Evaluations of Environmental Trace Gases Monitoring Instrument (EMI) Level 1 Data
CHENG Liang-xiao1, 2, TAO Jin-hua1*, ZHOU Hai-jin3, YU Chao1, FAN Meng1, WANG Ya-peng4, WANG Zhi-bao5, CHEN Liang-fu1
1. State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing Normal University, Beijing 100101, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
4. National Satellite Meteorological Center, China Meteorological Administration, Beijing 100081, China
5. School of Computer and Information Technology, Northeast Petroleum University, Daqing 163318, China
Abstract：The Environmental trace gases Monitoring Instrument (EMI) is the first high spectral resolution imaging spectrometer in China designed to monitor trace gas in tropospheric and stratospheric. To fully understand the instrument’s characteristics and make better use of the level 1 (L1) data, comprehensive evaluations of irradiance and radiance data measured by the EMI instrument were carried out in this study. For both ultraviolet-2 and visible-1 bands,the slit function change drastically as a function of detectors in the across-track direction of charge-coupled device (CCD), which is more than 6 times larger than that of TROP(OMI). The use of different slit functions according to the row can improve the calibration accuracy. Small wavelength shifts were found in both irradiance and radiance data with an average value of 0.015 and 0.03 nm, respectively. Although they meet the requirement of the design specification (0.05 nm), wavelength calibration is still needed during the trace gas retrieval. EMI irradiance data agree highly (r>0.95) with OMI and TROPOMI and the reference solar spectrum. EMI radiance data also shows a better agreement (r>0.93) with OMI and TROPOMI by averaging the cloud-free pixels over the clean Pacific region. This study demonstrates the good quality of current EMI L1 data for trace gas retrieval, and it provides a reference for the design and data quality evaluation scheme of subsequent instruments.
Key words：GF-5; EMI; Wavelength calibration; Slit function; Data quality
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