Comparison of Synthetic Nitric Acid Spectra with Laboratory and Atmospheric Measurements in the 11.3 Micron Region

Theoretical absorption spectra for nitric acid in the 11.3 micron spectral region were generated by line by line integration. Only the v5 and 2v9 bands were considered. The rotational line positions and intensities were calculated using Dana's constants for the v5 band; for the 2v9 band the rotational constants were based on the results given by Chevillard and Giraudet2 in conjunction with an ab initio calculation using the vibrational constants of McGraw et al3. A Lorentzian line shape was used with a self broadened semi-­half width of 0.75 cm-1 at 300K and 1 atm. The semi-half width was assumed constant and at this stage of the work the varia­tion with rotational quantum number was ignored. Fig. 1 shows a comparison between a calculated spectra and an experimental spectrum of Goldman et al4. The differences between the calcu­lations and the experimental results could be due to the following; (a) neglect of a large number of the weaker lines, it was necessary to eliminate a number of weaker lines for computational purposes; (b) neglect of possible hot bands, as will be noted below there could be hot bands in the vicinity of 885 cm−1 and 910 cm−1; (c) the possibility that the wings of the lines are super Lorentzian.