Pt. 50, App. D
Appendix D to Part 50
—Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere
1. Ambient air and ethylene are delivered simultaneously to a mixing zone where the ozone in the air reacts with the ethylene to emit light, which is detected by a photomultiplier tube. The resulting photocurrent is amplified and is either read directly or displayed on a recorder.
2. 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
and calibrated as follows:
1. Principle. The calibration procedure is based on the photometric assay of ozone (O3) concentrations in a dynamic flow system. The concentration of O3 in an absorption cell is determined from a measurement of the amount of 254 nm light absorbed by the sample. This determination requires knowledge of (1) the absorption coefficient (α) of O3 at 254 nm, (2) the optical path length (l) through the sample, (3) the transmittance of the sample at a wavelength of 254 nm, and (4) the temperature (T) and pressure (P) of the sample. The transmittance is defined as the ratio I/I0, where I is the intensity of light which passes through the cell and is sensed by the detector when the cell contains an O3 sample, and I0 is the intensity of light which passes through the cell and is sensed by the detector when the cell contains zero air. It is assumed that all conditions of the system, except for the contents of the absorption cell, are identical during measurement of I and I0. The quantities defined above are related by the Beer-Lambert absorption law,
α = absorption coefficient of O3 at 254 nm=308 ±4 atm−1 cm−1 at 0 °C and 760 torr.3
(1, 2, 3, 4, 5, 6, 7)
c = O3 concentration in atmospheres
l = optical path length in cm
In practice, a stable O3 generator is used to produce O3 concentrations over the required range. Each O3 concentration is determined from the measurement of the transmittance (I/I0) of the sample at 254 nm with a photometer of path length l and calculated from the equation,
The calculated O3 concentrations must be corrected for O3 losses which may occur in the photometer and for the temperature and pressure of the sample.
2. Applicability. This procedure is applicable to the calibration of ambient air O3 analyzers, either directly or by means of a transfer standard certified by this procedure. Transfer standards must meet the requirements and specifications set forth in Reference 8.
3. Apparatus. A complete UV calibration system consists of an ozone generator, an output port or manifold, a photometer, an appropriate source of zero air, and other components as necessary. The configuration must provide a stable ozone concentration at the system output and allow the photometer to accurately assay the output concentration to the precision specified for the photometer (3.1). Figure 1 shows a commonly used configuration and serves to illustrate the calibration procedure which follows. Other configurations may require appropriate variations in the procedural steps. All connections between components in the calibration system downstream of the O3 generator should be of glass, Teflon, or other relatively inert materials. Additional information regarding the assembly of a UV photometric calibration apparatus is given in Reference 9. For certification of transfer standards which provide their own source of O3, the transfer standard may replace the O3 generator and possibly other components shown in Figure 1; see Reference 8 for guidance.
3.1 UV photometer. The photometer consists of a low-pressure mercury discharge lamp, (optional) collimation optics, an absorption cell, a detector, and signal-processing electronics, as illustrated in Figure 1. It must be capable of measuring the transmittance, I/I0, at a wavelength of 254 nm with sufficient precision such that the standard deviation of the concentration measurements does not exceed the greater of 0.005 ppm or 3% of the concentration. Because the low-pressure mercury lamp radiates at several wavelengths, the photometer must incorporate suitable means to assure that no O3 is generated in the cell by the lamp, and that at least 99.5% of the radiation sensed by the detector is 254 nm radiation. (This can be readily achieved by prudent selection of optical filter and detector response characteristics.) The length of the light path through the absorption cell must be known with an accuracy of at least 99.5%. In addition, the cell and associated plumbing must be designed to minimize loss of O3 from contact with cell walls and gas handling components. See Reference 9 for additional information.
3.2 Air flow controllers. Devices capable of regulating air flows as necessary to meet the output stability and photometer precision requirements.
3.3 Ozone generator. Device capable of generating stable levels of O3 over the required concentration range.
3.4 Output manifold. The output manifold should be constructed of glass, Teflon, or other relatively inert material, and should be of sufficient diameter to insure a negligible pressure drop at the photometer connection and other output ports. The system must have a vent designed to insure atmospheric pressure in the manifold and to prevent ambient air from entering the manifold.
3.5 Two-way valve. Manual or automatic valve, or other means to switch the photometer flow between zero air and the O3 concentration.
3.6 Temperature indicator. Accurate to ±1 °C.
3.7 Barometer or pressure indicator. Accurate to ±2 torr.
4.1 Zero air. The zero air must be free of contaminants which would cause a detectable response from the O3 analyzer, and it should be free of NO, C2 H4, and other species which react with O3. A procedure for generating suitable zero air is given in Reference 9. As shown in Figure 1, the zero air supplied to the photometer cell for the I0 reference measurement must be derived from the same source as the zero air used for generation of the ozone concentration to be assayed (I measurement). When using the photometer to certify a transfer standard having its own source of ozone, see Reference 8 for guidance on meeting this requirement.
