The influence of air mass origin and ammonia emissions on new particle formation at a Mediterranean urban site (Athens, Greece)

New particle formation (NPF) is an important source of atmospheric aerosol particles and can affect air quality and climate, yet the processes controlling NPF in polluted urban environments, particularly in the Eastern Mediterranean, are not well understood. Here, a two-year dataset (January 2022–December 2023) of particle number size distributions from an urban background site in Athens, Greece, was combined with three-dimensional Potential Source Contribution Function (3D-PSCF) analysis and LOTOS-EUROS chemical transport modeling to investigate the role of altitude-dependent air-mass transport and regional NH 3 availability in NPF events. NPF took place at a frequency of 30%, with the highest occurrence observed during spring and autumn. While 65–78% of air mass trajectories associated with days comprising Class I NPF events had more than 50% of their points above the mixed layer (ML), similar or even higher fractions were observed on non-event days, indicating that residence in the free troposphere alone is not sufficient to distinguish NPF from non-event conditions. The spatial distribution of air-mass residence within the ML was particularly relevant to Class I NPF events, especially during the warm period, when enhanced PSCF probabilities, reaching 0.8, overlapped with regions of elevated NH 3 concentrations. These transport pathways traversed agricultural regions in western and central Greece, suggesting a potential contribution of regionally transported ammonia to precursor availability. However, contrasting NH 3 patterns between 2022 and 2023 suggest that ammonia alone cannot fully account for the observed interannual variability in NPF. Overall, the results highlight the importance of considering both the horizontal and vertical transport history of air masses, together with precursor abundance and condensation losses, when interpreting NPF at urban receptor sites.