Aerosol optical depth retrieval using ATSR-2 and AVHRR data during TARFOX

Satellite retrieved aerosol optical properties are compared to aircraft measurements for a case study during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Two satellite instruments are used: the Along Track Scanning Radiometer 2 (ATSR-2) and the advanced very high resolution radiometer (AVHRR). The aerosol optical depth in the mid-visible (0.555 mu m) retrieved from the ATSR-2 data agrees within 0.03 with colocated sunphotometer measurements. Also, the spectral behavior of the aerosol optical depth is retrieved accurately. Good correlation is found between aerosol optical depths for AVHRR channel 1 (0.64 mu m) and sunphotometer derived values, but the satellite retrieved values are 0.05 to 0.15 lower. The Angstrom wavelength exponent is determined both from the ATSR-2 and the AVHRR data. The ATSR-2 derived Angstrom exponents are in good agreement with the values computed from the sunphotometer data. The Angstrom exponents determined from AVHRR data show very large variations. Both the ATSR-2 and the AVHRR aerosol optical depth images show a large gradient. Vertical profile data of temperature, relative humidity, and particle scattering indicate that this gradient is probably caused by changes in the dry aerosol properties, rather than a change in the relative humidity.