5.1 General operation. The calibration photometer must be dedicated exclusively to use as a calibration standard. It should always be used with clean, filtered calibration gases, and never used for ambient air sampling. Consideration should be given to locating the calibration photometer in a clean laboratory where it can be stationary, protected from physical shock, operated by a responsible analyst, and used as a common standard for all field calibrations via transfer standards.
5.2 Preparation. Proper operation of the photometer is of critical importance to the accuracy of this procedure. The following steps will help to verify proper operation. The steps are not necessarily required prior to each use of the photometer. Upon initial operation of the photometer, these steps should be carried out frequently, with all quantitative results or indications recorded in a chronological record either in tabular form or plotted on a graphical chart. As the performance and stability record of the photometer is established, the frequency of these steps may be reduced consistent with the documented stability of the photometer.
5.2.1 Instruction manual: Carry out all set up and adjustment procedures or checks as described in the operation or instruction manual associated with the photometer.
5.2.2 System check: Check the photometer system for integrity, leaks, cleanliness, proper flowrates, etc. Service or replace filters and zero air scrubbers or other consumable materials, as necessary.
5.2.3 Linearity: Verify that the photometer manufacturer has adequately established that the linearity error of the photometer is less than 3%, or test the linearity by dilution as follows: Generate and assay an O3 concentration near the upper range limit of the system (0.5 or 1.0 ppm), then accurately dilute that concentration with zero air and reassay it. Repeat at several different dilution ratios. Compare the assay of the original concentration with the assay of the diluted concentration divided by the dilution ratio, as follows
E = linearity error, percent
A1 = assay of the original concentration
A2 = assay of the diluted concentration
R = dilution ratio = flow of original concentration divided by the total flow
The linearity error must be less than 5%. Since the accuracy of the measured flow-rates will affect the linearity error as measured this way, the test is not necessarily conclusive. Additional information on verifying linearity is contained in Reference 9.
5.2.4 Intercomparison: When possible, the photometer should be occasionally intercompared, either directly or via transfer standards, with calibration photometers used by other agencies or laboratories.
5.2.5 Ozone losses: Some portion of the O3 may be lost upon contact with the photometer cell walls and gas handling components. The magnitude of this loss must be determined and used to correct the calculated O3 concentration. This loss must not exceed 5%. Some guidelines for quantitatively determining this loss are discussed in Reference 9.
5.3 Assay of O
5.3.1 Allow the photometer system to warm up and stabilizer.
5.3.2 Verify that the flowrate through the photometer absorption cell, F allows the cell to be flushed in a reasonably short period of time (2 liter/min is a typical flow). The precision of the measurements is inversely related to the time required for flushing, since the photometer drift error increases with time.
5.3.3 Insure that the flowrate into the output manifold is at least 1 liter/min greater than the total flowrate required by the photometer and any other flow demand connected to the manifold.
5.3.4 Insure that the flowrate of zero air, Fz, is at least 1 liter/min greater than the flowrate required by the photometer.
5.3.5 With zero air flowing in the output manifold, actuate the two-way valve to allow the photometer to sample first the manifold zero air, then Fz. The two photometer readings must be equal (I=Io).
In some commercially available photometers, the operation of the two-way valve and various other operations in section 5.3
may be carried out automatically by the photometer.
5.3.6 Adjust the O3 generator to produce an O3 concentration as needed.
5.3.7 Actuate the two-way valve to allow the photometer to sample zero air until the absorption cell is thoroughly flushed and record the stable measured value of Io.
5.3.8 Actuate the two-way valve to allow the photometer to sample the ozone concentration until the absorption cell is thoroughly flushed and record the stable measured value of I.
5.3.9 Record the temperature and pressure of the sample in the photometer absorption cell. (See Reference 9 for guidance.)
5.3.10 Calculate the O3 concentration from equation 4. An average of several determinations will provide better precision.
[O3]OUT = O3 concentration, ppm
α = absorption coefficient of O3 at 254 nm=308 atm−1 cm−1 at 0 °C and 760 torr
l = optical path length, cm
T = sample temperature, K
P = sample pressure, torr
L = correction factor for O3 losses from 5.2.5=(1-fraction O3 lost).
Some commercial photometers may automatically evaluate all or part of equation 4. It is the operator's responsibility to verify that all of the information required for equation 4 is obtained, either automatically by the photometer or manually. For “automatic” photometers which evaluate the first term of equation 4 based on a linear approximation, a manual correction may be required, particularly at higher O3 levels. See the photometer instruction manual and Reference 9 for guidance.
