*2.4. Data Analysis Method*

As mentioned before, OMI data have been used for detecting the total column of NO2 levels in the atmosphere [31,70]. Previous studies [31,70–72] have also used timeaverage OMI tropospheric total column maps and area-average OMI tropospheric total column datasets for assessing the air quality, especially for macro scale. We selected China's mainland for this study and assessed the total column of NO2 before and after the outbreak of COVID-19. This study divided China's mainland into 145 grids (dimension 255 × 255 km2) for the micro-level study. While in the process, the work extracted the exact value of NO2 tropospheric total column of each grid for 15 cities using spatial attribute tools in ArcMap and then we divided 145 grids into 5 equal categories based on the maximum and minimum range. In the last stage, we calculated the total area of each category and compared the extracted datasets with before and after 60- and 10-day area-average columns of NO2 in total and in percentage.

We calculated the daily mean concentration of CO, O3, NO2, PM10, PM2.5, SO2, and AQI from ground monitoring datasets provided by MEPC. We also calculated the 50 days before and after average concentration of each pollutant in 14 major cities of China obtained from real-time datasets. This study conducted a compound bar graph and trend line analysis for monitoring the change in air quality before and after the lockdown of 14 cities of China, in terms of total and percentage.
