Ambient Air quality Monitoring Guidlines. - Maharashtra Pollution ...

2. Infrastructure for conducting Inter-Laboratory comparison on ambient air
quality methods.
The primary requirement for conducting inter-laboratory comparison on physical and
chemical methods for measurement of air pollutants is depicted in Fig. 5.2.
This system is primarily known as Ring Test Facility. It mainly consists of several mass
flow meters of different flow capacities. A mass flow controller, regulates these mass
flow meters. Provision for producing the dilute air is done by means of compressor. The
compressed air produced by the compressor is dried and purified by passing the air
through silica gel and activated carbon respectively. The standard gas stored in steel
cylinders is diluted to the desired concentration by the dilution air or zero air. It is difficult
to store nitrogen dioxide in pressurised cylinders due to its unstable nature. Therefore
nitric oxide stored in steel cylinders are converted to nitrogen dioxide by means of a
process called Gas Phase titration (GPT). In order to attain GPT, nitric oxide is allowed
to react with ozone ( produced by an ozone generator) to produce nitrogen dioxide. The
ozone generator used for GPT can also produce desired ozone gas mixtures for
calibration of continuous monitoring analysers and proficiency testing. In order to check
the mixing of gas mixtures it is advisable to install the continuous monitoring analysers
on the Ring Test Facility. The calibration of the continuous monitoring analysers is
accomplished by means of static dilution system. The static dilution system is shown in
Fig.5.3.
The standard gas mixtures produced by the Ring Test Facility are made available to
different participants through a glass tube for inter-laboratory comparison exercises.
(i) Choice Of Method/Procedure
Participants can use the method of their choice, which is consistent with routine
procedures used in their laboratories. However, in certain circumstances, the
coordinator may instruct participants to use a specified method. Such methods are
usually nationally or internationally accepted standard methods, and will have been
validated by an appropriate procedure ( e.g. collaborative trial).
Where calibration procedure is used, the assigned value will often be a reference value
obtained from measurements obtained by a reputed calibration laboratory which should
use a well defined and accepted procedure. It is desirable that participating laboratories
use the same or similar procedure, but this will not always be practicable for calibration
laboratories.
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