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

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 hotspot. 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 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. The observed time for the PBL height to reach the TRO altitude relative to the NPF event start time at AMX (1.73 h) is comparable with the time lag between peak particle number concentrations during concurrent NPF events (1.57 h). Additionally, the growth rates of smaller particles (3–7 nm) were similar, while larger particles (7–25 nm) exhibited higher growth rates at TRO. This suggests that particle growth occurred rapidly in air mass transported from lower altitudes, likely driven by vertical mixing or up-valley winds. Analysis of air mass trajectories supports this interpretation, indicating prior contact of air masses with the PBL before reaching TRO and highlighting the critical role of vertical dynamical mixing in 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.