Accurate measurement of seawater absorption coefficients is important for ocean radiative transfer simulations, biogeochemical parameter inversion, and calibration and validation of ocean color satellites. Reflective-tube absorption meters are the most commonly employed instruments for seawater absorption coefficient measurement, but the measured absorption coefficient must be corrected for scattering due to instrumental design limitations. There have been a few studies evaluating the ac-9/ac-s scattering correction methods. However, there is a lack of application evaluation of these correction methods applied to the coastal waters of China. We evaluate the ac-9/ac-s scattering correction methods based on field measurements from the coastal waters of China’s sea. Based on the evaluation, guidance is provided for the selection of scattering correction methods when reflective-tube absorption meters are employed in different water bodies.

We introduce five scattering correction methods to evaluate their performance on the field measured data from the Bohai Sea, Yellow Sea, East China Sea, and South China Sea. The data includes the absorption coefficient measured by the reflective-tube absorption meters of ac-9, ac-s, and the point source integrating cavity absorption meter (OSCAR), the backscattering coefficients measured by the backscattering instrument HS6, the volume scattering function measured by LISST-VSF and the temperature, salinity, and depth data measured by CTD. The absorption coefficients are measured by adopting ac-9 and ac-s for two cruises to independently evaluate the performance of different scattering correction methods applied to ac-9 and ac-s. In the South China Sea, the absorption coefficient measured by OSCAR is taken as the true value to evaluate the application of scattering correction methods to the ac-s/ac-9 measurements. In the Bohai Sea, Yellow Sea, and East China Sea, there is no synchronous OSCAR measurement. Meanwhile, the absorption coefficient corrected by the volume scattering correction method is utilized as a reference for evaluating other scattering correction methods. Since the volume scattering function is measured independently, the error obtained by integrating it in the limited angle is the scattering error measured by the reflection-tube absorption coefficient measurement instrument, thereby making this method a more accurate scattering correction method. Prior to each cruise, all instruments have undergone rigorous calibration, including pressurized flow ultra-pure water calibration for ac-s and ac-9 in the laboratory, integrator cavity reflectivity calibration for OSCAR, and calibration for HS6 to ensure the accuracy of field measurements.

The results show that for the clean water in the South China Sea, little difference is found among different scattering correction methods. The baseline method and volume scattering correction method have better correction effects, and the relative errors after data correction are 25.05% and 23.24% respectively. The relative error of the semi-empirical correction method is 36.01%. The performance of the proportional method and iteration correction method is poor. In the Bohai Sea, the proportional method and semi-empirical correction method perform better, with relative errors of 29.22% and 25.02% respectively. After the correction of the baseline method and iterative method, the correction results of each band have a large deviation from the reference value. In the Yellow Sea, the proportional method is relatively sound, with 23.17% of the relative error. The baseline method and the semi-empirical correction method are similar, and the relative error after data correction is 30.94% and 31.68% respectively. In the East China Sea, the semi-empirical correction method has the best correction effect, and the relative error after data correction is 14.71%, followed by the proportional method. Additionally, the relative error after data correction is 24.02%, and the baseline method performs slightly worse.

We evaluate five representative scattering correction methods for reflective-tube absorption coefficient measurement based on field data from several regions of China’s sea. These methods include the baseline method, proportional method, semi-empirical correction method, iterative correction method, and volume scattering function correction method. Generally, all these scattering correction methods can reduce the scattering error from reflective tubes, and make the absorption coefficient approximate to the true value. However, the performance of each method varies between different types of water. Based on the analysis results of all the data, our suggestions for selecting scattering correction methods for the reflective-tube absorption meters are as follows. The baseline method is more suitable for clean water bodies (the South China Sea), the semi-empirical correction method is suited for turbidity water (the Bohai Sea, the Yellow Sea, the East China Sea, and coastal water), and the volume scattering method is preferred if the measured volume scattering function of water is available.