Degree of linear polarization of light emerging from the cloudless atmosphere in the oxygen A band

We used radiative transfer calculations and model atmospheres for a theoret ical investigation of the behavior of the degree of linear polarization, P, of light emerging from the top or the bottom of the cloudless atmosphere i n the wavelength region of the O-2 A absorption band, between 755 and 775 n m. Results of P are shown for four model atmospheres and for various albedo s of the underlying Lambert surface. One of the model atmospheres contains only molecules, whereas the other atmospheres contain also aerosols. It is shown that when the molecular absorption optical thickness of the atmospher e is much smaller than one, which represents the continuum and the weak abs orption lines in the band, the state of polarization of the emerging radiat ion is mainly determined by low-order scattering in the lowest atmospheric layers and by reflection by the surface. In the strong absorption lines, wh ere the molecular absorption optical thickness is much larger than one, we distinguish the three following situations: (1) P of the reflected light is mainly determined by single scattering by molecules and aerosol in the upp er atmospheric layers, (2) for small solar zenith and/or viewing angles, P of the diffusely transmitted light is mainly determined by single scatterin g, and (3) for other geometries, P of the diffusely transmitted light is pr edominantly due to second-order scattering, with the first scattering takin g place in the upper atmospheric layers and the second in the lower layers. For a given surface albedo, the variation of P across an absorption line t hus depends on the scattering properties of the atmospheric particles and o n their vertical distribution. The surface albedo and P of light emerging f rom the atmosphere in the principal plane are shown to be related through a simple formula at each wavelength within the absorption band. It is conclu ded that high-resolution spectropolarimetry in wavelength regions with stro ngly varying molecular absorption optical thickness can provide valuable in formation on aerosol at various altitudes in the atmosphere.