1 MODIS Retrieval of Aerosol Optical Depth over 2 Turbid Coastal Water 3

We present a new approach to retrieve Aerosol Optical Depth (AOD) from Moderate 15 Resolution Imaging Spectroradiometer (MODIS) over the turbid coastal water. This approach 16 supplements the operational Dark Target (DT) aerosol retrieval algorithm that currently don’t 17 conduct any AOD retrieval in the regions with large water-leaving radiances in the visible spectrum. 18 Over the global coastal water regions in all cloud-free conditions, this unavailability of AOD 19 retrievals due to the inherent limitation in existing DT algorithm is ~20%. Here, we refine the MODIS 20 DT algorithm by considering that water-leaving radiance at 2.1 μm is negligible regardless of water 21 turbidity. This refinement, with the assumption that the aerosol single scattering properties over 22 coastal turbid water are similar to that over the adjacent open-ocean pixels, yields ~18% more of 23 MODIS-AERONET collocated pairs for six AEROENT stations in the coastal water regions. 24 Furthermore, comparison with these AERONET observations show that the new AOD retrievals are 25 in either equivalent or better accuracy than those retrieved by the MODIS operational algorithm 26 (over coastal land and non-turbid coastal water). Combining the new retrievals with the existing 27 MODIS operational retrievals not only yield an overall improvement of AOD over those coastal 28 water regions, but also successfully extend the spatial and temporal coverage of MODIS AOD 29 retrievals over the coastal regions where 60% of human population resides, and thereby, aerosol 30 impacts on regional air quality and climate are expected to be significant. 31