Simultaneous determination of aerosol optical thickness and water leaving radiance from multispectral measurements in coastal waters

Abstract

Abstract. Retrieval of aerosol optical properties and water leaving radiance over ocean is changeling since the latter mostly 10 accounts for ~10% of satellite observed signal and can be easily contaminated by the atmospheric scattering. Such an effort would be more difficulty in turbid coastal waters due to the existence of optically complex oceanic substances or high aerosol loading. In an effort to solve such problems, we present an optimization approach for the simultaneous determination of aerosol optical thickness (AOT) and normalized water leaving radiance (nL w ) from multi-spectral measurements. In this algorithm, a coupled atmosphere-ocean radiative transfer model combined with a comprehensive bio-optical oceanic module 15 is used to jointly simulate the satellite observed reflectance at the top of atmosphere and water leaving radiance just above the ocean surface. Then a full-physical nonlinear optimization method is adopted to retrieve AOT and nL w in one step. The algorithm is validated using Aerosol Robotic Network Ocean Color (AERONET-OC) products selected from eight OC sites distributed over different waters, consisting of observation cases covered both in and out of sun glint from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Results show a good consistency between retrieved and in situ 20 measurements in each site. It is demonstrated that more accurate AOT are determined based on the simultaneous retrieval method, particularly in shorter wavelengths and sun glint conditions, where the averaged percentage difference (APD) of retrieved AOT generally reduce by approximate 10% in visible bands compared with those derived from the standard atmospheric correction (AC) scheme. It is caused that all the spectral measurements can be used jointly to increase the information content in the inversion of AOT and the wind speed is also simultaneously retrieved to compensate the specular 25 reflectance error estimated from the rough ocean surface model. For the retrieval of nL w , over atmospheric correction can be avoided to have a significant improvement for the inversion of nL w at 412 nm. Furthermore, generally better estimates of band ratios of nL w (443)/nL w (554) and nL w (488)/nL w (554), which are employed in the inversion of chlorophyll a concentration (Chl), are obtained using simultaneous retrieval approach with less root mean square errors and relative differences than those derived from the standard AC approach in comparison to the AERONET-OC products, as a result that 30 the APD value of retrieved Chl decreases by about 5%. On the other hand, the standard AC scheme yields a more accurate retrieval of nL w at 488 nm, prompting a further optimization of oceanic bio-optical module of current model. Atmos. Chem. Phys. Discuss., https://doi