Schmale, JuliaBeck, Ivo Fabio2024-08-132024-08-132024-08-132024-08-2610.5075/epfl-thesis-10639https://infoscience.epfl.ch/handle/20.500.14299/240715Cloud condensation nuclei (CCN) and ice nucleating particles (INP) play an important role in the Arctic climate as they determine cloud properties and, thereby, the surface energy balance, which is strongly linked to the accelerated warming of the Arctic. However, measurements of aerosols in the Arctic, in particular over the central Arctic pack ice, are needed in order to improve our poor understanding of their interactions with clouds and their effects on the climate. This dissertation uses data collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC expedition) from October 2019 to September 2020 on the RV Polarstern in the central Arctic Ocean. The focus lies on exploring the seasonal variations in the properties of CCN and fluorescent aerosols, which are proxies for biological particles that can serve as INP, in the central Arctic Ocean, and potential drivers behind them. Our measurements were often influenced by emissions from the ship stack, vehicles or generators. Therefore, we first present an algorithm that allows fast, efficient, and uncomplicated detection and removal of contaminated periods in aerosol and trace gas measurements. It ensures that the observations represent the natural conditions in the Arctic. Measurements of fluorescent aerosols are used to observe biological aerosols, which can efficiently form ice crystals, thus influencing clouds. We find short-term "bursts" of hyper-fluorescent, potentially biological, aerosols during the polar night, a period which is generally referred to as biologically inactive, and high fractions of hyper-fluorescent particles during summer and autumn that coincide with INPs that can activate at relatively warm temperatures. While this work advances our understanding of central Arctic INP sources, further measurements are needed to investigate the exact nature and release mechanisms of the local biological aerosols. Moreover, we analyzed the first annual cycle of CCN concentrations and their hygroscopicity (i.e. their ability to absorb water) from the central Arctic Ocean at five supersaturations. We found that in the fall, CCN concentrations were low enough that they could limit cloud formation. High CCN concentrations were observed, in particular in winter and spring. Unexpectedly, CCN in winter were only slightly hygroscopic, contradicting our assumption of hygroscopic, long-range transported aerosols. We found that the low hygroscopicity could be due to the presence of externally mixed aerosol populations with different hygroscopicities. Furthermore, notable differences in the seasonal cycle of CCN hygroscopicity between MOSAiC and the Zeppelin Station on Svalbard, and the comparison to other measurements underscore the heterogeneous nature of CCN properties across the Arctic. This work delivers a methodology to remove the effects of local pollution on aerosol and trace gas measurements and offers insights into the seasonality of aerosols, which are important for understanding aerosol-cloud interactions and the Arctic climate system, especially during the polar night, from which to date no aerosol data are available from the central Arctic Ocean. These insights are valuable given the ongoing and upcoming changes in the Arctic climate system that will also lead to changes in the central Arctic CCN and INP.enAerosolsatmosphereclimateArcticArctic Oceanshippollutionfluorescent aerosolscloud condensation nucleiice nucleating particlesthesis::doctoral thesis