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

4.0 SCOPE
This method is applicable for measurement of oxides of nitrogen in ambient air.
5.0 RANGE AND SENSITIVITY
5.1 Instruments usually have multiple measurement ranges; typically 0 to 0.2, 0.5,
1.0 or 2.0 ppm v/v. For ambient monitoring, the most commonly used ranges are
the 0 to 0.5 and 0 to 1.0 ppm v/v.
5.2 The lower detectable limit is determined by the instrument range being used. For
the 0 to 0.5 ppm range a lower detectable limit of 0.01 ppm can be achieved (3).
6.0 INTERFERENCES
6.1 The chemiluminescent reaction of nitric oxide and ozone is not generally subject
to interferences from commonly found pollutant species such as ozone, carbon
monoxide and sulfur dioxide (4). However, any compound capable of being
converted to nitric oxide in the instrument converter (Section 8.1.3) could be a
possible interferent in the measurement of NOx. In this regard, gaseous
ammonia could present a problem at elevated temperatures (600 o C) in thermal
converters; also certain organic nitrates and nitrites have been shown to
decompose stoichiometrically to nitric oxide (5,6,7). Nitric acid reaching the
converter is also decomposed to NO. A continuous method for measuring nitric
acid at ambient levels based on a modified chemiluminescence analyzer has
been reported (8).
7.0 PRECISION AND ACCURACY
7.1 Precision is defined by the stability and repeatability of response to NO.
Response instability due to baseline drift should be equivalent to no more than +
0.020 ppm v/v over a 24-h period, and the noise level to no more than + 0.005
ppm v/v in the 0 to 0.5 ppm instrument range. Repeatability of response to a
standard NO-air mixture should have a coefficient of variation not greater than
2%. This latter precision figure represents the composite error that is associated
with both the NO measurement and the standardization operations (9).
7.2 Accuracy is dependent on the calibration equipment flow systems, and the
absolute concentration of the NO standard gas cylinder. The calibration flow
system specifications contained in the Section 10.2, together with a NO standard
gas cylinder of Standard Reference Material (SRM) or Certified Reference
Material (CRM) quality (Section 9.2) are capable of performing instrument
calibrations to an accuracy of + 5%.
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