Effect of restricted emissions during COVID-19 on atmospheric aerosol chemistry in a Greater Cairo suburb: Characterization and enhancement of secondary inorganic aerosol production.

To prevent the rapid spreading of the COVID-19 pandemic, the Egyptian government had imposed partial lockdown restrictions which led emissions reduction. This served as ideal conditions for a natural experiment, for study the effect of partial lockdown on the atmospheric aerosol chemistry and the enhanced secondary inorganic aerosol production in a semi-desert climate area like Egypt. To achieve this objective, SO, NO, and PM and their chemical compositions were measured during the pre-COVID, COVID partial lockdown, and post-COVID periods in 2020 in a suburb of Greater Cairo, Egypt. Our results show that the SO, NO, PM and anthropogenic elements concentrations follow the pattern pre-COVID > post-COVID > COVID partial lockdown. SO and NO reductions were high compared with their secondary products during the COVID partial lockdown compared with pre-COVID. Although, PM, anthropogenic elements, NO, SO, SO , NO , and NH decreased by 39%, 38-55%, 38%, 32.9%. 9%, 14%, and 4.3%, respectively, during the COVID partial lockdown compared with pre-COVID, with the secondary inorganic ions (SO , NO , and NH ) being the dominant components in PM during the COVID partial lockdown. Moreover, the enhancement of NO and SO formation during the COVID partial lockdown was high compared with pre-COVID. SO and NO formation enhancements were significantly positive correlated with PM concentration. Chemical forms of SO and NO were identified in PM based on their NH /SO molar ratio and correlation between NH and both NO and SO . The particles during the COVID partial lockdown were more acidic than those in pre-COVID.