STUDY OF PROCESSES AFFECTING AEROSOL SIZE DISTRIBUTION AND NEW PARTICLE FORMATION AT HIGH ALTITUDES

Atmospheric aerosols are tiny liquid or solid particles suspended in the atmosphere, yet they have a powerful impact threatening human health and shaping global climate. Their effects, strongly influenced by composition, size, and number, and their processes, including New Particle Formation (NPF), remain among the higest uncertainties in atmospheric science, especially in the free troposphere (FT), where observations are rare. Mountain regions, by virtue of their altitude and remoteness, provide a unique opportunity to advance our understanding of aerosol processes, offering valuable data that can improve climate models and air quality predictions. This thesis investigates almost two decades of particle number size distribution and NPF event at Monte Cimone (CMN), a GAW global high-altitude station in the Northern Italian Apennines (2165 m a.s.l.), representative of the Mediterranean Basin and the Southern European free troposphere (FT). Continuous measurements of particle size distribution using Differential/Scanning Mobility Particle Sizer (9 nm to 560/800 nm) and Neutral Air Cluster and Ion Spectrometers (2–40 nm for particles, 0.8–40 nm for ions) have been conducted since 2005 within EUSAAR, ACTRIS, ACTRIS-2 and ACTRIS RI providing, to our knowledge, one of the most extensive time series for a high-altitude site. The study also compares recent observations in extended size range with those from the Jungfraujoch GAW global station (JFJ) in the Swiss Alps (3454 m a.s.l.), using the same instrumental setup, to better characterize aerosol processes and NPF at two GAW high-altitude sites representing Southern Europe’s background conditions. Results show a long-term decrease in total particle concentrations at CMN of around 96 particles/cm3 since 2005, primarily driven by a decline in accumulation mode particles, while nucleation mode particles show an increasing tendency. NPF events occur on average 23% of days, with the highest frequency in spring. They are typically influenced by low condensation sink before nucleation, high solar radiation, calm winds, dry conditions, and higher concentrations of sulfur dioxide. Dynamic properties of stronger NPF events show moderate nucleation rates (dNNuc/dt = 0.51 cm−3s−1), particle growth rates (GRNuc = 4.5 nm/h), and formation rates (J10 = 0.121 cm−3s−1), with temporal trends indicating increasing nucleation rates but slightly decreasing growth rates. The comparison with JFJ reveals higher aerosol concentrations at CMN due to regional anthropogenic influences from the lower altitude and transport from the Po Valley. Although JFJ is characterized by a greater number of particles below 25 nm, CMN experiences more frequent and intense NPF events due to stronger regional influences. These events, classified using three complementary methods (Dal Maso, Dada, and Aliaga), show spring and autumn peaks at both locations, with CMN exhibiting higher formation and growth rates, particularly in spring and summer, reflecting differences in precursor availability. The comparison of two mountain sites reveals that NPF in the European FT can vary considerably depending on the measurement location. Mountain can act as a source of aerosol into the FT, both by vertically transporting primary aerosols and by creating conditions that trigger secondary aerosol formation. The latter may be influenced by the proximity to the FT/PBL (planetary boundary layer) interface. As NPF is considered to contribute to a major fraction of particle number and CCN concentrations in the FT, this new insight should be explored in greater depth and incorporated into global models.