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Show Table 3 - Minimwn Detectable Concentrations or Error Limits, ppm, for In Situ Measurements on Atmospheric Pressure Exhaust Streams Nonunal Concentration, UV Species ppm IDU40m FTIR/2m DOASl2m Conventional ""- ;:y.. ( Iln1) .J ~ H2O 30000 300 CO2 32000 320 CO 24 0.2 7 1 NDIR NO 40 0.4 3.5 0.4 1 Chemilum N20 1 0.01 2.5 N02 5 1 0.15 0.1 Chemil. OH 10 0.2 (0.003) CH4 <5 0.2 5 0.1 THe HN03 1 0.1 C2H2 1 0.02 S02 5 1 1 0.1 0.1 NDUV S03 1 0.4 (5) While further improvements in FTIR design, thermal source characteristics, source and collection optics, and especially data analysis software may bring detection limits down another order of magnitude, at prese~t it. is fair to say that each level of measurement difficulty using laser sources has its corresponding level of difficulty for broadband techniques at a minimum detectable absorption value about one order of magnitude larger, so the 10-3 value we used in assessing tunable diode laser experiments compares with a 10-2 minimum detectable optical depth for FTIR and DOAS measurements. Of course, one advantage to broadband techniques is their ability to measure several species simultaneously. However, as shown in Table 4, to some extent multiple species can be measured with a single laser diode, and quite a few can be measured (sequentially) using a single diode laser apparatus holdipg multiple lasers (the standard dewar holds four, but custom dewars are available). It has been seen in the example spectra that it is relatively easy to arrange that lines of a trace species appear in the same scan with water or C02 lines, a useful feature for absolute calibration of the measurement 11 |