MODIS Retrieval of Aerosol Optical Depth over Turbid Coastal Water

Yi Wang,Jeffrey S Reid,Xiaoguang Xu,Robert C Levy,Jun Wang

doi:10.3390/rs9060595

Yi Wang, Jeffrey S Reid + Show 3 more

Open Access

https://doi.org/10.3390/rs9060595

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Journal: Remote Sensing	Publication Date: Jan 1, 2017
Citations: 26	License type: CC BY 4.0

Affiliation: University of Iowa, Goddard Space Flight Center

Abstract

We present a new approach to retrieve Aerosol Optical Depth (AOD) using the Moderate Resolution Imaging Spectroradiometer (MODIS) over the turbid coastal water. This approach supplements the operational Dark Target (DT) aerosol retrieval algorithm that currently does not conduct AOD retrieval in shallow waters that have visible sediments or sea-floor (i.e., Class 2 waters). Over the global coastal water regions in cloud-free conditions, coastal screening leads to ~20% unavailability of AOD retrievals. Here, we refine the MODIS DT algorithm by considering that water-leaving radiance at 2.1 µm to be negligible regardless of water turbidity, and therefore the 2.1 µm reflectance at the top of the atmosphere is sensitive to both change of fine-mode and coarse-mode AODs. By assuming that the aerosol single scattering properties over coastal turbid water are similar to those over the adjacent open-ocean pixels, the new algorithm can derive AOD over these shallow waters. The test algorithm yields ~18% more MODIS-AERONET collocated pairs for six AERONET stations in the coastal water regions. Furthermore, comparison of the new retrieval with these AERONET observations show that the new AOD retrievals have equivalent or better accuracy than those retrieved by the MODIS operational algorithm’s over coastal land and non-turbid coastal water product. Combining the new retrievals with the existing MODIS operational retrievals yields an overall improvement of AOD over those coastal water regions. Most importantly, this refinement extends the spatial and temporal coverage of MODIS AOD retrievals over the coastal regions where 60% of human population resides. This expanded coverage is crucial for better understanding of impact of aerosol particles on coastal air quality and climate.

Highlights

Aerosols are a colloidal system of particles suspended in the atmosphere, and have significant impacts on weather, climate, and human health [1,2,3]
As only one band is used in the algorithm, aerosol single scattering pixel are the same as those used for the aerosol optical depth (AOD) retrieval by the standard Moderate Resolution Imaging Spectroradiometer (MODIS) algorithm over its properties need to be prescribed
The AOD retrieval algorithm for turbid coastal water takes advantage of the fact that water-leaving radiance is negligible at 2.1 μm, this band is only used to retrieve while other auxiliary information such as aerosol single scattering properties are obtained from Dark Target (DT) retrievals over the nearby non-turbid water surfaces

Summary

Introduction

Aerosols are a colloidal system of particles suspended in the atmosphere, and have significant impacts on weather, climate, and human health [1,2,3]. Because of its global observational coverage, satellite remote sensing has a critical role in quantifying these impacts. Such satellite remote sensing is being used to retrieve aerosol properties such as the aerosol optical depth (AOD), along with a well-characterized uncertainty envelope, at high spatial resolution across the globe. One persistent challenge to aerosol remote sensing is retrieval of environmental properties over coastal (or littoral) waters. Retrieving AOD in coastal waters is a much-desired part of new systems characterizing air quality and aerosol radiative effects. This study aims to address this observational gap by refining the MODIS DT-ocean algorithm to retrieve AOD over turbid coastal waters

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