A fast method for simulating observations of polarized light emerging from the atmosphere applied to the oxygen-A band

Abstract

Polarimetry of the atmosphere as a function of wavelength can provide valuable information on atmospheric aerosol and cloud properties. In particular, information can be obtained from polarization observations in wavelength regions where molecular absorption is prominent. Simulation of such observations is, however, extremely time-consuming when using line-by-line radiative transfer calculations. To address this problem, a fast method to simulate such observations was developed, using the adding–doubling method which is especially suited for polarized light, and the correlated k-distribution (c k) method. The part dealing with the c k-method was extended to include instrumental spectral response functions as well as the spectral distribution of the incident solar flux. In our calculations, we used spectral response functions representative for the GOME instrument on board the ERS-2, and focussed on the O 2-A band. We present the calculated radiance and the degree of linear polarization of light emerging at the top and at the bottom of an atmosphere that contains molecules as well as aerosol particles and that is bounded below by a Lambertian reflecting surface. Because the c k-method is an approximative method, special attention is paid to the errors in the degree of linear polarization of the emergent light resulting from the correlation assumption used in the c k-method. It appears that the errors in the polarization are in general smaller than the errors in the radiances. For the O 2-A band, the errors in the polarization are less than 0.25% for the reflected light and less than 0.05% for the transmitted light. This shows that our fast method can be used to simulate polarization observations in the O 2-A band accurately.