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First Retrieval of AOD at Fine Resolution Over Shallow and Turbid Coastal Waters From MODIS
Journal article   Peer reviewed

First Retrieval of AOD at Fine Resolution Over Shallow and Turbid Coastal Waters From MODIS

Yi Wang, Jun Wang, Robert C Levy, Yingxi R Shi, Shana Mattoo and Jeffrey S Reid
Geophysical research letters, Vol.48(17), e2021GL094344
09/16/2021
DOI: 10.1029/2021GL094344

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Abstract

The widely used Moderate Resolution Imaging Spectroradiometer (MODIS) Dark‐Target (DT) aerosol product fails to accurately retrieve Aerosol Optical Depth (AOD) over shallow and turbid Coastal Waters (CWs). To fill in gaps, and to improve land to ocean AOD continuity, we developed a coastal water retrieval algorithm at a spatial resolution of 1 km (CW‐1km). CW‐1km relies on observed top‐of‐atmosphere reflectance at 2.1 μm (ρ2.1), both to derive AOD and to perform a spatial variation test that enhances the existing DT masks for clouds and land. We show that the CW‐1km improves spatial continuity of AOD between land, coast, and open ocean, while also increasing AOD product availability by 47.0%. Comparing with 15 years of marine aerosol network measurements, CW‐1km AODs are validated to have a normalized mean bias of 1.0%, which is much smaller than 17.6% for the original DT product. Plain Language Summary The Moderate Resolution Imaging Spectroradiometer (MODIS)‐retrieved Aerosol Optical Depth (AOD) products have been widely used by the research community to study aerosol impacts on air quality, weather, and climate changes. However, AOD products are often missing or highly biased over Coastal Waters (CWs), due to CW being turbid, shallow, and mixed with the land. Here, we have created a new algorithm (CW‐1km) that retrieves AOD with a spatial resolution of 1 km. This new CW algorithm greatly improves spatial coverage and provides a smooth AOD transition from land to coastal to open waters and does not shown the spatial gaps and erroneous values as found in the MODIS AOD products over CW. The improvement in both accuracy and retrieval coverage can further enhance air quality and climate studies in coastal regions, which is also where over 60% of global human population reside. Our CW algorithm can be ported to existing and future satellite sensors to improve the retrieval of aerosol distribution over coastal and estuarine water areas. Key Points The Coastal Water (CW) algorithm for Moderate Resolution Imaging Spectroradiometer Aerosol Optical Depth (AOD) retrieval at 1 km over shallow and turbid CW is developed The 2.1‐μm Top‐of‐Atmosphere reflectance spatial variation test improves the land mask near coastline and cloud mask over turbid CW CW‐1km AOD agrees with in situ measurements, increases availability, and removes the artifacts of AOD from land to open ocean
 AERONET aerosol optical depth air quality and climate coastal water Maritime Aerosol Network (MAN) MODIS

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