Source apportionment of PM before and after COVID-19 lockdown in an urban-industrial area of the Lisbon metropolitan area, Portugal.

The lockdowns held due to the COVID-19 pandemic conducted to changes in air quality. This study aimed to understand the variability of PM levels and composition in an urban-industrial area of the Lisbon Metropolitan Area and to identify the contribution of the different sources. The composition of PM was assessed for 24 elements (by PIXE), secondary inorganic ions and black carbon. The PM mean concentration for the period (December 2019 to November 2020) was 13 ± 11 μg.m. The most abundant species in PM were BC (19.9%), SO (15.4%), NO (11.6%) and NH (5.3%). The impact of the restrictions imposed by the COVID-19 pandemic on the PM levels was found by comparison with the previous six years. The concentrations of all the PM components, except Al, Ba, Ca, Si and SO , were significantly higher in the winter/pre-confinement than in post-confinement period. A total of seven sources were identified by Positive Matrix Factorisation (PMF): soil, secondary sulphate, fuel-oil combustion, sea, vehicle non-exhaust, vehicle exhaust, and industry. Sources were greatly influenced by the restrictions imposed by the COVID-19 pandemic, with vehicle exhaust showing the sharpest decrease. Secondary sulphate predominated in summer/post-confinement. PM levels and composition also varied with the types of air mass trajectories.