Reduction in concentration of PM2.5 in India's top most polluted cities: with special reference to post-lockdown period.

Lockdown in India begins from 25 March and continues until 31 May 2020 due to the COVID-19 pandemic situation. Due to such an extended period of lockdown for about more than 2 months resulted in 1.38 billion populations restricted themselves from mass activities that contribute to air pollution. Thus, through our quantitative approach and trend analysis, the study aims to evaluate the changes in the level of PM2.5 as a major pollutant for the top ten polluted cities in India, with a special emphasis on finding what happened to its concentration after the lockdown ended. Thus, to better understand the nature of variation in PM2.5, we divide the entire 7 months into three periods for our analysis, i.e., before lockdown (1 January to 24 March), during lockdown (25 March to 31 May), and post-lockdown or unlock 1 and 2 (1 June to 31 July). Our investigation reveals that before lockdown, all the top polluted cities of India violating the national standard of PM2.5, as the lockdown begins interestingly, all cities show a momentous reduction in PM2.5 concentration. Further, surprisingly we found that after the post-lockdown period, the concentration of PM2.5 was reduced to minimal, as the average concentration of PM2.5 for all the cities is below the National Ambient Air Quality Standard (NAAQS). The study reveals that the lockdown has a consequence in improving overall air quality for the top polluted cities in India and further lockdown in the future with proper planning should be considered an alternative approach to restrain excessive emissions.