Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX

Radiation and in-situ measurements collected during the Tropospheric Aeroso l Radiative Forcing Observational Experiment (TARFOX) are used to test the method for remote sensing of aerosol properties and loading from the MODIS instrument. MODIS, a Moderate Resolution Imaging Spectroradiometer, will be launched in 1999 aboard the first EOS (Earth Observing System). Following the MODIS procedure [Tanre et al., 1997], the spectral radiance at the top of the atmosphere (TOA) measured over the ocean in a wide spectral range (0 .55-2.13 mu m) is used to derive the aerosol optical thickness (proportiona l to the aerosol total loading) and the aerosol size distribution (integrat ed over the vertical column) of the ambient (undisturbed) aerosol by compar ing measured radiances with values in look-up table (LUT). The LUT includes the gasphase oxidation accumulation mode, cloud-phase accumulation mode, a nd a coarse mode that represents maritime particles (salt) and dust. In eac h inversion, one accumulation and one coarse mode can be retrieved. The inv ersion retrieves the ratio of the contribution to the optical thicknesses o f the two particle modes and the mean particle size that best fits the meas urements. This algorithm is successfully applied to the data sets acquired during TARFOX. The MODIS airborne simulator (MAS) aboard the NASA ER-2 airc raft flew several times during the experiment above the University of Washi ngton C-131A research aircraft on which the six-channel Ames Airborne Track ing Sun Photometer (AATS-6) was mounted. It flew also above surface-based S un photometers. Optical thicknesses (at lambda = 550 nm) as well as the spe ctral dependence from the various data sets compare very well.