*3.2. Total NO2 Load over the Area Using air Quality Zonal Modeling*

A total 60-day average NO2 tropospheric column was demonstrated through the air quality zonal modeling (Figure 6). Here, the whole country was divided into 145 grids (255 × 255 km2) where the spatial distribution of NO2 (Figure 6a) was displayed for the micro-level understanding of the air quality changes in different regions of the country. The area was also calculated under each grid category. The NO2 concentration ranges between (1.7e+15–3.2e+15) had 131 grids and the area was 5.5 million sq.km before the lockdown but the grids have increased to 191 and the area has also increased to 80 million sq.km. Conversely, the areas under the high NO2 concentration category (5.5e+15–9.8e+15) have

significantly decreased from 1.1 to 0.2 million sq.km. Moreover, there was a significant decrease in NO2 concentration in the areas that come under a very high concentration category where the NO2 tropospheric column ranges between 1.6e+16 and 2.6e+16; 13 grids were accounting for 0.55 million km<sup>2</sup> before the lockdown but none of the areas were under this very high category after lockdown (Figure 6a).

**Figure 5.** Time series, area-average of NO2 tropospheric column (30% cloud screened) daily 0.25 deg. [OMI OMNO2d v003] 1/cm2 over 24 November 2019–22 March 2020, shape China. Source: OMI, Global Modeling and Assimilation Office, NASA, 2020.

**Figure 6.** (**a**) Time average map and zonal distribution of the NO2 tropospheric column, (**b**) declining rate (in total and percentage) of NO2, (**c**) trend analysis of the total NO2 tropospheric column of 145 random sites of China's mainland before and after pandemic. Source: OMI, Global Modeling and Assimilation Office, NASA.

The total column of NO2 decline rate in percentage was grouped into three categories, i.e., less than 50%, 51–70%, and more than 70%. The maximum decline rate (more than 70%) of the NO2 total column was measured in Guangzhou, Wuhan, and Chengdu city of China before and after the pandemic COVID-19 event (Figure 6b). Figure 6c reveals the trend of the total NO2 tropospheric column of 145 random sites before and after the pandemic where it is clearly visible that almost all the sample sites have experienced a low concentration of NO2 in the atmosphere.

In the graphs (Figure 7), steepness of the histogram revealed the degree of reduction in the NO2 concentration. Guangzhou city showed high steepness of NO2 reduction after the pandemic. The maximum (almost 60%) declining rate of NO2 was observed in Guangzhou, Wuhan, Chengdu, and Beijing; whereas the minimum (almost 40%) declining rate of NO2 total column in the atmosphere was found in Jinan, Shanghai, Anshan, Zhengzhou, Xinjiang, and Lanzhou.

**Figure 7.** Absolute trend line indicates 60-day average of NO2 tropospheric column (molecules/cm2), whereas the bar graph represents 10-day average concentration (X-axis) of NO2 in 15 major cities of China before (yellow) and after (purple) the COVID-19 lockdown period. Source: OMI, Global Modeling and Assimilation Office, NASA, 2020.
