Improving the coverage of MISR aerosol retrievals over shallow, turbid, and eutrophic waters

Shallow and coastal waters are often rich in nutrients (eutrophic) and biologically productive, turbidity from runoff is common, and the atmosphere above can be more aerosol-laden than over open ocean waters due to proximity to aerosol sources on land. Although the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR) on board the Terra satellite has been monitoring global aerosols for over 20 years, the current operational retrieval algorithm (V23) is not applied over shallow waters (less than 50 m in depth or within 5 km of land) due to the simplicity of the Dark Water algorithm, which assumes water surfaces are essentially black in the primarily-used red and near-infrared spectral bands. However, MISR measures radiances in a total of four spectral bands from nine different viewing directions and is thus able to distinguish atmospheric from surface contributions to the top-of-atmosphere signal. Here, we describe the implementation and validation of a ``Shallow Water'' aerosol retrieval algorithm for MISR, which uses all four spectral bands and includes a Lambertian surface reflectivity term to account for water-leaving radiance. This algorithm compares well to independent, surface-based observations and demonstrates better performance over shallow waters than the operational Dark Water retrieval algorithm. Globally, pixel-level aerosol retrievals over shallow waters increase the total number of MISR over-water measurements by more than 7%, over some of the most biologically productive parts of the ocean.