Impact of COVID-19 lockdown on air quality of Sri Lankan cities

The COVID-19 pandemic has led to the complete lockdown of many cities worldwide. Lockdowns have significantly changed human behaviour, such as fewer or no industrial activities and limited road and air transport, affecting the environment. In this study, we aimed to assess the variability and trends of PM2.5 (mass concentration of airborne particulate matter < 2.5µm) and carbon monoxide (CO) before and during the COVID-19 lockdown period in Sri Lanka. Data were collected in “Urban Background”, “Public & Mixed Residential”, and “Primary Residential” areas using small sensor technology, "KOALA" air quality sensor units, from five locations, three in Kandy, and two in Colombo city. Daily averages (24h) and daytime averages (08:00 AM to 8:00 PM) were calculated for the period before (before March 20th) and during (March 20th to May 10th) the lockdown. Air pollutions level before & during COVID-19 lockdown were compared, then Mann-Kendall and Sen's slope tests were performed to determine trends of PM2.5 and CO, and the magnitude of the trend. Meteorological parameters were fairly similar before and during both cities' lockdown periods, while both PM2.5 and CO concentrations declined. The highest average reductions of PM2.5 and CO were observed in Colombo's “Urban Background” area (52.4% and 46.7%, respectively). In Kandy, “Urban Background” site had a higher reduction of PM2.5 and CO (30.2% and 41.2%, respectively), compared to “Primary Residential” (10% and 9%, respectively). The daily averages of the pollutants' concentrations were higher before the lockdown period compared to during. Overall, a significant downward trend was observed of air pollutants over the entire study period. In Sri Lanka, the COVID-19 lockdown improved air quality significantly in urban areas.