Retrieval of Atmospheric Aerosol and Surface Properties Over Land Using Satellite Observations
It is difficult to retrieve aerosol and surface reflectance properties over land, simultaneously, from satellite observations. In the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol algorithm, 2.1-μm observations are essential to determine land surface reflectances. However, observations at 2.1 μm are not widely available for other sensors. To get rid of 2.1-μm observations in aerosol retrievals, we suggest a new method that makes use of the priori relationships among surface reflectances at different wavelengths in terms of the surface normalized difference vegetation index (sNDVI), which is defined by the surface reflectances at 0.87 and 0.67 μm. It has advantages to parameterize surface reflectances due to its independence from atmospheric condition. Based on these newly established surface reflectance relationships and the MODIS observations at 0.47, 0.67, 0.87, and 1.64 μm, we solve four variables simultaneously with the new method: aerosol optical depth (AOD) at 0.55 μm, proportion of fine mode AOD, surface reflectance at 0.67 μm, and sNDVI. The preliminary retrieval tests indicate that this method can successfully retrieve the aerosol properties and land cover features over East Asia. A comparison of the retrieved AODs at 0.55 μm with the AERONET observations shows a root-mean-square error (RMSE) of about 0.16, at the Beijing and Xianghe sites, in August 2007. The accuracy is the same order of magnitude as the MODIS aerosol algorithm (RMSE = 0.14) during the same period. The results demonstrate that the quality of aerosol retrieval in the new method is comparable to that of the MODIS aerosol algorithm over the testing area.