Daniele Contini, Daniela Cesari, Marianna Conte, Adelaide Dinoi
This study investigates the long-term (2015-2024) inter-annual and seasonal trends of PM2.5, coarse particles (PM10-2.5), particle number size distribution (PNSD, 10-10,000 nm), and equivalent black carbon (eBC) at the ECO Observatory in Lecce (Southern Italy) measured with online instruments. Organic (OC) and elemental (EC) carbon were analysed offline to evaluate the Mass Absorption Cross-section (MAC). Results showed a clear decoupling between fine and coarse mass fractions. PM2.5 decreased significantly by -2.58% yr-1, consistent with a reduction of local anthropogenic combustion sources. Conversely, PM10-2.5 remained stable due to the strong influence of natural Saharan dust events, which impacted 13.2% of the monitored days and induced opposite seasonal patterns between the two size fractions. eBC concentrations exhibited a statistically significant decline of -3.17% yr-1, consistent with European urban background trends. The mass absorption cross-section remained stable (12.4 ± 2.4 m2g-1). The reduction of absorption and a stable MAC suggest that the decrease of absorption is due to a decrease of eBC consistent with a decrease of combustion emissions. Seasonally, the eBC/PM10 ratio increases from 2.8% in summer to 4.2% in winter, tracking PM2.5 variations and highlighting the impact of wintertime emissions and boundary layer dynamics. Total particle number concentration (PNC) decreased by -2.99% yr-1, primarily driven by reductions in the Aitken and accumulation modes. While submicron particles peaked in winter due to combustion, the coarse mode peaked in summer due to dust. The nucleation mode lacked inter-annual seasonality because summer nucleation countered winter traffic inputs. Diurnal variations were governed by boundary layer dynamics, except for the nucleation mode, which peaked midday due to photochemical new particle formation events happening, on average, in 25% of the days.