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Present and upcoming regulations are creating increasing demands for instrumentation to monitor air quality and emissions from stationaiy and mobile sources. Remote sensing instruments using optical techniques are rapidly gaining popularity over existing point source monitors for most applications. Differential Optical Absorption Spectroscopy, (DOAS) based on optical absorption in the near ultraviolet and visible portion of the spectrum provides an attractive technique for simultaneous measurement of a number of important pollutants. In particular DOAS can measure and differentiate the so-called BETEX group of aromatic compounds, including benzene which cannot be readily measured with remote sensing FTIR techniques because of severe interferences from the ubiquitous CO2 and H2O. The traditional DOAS system1 uses a telescope to transmit a beam of broad band light through the atmosphere. The transmitted beam is then received by a similar telescope at the other end of a long path and disbursed by a spectrograph. Light paths of about 500 m are commonly used for most molecules at concentrations encountered in rural and urban conditions. The light captured by the receiver is brought by fibre optics to the analyzer, which consists of a grating spectrometer and a rapid scanning device. The purpose of the rapid scanning device is to overcome a problem that faces any remote sensing method, viz, the turbulence or scintillations of the air along the path being measured, which occurs on time scales of 0.1 to 1 second. The instrument must make its measurements in a time scale faster than this in order to prevent blurring of the spectrum. The traditional DOAS instrument overcomes this difficulty by using a rotating wheel, containing some 20 identical radial slits which sweep past the detector. Since the wheel rotates at a speed of 300 rpm, successive spectra are obtained at a frequency of about 100 Hz, which is much faster than the frequency of air turbulences. Thousands of scans are added in the computer over sufficient period of time to achieve get the desired sensitivity. The combined spectra is then analyzed to obtain the best fit to a combination of reference spectra of the gases believed to be present in the air being sampled
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