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Show In the above expressions the total line width ~Vtot is seen to have three contributions, the pressurebroadened width ~ v pb, the Doppler width ~ vD (due to the molecule 's distribution of speeds, and its limiting intrinsic linewidth at low pressures), and the laser linewidth. The pressure broadened width is estimated by multiplying the pressure of each major constituent of the gas mixture times an appropriate pressure broadening coefficient. An excellent source of line width, line strength, and line position infonnation is the HITRAN database.23 An earlier version of this compilation is also available in useful hardcopy plots.24 Although all the estimates presented in this paper were based on lists of individual lines, either from HITRAN or of our own devising, it is also possible to estimate absorption strengths and spectral feature widths for larger molecules for which complete line lists are impractical. A useful first reference for band strength infonnation is the review by Smith et al.25 Computer spectral databases are becoming an increasingly useful method of assessing the strength, shape and position of candidate molecular bands. An excellent place to start is the recently introduced NIST/EPA database26, which provides over 5000 spectra at 12 cm-1 resolution. A number of commercial compilations offer more spectra and/or higher resolution. Model Transmission Spectra At atmospheric pressure, pressure-broadened infrared linewidths are typically in the 0.05 to 0.1 cm-1 range. Many molecules (including most containing three or more atoms other than hydrogen) have line spacings of this order or smaller. Even if the target molecule has wellresolved lines, interfering lines may blend into a complex background. For this reason, it is often necessary to perform detailed computer simulations of the expected transmission in a spectral region, rather than simply relying on Equation 1. The sensitivity estimates reported in this paper are the results of such calculations, and we proceed to present a few example spectra. Figure 2 presents an example of an easy observation. The molecule of interest, NO, has strong, well-spaced lines in a spectral region where interfering gases do not cause serious problems. Differencing the curves with and without NO, one can compare the peak optical depth due to the strong NO line with our assumed minimum detectable peak absorption, 10-3, and derive a minimum detectable concentration. Although there are a number of possible trace gas measurements which would be this easy, or at least could be done without extraordinary effort, we will present no further such examples, but instead turn to a few examples of more difficult problems. The fust such example, shown in Figure 3, is that of nitric. acid. This molecule has many more absorption lines than does NO, so that only their occasional chance overlap forms a 5 |