Effect of Linewidth Narrowing on Radiative Forcing

Increasing greenhouse gas concentrations strongly affect the far-wing lineshape of molecular transitions. In particular, the finite time of molecular collisions as well as Dicke narrowing significantly reduce the absorption cross section in the far-wing region. It is known that this is essential to take into account when fitting atmospheric observations of the infrared radiance¹. Using incorrect line profiles results in an overestimate of radiative forcing by several tens of per cent. This work performed a line by line computation using data downloaded from the Hitran database. For each line, a Voigt profile was modified to correct the far-wing lineshape. Over 100,000 lines originating from the various CO₂ isotopomers and other greenhouse gases CH₄, N₂O and H₂O were considered. The computation developed fast numerical routines to solve the Schwarzschild equation. The calculations were done for the standard temperature profiles for the atmosphere² and also take into account the observed altitudinal dependence of the greenhouse gas concentrations³.

1. M. Hartmann, C. Boulet and D. Robert, Collisional Effects on Molecular Spectra, Elsevier, Amsterdam (2008)
2. NASA (1976), U.S. Standard Atmosphere supplements, 1976, U. S. Government Printing Office, Washington DC
3. Air Force Geophysics Laboratory Atmospheric Constituent Profiles (0-120 km), Project 7670, Environmental Research Papers No. 954, AFGL-TR-86-0110 (1986).