| Summary: | Aerosols are liquid or solid particles suspended in the atmosphere. Their size can range from a few nanometres to micrometres. Despite their small size, they have a significant impact on Earth’s radiation budget by scattering, absorbing and emitting radiation as well as serving as cloud condensation nuclei. Furthermore, aerosols can have severe health effects. The complexity of aerosols given by their vast amount of properties makes them challenging to characterise. A large fraction of aerosols is formed through a phase transition in the atmosphere via gas–to–particle conversion. This process involves a variety of precursor vapours, which are low enough in their volatility to be able to form clusters and further grow the particles. The objective of the studies presented in this thesis is to deepen our understanding of new particle formation mechanisms in the pristine Arctic atmosphere. The observations of two extensive field studies conducted in polar atmosphere showed various nucleation mechanisms. On Svalbard close to the open sea, the particle formation is initiated by negative ion–induced nucleation of sulfuric acid–ammonia with further growth due to condensation of methanesulfonic acid during springtime and organic compounds during summertime. In contrast, on Greenland close to sea ice, the nucleation mechanism is dominated by iodic acid during springtime. Furthermore, I aimed to characterise the chemical composition of atmospheric ions above the boreal forest. By developing an airborne setup for an Atmospheric Pressure interfaceTime–of–Flight mass spectrometer (APi–TOF) deployable in small aircraft, the temporal and spatial development of atmospheric ions in the boundary layer and free troposphere was measured. During daytime, organic compounds from above the canopy–level are mixed throughout the boundary layer, but are not abundant in the free troposphere. During nighttime, the ion chemistry within the boundary layer is determined by the vertical stability and air mass origin. For cases where the ...
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