Remote sensing of smoke from MODIS airborne simulator during the SCAR-B experiment

MODIS (moderate resolution imaging spectroradiometer) airborne simulator (MAS) data acquired during the SCAR-B (Smoke, Clouds, and Radiation-Brazil) experiment provide a test bed to evaluate aerosol retrievals for MODIS measurements over land. The SCAR-B MAS data covered forest and cerrado regions with varying smoke concentration as a result of fires in August-September, 1995. Excellent agreement is obtained by comparing the retrieved aerosol optical thickness from the MAS data with AERONET (Aerosol Robotic Network) ground-based Sun photometer observations. The evaluation of MODIS aerosol algorithm was performed for a range of smoke optical thickness from 0.1 to 2 at 0.66 mu m wavelength over cerrado and forest sites. All the retrieved values of optical thickness are found within the anticipated retrieval errors of Delta tau = +/-0.05+/-0.2 tau. This confirms for smoke aerosol the validity of the MODIS algorithms and the use of the new dynamic aerosol models in this algorithm (the algorithm was already validated previously for urban/industrial aerosol). The retrieved optical thickness is found to be very sensitive to the assumed value of the single-scattering albedo but not sensitive to the aerosol refractive index (real part). The single-scattering albedo of 0.90 suggested in the aerosol model yields the best results. Test of the MODIS use of a grid box of 10 x 10 km in the aerosol algorithm shows that the standard deviation of the retrieved aerosol should generally not be more than 10% of the average value for a large range of optical thickness values from 0.2 to 2.0, close and far from fires.