B. Suggested Definitions of Performance Specifications:

Range The minimum and maximum measurement limits.

Output-Electrical signal which is proportional to the measurement; intended for connection to readout or data processing devices. Usually expressed as millivolts or milliamps full scale at a given impedence. Full Scale-The maximum measuring limit for a given range.

Minimum Detectable Sensitivity-The smallest amount of input concentration that can be detected as the concentration approaches zero.

Accuracy-The degree of agreement between a measured value and the true value; usually expressed at ± percent of full scale. Lag Time-The time interval from a step change in input concentration at the instrument inlet to the first corresponding change in the instrument output. Time to 90 Percent Response The time interval from a step change in the input concentration at the instrument inlet to a

reading of 90 percent of the ultimate recorded concentration.

Rise Time (90 percent)-The interval between initial response time and time to 90 percent response after a step decrease in the inlet concentration.

Zero Drift-The change in instrument output over a stated time period, usually 24 hours, of unadjusted continuous operation, when the input concentration is zero; usually expressed as percent full scale.

Span Drift-The change in instrument output over a stated time period, usually 24 hours, of unadjusted continuous operation, when the input concentration is a stated upscale value; usually expressed as percent full scale.

Precision-The degree of agreement between repeated measurements of the same concentration. It is expressed as the average deviation of the single results from the mean.

Operational Period-The period of time over which the instrument can be expected to operate unattended within specifications. Noise Spontaneous deviations from a mean output not caused by input concentration changes.

Interference-An undesired positive or negative output caused by a substance other than the one being measured. Interference Equivalent-The portion of in

dicated input concentration due to the presence of an interferent.

Operating Temperature Range The range of ambient temperatures over which the instrument will meet all performance specifications.

Operating Humidity Range-The range of ambient relative humidity over which the instrument will meet all performance specifications.

Linearity-The maximum deviation between an actual instrument reading and the reading predicted by a straight line drawn between upper and lower calibration points.

1. Atmospheric concentrations of nitrogen dioxide (NO2) are measured indirectly by photometrically measuring the light intensity, at wavelengths greater than 600 nanometers, resulting from the chemiluminescent reaction of nitric oxide (NO) with ozone (O3). (1,2,3) NO2 is first quantitatively reduced to NO(4,5,6) by means of a converter. NO, which commonly exists in ambient air together with NO2, passes through the converter unchanged causing a resultant total NO, concentration equal to NO+NO2. A sample of the input air is also measured without having passed through the converted. This latter NO measurement is subtracted from the former measurement (NO+NO2) to yield

the final NO2 measurement. The NO and NO+NO2 measurements may be made concurrently with dual systems, or cyclically with the same system provided the cycle time does not exceed 1 minute.

2. Sampling considerations.

2.1 Chemiluminescence NO/NO/NO2 analyzers will respond to other nitrogen containing compounds, such as peroxyacetyl nitrate (PAN), which might be reduced to NO in the thermal converter. (7) Atmospheric concentrations of these potential interferences are generally low relative to NO2 and valid NO2 measurements may be obtained. In certain geographical areas, where the concentration of these potential interferences is known or suspected to be high relative to NO2, the use of an equivalent method for the measurement of NO2 is recommended.

2.2 The use of integrating flasks on the sample inlet line of chemiluminescence NO/ NO/NO2 analyzers is optional and left to couraged. The sample residence time between the sampling point and the analyzer

should be kept to a minimum to avoid erroneous NO2 measurements resulting from the reaction of ambient levels of NO and Os in the sampling system.

2.3 The use of particulate filters on the sample inlet line of chemiluminescence NO/ NO/NO2 analyzers is optional and left to the discretion of the user or the manufacturer. Use of the filter should depend on the analyzer's susceptibility to interference, malfunction, or damage due to particulates. Users are cautioned that particulate matter concentrated on a filter may cause erroneous NO2 measurements and therefore filters should be changed frequently.

3. An analyzer based on this principle will be considered a reference method only if it has been designated as a reference method in accordance with Part 53 of this chapter. Calibration

1. Alternative A-Gas phase titration (GPT) of an NO standard with 03.

Major equipment required: Stable O3 generator. Chemiluminescence NO/NO/NO2 analyzer with strip chart recorder(s). NO concentration standard.

1.1 Principle. This calibration technique is based upon the rapid gas phase reaction between NO and Os to produce stoichiometric quantities of NO2 in accordance with the following equation: (8)

The quantitative nature of this reaction is such that when the NO concentration is known, the concentration of NO2 can be determined. Ozone is added to excess NO in a dynamic calibration system, and the NO channel of the chemiluminescence NO/NO/ NO2 analyzer is used as an indicator of changes in NO concentration. Upon the addition of Oз, the decrease in NO concentration observed on the calibrated NO channel is equivalent to the concentration of NO2 produced. The amount of NO2 generated may be varied by adding variable amounts of Oз from a stable uncalibrated O3 generator. (9)

1.2 Apparatus. Figure 1, a schematic of a typical GPT apparatus, shows the suggested configuration of the components listed below. All connections between components in the calibration system downstream from the Os generator should be of glass, Teflon®, or other non-reactive material.

