In the present study, concentrations of different pollutants i.e., PM2.5 (diameter < 2.5 μm), PM10 (diameter < 10 μm), NO2, NH3, SO2, CO, ozone, and air quality index (AQI) were acquired from open access internet sources provided by the Central Pollution Control Board (CPCB), Ministry of Environment, Forests, and Climate Change (https://app.cpcbccr.com/AQI_India/). The data were recorded daily from January 1, 2020 to May 31, 2020, which is subdivided into two groups: (a) pre-lockdown period—January 1, 2020 to March 23, 2020, and (b) lockdown period—March 24, 2020 to May 31, 2020 at 17:00 IST among four different air quality monitoring stations of the CPCB for four major metropolitan cities in India i.e., site 1—ITO, Delhi, site 2—Worli, Mumbai, site 3—Jadavpur, Kolkata, and site 4—Manali Village, Chennai as shown in Fig. 2. For air quality assessment, % variations of air pollutants during the confinement period were compared with pre-lockdown values.
Fig. 2 The geography of monitoring stations among the populous sites of four major metropolitan cities in India