# Preliminary Global NO2 Retrieval from EMI-II Onboard GF5B/DQ1 and Comparison to Tropopmi
## 5 Conclusions  
This paper presents the retrieval of the total and tropospheric NO\({}_{2}\) VCD based on the EMI-II payloads onboard the GF5B and DQ1 satellites. The spectral fitting methods for the SCD retrieval followed the approach used for the GF5 EMI. During the de-stripe correction, the SCD correction values were modified by the climatology values of the clean ocean areas, so the SCD values were closer to reality with an uncertainty of 0.42 \(\times\) 10\({}^{15}\) molec./cm\({}^{2}\). The STREAM method was used for the STS step, and NO\({}_{2}\) profiles from the GEOS-cf CTM outputs were used for the tropospheric AMF calculations. The GF5B and DQ1 retrieval results were comprehensively compared considering different spatial scales (globally, regionally, and city) and temporal scales (daily and monthly). The results showed excellent consistency between the GF5B and DQ1 retrievals (R\({}^{2}\) > 0.8), except for a few specific emission sources that exhibited clear diurnal variations. Additionally, the EMI-II retrieval results were validated against TROPOMI products in four typical regions. Overall, the EMI-II retrieval results exhibited good spatial consistency with the TROPOMI results, where the correlation coefficients (R\({}^{2}\)) exceeded 0.85 and 0.57 for the total VCD and tropospheric VCD. The dual-satellite network of the GF5B and DQ1 enhanced the frequency of the atmospheric composition observations and reduced the impact of cloud coverage on the valid observation areas. The differences between the two satellites partially explain the diurnal variations in the NO\({}_{2}\) concentrations.  
Figure 9: Spatial distribution of average differences between DQ1 and GF5B on 5 July 2022 (**a**) and July 2022 (**b**–**d**).  
**Author Contributions:** Conceptualization, [PERSON]; formal analysis, L.C. ([PERSON]) and J.L.; methodology, L.C. ([PERSON]); resources, H.Y.; validation, J.T. and H.W.; writing--original draft, L.C. ([PERSON]) and Y.W.; writing--review and editing, J.X. and L.C. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was supported by the Beijing Natural Science Foundation (Grant No. 8244074), High-Resolution Earth Observation System Project (2) (32-Y30F08-901-20/22), Open Foundation of State Key Laboratory of Remote Sensing Science of China (OFSLRSS202203, OFSLRSS202211), and Fengyun Application Pioneering Project (FY-APP-2022.0501).  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Acknowledgments:** The authors are grateful to the EMI calibration team. We are thankful to ESA for the free release of the TROPOMI data. We also thank the contributors who prepared and provided the foundation database used in this study.  
**Conflicts of Interest:** Author [PERSON] is employed by China Siwei Surveying and Mapping Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.