Improvements in dark water, low light-level AOD retrievals in MISR operational algorithm

Satellite remote sensing of aerosols is taking bold steps towards higher spatial resolutions, as evidenced by the newly released MODIS 3 km product and the soon to be released MISR 4.4 km product. Finer horizontal resolution allows for a better aerosol characterization in proximity to clouds—which is important for studying indirect aerosol effects—but also poses additional challenges due to various cloud artifact effects. It is therefore imperative to refine satellite algorithms to correctly interpret aerosol behavior in the proximity of clouds. For instance, MISR aerosol optical depth (AOD) retrievals frequently overestimate AODs in pristine oceanic areas, in particular close to Antarctica, as evidenced by comparison with Maritime Aerosol Network (MAN) observations. We trace the origin of this overestimation to stray light, or veiling light, being scattered more or less uniformly over the camera's field of view and reducing the contrast of the primary image. We found that the MISR-MODIS radiance difference in dark areas correlates with average scene brightness within the whole MISR camera field of view. A simple, single parameter model is proposed to effect the corrections. Collocated MISR/MODIS pixels are used to fit the parameter in the MISR nadir camera. For the off-nadir cameras two alternative approaches are employed that are based on MISR radiances and radiative transfer model calculations. These two methods are prone to higher uncertainties, but suggest somewhat increasing correction values for the longer focal length cameras. Finally, the empirical corrections applied in the operational MISR retrieval algorithm substantially decrease AODs in analyzed cases, and lead to closer agreement with MAN and MODIS, proving the efficacy of the developed procedure.