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

1.2.2 NO flow controller. A device capable of maintaining constant NO flows within 12% of the required flowrate. Component parts in contact with the NO should be of a non-reactive material.

1.2.3 Air flowmeters. Calibrated flowmeters capable of measuring and monitoring air flowrates with an accuracy of 12% of the measured flowrate.

1.2.6 Ozone generator. The generator must be capable of generating sufficient and stable levels of Os for reaction with NO to generate NO2 concentrations in the range required. Ozone generators of the electric discharge type may produce NO and NO2 and are not recommended.

1.2.7 Valve. A valve may be used as shown in Figure 1 to divert the NO flow when zero air is required at the manifold. The valve should be constructed of glass, Teflon", or other nonreactive material.

1.2.8 Reaction chamber. A chamber, constructed of glass, Teflon®, or other nonreactive material, for the quantitative reaction of O3 with excess NO. The chamber should be of sufficient volume (VRC) such that the residence time (tr) meets the requirements specified in 1.4. For practical reasons, to should be less than 2 minutes.

1.2.9 Mixing chamber. A chamber constructed of glass, Teflon, or other nonreactive material and designed to provide thorough mixing of reaction products and diluent air. The residence time is not critical when the dynamic parameter specification given in 1.4 is met.

1.2.10 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.

1.3 Reagents.

1.3.1 NO concentration standard. Gas cylinder standard containing 50 to 100 ppm NO in N2 with less than 1 ppm NO2. This standard must be traceable to a National Bureau of Standards (NBS) NO in N2 Standard Reference Material (SRM 1683 or SRM 1684), an 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. A recommended protocol for certifying NO gas cylinders against either an NO SRM or CRM is given in section 2.0.7 of Reference 15. Reference 13 gives procedures for certifying an NO gas cylinder against an NBS NO2 SRM and for determining the amount of NO2 impurity in an NO cylinder.

PR-dynamic parameter specification, determined empirically, to insure complete reaction of the available O3, ppmminute

[NO]RC=NO concentration in the reaction chamber, ppm

R=residence time of the reactant gases in the reaction chamber, minute [NO]STD=concentration of the undiluted NO standard, ppm

FNO-NO flowrate, scm3/min

Fo-Os generator air flowrate, scm3/min

VRC Volume of the reaction chamber, scm3

1.4.2 The flow conditions to be used in the GPT system are determined by the following procedure:

(a) Determine Fr. the total flow required at the output manifold (F-analyzer demand plus 10 to 50% excess).

(b) Establish [NO]out as the highest NO concentration (ppm) which will be required at the output manifold. [NO]our should be approximately equivalent to 90% of the upper range limit (URL) of the NO2 concentration range to be covered.

1.5.1 Assemble a dynamic calibration system such as the one shown in Figure 1.

1.5.2 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.

1.5.3 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.

1.5.4 Select the operating range of the NO/ NO/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 NO, channels on higher ranges is desired, subsequent recalibration of the NO and NO, 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.

1.5.5 Connect the recorder output cable(s) of the NO/NO/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.

1.5.6 Determine the GPT flow conditions required to meet the dynamic parameter specification as indicated in 1.4.

1.5.7 Adjust the diluent air and O3 generator air flows to obtain the flows determined in section 1.4.2. 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 control common to all three channels.

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

1.5.8 Preparation of NO and NO, calibration curves.

1.5.8.1 Adjustment of NO span control. Adjust the NO flow from the standard NO cylinder to generate an NO concentration of approximately 80 percent of the upper range limit (URL) of the NO range. This exact NO concentration is calculated from:

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 1.5.7 and 1.5.8.1. Record the NO concentration and the analyzer's NO response.

1.5.8.2 Adjustment of NO, span control. When adjusting the analyzer's NO, span control, the presence of any NO2 impurity in the standard NO cylinder must be taken into account. Procedures for determining the amount of NO2 impurity in the standard NO cylinder are given in reference 13. The exact NOx concentration is calculated from:

If substantial adjustment of the NOx span control is necessary, it may be necessary to recheck the zero and span adjustments by repeating steps 1.5.7 and 1.5.8.2. Record the NO, concentration and the analyzer's NO, response.

1.5.8.3 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 NO, concentrations using equations (9) and (11) respectively. Record the analyzer's NO and NO, responses for each concentration. Plot the analyzer responses versus the respective calculated NO and NO, concentrations and draw or calculate the NO and NO, 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 NO, concentrations of approximately 80% of the

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