3.9  Determination of ozone

3.9.1  Introduction

Both chemiluminescence and UV-absorption based methods have been used in the past 20 years for the measurement of ozone in ambient air. After many years of discussion and intercomparison between different wet-chemical methods for calibration of ozone generators used for calibration of the monitors, it is now generally accepted to use UV-photometry as the primary calibration method. Since the UV-absorption method has proven to be reliable and robust in field operations, this method is recommended and described in this manual. The description follows the principles from the International Organization for Standardization (ISO) (ISO 13964:1998).

The general requirements to regional sites are valid for ozone. In addition ozone is a reactive gas that is taken up by vegetation; measurements should therefore be carried out well away from plant life. The Canadian networks recommend sampling at 3-5 m above ground level and more than 20 m away from trees.

Details in the operating procedures for the O3-instruments will be found in the Operator Manual for the different commercial monitors.

 3.9.2  Field of application

The described method is applicable for continuous monitoring of ozone (O3) in ambient air. The method can be used in the range 2 mg/m3 to 2 mg/m3 (1-1000 ppb).

3.9.3   Principle

Sample air is drawn continuously through an optical absorption cell where it is irradiated by monochromatic light at 253.7 nm from a stabilized low-pressure Hg discharge lamp. The absorption of this radiation by the sample air is a measure of the ambient air ozone concentration. To avoid interference from other gases absorbing light at the same wavelength and from instability in the light source, an ozone catalytic converter is used to selectively remove ozone from the sample stream either in the sample cell by alternately fill the sample cell with unscrubbed and scrubbed sample air, or by using another parallel sample cell.

The Beer-Lambert equation, shown below, is used to calculate the concentration of ozone from the ratio of the two light intensities measured:

          I/I0 = exp (-acd)

where

I0    is the light intensity measured with no ozone in the gas sample
I      is the light intensity measured with ozone in the gas sample
a     is the ozone absorption coefficient at 253.7 nm (1.44 x 10-5 m2/mg)
c     is the mass concentration of ozone in mg/m3
d     is the optical path length in m

UV photometry is also the recommended primary calibration procedure. The use of transfer standards (including non-UV methods) is possible if they have been previously calibrated against the primary UV photometric method.

3.9.4  Reagents and materials

Sampling line

The sampling line shall be made of material that is inert to ozone, such as glass or fluorocarbon polymer and shall be as short as possible to keep the residence time to a minimum. Any ambient nitric oxide present in the sample air will react with some of the ozone during the residence time in the sampling line. The sampling line or manifold shall be clean and should normally be replaced after one year. If high amount of dust deposition in the sampling line is expected, it should be cleaned or replaced more frequently.

Particle filter

The sample air has to be drawn through a filter before entering the absorption cell. The filter and its support shall be made of material inert to ozone, such as fluorocarbon polymer, and shall remove all particles likely to alter the performance of the analyzer. It shall be changed on a regular basis, depending on the ambient particle concentrations at the sampling site. This is necessary because excessive accumulation of particles on the filter can cause loss of ozone from the sample air and an excessive pressure drop across the filter.

Normally, a filter pore size of 5 µm is used.

Generally, new filters need some time to be conditioned, and the first 5-15 minutes data after the filter change should be discarded.

Zero air

Zero air is required in the analyzer calibration procedure. The zero air shall be free of ozone, nitrogen oxides and any other interfering substance that can cause a positive or negative response in the UV photometer. The zero air supplied to the photometer during the I0 measurement shall be the same as that used for generation of calibration ozone concentrations.

If synthetic air is used, the oxygen content shall be within 2% of the normal atmospheric concentration.

Details on a system for making zero air from ambient air is found in EPA (1979a).

3.9.5  Apparatus

3.9.5.1  UV photometric ambient ozone analyzer

The components of a typical UV photometric ozone measuring system are shown in Figure 3.9.1. The monitor should have specifications as listed below:

Range : 0.002–2 mg/m3
Output : 0–10V full scale
Lag time : 20 s
Rise time : 15 s
Fall time : 15 s
Warm-up time : 2h
Zero instability : ± 2 µg/m3 per week
Span instability : < 0.5% per week
Repeatability : ± 2 µg/m3
Period of unattended operation : 7 d
Sample flow rate : 1.5–2 l/min
Temperature range : 0–45 °C

Figure 3.9.1: UV photometric ozone measurement system.

Commercial instruments that meet these specifications are readily available.

One important component of the instrument is the ozone-specific scrubber. Normally, instrument manufacturers give an average lifetime of such scrubbers, however, the actual lifetime will depend on the concentration of other pollutants at the sampling site. A scrubber failure will give a decrease in response to ozone.

Since the ozone absorption coefficient given in the formula in Section 3.9.3 is dependent of temperature and pressure, it is important that the instruments have temperature and pressure indicators capable of measuring the temperature and pressure in the absorption cell with an accuracy of ±0.1°C and ±0.1kPa respectively.

3.9.5.2  Apparatus for calibration

A simplified scheme of a primary ozone calibration system is shown in Figure 3.9.2 and consists of an ozone generator and a UV calibration photometer.

