3.1 Air pollution
In Figure 1 and 2, the timeseries observed in 2020 is compared to the multiannual timeseries of daily mass loading for PM2.5and PM10 and for the gaseous pollutants NO2 and C6H6, measured at the urban background site and the urban traffic site identified above.
Air pollutants behavior reflects seasonal and interannual fluctuation in emission rates and climatological drivers. A detailed analysis of all these factors is beyond the scope of this work. For the purpose of this paper, we limited to exemplify changes in specific air pollution patterns that might have affected population exposure at the urban scale of Bologna.
Focusing on the first period of lockdown, March-May 2020 (lockdown-1), gaseous pollutants were strikingly reduced: NO2concentrations in 2020 decreased on average by 43% and 46% (P <0.01) at the urban background and urban traffic sites, respectively; and C6H6, only measured at the urban traffic site, decreased by 53% (P <0.01). When compared to historical levels, the differences of both gaseous pollutants gradually attenuated in late spring and summer, partially reflecting the slow, although not complete, return to regular activities that occurred in late-spring and summer. With the second outbreak, these divergences reappeared as a consequence of the new measures during the second (moderate) lockdown period from mid-October to December 2020 (lockdown-2), although those measures were less restricting than during lockdown-1.
In contrast, no significant impact of the lockdown measures is evidenced on the mass loading of PM10 and PM2.5, both at the urban background and traffic sites, consistently with other European cities21.
Despite the lack of evidence of reduction of the mass loading of PM10 and PM2.5, considering PM chemical composition, clear signals of reduced TRAP are observed. Figure 3A shows the contribution of BC mass to PM2.5 mass in 2019 vs 2020 (March to May). The average relative contribution of BC to PM2.5 significantly decreased from 11% to 6% (P <0.01). This is in line with a recent study reporting, for Europe, a more pronounced reduction of BC concentrations in the range of 20-40% during lockdown events in those countries that suffered more dramatically from the pandemic (e.g. Italy)22.
Accordingly, Figure 3B shows the TRAP contribution tracer, by comparing the measurements carried out during lockdown-1 with those one corresponding to intensive observation periods (IOPs) during previous years, covering a wide spectrum of seasonal conditions. The TRAP contribution tracer evidenced a marked decline up to 50% in 2020 with respect to previous IOPs (P <0.01).