Aerosol Optical Thickness (Ocean Color)

Aerosol optical thickness is the measure of light extinction by aerosols along the atmospheric path, and in ocean color processing it is the key unknown the atmospheric correction must solve. Over the open ocean the water is nearly black in the near-infrared, so sensors such as MODIS, VIIRS, and Sentinel-3 OLCI use NIR bands to estimate the aerosol type and optical thickness, then extrapolate that signal back across the visible bands to remove it. The atmosphere contributes roughly 90 percent of the top-of-atmosphere radiance over clear water, so a small error in aerosol optical thickness propagates into large errors in remote-sensing reflectance and downstream chlorophyll retrieval.