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

9.0 APPARATUS
8.1 Chemiluminescene Instrument - Two basic instrument designs has been
developed for the measurement of total oxides of nitrogen, nitric oxide and the
indirect determination of nitrogen dioxide. In both cases, the determination of
NOx (NOx = NO + NO2) and NO must be accomplished and the nitrogen dioxide
calculated by subtraction of NO from the NOx. The two instrument configurations
utilize the cyclic or dual mode of operation.
8.1.4 The cyclic mode instrument is shown schematically in Figure-1, and has a
single reaction chamber and detector. The incoming sample air is alternately
cycled directly to the reaction chamber to determine NO, or through the
instrument converter to determine NOx. A normal cycle, which is
approximately thirty (30) seconds, is accomplished by means of a timer
controlled solenoid valve. Separate NOx and NO values are determined
every thirty seconds. The photomultiplier tube outputs are amplified and
stored in memory circuits; the difference output, nitrogen dioxide, is updated
electronically after each cycle and similarly stored. Reorder outputs are
available for all three measurement channels, NO, NO2 and NOx.
8.1.5 The dual mode instrument is shown schematically in Figure-2, and has two
reaction chambers and a single detector. The incoming air sample is split into
two separate streams. The NO sample stream is routed directly to the
reaction chamber, while the NOx sample stream first passes through the
instrument converter before entering the second reaction chamber. An
optical chopper alternately exposes the detector to the respective
chemiluminescent outputs, generating continuous outputs for both NO and
NOx. The outputs are amplified and stored in memory circuits; real-time NO2
data obtained by difference are continuously generated. Reorder outputs are
available for the NO, NO2, and NOx concentration.
8.1.6 Converters - For the accurate determination of nitrogen dioxide it is essential
that the instrument converters have a high degree of efficiency (95%+) for the
conversion of NO2 to NO. The converters employed in commercially available
instruments are of two basic types.
8.1.6.1 Thermal Converters are made of a high grade stainless steel and operate at
elevated temperatures, 600-800 o C. At these temperatures the breakdown of
NO2 into NO and O2 occurs readily. These converters, though adequate for
the breakdown of NO2 to NO, have the obvious disadvantage of converting
ammonia into NO (See Section 6).
8.1.6.2 Chemical converters are to be found in the majority of chemiluminescence
instruments used for ambient monitoring. These converters have the
advantage of a much lower operating temperature, 200-400 o C, with efficient
NO2 conversion. Molybdenum and carbon converters have been in general
use and are available in commercial instruments.
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