Evaluation of in-situ metrics for determining the influence of the Planetary Boundary Layer at the Helmos Hellenic Atmospheric Aerosol & Climate Change (HAC)2 station

Understanding aerosol properties and their life cycle in regional air masses is essential for assessing their impacts on clouds, precipitation and climate. High-altitude mountain stations, often emphasized as free-tropospheric measurement sites, are ideal for cloud and climate research. However, depending on the season and time of day, high-altitude sites may be influenced by planetary boundary layer (PBL) air masses due to convective transport. The segregation between PBL-influenced and free-tropospheric (FT) air masses remains a challenging but critical issue. Being able to unravel the periods for which clouds are influenced by each air type can vastly expands the scientific value and relevance of aerosol-cloud studies at mountain tops because cloud formation and their susceptibility to aerosol and dynamic perturbations vary considerably with each air mass type; the types of droplets and ice nucleators also can vary significantly, which further expands the scope and relevance of the measurements.

The Helmos Hellenic Atmospheric Aerosol and Climate Change ((HAC)²) station in Greece (2314 m a.s.l.) is the only high-altitude station in the eastern Mediterranean, a region highly sensitive to climate change. It is located at the crossroads of different air masses; the station is well-suited for aerosol-cloud interaction studies. To enhance understanding of the processes driving the formation and evolution of warm and mixed-phase clouds, the CALISHTO (Cloud-Aerosol InteractionS in the Helmos Background TropOsphere) and CHOPIN (Cleancloud Helmos OrograPhic sIte experiment) campaigns were conducted at Mount Helmos during the autumn-winter periods of 2021–2022 and 2024–2025, respectively. During these campaigns, in-situ and remote sensing measurements at a number of sites located at the Kalavrita Ski Center and the (HAC)² were used to study the influence of the mixing layer (PBL), and their related aerosol and gases, at (HAC)². To achieve this, both in-situ and remote sensing instruments were employed. The permanent instrumentation of the (HAC)² station (e.g., GHGs, aerosol number size distributions, aerosol optical properties, meteorological data, and liquid water content) was supplemented with additional in-situ and remote sensing instruments operated at (HAC)² and the lower-altitude sites (about 1700 m a.s.l.). During the CHOPIN campaign, radiosonde measurements were conducted to measure critical atmospheric variables and provide further details about the structure of the atmospheric layers.

A set of aerosol and gaseous species and atmospheric metrics from in-situ measurements was established to indicate the presence of PBL air at the (HAC)² based on characteristic values of the water vapor mixing ratio, the accumulation mode (particles with a diameter greater than 95 nm) number concentration, and the ratio of eBC to CO. These thresholds were established by monitoring their values when the BL-FT boundary was at the (HAC)2 altitude, determined by remote sensing of atmospheric turbulence measurements. Application of these metrics to determine the presence (or not) of BL-influenced air agreed with the classification achieved by the remote sensing observations for up to 85 % of the time. Cloudy periods were studied separately from clear-sky periods owing to the substantially different gas and particle removal mechanisms occurring in each period.