This study investigated the concentrations of air pollutants (NO, NO2, NOx, SO2, CO, O3, PM10, and PM2.5) at three sites with different traffic loads (work, residential, and traffic sites) before, during, and after the COVID-19 lockdown. The main objective of this study was to evaluate the effects and associated potential pollution control implications of the lockdown on the quality of ambient air at three selected sites in the urban area of Riyadh City. The average concentrations of NO, NO2, NOx, and CO decreased during the lockdown period by 73%, 44%, 53%, and 32% at the work site; 222%, 85%, 100%, and 60% at the residential site; and 133%, 60%, 101%, and 103% at the traffic site relative to the pre-lockdown period, respectively. The average concentration of O3 increased by 6% at the work site, whereas the concentration of SO2 increased by 27% at the residential site and decreased by 6.5% at the work site. The changes in PM10 and PM2.5 varied and did not exhibit a clear pattern. The air quality index (AQI) results indicated that the contribution to "undesired" air quality by O3 was 35.29% of the lockdown period at the work site while contributions to undesired air quality by PM10 and PM2.5 were 75.6% and 100% at the work site, 94.5% and 100% at the residential site, and 96.7% and 100% at the traffic site, respectively. The findings of this study are useful for devising effective urban pollution abatement policies. Applying control measures comparable to the lockdown measures over one week will result in a decrease of approximately 19% and 15% in CO mean concentration and 25% and 18% in NO2 mean concentration at residential and traffic sites, respectively.
Keywords: AQI; COVID-19; Riyadh; air pollutants; air quality; lockdown; nitrogen oxides; ozone.