Variations and Source Apportionment of PM(2.5) and PM(10) Before and During COVID-19 Lockdown Phases in Delhi, India

Major cities across the globe including megacity Delhi have experienced considerable lower levels of air pollutants including particulate matter (PM) during COVID-19 lockdown. This study explores pre-lockdown and during lockdown air quality changes in PM(2.5), PM(10), PM(2.5)/PM(10) ratio along with meteorological effects. Selected sites with different pollution signatures in Delhi including Alipur (residential), Okhla (industrial) and Pusa Road (traffic) have experienced mean (S.D.) PM(2.5) as 87.56(± 54.06), 124.45(± 73.49) and 62.14(± 58.64) µg/m(3) and PM(10) as 163.01(± 77.37), 217.71(± 93.94) and 135.15(± 77.90) µg/m(3) before lockdown (BL), while for Lockdown 1 (L1), PM(2.5) concentrations decreased drastically as 39.26(± 16.31), 38.01(± 15.16) and 31.03(± 12.79) µg/m(3) and for PM(10) as 100.76(± 43.71), 79.47(± 30.97) and 66.53(± 22.78) µg/m(3), respectively, with gradual increase in both pollutants during successive lockdown phase—Lockdown 2, Lockdown 3, Lockdown 4 and Unlock phase 1. The percentage (%) decrease in PM(2.5) (69.46%) and PM(10) (63.49%) during lockdown was found well correlated with people mobility (Google and Apple mobility reports), as outdoor activities showed 70–80% decrease in L1 from BL phase. Source apportionment studies suggested both local and regional pollution contribution in Delhi. Comparison of PM(2.5) and PM(10) concentrations for the year 2020 with that of 2018 and 2019 and study on diurnal variations of PM(2.5) and PM(10) have been discussed.