Retrieving Dust Aerosols over Oceans from MODIS

The MODIS Dark Target aerosol algorithm over Ocean (DT-O) uses spectral reflectance in the visible, near-IR and SWIR wavelengths to determine aerosol optical depth (AOD) and Angstrom Exponent (AE). Dust aerosol presents unique spectral characteristics because of its absorption and non-spherical shape. Current “dust-like” models in DT-O are spherical and do not have enough absorption at short wavelengths, so retrieved AOD and AE for dusty regions tends to be biased.

In this work, we constructed two typical non-spherical dust models with medium volume size of 0.2 µm and 2 µm and assorted shape parameters. Assuming a lognormal size distribution, the optical properties of each subgroup are interpolated from a non-spherical optical database developed at the Texas A & M and integrated to create the lookup tables of a fine mode and coarse mode dust models for the DT-O. Dust retrieval is achieved with two steps: first, identifying dusty pixels using a combination of near-UV (Deep blue), visible, and thermal infrared (TIR) wavelength spectral tests; second, choosing the new dust models in the retrieval process. The retrieved dust AOD is compared to retrievals from a small set of AERONET stations.