The solution of radiative transfer problems in molecular bands without the LTE assumption by accelerated lambda iteration methods

An iterative method based on the use of approximate transfer (or Λ) operators, which was designed initially to solve multilevel NLTE line formation problems in stellar atmospheres, is adapted and applied to the solution of the NLTE molecular band radiative transfer in planetary atmospheres. The matrices to be constructed and inverted are much smaller than those used in the traditional Curtis matrix technique, which makes possible the treatment of more realistic problems (including rotational NLTE, overlapping of lines in the bands and overlapping of bands with continuua) using relatively small computers. This technique converges much more rapidly than straightforward iteration between the transfer equation and the equations of statistical equilibrium (Λ-iteration). A test application of this new technique to the solution of NLTE radiative transfer problems for optically-thick and thin bands (the 4.3 μm CO 2 band in the Venusian atmosphere and the 4.7 and 2.3 μm CO bands in the Earth's atmosphere) is described. The necessity for careful treatment of rotational NLTE in weak lines of the 4.3 μm CO 2 fundamental band is shown. The possible influence of neglecting or treating approximately the CO 2 10 μm laser transitions and also effects of the overlap of the 4.3 μm CO 2 with the continuum are investigated. The rotational NLTE effects in the 4.7 μm CO band in the upper Earth's atmosphere are estimated.