Space-based monitoring of NO(2) levels during COVID-19 lockdown in Cairo, Egypt and Riyadh, Saudi Arabia

The lockdown of COVID-19 pandemic has affected air quality due to the changes in human activities. Researchers worldwide observed reductions in NO(2) concentrations due to lockdown and related diminished human activities, notably the reduced industrial and vehicular use. However, limited information was available for the MENA Region. In this study, the impact of lockdown due to COVID-19 on NO(2) in two MENA major cities: Cairo, Egypt and Riyadh, Saudi Arabia was assessed. NO(2) column was retrieved from the Ozone Monitoring Instrument (OMI) on Aura for April 2017 to 2020. The monthly mean value of NO(2) concentrations of April 2017–2019 was used as a baseline. NO(2) concentration in April 2020 was compared to the baseline to assess the impact of lockdown on NO(2). The results demonstrated that the lockdown was associated with a reduction in NO(2) in both cities. NO(2) decreases by 40.3% and 23% in Riyadh and Cairo. By comparing the decrease of NO(2) at weekends and weekdays, it was found that in Cairo, the decrease in weekdays (16.3%) was lower than weekends (31.9%). While in Riyadh, the decrease in weekdays (43.9%) was higher than weekends (29.3%). Variation in the reduction rates appears to be related to the different lockdown regimens taken by the two countries, among other factors. The findings of the present investigation alert countries in the region about the impact of human activities on urban air population and urge them to take appropriate mitigation measures to maintain good ambient air quality to protect human health and the environment.