Nitrogen dioxide sensing using a novel gas correlation detector

A nitrogen dioxide point sensor, based on a novel nondispersive gas filter spectroscopic scheme, is described. The detection scheme relies on the fact that the absorotion spectrum of nitrogen dioxide in the 400-550 nm region consists of a complicated Line structure superimposed on an average broadba nd absorption. A compensating filter is used to remove the effect of the br oadband absorption, making the sensor insensitive both to small particles i n the optical path and to potentially interfering gases with broadband abso rption features in the relevant wavelength region. Measurements are obtaine d using a remote optical absorption cell that is linked via multimode fibre : optics to the source and detection optics. The incorporation of blue ligh t emitting diodes which spectrally match the nitrogen dioxide absorption al lows the employment of electronic (instead of mechanical) switching between optical paths. A sensitivity of better than 1.0 ppm m column density (1 s integration time) has been observed: improvements in electronics and therma l stabilization should increase this sensitivity.