The COVID-19 lockdown measures gradually implemented in Lombardy (northern Italy) from 23 February 2020 led to a downturn in several economic sectors with possible impacts on air quality. Several communications claimed in the first weeks of March 2020 that the mitigation in air pollution observed at that time was actually related to these lockdown measures without considering that seasonal variations in emissions and meteorology also influence air quality. To determine the specific impact of lockdown measures on air quality in northern Italy, we compared observations from the European Commission Atmospheric Observatory of Ispra (regional background) and from the regional environmental protection agency (ARPA) air monitoring stations in the Milan conurbation (urban background) with expected values for these observations using two different approaches. On the one hand, intensive aerosol variables determined from specific aerosol characterisation observations performed in Ispra were compared to their 3-year averages. On the other hand, ground-level measured concentrations of atmospheric pollutants (NO.sub.2, PM.sub.10, O.sub.3, NO, SO.sub.2) were compared to expected concentrations derived from the Copernicus Atmosphere Monitoring Service Regional (CAMS) ensemble model forecasts, which did not account for lockdown measures. From these comparisons, we show that NO.sub.2 concentrations decreased as a consequence of the lockdown by -30 % and -40 % on average at the urban and regional background sites, respectively. Unlike NO.sub.2, PM.sub.10 concentrations were not significantly affected by lockdown measures. This could be due to any decreases in PM.sub.10 (and PM.sub.10 precursors) emissions from traffic being compensated for by increases in emissions from domestic heating and/or from changes in the secondary aerosol formation regime resulting from the lockdown measures. The implementation of the lockdown measures also led to an increase in the highest O.sub.3 concentrations at both the urban and regional background sites resulting from reduced titration of O.sub.3 by NO. The relaxation of the lockdown measures beginning in May resulted in close-to-expected NO.sub.2 concentrations in the urban background and to significant increases in PM.sub.10 in comparison to expected concentrations at both regional and urban background sites.