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).