Evaluation of a QAA-based algorithm using MODIS land bands data for retrieval of IOPs in the Eastern China Seas

A quasi-analytical algorithm (QAA)-based algorithm which is applied to moderate imaging spectroradiometer (MODIS) land band data (469, 555, and 645 nm) is proposed and named QAA-RGR (Red-Green-bands-Ratio). The performance has been evaluated using in situ measurements data and MODIS data from the Eastern China Seas (ECS). The QAA-RGR algorithm uses the ratio of the remote sensing reflectance at 645nm (R_rs645) to the R_rs555 to estimate the absorption coefficient at 555nm. In addition, the spectral slope of the backscattering coefficient (b_b) is estimated using a statistical relationship based on b_b555. The other steps of the retrieval algorithm are the same as those of the extensively used QAA version 5 (QAAv5). First, the QAA-RGR algorithm was applied to an in situ measurement data set for the ECS to retrieve inherent optical properties (IOPs), and the results were compared with the QAAv5. The results demonstrate that, the two algorithms exhibit similar performance for in situ measurements. Second, the algorithm was applied to the land bands data and ocean bands data of MODIS over the ECS to obtain the distribution of IOPs at 500m and 1000m resolutions. The results of the retrieval algorithm were evaluated against the corresponding in situ measurements and compared to those from QAAv5. The results demonstrate that, the QAA-RGR algorithm is characterized by a better performance than QAAv5 for MODIS data. For QAAv5, the averaged mean absolute percentage error (MAPE) values of retrieval results of absorption coefficients and backscattering coefficients compared with in situ match-up measurements are 25.2% and 22.2%, respectively. For QAA-RGR, the averaged MAPE values are 15.9% and 18.3%, respectively. The QAAv5 retrieval results are often significantly underestimated especially for turbid coastal waters because of the easy saturation at 667nm band in addition to a large uncertainty in the estimation of R_rs of the blue bands. The QAA-RGR algorithm may be used to retrieve IOPs from MODIS measurements over the ECS for the measurement periods used in the study.