# Preliminary Global NO2 Retrieval from EMI-II Onboard GF5B/DQ1 and Comparison to Tropopmi  
Liangxiao Cheng  
1 China Siwei Surveying and Mapping Technology Co., Ltd., Beijing 100089, China; [EMAIL_ADDRESS]  
[PERSON]  
2 National Satellite Meteorological Center (National Center for Space Weather), Beijing 100081, China; [EMAIL_ADDRESS]  
[PERSON]  
2 National Satellite Meteorological Center (National Center for Space Weather), Beijing 100081, China; [EMAIL_ADDRESS]  
[PERSON]  
5 [PERSON]  
6 University of Chinese Academy of Sciences, Beijing 101408, China 6  
[PERSON]  
7 University of Chinese Academy of Sciences, Beijing 101408, China 7  
[PERSON]  
6 University of Chinese Academy of Sciences, Beijing 101408, China 6  
[PERSON]  
5 [PERSON]  
6 University of Chinese Academy of Sciences, Beijing 101408, China 6  
[PERSON]  
affects the ozone layer by acting as a catalyst for ozone destruction [2; 3] or by suppressing ozone depletion [4]. Thus, it is essential to monitor these species on a global scale. Since the mid-1990s, satellite observations of NO\({}_{2}\) have been used by atmospheric chemistry researchers to monitor the global daily NO\({}_{2}\) pollution, study the long-term and short-term changes in NO\({}_{2}\), locate the sources of NOx emissions, and support the formulation of emission control policies and pollution prevention measures. Many space-based sensors onboard polar orbit satellites can provide NO\({}_{2}\) information via VIS band observations, e.g., the Global Ozone Monitoring Experiment (GOME) [5], Scanning Imaging Spectrometer for Atmospheric Cartography (SCIAMACHY) [6], Ozone Monitoring Instrument (OMI) [7], GOME-2 [8; 9], and the TROPOspheric Ozone Monitoring Instrument (TROPOMI) [10]. With the advantage of a finer spatial resolution and higher signal-to-noise ratio compared to other similar satellite missions, TROPOMI products have been widely used for detecting emission sources [11]. The EMI sensor onboard the GaoFen-5 satellite, which has a spatial resolution of 13 \(\times\) 48 km\({}^{2}\), has also demonstrated the capability for satellite-based global NO\({}_{2}\) monitoring [12; 13]. In addition, [PERSON] et al. (2014) investigated the feasibility of NO\({}_{2}\) retrieval based on the OMPS Nadir Mapper (NM) using the UV band of 345-390 nm [14], and the algorithm was implemented in the NOAA-20 OMPS-NM [15].Alternatively, geostationary satellite missions like the GEMS (Geostationary Environment Monitoring Spectrometer) onboard the Geostationary Korea Multi-Purpose Satellite-2B (GK-2B), which was launched in February 2020, and the Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument, launched in April 2023, can provide measurements of NO\({}_{2}\) and other pollutants in the daytime on an hourly basis [16; 17]. The Environmental Trace Gases Monitoring Instrument (EMI-II) aboard the Chinese GaoFen-5B (GF5B) and DaQi-1 (DQ1) satellites, which were launched in September 2021 and April 2022, respectively, is the successor of the EMI aboard the Chinese GaoFen-5 (GF5) satellite under the 13 th Five-Year Plan for Space Infrastructure Development. Because the EMI data have worse signal-to-noise ratios, calibration accuracy, and spatial resolution than TROPOMI, several methods such as pre-calibration have been conducted to improve the spectral quality of EMI data before retrieval [18]. In addition, the earth radiance over the remote Pacific is used as a reference spectrum instead of solar irradiance in the EMI NO\({}_{2}\) retrieval setting, which is different from traditional DOAS retrieval, to eliminate the cross-track stripes in the retrieval as a result of imperfect irradiance measurements [12]. As a new data source for global NO\({}_{2}\) monitoring, EMI-II has a better calibration accuracy and spectral quality than EMI [19], and its detection performance and comparison with similar international instruments must be further evaluated.