Simultaneous retrieval of aerosol and surface optical properties using data of the Multi-angle Imaging SpectroRadiometer (MISR)

An algorithm for retrieving simultaneously aerosol and surface optical properties from radiance data of the Multi-angle Imaging SpectroRadiometer (MISR) was developed. It uses MISR subregion radiance data of the 9 cameras and 4 spectral bands (spatial resolution 1.1 km) to generate characteristic functions ρsurf = fλ,cam (βe (550 nm)), where βe (550 nm) and ρsurf are the ground level aerosol extinction coefficient at 550 nm, and the surface reflectance, respectively. The analysis of the mutual intersections of those functions yield optimum values for βe (550 nm), aerosol optical depth (AOD) at 550 nm and ρsurf. MODTRAN 4 v3r1 was used to create radiance look-up tables. The algorithm was tested for MISR paths covering the complex terrain of Switzerland and northern Italy. Results of 2 days (low and high aerosol loads on May 14 and June 17, 2002, respectively) were analyzed and compared with sun photometer measurements. First, ground level aerosol extinction coefficients and optical depth over water were derived. From those data it was possible to retrieve a best-fit aerosol mixture. AOD (550 nm) over water is in satisfactory agreement with both sun photometer data and the operational aerosol product of MISR. Second, the Ross–Li approach for the bidirectional reflectance factor (BRF), supported by MODTRAN 4, was applied to simulate the radiance over vegetation surfaces. In this case, the retrieved aerosol extinction coefficients and optical depths over vegetation are significantly lower than the values derived over water. We assume that the incomplete coupling of BRF and radiation in MODTRAN 4 is at least partly responsible for this discrepancy.