Concentration and size distribution of atmospheric particles in southern Italy during COVID-19 lockdown period

Abstract

Many countries imposed lockdown (LD) to limit the spread of COVID-19, which led to a reduction in the emission of anthropogenic atmospheric pollutants. Several studies have investigated the effects of LD on air quality, mostly in urban settings and criteria pollutants. However, less information is available on background sites, and virtually no information is available on particle number size distribution (PNSD). This study investigated the effect of LD on air quality at an urban background site representing a near coast area in the central Mediterranean. The analysis focused on equivalent black carbon (eBC), particle mass concentrations in different size fractions: PM_2.5 (aerodynamic diameter D_a < 2.5 μm), PM₁₀ (D_a < 10 μm), PM_10-2.5 (2.5 < D_a < 10 μm); and PNSD in a wide range of diameters (0.01-10 μm). Measurements in 2020 during the national LD in Italy and period immediately after LD (POST-LD period) were compared with those in the corresponding periods from 2015 to 2019. The results showed that LD reduced the frequency and intensity of high-pollution events. Reductions were more relevant during POST-LD than during LD period for all variables, except quasi-ultrafine particles and PM_10-2.5. Two events of long-range transport of dust were observed, which need to be identified and removed to determine the effect of LD. The decreases in the quasi-ultrafine particles and eBC concentrations were 20%, and 15-22%, respectively. PM_2.5 concentration was reduced by 13-44% whereas PM_10-2.5 concentration was unaffected. The concentration of accumulation mode particles followed the behaviour of PM_2.5, with reductions of 19-57%. The results obtained could be relevant for future strategies aimed at improving air quality and understanding the processes that influence the number and mass particle size distributions.

Keywords: COVID-19; Coarse particles; Equivalent black carbon; Long-range transport; Ultrafine particles.

Graphical abstract

Fig. 1

Relative impact of COVID-19 on community public transport, working and residential places for the Puglia region (Italy). Data taken from Google COVID-19 Community Mobility Reports (https://www.google.com/covid19/mobility/). Stringency index SI was taken from https://ourworldindata.org/.

Fig. 2

Box plots of the (left column) eBC, PM_2.5, and PM_10-2.5 concentrations during 2015–2019 (blue boxes) and 2020 (orange boxes). Q-UFP (0.01 μm < D_p < 0.3 μm), accumulation (0.3 μm < D_p < 2.5 μm) and coarse (2.5 μm < D_p < 10 μm) particle concentrations (left column) for the same periods. Horizontal lines in each box are the median (yellow lines) and mean values (purple lines). The lower and upper boundaries of each box represent the 25th and the 75th percentiles, respectively. The lower and upper whiskers represent the 5th and the 95th percentiles, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Mean mass particle and eBC concentrations (left column) and number particle concentrations (right column) during the two dust events (red colour represents event data and the blue colour represents no event data). The lower and upper boundaries of the whiskers represent the maximum and the minimum values. Back trajectories (NOAA HYSPLIT Model, GDAS Meteorological Data) representing the first event and the second one, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

(top) Particle number size distributions of the combined MPSS (red) and OPC (blue) data during the event days and non-event days. (bottom) Ratio of concentrations during event and non-event days as function of particle diameter. D_p indicates mobility diameter for MPSS data and optical diameter of OPC data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Relative differences of mean concentrations measured in 2020 and those measured in the reference years (2015–2019) during LD and POST-LD periods. Error bars represent the standard errors. (*) Indicate differences statistically significant (p < 0.05).

Fig. 6

Comparison of PNSD, obtained combining MPSS (red) and OPC (blue) data, during the reference years (2015–2019) and the pandemic year (2020). PNSD were compared for the LD period (left column) and the POST-LD period (right column). Ratio of the PNSD during 2015–2019 and during 2020 for the two periods. D_p indicates mobility diameter for MPSS data and optical diameter of OPC data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7

Average daily patterns of eBC, Q-UFP, accumulation, and coarse fraction particles (from top to bottom) during LD (left column) and POST-LD (right column) periods. Error bars represent the standard errors.