Open Path Optical Remote Sensing Systems

Table 5.13.1 describes several sensing techniques. Of the techniques listed, the FTIR and UV differential optical absorption spectroscopy (UV-DOAS) are the most widely publicized. They both measure multiple compounds in open areas in near real time.

These systems use a high-pressure xenon lamp in the UV-DOAS or a grey-body source in the FTIR to produce a broadband light that is collimated by a telescope into a narrow beam. This beam is then transmitted through the area of contamination to a receiver. The transmission path can range from several feet, in stacks, to 1 to 3 km, in ambient air over large areas (see Figure 5.13.1).

In route to the receiver, a portion of the light energy is absorbed by the pollutants. The light that reaches the receiver is sent to a spectrometer or interferometer, which generates a spectrum. The computer analyzes the spectrum by comparing it with a precalibrated reference spectrum for both the measured and the interfering components. Because each pollutant absorbs a unique pattern of wavelengths, missing light reveals the presence of a pollutant. The amount of missing light determines its concentration. Measurements are taken as often as every minute and averaged at any interval.

UV-DOAS systems are best suited to monitor criteria pollutants such as SO2, NO, NO2, O3 and benzene, toluene, and xylene (BTX). FTIR systems are better suited to measure VOCs. FTIR systems also detect criteria pollutants but not with the same degree of sensitivity as UV-DOAS. These limitations stem from interferences of water vapor, which absorbs strongly in the IR region but does not absorb in the UV region. However, powerful software algorithms are becoming available to filter out the interference signals from water.

In general, more chemicals absorb light in the IR spectrum than in the UV spectrum. Thus, FTIR systems measure a broader range of compounds (more than fifty) than UV-DOAS systems, which detect about twenty-five (Nudo 1992). With the aid of a contractor, the EPA has developed reference IR spectra for approximately 100 of the 189 HAPs listed under Title III. The maximum number of HAPs to which FTIR might ultimately be applicable is about 130 (Schwartz, Sample, and McIlvaine 1994). The EPA plans to initiate its own investigation into this area. Additional investigation into the potential of FTIR in continuous applications is underway at Argonne National Labs.

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