We studied the influence of the Planetary Boundary Layer (PBL) on the air masses sampled at the mountaintop Hellenic Atmospheric Aerosol and Climate Change station ((HAC) 2 ) at Mount Helmos (Greece) during the Cloud-AerosoL InteractionS in the Helmos background TropOsphere (CALISTHO) Campaign from September 2021 to March 2022. The PBL Height (PBLH) was determined from the standard deviation of the vertical wind velocity ( sigma w ) measured by a wind Doppler lidar (over a 30 -min time window with 30 m spatial resolution); the height for which sigma w drops below a characteristic threshold of 0.1 m s -1 corresponds to the PBLH. The air mass characterization is independently carried out using in situ measurements sampled at (HAC) 2 (equivalent black carbon, eBC ; fluorescent particle number, aerosol size distributions, absolute humidity). We found that a distinct diurnal cycle of aerosol properties is seen when the station is inside the PBL (i.e., PBLH exceeds the (HAC) 2 altitude); and a complete lack thereof when it is in the Free Tropospheric Layer (FTL). Additionally, we identified transition periods where the (HAC) 2 site location alternates between the FTL (usually during the early morning hours) and the PBL (usually during the midday and late afternoon hours), during which the concentration and characteristics of the aerosols vary the most. Transition periods are also when orographic clouds are formed. The highest PBLH values occur in September [400 m above (HAC) 2 ] followed by a transition period in November, while the lowest ones occur in January [200 m below (HAC) 2 ]. We found also that the PBLH increases by 16 m per 1 degrees C increase of the ground temperature.