Aerosol Size Distribution and New Particle Formation in High Mountain Environments: A Comparative Study at Monte Cimone and Jungfraujoch GAW Stations

Abstract. Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.