Characterization of urban aerosol pollution before and during the COVID-19 crisis in a central-eastern European urban environment

The worldwide restrictions due to the COVID-19 pandemic induced a radical change in urban air quality. The objective of this work was to study and understand the variation of PMcoarse and PM2.5 pollution besides other air quality (AQ) parameters in the city of Debrecen, Hungary from 2018 to 2022. To achieve this goal, we introduced an integrated approach that combines source apportionment by receptor modelling, trajectory-based statistical methods and takes into account local and regional meteorology. Concentration, elemental composition, and sources of atmospheric particulate matter (APM) were determined for four lockdown periods with varying levels of restrictions, two transition intervals and two relaxation periods in 2020-22, and they were compared to corresponding baseline values from 2018-19. The concentration, composition and sources of PM2.5, PMcoarse showed strong seasonality. The main component of PMcoarse was mineral dust (32%), while BC (15%), sulphate aerosols (19%) and mineral dust (13%) were the major constituents of PM2.5. Source apportionment by Positive Matrix Factorization identified 8 sources on both size fractions: two types of soil, traffic, combustions, biomass burning, biogenic emission, sea salt, construction, roadworks, secondary sulphate. The relative contribution of sources did not change during the pandemic compared to the previous two years. However, considering the whole 2-year-long period affected by restrictions, a 20-25% reduction was detected in the concentrations and source contributions, apart from temporary sources like roadworks or sea salt episodes. The changed habits of the population could be traced in the evolution of APM. Before the pandemic, increased concentration of traffic related pollutants was characteristic of the school year, while in 2020-21 it was more typical in the summers, the relaxation periods. On the other hand, staying at home during lockdowns led to a significant increase in biomass burning from domestic heating in spring 2020 and 2021. By studying the individual lockdown and relaxation periods, we have shown that the evolution of APM pollution in relatively short, 2-3-month intervals was significantly influenced by local and regional meteorological parameters, the origin of air masses, desert dust episodes and periodic construction works.