Dependence of the spectral aerosol optical thickness retrieval from space on measurement errors and model assumptions

The influence of measurement errors and aerosol model assumptions was investigated on the accuracy of retrieving the optical thickness at 550 nm and the Angstrom exponent of the aerosols over the ocean from space using visible (6W70 nm) and near-infrared (865 nm) measurements. Two methods (the radiance method and the polarization method) were compared. The retrieval errors were investigated for the measurement error of the degree of polarization by 0.5% in polarization units and for the radiance error by 5%. A moderate optical thickness case over the ocean (0.16) was considered.

The influence due to these errors was not very different between the radiance and the polarization methods for retrieving the aerosol optical thickness at 550 nm. The retrieval of the Angstrom exponent by the radiance method was more influenced by the measurement errors than retrieval by the polarization method.

The polarization method was more affected by the aerosol model assumptions (the size distribution parameters and refractive index) than the radiance method. The polarization method was more influenced by the aerosol model assumptions than by the measurement errors. On the other hand, the radiance method was less influenced by the aerosol model assumptions than by the measurement errors for retrieving the Ångström exponent, whereas the effect of aerosol model assumptions (refractive index) were larger for retrieving the optical thickness.

The accuracy of measurement is important for the use of radiance to retrieve spectral aerosol optical thickness, whereas appropriate aerosol models are essential for the use of the degree of polarization.