Atmospheric Correction Using the Information From the Short Blue Band

We describe our effort to use normalized water-leaving radiance at the short blue band 410 nm, nLw (410), derived from the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership to improve the VIIRS ocean color products over coastal and inland waters, in particular, over the regions contaminated by strongly absorbing aerosols. The current standard atmospheric correction algorithm has significant issues when dealing with cases of strongly absorbing aerosols, e.g., dust, smoke, and air pollution from nearby cities. For such cases, satellite-derived normalized water-leaving radiance spectra nLw ( <tex-math notation="LaTeX">λ </tex-math> ) are usually biased low and may be negative, particularly for the short blue bands, e.g., for the VIIRS nLw (410). In addition, for extremely turbid waters and waters dominated by colored dissolved organic matter (CDOM) that is strongly absorbing at the short blue band, slightly negative nLw (410) data are also sometimes observed. Obviously, cases with nLw (410) < 0 are unphysical and incorrect. However, VIIRS-derived nLw (410) < 0 do provide useful information to further improve satellite-derived ocean color products in such cases. In this paper, we develop a technique to improve VIIRS ocean color products in cases with strongly absorbing aerosols, CDOM-dominated and extremely turbid waters using VIIRS-derived nLw (410) information. Specifically, for cases with nLw (410) < 0, atmospheric correction can be carried out again to ensure that nLw (410) <tex-math notation="LaTeX">≥ 0 </tex-math> [e.g., assuming nLw (410) = 0], thereby removing unphysical retrievals for VIIRS-derived nLw ( <tex-math notation="LaTeX">λ </tex-math> ) spectra. Using this technique, VIIRS-derived nLw ( <tex-math notation="LaTeX">λ </tex-math> ) spectra are now all ≥ 0, showing considerable improvements for VIIRS ocean color products, particularly for biological and biogeochemical products [e.g., chlorophyll-a concentration and diffuse attenuation coefficient at 490 nm <tex-math notation="LaTeX">Kd </tex-math> (490)] which are derived using VIIRS nLw ( <tex-math notation="LaTeX">λ </tex-math> ) spectra. Several detailed examples from VIIRS measurements over various coastal and inland waters are provided and discussed. The new technique has been implemented in the Multi-Sensor Level-1 to Level-2 (MSL12) ocean color data processing system, which has been used for the routine production of VIIRS global ocean color products.