First high temporal resolution retrievals of AOD over shallow and turbid coastal waters for Himawari-8

Although the operational Himawar-8 Aerosol Retrieval Product (ARP) can resolve diurnal variation of aerosol loading, the ARP Aerosol Optical Depth (AOD) retrievals are significantly overestimated over Shallow and Turbid Coastal Waters (STCW) as water-leaving radiance is not explicitly considered in the retrieval process. Taking the advantage that 2.3 μm water-leaving radiance from STCW is insignificant, we developed a Himawari-8 Coastal Water AOD retrieval approach (CW) by using 2.3 μm Top of Atmosphere (TOA) reflectance and the aerosol properties of the nearest open ocean pixel. Unlike the abnormally large ARP AOD retrievals over STCW, the CW algorithm yields a smooth transition among land, coast, and open ocean over Asia and increases the number of AOD retrievals by more than 10 %, depending on the variation of coastal regions. Validation against Marine Aerosol Network measurements over STCW show that 65.5 % of CW retrievals are within the expected error envelope (± (0.05 + 15 %AOD)), compared with 56.5 % for ARP, and this improvement is independent of observational geometry and terrain. Moreover, CW AOD retrievals over STCW can well capture the diurnal variation observed by Aerosol Robotic Network while ARP cannot. As to seasonal variation, CW AOD over the STCW of East China increases in spring and peaks in June, while the coverage of abnormally large ARP AOD follows the temporal change of turbid coastal waters area with a maximum occurring in winter.