基于EMI-Ⅱ的火山区域二氧化硫反演研究

Research on Retrieval of Sulfur Dioxide in Volcanic Region from EMI-Ⅱ

利用差分吸收光谱技术(DOAS)反演了我国第二代星载大气痕量气体差分吸收光谱仪(EMI-Ⅱ)的SO_(2)斜柱浓度(SCD),并通过辐射传输模型SCIATRAN建立了SO_(2)大气质量因子(AMF)的查找表,经去条带处理后获得SO_(2)的垂直柱浓度(VCD)。以2021年10月底拉帕尔马岛火山区域为研究对象,基于EMI-Ⅱ数据反演的SO_(2)VCD与国外同类型载荷TROPOMI的结果一致,相关性系数R分别为0.89、0.90、0.92。此外,还将汤加海底火山的SO_(2)反演结果与TROPOMI的监测数据进行对比,结果表明,EMI-Ⅱ观测结果与TROPOMI一致,都观测到此次SO_(2)羽流的自东向西的传输过程。结合风场数据,计算了2022年1月14—15日汤加海底火山爆发产生的SO_(2)排放通量,结果表明,利用EMI-Ⅱ载荷反演的火山区域SO_(2)VCD可靠性高,可实现全球火山爆发预警。

The SO_(2) slant column density(SCD)of environmental trace gases monitoring instrumentⅡ(EMI-Ⅱ)from China was firstly retrieved using the differential optical absorption spectroscopy(DOAS)method.The air mass factor(AMF)look-up table of SO_(2) was established using the SCIATRAN radiative transfer model.The vertical column density(VCD)was obtained after deducting the stripe effect.Taking the La Palma volcano at the end of October as an example,the SO_(2) VCD from EMI-Ⅱwas consistent with that from TROPOMI,with the correlation coefficients(R)of 0.89,0.90,and 0.92,respectively.In addition,the SO_(2) VCD from EMI-Ⅱwas also compared with that from TROPOMI in Tonga submarine volcano.The EMI-Ⅱresults showed similar spatial distributions with TROPOMI,and the transmission process(from the east to west direction)of SO_(2) plume was monitored.Combined with the wind field data,the fluxes of SO_(2) from 14 and 15 January 2022 in Tonga submarine volcano were calculated.The results of this paper show that EMI-Ⅱcan retrieve the reliable SO_(2) VCD results in volcanic regions and realize the earth warning of global volcano eruptions.Objective SO_(2) not only affects human health(such as acid rain,respiratory diseases,etc.),but is also closely related to climate and environment.The oxidation of SO_(2) may lead to the formation of aerosol and photochemical smog.SO_(2) is an important indicator of air quality,and closely related to volcanic eruptions.The SO_(2) VCD can provide a data basis for tracing the SO_(2) pollution caused by the industrial emissions and early warning signals for volcanic eruptions around the world.Therefore,it’s extremely important for us to obtain the daily global SO_(2) VCD.In this study,we report the first SO_(2) VCD results from EMI-Ⅱ,and validated the retrieved results with that from TROPOMI in volcanic region.In addition,the fluxes of Tonga submarine volcano were calculated,which may be helpful for the dynamics of degassing magma.We hope that our results can be helpful for the development and global validations of the EMI-ⅡSO_(2) VCD.Methods The SO_(2) SCD was calculated using the QDOAS software based on the DOAS method.The DOAS method retrieves concentrations of trace gases based on their characteristic absorption and the measured intensity,which is based on the Lambert-Beer’s law.Then,the corresponding SO_(2) AMF of the EMI-Ⅱwas calculated using the established AMF look up table,which was simulated in the radiative transfer model of SCIATRAN.The SO_(2) VCD was then obtained by SCD and AMF.We used the spatial filtering following the Fourier transform method to remove the obvious stripes caused by irradiance calibration error when retrieving the SO_(2) VCD of EMI-Ⅱ.The fluxes of SO_(2) from satellite-based measurements can be calculated using the Theys’method.For Tonga submarine volcano,the effect of distance can be omitted for the long lifetime of stratospheric SO_(2) plume.Results and Discussions To validate the retrieved SO_(2) VCD results,we compared the EMI-ⅡSO_(2) VCD with that from TROPOMI at La Palma volcano on 27,29 and 31 October 2021.The SO_(2) VCD from EMI-Ⅱshows similar spatial distributions with that from TROPOMI(Fig.4),with the correlation coefficients(R)of 0.891,0.901 and 0.915(Fig.5),respectively.In addition,the SO_(2) VCD of EMI-Ⅱwas also compared with TROPOMI in Tonga submarine volcano from 14 to 18 January 2022(Fig.6).As shown in Fig.6,EMI-ⅡSO_(2) VCD shows similar spatial distributions with TROPOMI.The SO_(2) plume transported from Tonga to Australia,which corresponds the wind field results from HYSPLIT model(Fig.7).However,the SO_(2) VCD from EMI-Ⅱis lower than that from TROPOMI in the grid with high SO_(2) SCD,which is mainly because the a priori profile of TROPOMI is different with that from EMI-Ⅱin radiative transfer model.Combined with the calculated SO_(2) VCD and wind field data,the fluxes of SO_(2) from 14 and 15 January 2022 in Tonga submarine volcano were 345.83 and 504.85 tons per second,respectively.Conclusions In this paper,the SO,VCD is retrieved from EMI-II and validated in volcanic regions.With the La Palma volcano and the Tonga submarine volcano as examples,the SO,VCD from EMI-II presents similar spatial distributions to those of the SO,VCD from TROPOMI.In addition,the transmission process of SO_(2) plume in a volcanic region can be monitored using the retrieved SO,VCD from EMI-II.The results of this study confirm that EMI-II can monitor SOz in volcanic regions and realize the early warning of global volcanic eruptions.This paper is of great importance for the development and global validation of SO,VCD from EMI-Ⅱ.