*3.1. Spatio-Temporal Distribution of NO2 before and after the Pandemic*

The spatial distribution of 10-day time-average maps of NO2 tropospheric columns before and after the COVID-19 pandemic exhibited that the cities experienced a low concentration of NO2 and improved quality of air. Among these, the period 2 February to 11 February observed the lowest level of NO2 in all 15 major cities (Figure 1).

The post-lockdown maps portray that NO2 concentration has increased from 22 February to 12 March and then again started decreasing from 13 March to 22 March. The 10-day average NO2 tropospheric column (Figure 2) reveals that the level of NO2 has tremendously reduced in major cities such as Guangzhou, Beijing, Anshan, Zhengzhou, and Chengdu after the pandemic. The air quality has started improving from the 2nd week of January just after the lockdown throughout the country.

Similarly, the 60-day time-average map (Figure 3) of NO2 tropospheric column also displayed that the lowest range of NO2 concentration was 2e+14 and the maximum concentration was 3.5e+16 before imposition of lockdown whereas the lowest range of NO2 concentration came down to 1.4e+15 and the maximum to 1.8e+16 after the lockdown.

**Figure 1.** Spatial distribution of 10-day time-average map of NO2 tropospheric column of China before (24 November 2019 to 22 January 2020) and after the pandemic (23 January to 22 March 2020). Source: OMI, Global Modeling and Assimilation Office, NASA, 2020.

**Figure 2.** Temporal variation (10 days average) in NO2 tropospheric column of 15 major cities of China before and after the pandemic. Sources: Calculated by author, 2020.

The absolute concentration of NO2 tropospheric column (Figure 4) also revealed that all 15 major cities of China experienced lower emissions of NO2 just after lockdown. The maximum concentration of NO2 tropospheric column was 2.8e+16 before lockdown (24 November–22 January) and the minimum concentration was 1e+16 after lockdown (23 January–22 March) (Figure 4a). While we look at the proportion of NO2 concentration reduction, Guangzhou received only 37% of NO2 in the atmosphere after the pandemic. Similarly, the air quality of other cities also improved because of low emissions of NO2 after lockdown. The graph (Figure 4b) revealed that Wuhan received only 35% of NO2

concentration; whereas, Chengdu received 39%, Beijing 41%, Nanjing 48%, Xiamen 50%, Chongqing 53%, and Harbin 57% after the pandemic outbreak (23 January–22 March). On the other hand, the minimum reduction in NO2 concentration was found in cities of Anshan (67%), Lanzhou (67%), Jinan (66%), Xinjiang (65%) where NO2 concentration was comparatively low even before the pandemic in these same regions (Figure 4b).

**Figure 3.** Time-average map of NO2 tropospheric column (molecules/cm2) of 15 major cities of China before and after the pandemic. Source: OMI, Global Modeling and Assimilation Office, NASA, 2020.

**Figure 4.** NO2 tropospheric column of 15 major cities of China before and after the pandemic. (**a**) Absolute concentration and (**b**) decline in percentage. Source: Calculated by author, 2020.

The time-series of NO2 tropospheric column graph (30% cloud screened) displays that the total mass column of NO2 in China was highest during the first two weeks of December 2019 and remained as high as 4.3e+15 till the second week of January. After that, the NO2 mass column suddenly dropped till March 22 (Figure 5).
