Sources and chemical composition of natural and anthropogenic aerosols in the central Arctic

The Arctic is warming much faster than the global average, with far-reaching climatic, ecological, and socioeconomic implications. Because of the way aerosols interact with radiation and clouds, they are an important component of the Arctic climate system. They can influence the surface energy budget directly, by scattering and absorbing solar and terrestrial radiation, and indirectly, by acting as cloud condensation nuclei (CCN) and ice nucleating particles, shaping cloud occurrence, phase, and radiative properties. Over the central Arctic Ocean, however, aerosol sources, chemical composition, and CCN-relevant properties remain poorly constrained, because year-round in situ observations are extremely scarce. This thesis addresses this gap by providing and analyzing the first comprehensive, year-long, high-time resolution dataset of CCN-relevant aerosols physicochemical properties over the central Arctic Ocean, based mainly on measurements collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The annual cycle of submicron aerosol composition over the central Arctic was characterized and compared to records from lower-latitude Arctic coastal stations, showing that central Arctic aerosols follow the typical Arctic aerosol seasonal regimes. However, important differences emerged, notably year-round lower ammonium levels and distinct organic aerosol (OA) sources and molecular composition from spring to summer. Central Arctic OA was more mixed and more oxidized than at coastal sites, consistent with longer transport times and different processing and source contributions. These compositional differences are likely to affect aerosol acidity, hygroscopicity and, consequently, CCN-relevant properties. High-time-resolution measurements were then used to investigate how synoptic and local processes shaped the central Arctic CCN populations on hourly to seasonal timescales. Cyclones emerged as key modulators of aerosol and CCN variability from autumn to spring, by both transporting pollution from lower latitudes and by promoting local wind-driven emissions of sea-salt particles from blowing snow and open leads. A detailed source-apportionment of OA during the transition from polar night to polar day further separated the contribution of anthropogenic versus natural and local versus remote aerosols to the central Arctic aerosol budget. Seasonal shifts in OA sources following polar sunrise and the collapse of the polar vortex in spring were associated with changes in aerosol oxidation state, which appeared to be a key property conducive to OA activation in cloud and fog droplets. Finally, the full year of ship-influenced MOSAiC observations, combined with model simulations, were used to provide the first year-long characterization of ship-aerosol-cloud interactions in the central Arctic. Importantly, we showed that shipping emissions, which are expected to be an important emerging source of aerosol in