Source apportionment of fine and ultrafine particle number concentrations in a major city of the Eastern Mediterranean

Ultrafine particles (UFP) are recognized as an emerging pollutant able to induce serious health effects. However, quantitative information regarding the contributions of UFP sources is generally limited. This study evaluates statistical (k-means clustering) and receptor models (Positive Matrix Factorization - PMF) using particle number size distributions (PNSD), along with chemical speciation data, measured at an urban background supersite in Athens, Greece, aiming to characterize their sources. PNSD measurements (10-487 nm) were performed during three distinct periods (warm, cold, and lockdown cold). Traffic and residential biomass burning (BB) produced high UFP number concentrations (N-UFP) in the cold period (+107 % compared to summer), while the lockdown restrictions reduced N-UFP (-42 %). The five groups produced by cluster analysis that were common among periods were linked to high- and low-traffic, new particle formation (NPF), urban background and regional aerosols. PMF source apportionment identified 5 and 6 factors during warm and cold periods, respectively, indicating that traffic particles dominated N-UFP (64-78 % in all periods), while accumulation-mode particles and volume concentrations were controlled by processed aerosol, and especially in the cold periods by BB emissions. A nucleation factor linked to NPF contributed 7-11 % to N-UFP. Comparing the two cold periods (business-as-usual, lockdown), important lockdown reductions (-46 %) were seen for fresh traffic contributions to total number concentration (N-total). The impact of the source attributed to NPF also eroded (-41 % for N-total). Due to the large reduction (-47 % for N-total) observed also for the BB source during the lockdown (reduced wood usage due to a milder winter), the relative contributions of all sources did not change considerably (fractional reductions <7 % for N-total). The quantitative results, bolstered by source apportionment combining PNSD and online chemical composition measurements, indicate the potential to constrain UFP levels by regulating traffic and residential emissions, with a large upside for population exposure control.