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Traceability of ozone measurements

Traceability of ozone measurements
L. Konopeiko, I. Nekhlioudov and V. Chelibanov

Abstract. The hierarchy of references used in Russia to transfer a unit value for ozone concentration measurements is described. The equipment and methods used in its practical implementation are listed.

1. Introduction
t Ozone, one of the major components of atmospheric air, is produced in the top layers of the atmosphere and in many respects determines the energy flow in the biosphere. Ozone can be formed in significant quantities as a product of photochemical reactions which take place, with the participation of the oxides of nitrogen and of hydrocarbons, in the lowest part of the troposphere. The resulting ozone perturbs the balance of minor gas components in atmospheric air, with the effect that ozone should be considered as an atmospheric pollutant.

Ozone is the strongest oxidizer known and is increasingly used in industry and medicine. Water purification, bactericidal treatment of instruments and bandaging material, deodorization and bactericidal treatment of warehouses containing agricultural and industrial products, synthesis of materials of new composition and the chemical analysis of organic substances, are just a few of the spheres in which ozone and ozone technologies are used commercially. Its chemical properties are such that ozone is considered toxic and is listed as an atmospheric pollutant of the most dangerous kind. For this reason stringent conditions apply to the use of this substance in the bottom layers of the atmosphere. (In Russia, the maximum permissible concentration is 160 p-g/m3).

2. Russian standards
At present, several kinds of ozonometer are used in Russia [I], but those based on solid-state chemi-luminescent sensors are of particular interest for the applications listed above.

Chemiluminescence observed during the oxidation of organic substances in heterogeneous conditions provides the basis for building inexpensive, yet rather

L. Konopeiko. I. Nekhlioudov and V. Chelibanov: D. I. Mendeleyev Institute for Metrology, St Petersburg, Russian Federation.

Metrologia, 1997, 34, 97-99

sensitive, contact-type gas analysers for ozone. Such devices are portable, have high speed and sensitivity, and are a significant and useful resource in this kind of work [2, 3].

Heterogeneous Chemiluminescence is a process in which light is emitted as a result of chemical reaction, in this case the reaction of certain organic compounds with ozone (03). To control and compensate for the change in sensor sensitivity, the gas-analyser circuit is equipped with an in-built calibration mixture source (calibrator) which takes periodic readings of a constant concentration of the gas to be analysed whenever the device is in operation.

The gas mixture is periodically supplied from the in-built calibrator to the sensor, and the microcontroller of the gas analyser normalizes the operations of the instrument so that it functions according to the level set for the sensor sensitivity.

A peculiarity of chemi luminescent sensors is that the rate of decrease of sensitivity mainly depends on the quantity of gas which has interacted with the sensor surface rather than on its absolute concentration. Here, we understand the expression "quantity of gas" to refer to the product of the mean concentration of a gas to be analysed with the time for which it blows through the chemilurninescent reactor. The stability of operation of a chemihiminescent sensor depends critically on the operating temperature.

It should be noted that, due to the chemical specific character of these instruments, there are practically no observed limitations on the speed of sensor operation. This makes it possible to use chemilurninescent sensors in the pulse mode ensuring extended operation of the instrument as a whole.

Investigations of chemiluminescent sensors show that, with a suitable choice of measurement procedure, they provide metrological characteristics appropriate to the operation of gas analysers in atmosphere monitoring. Some general information about solid-state chemiluminescent sensors produced in Russia is given in Table 1,

In the calibration of ozonometers, use is made of various types of ozone-air and ozone-oxygen generators 97

L. Konopeiko et al.

Table 1

Sensor type Minimum detectable concentration, (uLg/m3) Measured concentration range, (mg/m3) Service life, hours Response lime, W Concentn at which operation ttion of other components no impact on the sensor is detected
SO; mg/m3 NO; NHa Mg/m3 Mg/m3
K-3 DPA 0,3 1,0 0,1 I 0,001 -> 1,000 0.005-0,5 0.001-0.5 0,001-10,0 10000 40000 > 40 000 5000 0,05 0,1 1,0 20 <5 <5 <5 <5 < 10 < 10 < 10 < 10 < 10 < 10 < 5 < 5
whose operating principles are based on photolysis of molecular oxygen or on electric discharge.

The traceability of measurements of ozone content is currently based on a combination of methods and means, their relationship being illustrated in Figure 1.

At present a new generation of standard instruments is being developed in Russia.

The primary standard takes the form of an optical absorption ozone analyser based on measurement of the ozone-absorbed optical radiation fraction passing through a cell at wavelength 254 nm. This is a high-precision UV instrument. A gas sample to be analysed is delivered to the measuring cell and purified air from which ozone has been removed in the deozonator is delivered to the comparison cell. Photodetectors simultaneously measure the light intensity in the






Figure 1. Traceabilhy of ozone measurements.

measurement and comparison channels. With such a design for the optical and pneumatic aspects of the instrument the relative attenuation of the light flux in the cell with the sample to be analysed is proportional to the ozone content and is practically independent of other components present in the sample. The radiation source used is a specially designed spherical mercury (Hg) no-electrode lamp with a buffer gas, excited by a high-frequency oscillator, which makes it possible to release up to 98% of the visible and UV radiation energy at the desired wavelength. This makes it possible to consider it as a monochromatic source.

The high-precision ozone-air generator which produces the primary mixture is of traditional design, receiving ozone-air or ozone-oxygen mixtures from the photolysis of molecular oxygen. We use ozone generators FC-024, FC-2 manufactured by OPTEC Company Ltd for this purpose. Distinctive features of these generators are the high stability of the plasma temperature of the photolytic lamp and rigid control of the light flux in
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