Nondispersive infrared monitoring of NO emissions in exhaust gases of vehicles

Road traffic is one of the most important contributors to air pollution, being that a small fraction of the running vehicles is responsible for more than a half of the emissions. Roadside emission monitoring of individual cars appears to be an efficient way to identify these gross polluters. In this sense, nondispersive infrared (NDIR) systems have been developed to monitor the gas emissions of individual vehicles. However, these systems do not include NOx detection because of the strong interference of NO and NO2 absorption bands with the water band. This work is focused on the roadside monitoring of NO emissions by NDIR techniques. A theoretical study of the interference between NO and H2O absorption bands in the 1800-1950 cm(-1) spectral region has been performed. Two absorption lines, centered at 1876 and 1900 cm(-1) have been selected due to the very low water interference. The development of a new application based on the buildup of a high order interference filter, the solid state Fabry-Perot filter, is presented. Design of the filter system has been done, optimizing the transmittance at these two absorption lines. Finally, the ability of such a filter to discriminate NO absorption has been tested by using experimental absorption spectra measured by a commercial Fourier transform infrared spectroradiometer working in the active mode. The buildup of such a filter would permit us to increase the capabilities of on road exhaust monitoring systems using the NDIR technique, extending the range of analyzed gases to the nitrogen oxides. (C) 1999 American Institute of Physics. [S0034-6748(99)01907-3].