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,O_3 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 region306–330 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.