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(2), NO(2), and PM(2.5) 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(2), NO(2), PM(2.5) and anthropogenic elements concentrations follow the pattern pre-COVID > post-COVID > COVID partial lockdown. SO(2) and NO(2) reductions were high compared with their secondary products during the COVID partial lockdown compared with pre-COVID. Although, PM(2.5), anthropogenic elements, NO(2), SO(2), SO(4)(2−), NO(3)(−), and NH(4)(+) 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(4)(2−), NO(3)(−), and NH(4)(+)) being the dominant components in PM(2.5) during the COVID partial lockdown. Moreover, the enhancement of NO(3)(−) and SO(4)(2−) formation during the COVID partial lockdown was high compared with pre-COVID. SO(4)(2−) and NO(3)(−) formation enhancements were significantly positive correlated with PM(2.5) concentration. Chemical forms of SO(4)(2−) and NO(3)(−) were identified in PM(2.5) based on their NH(4)(+)/SO(4)(2−) molar ratio and correlation between NH(4)(+) and both NO(3)(−) and SO(4)(2−). The particles during the COVID partial lockdown were more acidic than those in pre-COVID.