Ten-year trends of size distributions and black carbon at the Environmental-Climate Observatory in Lecce (Italy)

This study investigates long-term (2015–2024) interannual and seasonal trends of PM_2.5, coarse fraction (PM_10–2.5), particle number size distribution (PNSD, extended range 10–10,000 nm), and equivalent black carbon (eBC) at the ECO Observatory in Lecce, Southern Italy, using online measurements. The dataset provides a decadal perspective for a Mediterranean background site influenced by anthropogenic emissions, boundary-layer dynamics, and natural aerosol inputs. Results reveal a decoupling between fine and coarse particulate mass fractions. PM_2.5 showed a significant decrease of −2.58% yr⁻¹, attributable to reduced local and regional combustion sources. Conversely, PM_10–2.5 displayed no significant long-term trend, reflecting the influence of natural coarse particles, especially Saharan dust. Dust events affected 13.2% of monitored days and were responsible for contrasting seasonal behaviours between fine and coarse fractions, with coarse particles peaking in summer and fine particles increasing during the cold season. eBC concentrations decreased by −3.17% yr⁻¹, in agreement with European background trends. The stable mass absorption cross-section (12.4 ± 2.4 m² g⁻¹) indicates that this decline represents a genuine reduction in primary combustion emissions, rather than changes in aerosol optical properties. Seasonally, the eBC/PM₁₀ ratio increased from 2.8% in summer to 4.2% in winter, following PM_2.5 variability, highlighting the effects of heating emissions, traffic contributions, and reduced boundary-layer mixing. Total particle number concentration (PNC) declined by −2.99% yr⁻¹, driven by reductions in the Aitken and accumulation modes, which correlated with eBC and were linked to improved vehicular emission control. Submicron particles exhibited winter maxima associated with combustion, whereas the coarse mode peaked in summer due to dust transport. The nucleation mode showed no clear seasonality because summertime photochemical new particle formation compensated for winter traffic inputs. Diurnal patterns were governed by boundary-layer dynamics, except for nucleation particles, which peaked midday, consistently with new particle formation events occurring on 25% of days.