Abstract
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, SO2, NO2, and PM2.5 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 SO2, NO2, PM2.5 and anthropogenic elements concentrations follow the pattern pre-COVID > post-COVID > COVID partial lockdown. SO2 and NO2 reductions were high compared with their secondary products during the COVID partial lockdown compared with pre-COVID. Although, PM2.5, anthropogenic elements, NO2, SO2, SO42−, NO3−, and NH4+ 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 (SO42−, NO3−, and NH4+) being the dominant components in PM2.5 during the COVID partial lockdown. Moreover, the enhancement of NO3− and SO42− formation during the COVID partial lockdown was high compared with pre-COVID. SO42− and NO3− formation enhancements were significantly positive correlated with PM2.5 concentration. Chemical forms of SO42− and NO3− were identified in PM2.5 based on their NH4+/SO42− molar ratio and correlation between NH4+ and both NO3− and SO42−. The particles during the COVID partial lockdown were more acidic than those in pre-COVID.
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•PM2.5, elements, NO2, and SO2 decreased by 32.9–55% during COVID partial lockdown.•Emission reduction enhance oxidizing capacity and secondary PM formation.•SO42−, NO3−, and NH4+ were the dominant in PM2.5 during COVID partial lockdown.•SO42− and NO3− formation were enhanced via droplet phase and gas-phase reactions.•Particle was more acidic during COVID partial lockdown than during pre-COVID period.