Abstract

It has long been recognized that atmospheric transmittance through columns of trace gases cannot be determined based upon laboratory measurements at room temperature alone. For example, through an inhomogeneously and non-isothermally distributed column of chlorofluorocarbons (CFCs) in the real atmosphere the spectral transmittance would show a shifting of the spectral peaks from their positions observed at room temperature in the laboratory. The extent to which the transmittance profiles would be different from the room temperature observations can only be determined by laboratory measurements of absorption coefficients at lower temperatures. Examples of such measurements, which are currently in progress in our laboratory, are illustrated in Figs. 1 and 2. At lower temperatures, the contributions from the “hot bands”, which represent absorption of photons that results in transitions from an excited state with nearly the same energy as involved in the ground state bands, are smaller. If hot bands were not present, the band strength per atmosphere of the absorbing gas would be inversely proportional to the temperature making S_VT a constant. Such is not the case with the CFC bands as can be seen clearly in Table 1.

© 1990 Optical Society of America