Analysis of aerosol and cloud measurements from a tethered balloon during the Pallas Cloud Experiment 2022

The contribution of aerosol-cloud interactions to the radiative forcing of the Earth’s climate remains largely uncertain, especially at the poles, which are highly sensitive to climate change. This thesis examined the influence of clouds and atmospheric processes on the distribution of aerosols over subarctic Finnish Lapland. The Pallas Cloud Experiment was conducted in 2022 with the objective of better understanding atmospheric processes. During this campaign, a tethered balloon, equipped with instruments measuring meteorological variables and aerosol concentration for two ranges of diameters, performed numerous flights to collect data on different days and under various weather conditions. This study investigated the nineteen flights carried out on eleven different days between September 28 and October 13 2022 in Pallas. The airborne measurements were associated with multiple ground measurements and remote sensing data. Vertical profiles were analyzed, along with normalized particle size distributions derived from aerosol number concentrations and the instrument bin diameters. First, a comparison between the different air mass sources observed for the nineteen flights was made, to understand the characteristics of the air coming and its influence on aerosol properties. Next, a comparison between the measurements taken below clouds and inside clouds was performed, to highlight the effects of cloud and thermodynamics processes on aerosol distribution. Finally, two case studies were selected to examine specific aerosol features in greater detail. Analysis of the vertical profiles and particle size distribution revealed that air masses originating from polluted areas were mainly associated with a bimodal particle distribution, dominated by Aitken mode particles. Number concentrations of large particles, with diameters between 186 nm to 3.37 μm, were 2 to 10 times higher in polluted air masses than those measured in cleaner air masses. In comparison, the number concentrations of small particles, with diameters between 8 nm to 236 nm, were 2 to 5 times higher in clean air than in polluted air. Additionally, the more polluted air masses typically originated from the continent and were accompanied by cloudy conditions, whereas the cleaner air had a marine origin and was cloud-free about half the time. Ultimately, clean and marine air masses were prone to new particle formation events. Clouds and thermodynamics also affected the aerosol properties inside and below the clouds, through various mechanisms such as cloud processing, droplet activation and evaporation, particle scavenging, and downdrafts. Finally, the case studies highlighted the importance of vertical exchanges in two different scenarios, firstly in the presence of a multilayer cloud system and secondly through a mixed-phase cloud. This work showed, on the one hand, the importance of combining high-resolution airborne measurements with ground measurements and remote sensing data to describe