Comparison of SO 2 column retrievals from BRD and DOAS algorithms

Atmospheric SO 2 has a significant impact on the urban environment and global climate. Band Residual Difference Algorithm (BRD) and Differential Optical Absorption Spectroscopy (DOAS) were used respectively by NASA and ESA science team to derive SO 2 columns from satellite observations, but there are few studies on the comparison and validation of BRD and DOAS SO 2 retrievals under the same observation conditions. In this study, the radiative transfer model SCIATRAN was firstly used to validate the accuracies of BRD and DOAS SO 2 retrievals, and analyse the uncertainty of SO 2 retrieval caused by band selection, O3 absorption, aerosol, surface reflectance, solar and viewing zenith angle. Finally, BRD and DOAS algorithms were applied to the same radiances from satellite observations, and comparisons of BRD and DOAS SO 2 retrievals were conducted over volcanic eruption and North China. Results show that, for the case with low SO 2 columns, BRD SO 2 retrievals have higher retrieval accuracy than DOAS, but typical seasonal variation with high SO 2 column in winter and low in summer can be more clearly discernible in DOAS SO 2 retrievals than BRD from satellite observations. For the case with high SO 2 columns, the differences between BRD (310.8–314.4 nm) and DOAS (315–327 nm) retrievals are large, and the value and accuracy of BRD (310.8–314.4 nm) SO 2 retrievals are lower than those of DOAS (315–327 nm) retrievals. Compared with the SO 2 inputs in forward model, both BRD (310.8–314.4 nm) and DOAS (315–327 nm) SO 2 retrievals are underestimated for the case with high SO 2 columns. The selection of wavelength range can significantly affect the accuracy of SO 2 retrieval. The error of BRD SO 2 retrieval from 310.8–314.4 nm is lower than other bands in the ultraviolet spectral region (306–327 nm). The increase of wavelength in the ultraviolet spectral region 306–330 nm can reduce the underestimation of DOAS SO 2 retrievals in the case of high SO 2 column, but slight overestimation of SO 2 retrieval is found from the 315–327 nm range in the case of low SO 2 column. The values of BRD and DOAS SO 2 retrieval decrease with atmospheric O 3 column and aerosol optical depth increasing, but increase with surface reflectance increasing. Large solar zenith angle and viewing zenith angle can introduce more errors to the BRD and DOAS SO 2 retrievals. This study is important for the improvement of retrieval algorithm and the application of SO 2 products from satellite observations.