Frequency distribution of pollutant concentrations over Indian megacities impacted by the COVID-19 lockdown

The megacities experience poor air quality frequently due to stronger anthropogenic emissions. India had one of the longest lockdowns in 2020 to curb the spread of COVID-19, leading to reductions in the emissions from anthropogenic activities. In this article, the frequency distributions of different pollutants have been analysed over two densely populated megacities: Delhi (28.70 degrees N; 77.10 degrees E) and Kolkata (22.57 degrees N; 88.36 degrees E). In Delhi, the percentage of days with PM2.5 levels exceeding the National Ambient Air Quality Standards (NAAQS) between 25 March and 17 June dropped from 98% in 2019 to 61% in 2020. The lockdown phase 1 brought down the PM10 (particulate matter having an aerodynamic diameter <= 10 mu m) levels below the daily NAAQS limit over Delhi and Kolkata. However, PM10 exceeded the limit of 100 mu gm(-3) during phases 2-5 of lockdown over Delhi due to lower temperature, weaker winds, increased relative humidity and commencement of limited traffic movement. The PM2.5 levels exhibit a regressive trend in the highest range from the year 2019 to 2020 in Delhi. The daily mean value for PM2.5 concentrations dropped from 85-90 mu gm(-3) to 40-45 mu gm(-3) bin, whereas the PM10 levels witnessed a reduction from 160-180 mu gm(-3) to 100-120 mu gm(-3) bin due to the lockdown. Kolkata also experienced a shift in the peak of PM10 distribution from 80-100 mu gm(-3) in 2019 to 20-40 mu gm(-3) during the lockdown. The PM2.5 levels in peak frequency distribution were recorded in the 35-40 mu gm(-3) bin in 2019 which dropped to 15-20 mu gm(-3) in 2020. In line with particulate matter, other primary gaseous pollutants (NOx, CO, SO2, NH3) also showed decline. However, changes in O-3 showed mixed trends with enhancements in some of the phases and reductions in other phases. In contrast to daily mean O-3, 8-h maximum O-3 showed a reduction over Delhi during lockdown phases except for phase 3. Interestingly, the time of daily maximum was observed to be delayed by similar to 2 h over Delhi (from 1300 to 1500 h) and similar to 1 h over Kolkata (from 1300 to 1400 h) almost coinciding with the time of maximum temperature, highlighting the role of meteorology versus precursors. Emission reductions weakened the chemical sink of O-3 leading to enhancement (120%; 11 ppbv) in night-time O-3 over Delhi during phases 1-3.