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° N; 77.10° E) and Kolkata (22.57° N; 88.36° E). In Delhi, the percentage of days with PM 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 PM (particulate matter having an aerodynamic diameter ≤ 10 μm) levels below the daily NAAQS limit over Delhi and Kolkata. However, PM exceeded the limit of 100 μgm during phases 2-5 of lockdown over Delhi due to lower temperature, weaker winds, increased relative humidity and commencement of limited traffic movement. The PM levels exhibit a regressive trend in the highest range from the year 2019 to 2020 in Delhi. The daily mean value for PM concentrations dropped from 85-90 μgm to 40-45 μgm bin, whereas the PM levels witnessed a reduction from 160-180 μgm to 100-120 μgm bin due to the lockdown. Kolkata also experienced a shift in the peak of PM distribution from 80-100 μgm in 2019 to 20-40 μgm during the lockdown. The PM levels in peak frequency distribution were recorded in the 35-40 μgm bin in 2019 which dropped to 15-20 μgm in 2020. In line with particulate matter, other primary gaseous pollutants (NO, CO, SO, NH) also showed decline. However, changes in O showed mixed trends with enhancements in some of the phases and reductions in other phases. In contrast to daily mean O, 8-h maximum O showed a reduction over Delhi during lockdown phases except for phase 3. Interestingly, the time of daily maximum was observed to be delayed by ~ 2 h over Delhi (from 1300 to 1500 h) and ~ 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 leading to enhancement (120%; 11 ppbv) in night-time O over Delhi during phases 1-3.