Multiseasonal aerosol pH variations between boundary layer and free tropospheric airmasses in the East Mediterranian during the CleanCloud CHOPIN Campaign

Atmospheric acidity is a major aerosol parameter that influences atmospheric chemistry, nutrient availability and deposition rates, aerosol formation and growth rates, nutrient availability, aerosol toxicity and the ability of aerosol to nucleate ice crystals and cloud droplets. Aerosol acidity depends on the concentration and volatility of precursor gases/bases, the amount of non-volatile cations (such as Ca, K, Mg), sulfate, temperature and humidity. Understanding how aerosol acidity changes between airmasses and its vertical evolution from moist, warm boundary layer conditions (close to source regions), into the dry, cold and clean free tropospheric air is highly unconstrained from observations. High altitude mountaintop sites observations offer a unique opportunity to address this uncertainty, as observations required to constrain aerosol pH can be carried out for extensive periods of time, and can sample both free tropospheric and boundary layer air from a variety of sources and over different seasons.This study addresses the need for vertical profiling of aerosol pH by utilizing the extensive dataset available from the CleanCloud CHOPIN field campaign (https://go.epfl.ch/chopin-campaign) at Mount Helmos, Greece from Fall 2024 to Spring 2025. pH is calculated with the ISORROPIA-Lite thermodynamic model applied to the aerosol chemical composition and gas-phase NH3 measurements carried out at the Helmos Hellenic Atmospheric Aerosol and Climate Change ((HAC)²) station (2314 m a.s.l.) at mount Helmos. Airmass origin is identified through a series of chemical and turbulence metrics (to identify when observations correspond to boundary layer or free tropospheric conditions) and backtrajectory analysis when the site is residing in the free troposphere. We observed a clear daily pH cycle at the site, with lower pH values between 7 am and 1 pm, where the airmass is predominantly influenced by free tropospheric air. Higher pH values tend to be observed in the afternoon when ammonia associated with anthropogenic emissions from nearby urban and agricultural activities reached the station, which together with higher humidity and ammonia levels end up reducing acidity. Seasonal variations and the influences of dust episodes, biomass burning and temperature are all analyzed to determine "characteristic" acidity levels associated with each airmass type and infleunce. We then conclude by discussing the implications of the acidity levels for nutrient availability and deposition in each regime, and discuss the ability of models to reproduce the observed acidity patterns.