Improved MODIS Dark Target Aerosol Optical Depth algorithm
over land: Angular effect Correction
Abstract. Aerosol Optical Depth (AOD) retrieved from MOderate Resolution Imaging Spectroradiometer (MODIS) measurements over land, can be improved by taking into account the surface Bidirectional Reflectance Distribution Function (BRDF), as shown in a previous study (Wu et al., 2016). However, the relationship of the surface reflectance between visible and short wave Infrared band that applied in the previous study, can lead to an angular dependence of the AOD retrieval. This has at least two reasons. The relationship based on the assumption of isotropic reflection or Lambertian surface is not suitable for the surface directional-directional reflectance. On the other hand, although the relationship varies with the surface cover type by considering the vegetation index NDVI_SWIR, this index itself has a directional effect and affects the estimation of the surface reflection, and finally can lead to some errors in the AOD retrieval. To improve this situation, we derived a new relationship for the spectral surface directional-directional reflectance in this study, using 3 years of dataset from AERONET-based Surface Reflectance Validation Network (ASRVN). To test the performance of the new algorithm, three case studies were used: 2 years of data from Eastern China and North America, and 4 months of data from the global land. The results show that the angular effects of the AOD retrieval are largely reduced in most cases. Particularly, for the global land case, the AOD retrieval was improved by the new algorithm compared to the previous study and MODIS collection 6 dark target algorithm, with the increase of 2.5 % and 5 % AOD retrievals falling within the expected accuracy level ±(0.05 + 15 %), respectively.