Emerging Investigator Series: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy.

The lockdown measures implemented to curb the COVID-19 epidemic in Italy reduced human mobility dramatically, which resulted in a marked decline in traffic intensity. In this study, we present the effect of lockdown measures on several air pollutants, particle number size distribution as well as on regional new particle formation (NPF) frequency in the Po Valley (northern Italy). The results show that during the lockdown period, concentrations of nitrogen dioxide (NO), nitric oxide (NO), benzene (CH), and toluene (CH) decreased, while ozone (O) concentrations mildly increased as compared to the corresponding period in 2016-2019.