Retrieval of Aerosol Fine Mode Fraction over land from MODIS in a new algorithm

The MODerate Imaging Spectrometer (MODIS) has been successfully retrieving aerosol properties over land, beginning in early 2000 from Terra and from mid 2002 from Aqua. The aerosol optical depth (AOD) has been validated with known errors bars, however the fine mode weighting (FMW) of the AOD is accurate only under certain conditions. The retrieval of FMW suffers because: A) spectral AOD is derived in only two close visible channels (0.47 and 0.66 μm) and B) that the 2.1 μm IR channel, used to estimate the surface properties in the visible, is assumed to be transparent to aerosol. When coarse mode aerosol is present the 2.1 μm channel is not transparent to aerosol, leading to errors when estimating the visible surface reflectance used as the boundary condition. A new algorithm has been developed to derive aerosol properties, using simultaneous inversion of the three channels (0.47, 0.66 and 2.1 μm). Even though visible reflectance must still be estimated from that in the IR (the assumed relationship has been updated), the IR is not assumed transparent to aerosol. Through an iterative process, a lookup table (representing updated aerosol models and vector radiative transfer) is interpolated to find the total AOD, FMW and IR surface reflectance that best simulates the MODIS spectral reflectance. Results from a test-bed of MODIS granules show improved retrieval of AOD and FMW. Specific case studies have also been analyzed.