Oil spills in China Seas revealed by the national ocean color satellites

Oil spill is one of the key targets in marine environment monitoring. In the process of weathering (e.g., spreading, drift, emulsification, evaporation, dispersion, and biological degradation), various weathered oils with different visual characteristics can form, such as non-emulsified oil slicks, water-in-oil (WO) emulsions and oil-in-water (OW) emulsions. Timely and accurate information about the spatial coverage, concentration, and thickness of various oil types is important for the recovery and cleanup. Satellite observation can serve as an effective technique for the detection and monitoring of spilled oils. Synthetic aperture radar (SAR) and optical remote sensing are the most-frequently used techniques. SAR provides data under all-weather conditions, but it is often restricted to presence/absence detection without classifying oil types. Passive optical remote sensing is another popular tool as it is able to classify different weathered oils and quantify the thickness, concentration, or volume of them. The operational monitoring of marine oil spills via remote sensing requires optical sensors for wide coverage, high spatial resolution, high signal-to-noise ratio (SNR), large radiative dynamic range, and short revisit period. Haiyang-1C (HY-1C) and Haiyang-1D (HY-1D), launched on 7 September 2018 and 11 June 2020, are the first operational ocean color satellites of China. They are equipped with five sensors, including Chinese ocean color and temperature scanner (COCTS), coastal zone imager (CZI), ultraviolet imager (UVI), calibration spectrometer, and automatic identification system. The integrated imaging system composed of HY-1C/D sensors can provide more frequent observations. Particularly, the CZI sensors onboard, which have a spatial resolution of 50 m with high SNR, and ensure double coverage in China Seas every three days, have demonstrated excellent performance in optical remote sensing of marine oil spills; thus, they can be used for the operational monitoring of oil spills. The results indicate that 57 oil spill incidents were detected by the CZI sensors over the past three years, the image area of spilled oils can be up to 1291.63 km2. More than 47 oil spills occurred in China Seas, particularly in the Bohai Sea, Yellow Sea, and South China Sea, which require further attention. Among these, 13 oil spills could be detected by both HY-1C and HY-1D, indicating that the satellites network can increases observation times and is helpful the oil spill detection. Meanwhile, owing to the wide coverage and high temporal resolution, the CZI sensors can effectively distinguish oil types (i.e., non-emulsified oil slicks and oil emulsions) under different sunglint conditions. The results indicate that almost half of the oil spills have emulsified, and these emulsified spilled oils tend to have larger length and image area, posing a great threat to the marine environment. Moreover, based on the identification result, the thickness and volume of the spilled oils can be estimated preliminarily according to Bonn Agreement, and the damage caused by spilled oils can be assessed with less uncertainties. Therefore, the HY-1C/D satellites can provide reliable data and improve the monitoring of marine oil spills in China Seas.