Effect of planetary boundary layer evolution on new particle formation events over Cyprus

Abstract. Atmospheric new particle formation (NPF) occurs ubiquitously in the atmosphere, but more often in the planetary boundary layer (PBL). However, particle formation and early growth are poorly understood processes in aerosol science, particularly over the Eastern Mediterranean and Middle East (EMME) region, which has been recognised as a global climate change hot spot. Here, we present semi-continuous concurrent measurements of ion and particle size distributions in Cyprus for the year 2022 from a lower-altitude rural background site (Agia Marina Xyliatou, AMX, 532 m a.m.s.l.) and a higher-latitude mountain background site (Troodos, TRO, 1819 m a.m.s.l.) with only about 20 km distance between the sites. We also used concurrent measurements of sulfur dioxide, ozone, and meteorological parameters from both sites. The boundary layer evolution and its impact on the occurrence of NPF events at a mountain site were investigated using a combination of water vapour mixing ratio, a passive tracer of PBL dynamics, at both sites and the Vaisala ceilometer estimated and screened PBL height from AMX. We found that NPF event frequencies are comparable between AMX (60 %) and TRO (54 %), however only half of the observed NPF events at both sites were observed concurrently. The smaller mode diameter at AMX than at TRO indicates that NPF was initiated near AMX. This is supported by peaks in ion and particle concentrations that were first observed at AMX and followed by a 1–2 hour delay at TRO. This indicates that transported precursor vapour-laden air from lower-altitudes, likely driven by vertical mixing or up-valley winds, significantly contributes to secondary aerosol formation at the mountain site. Airmass history analysis further revealed that significant trajectories had been in contact with the PBL before reaching TRO, underscoring the influence of vertical dynamical mixing on NPF processes. The TRO site is within the PBL for about 25 % of days during late winter and early spring, increasing to >80 % for the rest of the year, which supports our findings. Our results highlight the significant impact of secondary aerosol production in the evolving PBL on higher-altitude environments, though the vertical extent of nucleation processes remains unclear. Understanding these processes is crucial for climate models, as the PBL drives the exchange of energy, moisture and atmospheric constituents, including aerosols, with the atmosphere above.