Continuous monitoring methods cover various instrumental techniques. However, the
chemiluminescence method is the recommended and most commonly used method.
The instrument signals are recorded continuously intervals and typically integrated to
Chemiluminescence. The technique is based on the gas phase chemiluminescence
reaction of NO with O3, which produces stimulated NO2 emitting light
(chemiluminescence) at about 1200 nm wavelength (ISO, 1985). The different types
of chemiluminescence monitors measure NOx and NO concentrations on the air,
using a somewhat different design. The air sample passes a heated catalytic converter
where NO2 is reduced to NO. The signal from the air passing the converter gives the
NOx concentration, while the signal from the air drawn directly to the detector gives
the NO concentration. NO2 is determined as the difference between the two signals.
The instrument is commonly used in urban air and is the reference method
recommended by United States Environmental Protection Agency, US EPA, 1991).
The sensitivity of the traditional commercial monitors is usually not sufficient for
rural, low-polluted areas. The method is also unspecific. Other reducible nitrogen
compounds (such as HNO3 and PAN) may interfere. This is of less importance in
urban air, where concentrations of NO and NO2 dominate.
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