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