5.3.11 Obtain additional O3 concentration standards as necessary by repeating steps 5.3.6 to 5.3.10 or by Option 1.
5.4 Certification of transfer standards.
A transfer standard is certified by relating the output of the transfer standard to one or more ozone standards as determined according to section 5.3
. The exact procedure varies depending on the nature and design of the transfer standard. Consult Reference 8 for guidance.
5.5 Calibration of ozone analyzers.
Ozone analyzers are calibrated as follows, using ozone standards obtained directly according to section 5.3
or by means of a certified transfer standard.
5.5.1 Allow sufficient time for the O3 analyzer and the photometer or transfer standard to warmup and stabilize.
5.5.2 Allow the O3 analyzer to sample zero air until a stable response is obtained and adjust the O3 analyzer's zero control. Offsetting the analyzer's zero adjustment to 5% of scale is recommended to facilitate observing negative zero drift. Record the stable zero air response as “Z”.
5.5.3 Generate an O3 concentration standard of approximately 80% of the desired upper range limit (URL) of the O3 analyzer. Allow the O3 analyzer to sample this O3 concentration standard until a stable response is obtained.
5.5.4 Adjust the O3 analyzer's span control to obtain a convenient recorder response as indicated below:
recorder response (%scale) =
URL = upper range limit of the O3 analyzer, ppm
Z = recorder response with zero air, % scale
Record the O3 concentration and the corresponding analyzer response. If substantial adjustment of the span control is necessary, recheck the zero and span adjustments by repeating steps 5.5.2 to 5.5.4.
5.5.5 Generate several other O3 concentration standards (at least 5 others are recommended) over the scale range of the O3 analyzer by adjusting the O3 source or by Option 1. For each O3 concentration standard, record the O3 and the corresponding analyzer response.
5.5.6 Plot the O3 analyzer responses versus the corresponding O3 concentrations and draw the O3 analyzer's calibration curve or calculate the appropriate response factor.
5.5.7 Option 1: The various O3 concentrations required in steps 5.3.11 and 5.5.5 may be obtained by dilution of the O3 concentration generated in steps 5.3.6 and 5.5.3. With this option, accurate flow measurements are required. The dynamic calibration system may be modified as shown in Figure 2 to allow for dilution air to be metered in downstream of the O3 generator. A mixing chamber between the O3 generator and the output manifold is also required. The flowrate through the O3 generator (Fo) and the dilution air flowrate (FD) are measured with a reliable flow or volume standard traceable to NBS. Each O3 concentration generated by dilution is calculated from:
[O3]′OUT = diluted O3 concentration, ppm
F0 = flowrate through the O3 generator, liter/min
FD = diluent air flowrate, liter/min
1. E.C.Y. Inn and Y. Tanaka, “Absorption coefficient of Ozone in the Ultraviolet and Visible Regions”, J. Opt. Soc. Am., 43, 870 (1953).
2. A. G. Hearn, “Absorption of Ozone in the Ultraviolet and Visible Regions of the Spectrum”, Proc. Phys. Soc. (London), 78, 932 (1961).
3. W. B. DeMore and O. Raper, “Hartley Band Extinction Coefficients of Ozone in the Gas Phase and in Liquid Nitrogen, Carbon Monoxide, and Argon”, J. Phys. Chem., 68, 412 (1964).
4. M. Griggs, “Absorption Coefficients of Ozone in the Ultraviolet and Visible Regions”, J. Chem. Phys., 49, 857 (1968).
5. K. H. Becker, U. Schurath, and H. Seitz, “Ozone Olefin Reactions in the Gas Phase. 1. Rate Constants and Activation Energies”, Int'l Jour. of Chem. Kinetics, VI, 725 (1974).
6. M. A. A. Clyne and J. A. Coxom, “Kinetic Studies of Oxy-halogen Radical Systems”, Proc. Roy. Soc., A303, 207 (1968).
7. J. W. Simons, R. J. Paur, H. A. Webster, and E. J. Bair, “Ozone Ultraviolet Photolysis. VI. The Ultraviolet Spectrum”, J. Chem. Phys., 59, 1203 (1973).
8. Transfer Standards for Calibration of Ambient Air Monitoring Analyzers for Ozone, EPA publication number EPA-600/4-79-056, EPA, National Exposure Research Laboratory, Department E, (MD-77B), Research Triangle Park, NC 27711.
9. Technical Assistance Document for the Calibration of Ambient Ozone Monitors, EPA publication number EPA-600/4-79-057, EPA, National Exposure Research Laboratory, Department E, (MD-77B), Research Triangle Park, NC 27711.
[44 FR 8224, Feb. 8, 1979, as amended at 62 FR 38895, July 18, 1997]