## 5 Summary and Conclusions  
EMI onboard the GF5 satellite was designed for trace gas monitoring in the troposphere and stratosphere. The measured earth radiance and solar irradiance in the spectral range of 401-550 nm (VIS1 band) were used for NO\({}_{2}\) retrieval in this study. The EMI NO\({}_{2}\) retrieval algorithm, as well as some results and their comparison with OMI and TROPOMI, were presented in this paper. The comparison of the global distribution of the monthly averaged NO\({}_{2}\) column density shows that OMI, EMI, and TROPOMI agree with each other with r \(>\) 0.88; however, the discrepancy of the regional average between EMI and OMI (and TROPOMI) is within 27.3%. Using one day data over the North China Plain on 22 January, 2019, the correlation coefficient between EMI and OMI (TROPOMI) is 0.93 (0.95), but the region's averaged value of EMI is 24.5% and 19.9% higher than that of OMI and TROPOMI, respectively. The SCD uncertainty of EMI is about \(0.79\times 10^{15}\) molec cm\({}^{-2}\). The spatial distribution of tropospheric AMF and VCD of EMI also agree well with OMI and TROPOMI. The comparison of the daily variation using one month data also shows a very good agreement between EMI and TROPOMI in Beijing (r = 0.96). The good agreement of EMI products with OMI and TROPOMI makes us confident that EMI can be used to retrieve tropospheric NO\({}_{2}\) and provide the daily NO\({}_{2}\) global distribution. It should be noted that the results presented in this study are preliminary, and many more works related to the EMI NO\({}_{2}\) retrieval algorithm require further study to improve the result. For example, the cloud information is currently obtained from TROPOMI cloud products; however, due to the difference in the satellite pass time and spatial resolution, the external cloud information cannot meet the requirements of near real-time NO\({}_{2}\) retrieval. Cloud parameters with a sufficient precision and consistency need to be updated in the future. In addition, because of the insufficient solar irradiance measurements, the reference spectra used in this study are individual measurements from June 12, 2018. The SCD uncertainty caused by the use of single irradiance needs to be further analyzed. It is expected that, at least, weekly-to-monthly averaged reference spectra should be used in the future to reduce the retrieval uncertainty. Besides, there is an urgent need for more irradiance data, not only for trace gas retrieval, but also for the monitoring of EMI degradation. In spite of the existing shortcomings, this study may serve as a reference for the design of subsequent satellite payloads, i.e., EMI-02 and EMI-03. Conceptualization, [PERSON] and [PERSON] (L.C. ); formal analysis, [PERSON] (L.C. ); methodology, [PERSON] and S.L. ; resources, [PERSON]; validation, C.Y. ; writing--original draft preparation, [PERSON] (L.C.) and [PERSON]; writing--review and editing, [PERSON] and [PERSON] (L.C.). This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA19040201) and the National Key Research and Development Program of China (Grant No. 2017 YFB0503901). We would like to thank the following agencies for providing the satellite data: The EMI level 1 data is provided by the Ministry of Ecology and Environment Satellite Application Center for Ecology and Environment (SACEE). The OMI OMI OMNO2 product is archived and distributed by the Goddard Earth Sciences Data & Information Services center (NASA). The Sentinel 5 Precursor TROPOMI Level 2 NO\({}_{2}\) product is developed by KNMI with funding from the Netherlands Space Office (NSO) and processed with funding from the European Space Agency (ESA).