高分五号大气痕量气体差分吸收光谱仪对流层NO_(2)柱浓度遥感反演研究

Tropospheric NO_(2) column density retrieval from the GF-5 EMI data

搭载于"高分五号"(GF-5)卫星上的痕量气体差分吸收光谱仪(EMI)是一台星下观测的高光谱载荷,测量紫外和可见光光谱范围的地球后向散射辐射,设计用于大气痕量气体的探测。本研究基于EMI在VIS1通道的实测光谱,利用差分光学吸收光谱(DOAS)方法进行了对流层NO_(2)柱浓度反演,展示了基于EMI载荷的对流层NO_(2)柱浓度反演结果,与同类载荷产品进行交叉验证,并利用地基观测结果进行了地基验证。研究表明,EMI反演结果与OMI、TROPOMI具有较好的空间分布一致性和较低的相对偏差,与TROPOMI具有较好的时间变化一致性。地面验证结果表明EMI NO_(2)反演结果具有较高的精度。本研究证明了EMI在全球NO_(2)监测方面的能力,可以为中国后续污染气体探测载荷的设计和反演算法的开发提供参考。

Significant impact of NO_(2)on global atmospheric environment and human health necessitate accurate monitoring of NO_(2).On the one hand,people can study and analyze their generation and extinction laws,distribution characteristics,diffusion,and transmission characteristics.On the other hand,it can provide decision-making basis for the formulation of pollutant discharge policy and pollution control program.However,the number of ground-based air quality monitoring stations has been increasing,providing abundant NO_(2)ground observation data.Large-scale monitoring of NO_(2)emissions requires the development of other monitoring methods.Satellite instruments covering the ultraviolet and visible spectrum have been widely used to detect the concentration of NO_(2)column in the atmosphere with the advantage of wide-range observation.to further strengthen the domestic air quality monitoring,and improve the air quality in China,the Environmental Trace Gas Monitoring Instrument(EMI)onboard the Chinese GaoFen-5(GF-5)satellite was launched on May 9,2018.It is a nadir-viewing wide-field hyperspectral spectrometer,which measures the earth’s backscattered radiation in the ultraviolet and visible spectrum and is designed for atmospheric trace gas detection.Based on the measured spectrum of EMI VIS1 channel,the tropospheric NO_(2)Vertical Column Density(VCD)was retrieved by Differential Optical Absorption Spectrometry(DOAS)method,which consists of three key steps,namely,spectral fitting,Stratosphere-Troposphere Separation(STS),and tropospheric Air Mass Factor(AMF)calculations.After spectral fitting,a stripe correction scheme was developed for the stripe phenomenon that appears in the initial fitted NO_(2)SCD.The current advanced STREAM algorithm was used to estimate the stratospheric NO_(2)concentration,and the TM5 NO_(2)profile with higher spatial resolution was used in the calculation of tropospheric AMF.The retrieval results of tropospheric NO_(2)VCD based on EMI were presented,and the results were cross-verified with NO_(2)products from international similar instruments,i.e.,OMI and TROPOMI.From a larger spatial scale,EMI can reflect the global distribution of typical NO_(2)pollution city sources.In terms of regional scale,the daily spatial distribution correlation coefficients between EMI and TROPOMI in different regions are greater than 0.9.On a monthly time scale,EMI and OMI(TROPOMI)show consistent spatial distribution in the four urban agglomerations of China,and the average spatial correlation coefficient is0.8(0.87).The regional mean bias between EMI and OMI(TROPOMI)is within 11.3%(9.5%).The time series analysis of the Pearl River Delta region shows that EMI has high consistency(r=0.89)with TROPOMI.The ground-based MAX-DOAS observation results are also used for validation.The ground validation results show that the EMI retrieval results have high correlation coefficient(0.96)and approximately 35%underestimated.This study proves EMI’s ability in global NO_(2)monitoring.In the future,domestic instruments similar to EMI are carried out on the GF-5(02)satellite and the atmospheric environmental monitoring satellite(AEMS),which contributes continuously to China’s trace gas detection.Therefore,this study can provide reference for the design of next similar instruments and the development of corresponding NO_(2)retrieval algorithm in China.