 

Figure 3.9.2: Primary ozone calibration system.

Primary UV calibration photometer

The primary UV calibration photometer shall be used for only that purpose and shall be carefully maintained under stable laboratory conditions. The different parts of the calibration photometer are the same as shown in Fig. 3.9.1 but without the catalytic ozone scrubber. Every part shall be manufactured optimum care.

When using the expression primary calibration photometer in this section, it will normally mean a laboratory or e.g. a country’s primary calibration photometer. Such primary calibration photometers should preferably be traceable to the NIST (US National Institute of Standards and Technology) UV-photometer. One way of obtaining this is to buy from NIST a NIST-UV photometer which periodically will be checked by NIST. Another way is to have the laboratory or national primary calibrator calibrated against a NIST-UV photometer in another country e.g. once per year. To the CCCs knowledge there are at least three NIST-UV photometers in Europe:

          Institute of Applied Environmental Research at the University of Stockholm,
          Air Pollution Laboratory, Stockholm, Sweden
          Phone  :    + 46 8 674 70 00
          Fax      :    + 46 8 612 08 49

          Swiss Federal Laboratories for Materials Testing and Research (EMPA),
          Dübendorf, Switzerland
          Phone  :    + 41 1 823 55 11
          Fax      :    + 41 1 821 62 44

          Czech Hydrometeorological Institute
          Prague, Czech Republic
          Phone  :    +42 2 401 6719
          Fax      :    +42 2 401 0800

Ozone generator

This apparatus generates the stable ozone concentrations that are measured by the calibration photometer described above. It shall be capable of producing steady ozone concentrations in the range of interest at the required flow rate and throughout the calibration period. If a variable ozone generator is not available, the calibration system shall include a means of diluting the ozone with additional zero air, and a mixing chamber shall be installed before the output manifold.

Output manifold

The output manifold shall be of materials inert to ozone, such as glass or fluorocarbon polymer. It shall be of sufficient diameter and be vented to ensure equal pressure inside and outside the manifold. The vent shall be located so as to prevent intrusion of ambient air.

Procedure

In the next paragraphs the principles for the measurement and calibration procedures are described.

Operation of the ambient ozone analyzer

Install the instrument in a suitable location. The room for the analyzer shall be temperature controlled. For background ozone measurements, local sources of nitric oxide have to be avoided as it readily react with ozone. The air intake should be positioned at least 2 m above ground.

Follow the manufacturers instructions for the specific analyzer to set correctly the various parameters, including UV source lamp intensity, sample flow rate and activation of the electronic temperature/pressure compensation.

Introduce sample air into the instrument and record the ozone concentrations by means of a suitable recording device.

During continuous operation, checks on the instrument zero and operational parameters shall be made at least once per week. If an internal ozone source is available in the monitor, this can be used for a span check, but is normally not stable enough for making calibrations. It is recommended that a multipoint calibration be carried out every 3-4 months.

3.9.5.3  Calibration of the ambient ozone analyzer

Principle

The calibration system is shown in Figure 3.9.2. Various ozone concentrations (in air) are generated and accurately measured with the primary UV calibration photometer. These ozone calibration concentrations are simultaneously sampled by the ambient ozone analyzer to be calibrated via a common manifold. The analyzers response is plotted against the ozone concentrations measured by the primary UV calibration photometer.

Alternatively, a transfer standard calibration method can be calibrated against the primary ozone standard and then used to calibrate the ambient analyzer at the sampling location.

More details for the calibration procedure are given in EPA (1979a) and in the instrument manuals.

Transfer standard calibration procedure

A transfer standard has to be used whenever the primary calibration photometer is not readily available such as at the sampling site.

The transfer standard shall be calibrated against the primary UV photometric standard at least once per year and its accuracy shall be maintained within ±5% between successive primary calibrations.

The recommended (portable) transfer standard calibration method is a second UV photometer system with its own zero air and ozone supply. The other acceptable transfer standard calibration methods are the gas-phase titration of excess nitric oxide by ozone (or vice versa) and the laboratory-based neutral buffered iodide-potassium bromide (KIBR) method. For details see EPA (1979b).

3.9.6  Co-operation with WMO GAW on surface ozone measurements

WMO has produced a quality assurance plan for surface ozone including standard operating procedures (WMO GAW No. 97). WMO GAW and EMEP are trying to harmonize and will hopefully adopt the same SOPs, and co-operate with respect to audit and calibration.

3.9.7  References

EPA (1979a) Technical assistance document for the calibration of ambient ozone monitors. Research Triangle Park, N.C., United States Environmental Protection Agency (EPA-600/4-79-057).

EPA (1979b) Transfer standards for calibration of air monitoring analyzers for ozone. Research Triangle Park, N.C., United States Environmental Protection Agency (EPA-600/4-79-056).

WMO (1994) Quality assurance project plan for continuous ground based ozone measurements. Prepared by GAW QA/SAC. Geneva (WMO GAW No. 97).


Last revision: November 2001