# Preliminary Global NO2 Retrieval from EMI-II Onboard GF5B/DQ1 and Comparison to Tropopmi
## 4 Discussion  
The use of the GF5B and DQ1 dual-satellite constellations for atmospheric composition observations can significantly increase the frequency of measurements and partially compensate for the impact of clouds on the effective observation coverage of single satellites. To evaluate the ability of the dual-satellite network to improve the observation frequency, we applied a grid to the EMI-II NO\({}_{2}\) data with a resolution of 0.25 degrees for the statistical analysis. Because there are currently no officially released cloud products for EMI-II, the criteria for cloud identification in EMI-II were set here with radiance at a wavelength of 480 nm exceeding 20 \(\upmu\)W\(\cdot\)cm\({}^{-2}\cdot\)nm\({}^{-1}\).  
As depicted in Figure 8a,b show the cloud coverage distributions from single-satellite observations of the GF5B and DQ1 on July 5, 2022. The grid percentages with cloud coverage for the GF5B and DQ1 were 17.08% and 17.68%, respectively. The grid percentages with clear-sky coverage accounted for 60.45% and 60.83% for the GF5B and DQ1, respectively, whereas the grid percentages with no observations accounted for 22.47% and 21.49%. Figure 8c displays the coverage types from the dual-satellite observations of the GF5B and DQ1 on the same day. Compared with single satellites, the grid percentage with cloud coverage from both satellites was 10.22%, i.e., decreases of 6.86% for the GF5B and 7.46% for the DQ1. Additionally, 49.95% of the global regions was observed twice by the dual satellites, which effectively increased the number of observations. Figure 8d shows the distribution of effective observation counts for the GF5B and DQ1 for the entire month of July in 2022. The dual-satellite network significantly enhances the number of observations in regions including northern and southern Africa, the Middle East, western Australia, and western North America during this month.  
Figure 8: Statistical analysis of the cloud coverage. Cloud coverage distributions from the single-satellite observations of GF5B (**a**) and DQ1 (**b**) on July 5, 2022. (**c**) Coverage types from the dual-satellite observations of GF5B and DQ1 on the same day. (**d**) Distribution of effective observation counts for GF5B and DQ1 for the entire month of July in 2022.  
Because the GF5B and DQ1 uses identical retrieval algorithms, input parameters, and auxiliary data in the EMI-II retrievals, we assumed little variation in the stratospheric NO\({}_{2}\) levels. Then, the differences in the retrieved VCDs between the two instruments can be attributed to NO\({}_{2}\) signal discrepancies. In other words, the disparity between the DQ1 and GF5B can represent the NO\({}_{2}\) diurnal variation information. Figure 9a,b illustrate the spatial distribution of the daily (5 July 2022) and monthly (July 2022) average differences between the DQ1 and GF5B. At the daily scale, significant instances of the DQ1 measurements that are higher than those of the GF5B are at the border between Congo and Angola, which may indicate that NO\({}_{2}\) emissions from this source predominantly occur in the morning. On the monthly scale, many emission sources in the Middle East, Iran, and China present higher values for the GF5B than the DQ1. This result may shed some light on the timing of NO\({}_{2}\) emissions from these sources.