The Uncertainty in Aerosol Optical Depth Retrieved by the MODIS Dark Target Algorithm

Abstract:

Aerosol optical depth (AOD) retrieved by the Moderate Resolution Imaging Spectrometer (MODIS) Dark Target (DT) algorithm now has a record of nearly 15 years. During that time, there have been many studies to evaluate the retrieved AOD product, including direct comparison with the AOD reported by ground-based sun photometers. The result has been the rigorous “validation” of the DT products, in that the agreement between retrieved and observed AOD has been quantified on a global scale. For example, validation means that >68% of the MODIS data (or one standard deviation) agrees to the ground-truth, within a specified error envelope. However, validation does not tell a user anything about specific aerosol retrieval cases. Therefore, our team is taking the next logical step and developing the framework for deriving per-pixel retrieval uncertainty. The retrieval uncertainty in AOD comes from (1) non-linearity the algorithm’s look-up-table approach and (2) various assumptions made prior to and during the retrieval. Some of these assumptions include errors in the observed reflectance (calibration or missing data), standard deviation of the reflectance over the retrieval box, non-aerosol (e.g. gas absorption and Rayleigh) atmospheric corrections, and characterization of the surface reflectance. We will discuss the methodology of our framework and present a year’s worth of global and regional analysis of the uncertainty our AOD retrievals.