Sciamachy Toa Reflectance Correction Effects on Aerosol Optical Depth Retrieval

Aerosol retrieval from SCIAMACHY data processing at ISAC-CNR has been carried-out to retrieve Aerosol Optical Depth (AOD) at 500 nm wavelength together with a parameter (class) which defines a set of chemico-physical properties pertaining to suspended particles. SCIAMACHY Level 1B data from nadir measurements both over ocean and land are used as input for this processor. SCIAMACHY, onboard ENVISAT, is a remote sensing spectrometer measuring sunlight scattered, transmitted and reflected by the Earth's surface-atmosphere system. Its wavelength range spans from about 240 nm to about 2440 nm, with a spectral resolution varying from 0.24 nm in the UV to 1.48 nm in the NIR. In this aerosol retrieval algorithm, working wavelengths are selected in the UV-VIS-NIR range characterized by negligible gases absorption, i.e. lower than 1%. At these wavelengths, measured TOA reflectance are corrected taking into account that SCIAMACHY underestimates the reflectance in the UV and VIS range by 10 to 25% and by 10 to 20%, respectively. Once reflectances, presenting values of cloud coverage fraction lower than 0.05, are properly corrected, they are fitted with modelled reflectances. The couple formed by AOD and class that minimizes the merit function represent the retrieved aerosols parameters for the spectrum being analysed. The effect on the aerosol retrieval of the applied reflectance correction factors will be critically discussed and quantified. Then, SCIAMACHY data relative to 2003 and 2004 and in spatio-temporal coincidence with AERONET measurements have been considered.