OCR Text |
Show distinguishable spectral feature in an atmospheric pressure spectrum. Still, I ppm HN03 gives rise to a spectral feature which rises more than 0.005 optical depth units above the surrounding baseline. In Figure 4, S02 is seen to be an even more difficult case. For one, its lines are weaker and even less clumped, such that a difference optical depth of 0.005 is only attained with a concentration of 5 ppm. Furthennore, its absorption lines lie in a region with relatively strong absorption by water, an unavoidable component of combustion gases. Fast laser scanning and realtime spectral fitting will allow the removal of most of the effects of the high background absorption-a procedure which will clearly be more successful in the relatively flat baseline area on the left of the figure than in the vicinity of the strong water absorption line on the right. Finally, we consider the case of S03 shown in Figure 5. In this molecule, lines do not pile up to make peaks which can be detected by their contrast against a relatively featureless background absorption. Instead, they pile up such that the S03 optical depth changes in a stepwise fashion, as seen in the middle of the figure. This sudden change is still a detectable spectral feature, and in this particular case happens to occur at a relatively benign place in the interfering water background. At the left of the figure it can be seen that strong water absorption lines throughout this region make much of it unusable at path lengths even much shorter than 40 m. In Figure 6 we see a dramatic demonstration of the improved spectral discrimination at low pressures which was mentioned above. Here, the dashed lines are those of the added S03, while the solid lines are interfering water lines. The source of the step like change in the S03 optical depth in Figure 5 is seen to be the steadily increasing progression of lines which comes to an abrupt end at 1413.1 cm-I . It can be seen that although the peak optical depths in the two cases are very similar, the contrast or difference optical depth which could be picked out of an experimental spectrum is over three times larger in the low pressure case. Sensitivity Predictions Table 2 exhibits minimum detectable concentrations for a variety of molecules for both in situ and extractive sampling implementations of tunable diode laser absorption. As could be deduced from the above examples, a generalization which can be made is that diatomic molecules can be most sensitively detected, with larger molecules being more difficult to detect In fac~ in our aircraft engine exhaust study, we concluded that the H2S04 gas phase molecule was not detectable at concentrations at which smaller molecules would be easily seen, because its absorption spectrum is featureless on the spectral scale of diode laser tunability (order 1 cm- 1). 7 |