The Arctic is a place particularly vulnerable to climate change, warming at an accelerated rate compared to the rest of the Earth. In this remote environment, the atmosphere, the ocean, the ice, and the land are all interlinked and are shaping a very complex system. This is why the interplay between aerosols and clouds and their role in the warming Arctic is still not fully understood.

To address this issue, a better understanding of the sources, properties, and fate of aerosol particles in the Arctic is needed. By means of in situ observations of aerosols and clouds at the Zeppelin Observatory on Svalbard, this thesis aims to shed light on aerosol-cloud interactions in the Arctic. These measurements were conducted within the framework of the one-year long Ny-Ålesund Aerosol Cloud Experiment (2019-2020). A special focus of this thesis is on the chemical composition of aerosol particles from a molecular-level perspective, where measurements from a filter inlet for gases and aerosols coupled to a chemical ionization mass spectrometer were used.

To identify the properties of the aerosol particles serving as cloud condensation nuclei (CCN) or ice nucleating particles (INP), cloud droplets and ice crystals were sampled with a ground-based counterflow virtual impactor inlet. The measured particles are called cloud residuals. The observations show that the cloud residuals have sizes in the Aitken and accumulation mode (as small as 10 nm in diameter). The chemical composition of these cloud residuals followed largely the expected annual cycle of aerosol particles in the Arctic, suggesting that most of the aerosol particles can act as CCN or INP in the Arctic. Anthropogenic signatures were present in the cloud residuals in the winter and spring, whereas in the summer a large contribution from methanesulfonic acid (MSA) was present, indicating natural source regions.

The thesis also investigated how the oxidation products of dimethyl sulfide, MSA, sulfuric acid, and hydroperoxymethyl thioformate (HPMTF) are related to each other in the gaseous and particulate phase. HPMTF was observed to be present mainly in the gas phase, where it followed the gas phase signal of MSA in the summer. However, it was not present in significant amounts in the particle phase. In the presence of clouds, the gas phase levels of HPMTF decreased, indicating the uptake by cloud droplets.

Another source of aerosol particles investigated are those from biomass burning (BB) emissions. The BB aerosol showed a largely similar molecular-level chemical composition of the organic aerosol compared to the rest of the year; however, a clear change to a largely organic dominated bulk aerosol composition was observed. Back trajectories suggested mainly Eastern Europe and Siberia as the source regions for the BB events. Using BB tracer compounds in combination with the back trajectories suggested that agricultural fires from Eastern Europe have a larger impact on the Arctic aerosol population, where mass and number enhancements compared to times not influenced by BB were found to reach up to one order of magnitude.

The results from this thesis show that aerosol particles from natural emissions are an important source for Arctic aerosol particles. Especially, emissions from marine biological activity are relevant for the growth of aerosol particles to sizes in the CCN active regime in the summer.

This PhD thesis investigates the atmospheric life cycle of Black Carbon (BC) in the Arctic. The Arctic region has been rapidly changing in the last decades and the role of BC aerosols in this is still uncertain. BC aerosols are mainly produced by incomplete combustion of biomass burning and fossil fuel and stand out from other aerosol species due to their strong ability to absorb solar radiation. The impact of BC on the Earth’s radiation budget is estimated to be overall warming. While the indirect effect, interactions with clouds, is estimated to be negative, the direct radiation effect is positive because of the absorption ability of the BC. These estimates are uncertain, especially for aerosol-cloud interactions. To estimate the role of BC in the Arctic, it is necessary to know the size distribution of BC, the transport pattern and the loss processes that affect the BC concentration. In this thesis, in-situ observations from the Zeppelin observatory in the Arctic, as well as global modelling tools, are used to answer the following research questions: 1. What kind of new insights about BC size distributions can be gained from simultaneous long-term measurements of absorption and aerosol number size distributions? 2. How do source regions impact BC size distributions measured at Zeppelin? 3. How are observations of biomass burning tracers at Zeppelin connected to transport from source regions with active fires? 4. How do emissions, as well as, wet and dry removal pathways drive the diversity of the BC life cycle in General Circulation Models (GCMs)?

A statistical method to derive BC size distributions from filter-based absorption measurements was developed and applied to long-term data from the Arctic measurement station Zeppelin on Svalbard. Promising results were obtained for inferring BC number size distributions from absorption and size distribution data, except for the most polluted conditions with the air masses arriving from Northern Eurasia and Russia - as identified from an analysis using back trajectories. Trajectory analysis was also used to link events with elevated biomass burning tracers and BC at Zeppelin to fire activity measured by satellites on the continents around the Arctic. To investigate the interplay of emissions and removal processes of BC in models and to understand the diversity in model representation of BC in the Arctic, a detailed analysis of processes in four GCMs was performed. The BC concentrations in the Arctic were compared and their response to removal processes during long-range transport to Zeppelin. The results underline the importance of BC sources and processing far away from the Arctic.

The knowledge gained about the BC life cycle will facilitate a better assessment of the large-scale influence of BC on the Arctic climate and environment.