## 4 Results  
### Total Vcd  
Figure 6 displays the global monthly averaged NO2 column density from OMI, EMI, and TROPOMI in August 2018 and January 2019, respectively. Overall, the spatial patterns are similar, particularly the low values in the tropics in January/August and high values in the northern latitudes in August, the enhanced zone in the mid-high latitudes of the northern hemisphere in August relative to January, and the huge pollution cluster in the North China Plain region in January. The spatial correlation between them is very good, with the correlation coefficient between EMI and OMI (TROPOMI) being 0.92 (0.93) in August and 0.89 (0.91) in January. However, the stripes are faintly visible in EMI, which could be due to imperfect de-striping correction. In addition, some noisy spots scattered in the ocean area are visible, which may have been caused by incomplete cloud screening. Figure 5: Cloud fractions and tropospheric AMF over the North China Plain region on 22 January, 2019 for OMI (**a**,**d**), EMI (**b**,**e**), and TROPOMI (**c**,**f**). Overall, like OMI and TROPOMI, EMI can detect the spatial distribution of NO\({}_{2}\) and is capable of NO\({}_{2}\) pollution monitoring. A detailed comparison of EMI with OMI and TROPOMI was made by selecting five regions, i.e., Pacific, North China, South Africa, Middle East, and India, as marked in Figure 6. Figure 7 shows the statistical values of the five regions. Figure 6: Monthly averaged NO\({}_{2}\) column density from OMI (**top** row), EMI (**middle** row), and TROPOMI (**bottom** row) in August 2018 (**left** panel) and January 2019 (**right** panel). Figure 7: Regional statistics of NO\({}_{2}\) column density from OMI, EMI, and TROPOMI for August 2018 (**top**) and January 2019 (**bottom**). The mean value of EMI in the Pacific region is consistent with TROPOMI in both August and January, but is lower than OMI by 10% in August and 7.5% in January, indicating that EMI can accurately reflect the background concentration in clean regions. In the other polluted regions, the mean value of EMI is, generally, higher than TROPOMI, but lower than OMI in August, and in January, EMI is higher than both OMI and TROPOMI. This is particularly evident in the North China region in January. On average, the discrepancies between EMI and OMI are within 10.5% (27.3%) in August (January), with the corresponding value between EMI and TROPOMI being 9.3% (17.2%). Figure 8 shows a map of the NO\({}_{2}\) total VCD over the North China Plain region from EMI, OMI, and TROPOMI, respectively, using data from 22 January, 2019. The selection of this date is due to the severe NO\({}_{2}\) pollution on this day, and it is clear, so there are few pixels contaminated by cloud and few OMI pixels affected by \"row anomaly\". The EMI results (Figure 8a) are consistent with OMI (Figure 8b) and TROPOMI (Figure 8c) in terms of the spatial distribution, particularly over regions where high values of NO\({}_{2}\) are clearly visible in the EMI results. EMI gives finer NO\({}_{2}\) column information over polluted regions than OMI due to its relatively higher spatial resolution, e.g., the hot-spots in northern areas of Shanxi province (113 degE, 37-40 degN). Generally, as a supplement of OMI who has passed its designed mission lifetime, EMI can provide better spatial coverage than OMI, and, as a new sensor, it plays an important role in NO\({}_{2}\) monitoring. In order to further examine the consistency in spatial distribution, this region was divided into 100 boxes with a 1 deg \(\times\) 1 deg size. The mean values in each box are compared in Figure 9. As expected, there is a good correlation between them, with a correlation coefficient of 0.93 (0.95) between EMI and OMI (TROPOMI). The regional averaged total VCD values of OMI and TROPOMI are \(6.34\times 10^{15}\) and \(6.59\times 10^{15}\) molec cm\({}^{-2}\), respectively.