Absorption by Atmospheric Gases

Abstract

Digital archives offer the investigator an up-to-date analysis of an extensive and specialized literature. Periodic revisions are reported in the open literature and it seems unlikely that future investigators will attempt to use any other source where archives can provide the required data. For this reason, we shall confine our comments on permitted vibration-rotation transitions to describing the AFGL tape contents, but we shall add two areas not contained in it: first, electronic bands, and second, the related topics of forbidden transitions, collision-induced transitions, and polymer spectra. The AFGL tape lists data on one important set of electronic transitions, those giving rise to the near-infrared atmospheric bands of molecular oxygen. These bands behave in the same way as vibration rotation bands, except for the frequency displacement caused by the change in electronic energy and the symmetry conditions imposed by the electronic wave functions. Other electronic transitions usually involve larger differences between energy levels and cannot be understood as completely as the lower energy, vibrational and rotational transitions. Fortunately, visible and ultraviolet bands of importance for atmospheric problems are less complicated than vibration—rotation bands and they are usually less affected by state parameters. Atmospheric absorption calculations in the visible and ultraviolet spectrum are commonly made on the basis of empirical data without requiring the level of understanding developed in Chapters 3 and 4 for vibration-rotation bands. The altitude of unit optical depth for ultraviolet atmospheric bands is illustrated in Fig. 5.1. The intensity of solar radiation falls off rapidly with decreasing wavelength in the spectral region shown (the irradiance at 2000 Å compared to that at 3000 Å is 10-2 whereas at 1000 Å it is 10-5, see Appendix 9). For heating rate calculations at altitudes less than 100km, only O2 and O3 are important, except under special conditions when the atmosphere contains large amounts of volcanic aerosols, or polar stratospheric clouds at high latitudes. All of the absorptions shown in Fig. 5.1 are important for other reasons that do not directly concern us here.