Satellite Retrieval of Water Quality Indicators Under High Solar Zenith Angles

Accurate and high spatiotemporal resolution water quality data are critical for the effective management of marine and coastal ecosystems. However, accurate atmospheric correction under high solar zenith angles (SZAs) remains a challenge, introducing substantial uncertainties in satellite-derived water quality indicators (WQIs) under high SZA. With an attempt to fill the gap, this study evaluated three types of strategies for satellite retrieval of suspended particulate matter (SPM) and chlorophyll-a (Chl-a) concentrations from top-of-atmosphere reflectance (ρ_t), Rayleigh-corrected reflectance (ρ_rc), and remote sensing reflectance (R_rs). The models, named XGBWQI, based on three types of remote sensing data were tested with in situ data and compared with the geostationary ocean color imager (GOCI) standard algorithms. Results showed that: 1) ρ_t-based XGBWQI had the best accuracy (R² = 0.90 and mean absolute percentage deviation (MAPD) = 14.65% for SPM, and R² = 0.85 and MAPD = 5.34% for Chl-a); 2) model testing results with in situ data also confirmed the advantage of ρ_t-based XGBWQI over other models (R² = 0.88 , MAPD = 26.9%, and mean relative percentage deviation (MRPD) = 11.8% for SPM; R² = 0.78 , MAPD = 43.3%, and MRPD = -15.5 % for Chl-a); and 3) the XGBWQI models obtained more valid WQI values for GOCI images under high SZA and successfully revealed the diurnal variations of a red tide event in the Yellow Sea and the SPM dynamics in the East China Sea. Therefore, ρ_t-based XGBWQI models were recommended as the best strategy for satellite retrievals of WQI under high SZA. The methods can serve as an effective tool in retrieving WQI in coastal waters under high SZA and thus contribute to better and high-frequency water quality monitoring.