## 1 Introduction  
Nitrogen oxides (NOx=NO+NO2) play a key role in both stratospheric and tropospheric chemistry. In the stratosphere, they lead to ozone destruction by direct reactions with atomic oxygen and through being in the reaction cycles of halogen compounds [1]. In the lower troposphere, NOx is a critical precursor to surface ozone [2] and the titration effect of NOx also consumes O3 [3]. Both NOx and volatile organic compounds (VOCs) impact the ozone photochemically [4]. Secondary organic aerosol (SOA) formation from aromatic hydrocarbon photooxidation is highly sensitive to the NO concentration [5]. Nitrogen dioxide (NO2) is more stable than NO in the atmosphere, which is primarily located in the stratosphere over remote regions. In contrast, up to 90% of the total NO\({}_{2}\) column may be located in the troposphere over polluted regions [6]. For instance, it has been shown that there was a continuous increasing pattern for NO\({}_{2}\) from 2005 to 2012 over China [7]. Thereafter, it showed a decreasing trend in China due to regulations [8; 9; 10]. Over less polluted regions, natural emissions from microbiological processes in soils, wildfires, and lightning processes also influence the ambient NOx concentration and contribute to the vertical distribution of NOx. Besides, NOx is also a precursor to secondary aerosol (nitrate) through gas-to-particle conversion [11]. Satellite observations of the vertical column density (VCD) of tropospheric NO\({}_{2}\) from nadir-viewing UV/VIS backscatter instruments have been widely used for air quality monitoring since the mid-1990s. The Global Ozone Monitoring Experiment (GOME) onboard the European Space Agency (ESA) [12], with a spatial resolution of \(320\times 40\) km\({}^{2}\), revealed the weekly cycles of tropospheric NO\({}_{2}\) VCDs over specific areas [13]. The Scanning Imaging Spectrometer for Atmospheric Cartography (SCIAMACHY) onboard the ENVISAT satellite had a spatial resolution of \(60\times 30\) km\({}^{2}\)[14], which met the needs of urban-scale air quality monitoring. The better spatial resolution of the Ozone Monitoring Instrument (OMI), i.e., \(13\times 24\) km\({}^{2}\) at nadir [15], made it possible to monitor the stationary anthropogenic point sources (e.g., coal-fired power plants) [16]. GOME-2 onboard the MetOp-A/B/C satellites provided NO\({}_{2}\) information mid-morning, with a resolution of \(40\times 80\) km\({}^{2}\)[17; 18]. The new generation sensor, the TROpospheric Ozone Monitoring Instrument (TROPOMI), onboard Sentinel-5-Precursor, was launched on October13, 2017 [19]. With an unprecedented spatial resolution of \(3.5\times 5.5\) km\({}^{2}\) at band 4 and significantly improved signal to noise ratio, TROPOMI can provide more detailed information of emission sources [20]. In order to further monitor the air pollution, as well as improve the air quality on a global scale, the Environmental trace gases Monitoring Instrument (EMI), which is the main focus of this study, is located onboard the Chinese GaoFen-5 (GF5) satellite launched on 9 May, 2018 [21; 22]. It is the first sensor designed in China for trace gas monitoring with a relatively higher spatial resolution (\(13\times 12\) km\({}^{2}\)) than OMI. Tropospheric NO\({}_{2}\) VCD retrieval from satellites consists of three key steps: spectral fitting, stratosphere-troposphere separation (STS), and tropospheric air mass factor (AMF) calculations [23; 24; 25; 26; 27]. The spectral fitting step obtains the total NO\({}_{2}\) slant column density (SCD; the integral of target trace gas along the effective light path from the sun through the atmosphere to the instrument) from the satellite-measured radiance spectrum by means of the differential optical absorption spectrometry (DOAS) method. Then, the STS step estimates the stratospheric NO\({}_{2}\) concentration and removes it from the total SCD to get the tropospheric SCD. Finally, the tropospheric AMF is calculated to convert the tropospheric SCD to tropospheric VCD.