**5. Conclusions**

The assessment of major atmospheric pollutants during lockdown due to the COVID-19 pandemic has markedly influenced the air quality in China. The cities such as Guangzhou, Wuhan, and Chengdu that have shown a very high concentration of NO2 tropospheric column (molecules/cm2) before the pandemic and have experienced a sudden decline after the lockdown. The air quality zonal model has also displayed about 390,150 km<sup>2</sup> areas that were under the high level of air pollutants have come under low concentration areas. In addition, the spatio-temporal evaluation of the NO2 and other main pollutants in the major 15 cities of China exhibited a remarkable reduction which in turn, facilitated to improve the overall air quality except in Beijing where the air quality has been degraded due to the overwhelming concentration of PM2.5 and PM10.

The overall air quality of eastern coastal and industrial cities such as Jinan, Zhengzhou, Wuhan, Guangzhou, Chengdu, and Xian have recorded low concentrations of air pollutants and such reduction in atmospheric pollutants has improved the air quality. The overall air quality change in 50 days reveals that Harbin has recorded the highest reduction (43%) in air pollutants followed by Guangzhou (42%), Wuhan (36%), Jinan (33%), Chengdu (32%), Nanjing (31%), Shanghai (30%), Zhengzhou (29%), Xian (25%), Xiamen (23%), Chongqing (23%), Lanzhou (17%), and Anshan (4%). Since most of the industrial productions, vehicle

movements, and other anthropogenic activities in the cities were restricted, the study revealed that the levels of NO2, CO, PM2.5, PM10, and SO2 have remarkably reduced. It is noteworthy that the reduction in atmospheric pollutants and consequently improved air quality would have a positive impact on the environment and human health. To address the emission of atmospheric pollutants, particularly NO2, CO, PM2.5, PM10, and SO2, the government needs to establish an environmental policy framework involving central, provincial, and local governments with stringent laws for environmental protection. The restriction and regulation of all anthropogenic sources of pollutants due to the COVID-19 outbreak was an example of such stringent enforcement of the law to protect human health. Similarly, to protect the environment from atmospheric pollutants strong political commitment, technological development, and policy enforcement are essential to make policies, such as 11th and 12th five-year plans of China, successful towards improving and protecting the environment and human health.

**Author Contributions:** Data collection, S.R. and R.C.; conceptualization, S.R., S.J., P.K. and R.C.; methodology, S.R.; software, S.R.; validation, S.R. and R.C.; formal analysis, S.J.; investigation, P.K. and R.C.; resources, S.R., S.J., P.K. and R.C.; data curation, S.R.; writing introduction, P.K. and S.J.; writing data source and methodology, S.R.; writing results discussion and conclusion, S.J.; referencing, S.R. and S.J.; review and editing, S.R.; supervision, P.K. and R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are openly available at Global Modeling and Assimilation Office, NASA. The hourly and daily average concentrations of CO, O3, NO2, PM10, PM2.5, SO2, and AQI data on China Ministry of Ecology and Environment (https://aqicn.org/map/china/cn/, accessed on 20 March 2020).

**Acknowledgments:** We would like to thank the Ministry of Environmental Protection of China (MEPC), Chinese Ministry of Ecology and Environment, 2020 and OMI datasets, Global Modeling and Assimilation Office, NASA for providing data.

**Conflicts of Interest:** The authors declare no conflict of interest.
