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<p>To control the spread of COVID-19, exceptional restrictive measures were taken in March 2020 that imposed a radical change on the lifestyle of millions of citizens around the world, albeit for a short period. The national lockdown, which lasted from 10 March to 18 May 2020 in Italy, was a unique opportunity to observe the variation in air quality in urban environments under conditions comprising almost total traffic restriction and a strong reduction in work activities. In this paper, the data from 17 urban monitoring sites in Tuscany are presented, and the PM and <span class="inline-formula">NO₂</span> concentrations in the 2 months before the start of the lockdown and the 2 months after lockdown are compared with the corresponding months of the previous 3 years. The results show that the total loads of PM<span class="inline-formula">_2.5</span> and PM<span class="inline-formula">₁₀</span> (particulate matter with an aerodynamic diameter smaller than 2.5 and 10 <span class="inline-formula">µm</span>, respectively) decreased, but they did not exhibit significant changes compared to previous years, whereas <span class="inline-formula">NO₂</span> underwent a drastic reduction. For three of these sites, the chemical composition of the collected samples was measured using thermal–optical techniques, ion chromatography, and particle-induced X-ray emission analysis, and the application of multivariate positive matrix factorization analysis also allowed for PM<span class="inline-formula">₁₀</span> source identification and apportionment. Using these analyses, it was possible to explain the low sensitivity of PM<span class="inline-formula">₁₀</span> to the lockdown effects as being due to different, sometimes inverse, behaviors of the different sources that contribute to PM. The results clearly indicate a decline in pollution levels related to urban traffic and an increase in the concentration of sulfate for all sites during the lockdown period.</p>