REMOTE-SENSING OF AEROSOL PROPERTIES OVER OCEANS USING THE MODIS EOS SPECTRAL RADIANCES/

Spectral radiances measured at the top of the atmosphere in a wide spe ctral range (0.55-2.13 mu m) are used to monitor the aerosol optical t hickness and the aerosol size distribution (integrated on the vertical column) of the ambient (undisturbed) aerosol over the oceans. Even fo r the moderate resolution imaging spectrometer (MODIS) wide spectral r ange, only three parameters that describe the aerosol loading and size distribution can be retrieved. These three parameters are not always unique. For instance, the spectral radiance of an aerosol with a bilog normal size distribution can be simulated very well with a single logn ormal aerosol with an appropriate mean radius and width of distributio n. Preassumptions on the general structure of the size distribution ar e therefore required in the inversion of MODIS data. The retrieval of the aerosol properties is performed using lookup table computations. T he volume size distribution in the lookup table is described with two lognormal modes: a single mode to describe the accumulation mode parti cles (radius <0.5 mu m) and a single coarse mode to describe dust and/ or salt particles (radius >1.0 mu m). Note that two accumulation modes may be present, one dominated by gas phase processes and a second dom inated by cloud phase processes. The coarse mode can also be split int o several partially overlapping modes describing maritime salt particl es and dust. The aerosol parameters we expect to retrieve are eta, the fractional contribution of the accumulation mode to scattering; tau, the spectral optical thickness; and r(m), the mean particle size of th e dominant mode. Additional radiative quantities such as asymmetry par ameter and effective radius are derived subsequently. The impact of th e surface conditions, wind speed and chlorophyll content on the retrie val is estimated, the impact of potential sources of error like the ca libration of the instrument is also tested. The algorithm has been app lied successfully to actual data sets provided by the Thematic Mapper on Landsat 5 and by the MODIS airborne simulator on the ER-2 and teste d against ground and airborne measurements. A first estimate of the ge neral accuracy is Delta tau = +/-0.05 +/- 0.05 tau (at 550 nm), Delta r(m) = 0.3r(m), Delta eta = 10.25.