Changes in the concentration and composition of urban aerosols during the COVID-19 lockdown.

This work investigates the impact of COVID-19 restrictive measures on the mass concentrations of PM1 and PM10, and their chemical components (water-soluble ions, organic and elemental carbon, and major and trace metals) at an urban site in the western Mediterranean. The evolution of gaseous pollutants (NOx, O3 and some volatile organic compounds) was also analyzed. The concentrations measured during the lockdown in 2020 were compared to those obtained during the same period over the preceding five years. The average decrease in the levels of NOx and traffic-related volatile organic compounds was higher than 50 %, while O3 concentrations did not exhibit significant variations during the study period. Our results show that temporal variations in PM1 and PM10 concentrations were strongly affected by the frequency of Saharan dust events. When these episodes were excluded from the analysis period, a 35 % decrease in PM1 and PM10 levels was observed. Traffic restrictions during the lockdown led to important reductions in the concentrations of elemental carbon and metals derived from road dust (e.g. Ca and Fe) and break wear (e.g. Cu). Regarding secondary inorganic aerosols, nitrate showed the largest reductions as a consequence of the drop in local emissions of NOx.