## 2 Data  
As part of the China high-resolution earth observation system (CHEOS) project [37], GF5 is flying in a sun-synchronous polar orbit, with an orbital altitude of 705 km and the ascending equator crossing time of 13:30 [22]. EMI has two ultraviolet bands--UV1 (240-315 nm) and UV2 (311-403 nm)--and two visible bands--VIS1 (401-550 nm) and VIS2 (545-710 nm). Each band adopts an Offner imaging spectrometer and a two-dimensional charge-coupled device (CCD) with the spectral resolution \(\sim\)0.3-0.5 nm. As shown in Figure 1, the swath width of the VIS1 band is approximately 2600 km, which enables it to obtain a daily global coverage. The CCD in the VIS1 band has 111 row detectors and 1286 spectral detectors, and with the binning factor of 4, the nadir spatial resolution is 12 km (across-track direction) \(\times\) 13 km (along-track direction). The designed SNR for the VIS1 band is better than 1300 [22] and enables it to meet the requirements of NO\({}_{2}\) retrieval. Figure 1: Environmental trace gases Monitoring Instrument (EMI) observation sketch map. To evaluate the first EMI NO\({}_{2}\) results, NO\({}_{2}\) products from OMI and TROPOMI were used in this study for intercomparison. OMI is the Dutch-Finnish UV-Vis spectrometer aboard the EOS-Aura satellite that was launched on 15 July, 2004, with a local overpass time of 13:40 (ascending node). It has two ultraviolet bands--UV-1 (270-310 nm) and UV-2 (310-365 nm)--and one visible band (VIS, 365-500 nm). Each band is a two-dimensional CCD that simultaneously observes the direct and atmospheric backscattered sunlight for 60 individual rows [15]. With a spectral resolution of 0.63 nm and a nadir spatial resolution of 13 km \(\times\) 24 km, measurements from the OMI VIS band could be used for NO\({}_{2}\) retrieval. TROPOMI is a single payload developed by The Netherlands and European Space Agency (ESA) onboard the SSP satellite, which was launched on 13 October, 2017. It is flying in a sun-synchronous orbit with an ascendant overpass time of 13:30 [19]. It has four spectrometers that cover the UV-VIS-NIR-SWIR with seven bands, wherein the spectral range of the fourth band is 405-500 nm, which can be used for NO\({}_{2}\) monitoring. The spectral resolution and spatial resolution of band 4 are 0.55 nm and 5.5 km \(\times\) 3.5 km, respectively. With similarly designed specifications as OMI, EMI has a slightly higher spatial resolution than OMI (13 km \(\times\) 24 km), and the local solar time at the ascending node (LTAN) of EMI is 10 minutes earlier than OMI (13:40). TROPOMI has a better spatial resolution (5.5 km \(\times\) 3.5 km), and the LTAN (13:30) is nearly the same as EMI. Since these three instruments are similar, the OMI and TROPOMI NO\({}_{2}\) products are ideal data for the evaluation of EMI results. The OMI NO\({}_{2}\) product used in this study is the latest version (SPv3) obtained from the NASA website ([[https://disc.gsfc.nasa.gov/datasets](https://disc.gsfc.nasa.gov/datasets)]([https://disc.gsfc.nasa.gov/datasets](https://disc.gsfc.nasa.gov/datasets))). The OMI NO\({}_{2}\) SCD fitting adopts the method presented by [PERSON] et al. (2015), with a bias of \(1.2\times 10^{15}\) molec cm\({}^{-2}\) and noise of 0.9 \(\pm\) 0.3 \(\times\) 10\({}^{15}\) molec cm\({}^{-2}\). The a priori NO\({}_{2}\) profiles used in the AMF calculations are from the GMI, which is a three-dimensional CTM, with 72 atmospheric vertical levels from the surface to 0.01 hPa. The stratosphere-troposphere separation (STS) adopted in the current NASA algorithm is the modified reference sector method (MRSM) proposed by [PERSON] et al. (2013). This STS method determines the \"unpolluted\" observation pixels based on the monthly mean NO\({}_{2}\) profile rather than a fixed \"regional mask\", which has the advantage that it reserves more observation in the continental area in cases of cloud shielding, thus reducing the interpolation error. The TROPOMI NO\({}_{2}\) product (offline v.101) is available from [https://s5 pub.copernicus.eu/](https://s5 pub.copernicus.eu/).