Mumbai, the sixth most populous city in the world, is located on India’s west coast and is the capital of Maharashtra. It is the financial, entertainment, and commercial center of India. During COVID-19 pandemic confinement, the second most populated city of India i.e., Mumbai has moved from poor to a satisfactory level of air quality. As initially at site 2, the values of the pollutants which were scattered around 200–300 μg/m3 before confinement fallen to less than 60 μg/m3 during the confinement period (Fig. 4). The mean concentration of PM2.5, PM10, NO2, NH3, SO2, and CO, significantly reduced with a percentage of 73, 47, 86, 58, 58, 55, and 60 respectively due to shutdown of navigation activities and other industrial sectors with automobile transportation (Table 1). The drastic decline in nitrogen oxide levels over Mumbai is the result of reduced carbon-emission hotspots, industrial and coal combustion-dominated areas. A decrease in the concentration of urban ground-level ozone was recorded by 60% due to high reduction in nitrogen oxide concentration in the atmosphere.
Fig. 4 The concentration of air pollutants (PM2.5 in μg/m3, PM10 in μg/m3, CO in μg/m3, NH3 in μg/m3, NO2 in μg/m3, SO2 in μg/m3, and O3 in μg/m3) during pre-lockdown and lockdown period at 17:00 IST among four different air quality monitoring stations of the CPCB for four major metropolitan cities in India (site 1—ITO, Delhi, site 2—Worli, Mumbai, site 3—Jadavpur, Kolkata, and site 4—Manali Village, Chennai)