# MODIS Retrieval of Aerosol Optical Depth over Turboid Coastal Water
## 1 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. Indeed, since the launch of the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra satellite in 1999, AOD retrievals derived by Dark Target (DT) algorithm [4] or Deep Blue (DB) algorithm (land only) [5; 6] have been used widely in the research community [7]. One persistent challenge to aerosol remote sensing is retrieval of environmental properties over coastal (or littoral) waters. Variable ocean color, along with the presence of visible sediments and sea floor along coastlines, has strong spectral and spatial variability leading to poorly constrained lower boundary conditions for aerosol retrievals. Yet, retrieving AOD in coastal waters is a much-desired part of new systems characterizing air quality and aerosol radiative effects. As \(\sim\)60% of the human population lives in coastal areas [8], it is crucial to expand the satellite-remote sensing datasets to include these areas. This study aims to address this observational gap by refining the MODIS DT-ocean algorithm to retrieve AOD over turbid coastal waters. The MODIS DT-ocean algorithm uses top of atmosphere (TOA) reflectance in six wavelength bands, ranging from 0.55 to 2.1 \(\upmu\)m, to simultaneously retrieve AOD and Fine Mode Fraction (FMF) based on a lookup table approach. The lookup table is constructed by assuming aerosol optical properties of four fine aerosol modes and five coarse aerosol modes, coupled with the boundary conditions of molecular scattering, and a rough ocean surface (glitter, whitecaps, foam). Additionally, the lookup table assumes zero water-leaving radiance at all bands except 0.55 \(\upmu\)m (at which a fixed water-leaving reflectance of 0.005 is used) [4; 9]. The retrieval process searches the lookup table to find the best combination of fine mode and coarse mode aerosol type (out of possibly 20 combinations) such that the AOD and FMF retrievals render the best match between observed and simulated (e.g., those from the lookup table) radiances. However, while the assumption of nearly-zero water-leaving radiance may be suitable for open ocean, it is clearly not applicable to turbid coastal waters. There, the water-leaving radiances can be contributed by reflection of shallow-water sea floor (particularly in blue wavelengths) and either suspended or dissolved particulate matter in the water, especially in green to red wavelengths such as 0.55, 0.66, and 0.86 \(\upmu\)m [10]. Hence, as part of the DT-ocean algorithm, the turbid water pixels are masked and not considered for retrieval. The method used for such masking compares the TOA reflectance at 0.55 \(\upmu\)m with the expected counterpart from the power-law fitting using the TOA reflectance at 0.47, 1.2, 1.6, and 2.1 \(\upmu\)m; if a significant difference (larger than 0.01) is found, the corresponding pixel is masked out for AOD retrieval [8]. We analyze the MODIS AOD retrieval unavailability over the cloud-free conditions at both global and regional scale to reveal how often the AOD is not retrieved only because of water turbidity (and not other factors such as cloud cover). As shown in Figure 1, this data availability is near total over all open ocean and decreases dramatically (by 90-100%) toward coastlines. In a global average, the unavailability of AOD is \(\sim\)20% over coastal water region which is labelled as shallow ocean (within 5 km of coastline or with water depth less than 50 m) in the MODIS geolocation product [11]. In other words, in these 20% of cloud-free cases, the AOD should have been retrieved if the water was not turbid.