, E. E. Daby, F. Stuhl, and H. Niki, "Analysis of Ozone and Nitric Oxide

by a Chemiluminiscent Method in Laboratory and Atmospheric Studies of Photochemical Smog," J. Air Poll. Control Assoc., 22, 260 (1972).

3. B. E. Martin, J. A. Hodgeson, and R. K. Stevens, "Detection of Nitric Oxide Chemiluminescence at Atmospheric Pressure, Presented at 164th National ACS Meeting, New York City, August 1972.

4. J. A. Hodgeson, K. A. Rehme, B. E. Martin, and R. K. Stevens, "Measurements for Atmospheric Oxides of Nitrogen and Ammonia by Chemiluminescence,' Presented at 1972 APCA Meeting, Miami, FL, June 1972.

5. R. K. Stevens and J. A. Hodgeson, "Applications of Chemiluminescence Reactions to the Measurement of Air Pollutants," Anal. Chem., 45, 443A (1973).

6. L. P. Breitenbach and M. Shelef, "Development of a Method for the Analysis of NO2 and NH3 by NO-Measuring Instruments," J. Air Poll. Control Assoc., 23, 128 (1973).

7. A. M. Winer, J. W. Peters, J. P. Smith, and J. N. Pitts, Jr., "Response of Commercial Chemiluminescent NO-NO2 Analyzers to Other Nitrogen-Containing Compounds," Environ. Sci. Technol., 8, 1118 (1974).

8. K. A. Rehme, B. E. Martin, and J. A. Hodgeson, Tentative Method for the Calibration of Nitric Oxide, Nitrogen Dioxide, and Ozone Analyzers by Gas Phase Titration," EPA-R2-73-246, March 1974.

9. J. A. Hodgeson, R. K. Stevens, and B. E. Martin, "A Stable Ozone Source Applicable as a Secondary Standard for Calibration of Atmospheric Monitors," ISA Transactions, 11, 161 (1972).

10. A. E. O'Keeffe and G. C. Ortman, "Primary Standards for Trace Gas Analysis," Anal. Chem., 38, 760 (1966).

11. F. P. Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. P. Bell, "Preparation of Known Concentrations of Gases and Vapors with Permeation Devices Calibrated Gravimetrically," Anal. Chem., 42, 871 (1970).

12. H. L. Rook, E. E. Hughes, R. S. Fuerst, and J. H. Margeson, "Operation Characteristics of NO2 Permeation Devices," Presented at 167th National ACS Meeting, Los Angeles, CA, April 1974.

13. E. C. Ellis, "Technical Assistance Document for the Chemiluminescence Measurement of Nitrogen Dioxide," EPA-E600/4-75-003 (Available in draft form from the United States Environmental Protection Agency, Department E (MD-76), Environmental Monitoring and Support Laboratory, Research Triangle Park, NC 27711).

14. A Procedure for Establishing Traceability of Gas Mixtures to Certain National Bureau of Standards Standard RefJoint erence Materials. EPA-600/7-81-010, publication by NBS and EPA. Available from the U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory (MD-77), Research Triangle Park, NC 27711, May 1981.