Long-term (10-year) trends in ultrafine particle number concentrations and size distributions in Athens, Greece

This study presents a decade-long record of particle number size distribution measurements (14‒487 nm) collected at the Thissio urban background station in the Greater Athens Area, Greece, between June 2015 and October 2024. This dataset represents one of the longest continuous records of ultrafine (≤ 100 nm) and submicron (< 500 nm) particle number concentrations (PNCs) in the Eastern Mediterranean and enables modal analysis of particles across the nucleation, Aitken, and accumulation modes, as well as an assessment of the factors controlling their variability. Ultrafine particles (UFPs) accounted for 79±3% of the total PNCs, with an average (median) of 6,224 ± 4,901 cm -3 (4,885 cm -3 ). Pronounced seasonal variability was observed, with winter maxima driven by traffic emissions and residential wood burning under shallow boundary-layer conditions, and summer minima associated with reduced local emissions and enhanced atmospheric mixing. Diurnal patterns exhibited clear bimodal peaks linked to traffic activity, along with a persistent midday enhancement attributed to new particle formation, particularly in spring. The lockdown provided a unique opportunity to assess the sensitivity of PNCs to abrupt emission reductions, resulting in a 40–45% decrease across all size modes while preserving their characteristic diurnal and modal structures. Long-term trend analysis revealed statistically significant ( p 0.05). Overall, the decrease in UFP levels coincided with reductions in traffic-related combustion tracers, while additional analyses suggested an enhanced influence of regional transport on accumulation-mode particle concentrations, which showed an increment during the 2022-2024 period. These findings highlight the importance of sustained long-term measurements for robustly assessing inter-annual trends, supporting urban air quality management policies and epidemiological research.