Atmospheric aerosol particles are small liquid or solid particles suspended in the air. They are present in the atmosphere all around us and affect the planetary energy balance by scattering and absorbing radiation and by interacting with clouds. In model projections of future climate, aerosol–cloud interactions contribute a lot of uncertainty. Large-scale climate models particularly struggle with simulating low-level clouds in the Arctic, which is a region that is not only warming at twice the global average rate or higher but also where natural aerosol emissions are expected to change most as a result of the warming. The goal of this thesis was to study aerosol–cloud interactions to help improve our understanding of what role clouds play in the Arctic climate and how they will respond to climate change. Specifically, the project focused on studying the microphysical properties of aerosol particles and cloud nucleating particles—the subset of aerosol particles that participate in cloud formation. This was done both through field experiments in the high Arctic over the pack ice and by analysis of an existing two-year data set from an Arctic research station on Svalbard.

The main instrument used in this thesis was a ground-based counterflow virtual impactor (GCVI) inlet, which dries cloud droplets and ice crystals and allows us to characterise the particles that were inside. The Svalbard study is the longest GCVI study to date, and the first to cover more than a full annual cycle. It also involved a detailed evaluation of the GCVI. Using the GCVI inlet and a large array of other instruments, we were able to show that small, so-called Aitken mode particles act as cloud nucleating particles, supporting results from previous studies. However, our measurements showed these particles to be more abundant in the cloud droplets and ice crystals than expected, both over the pack ice and on Svalbard. While some uncertainties remain, these datasets can potentially be used to evaluate and improve model representations of low-level Arctic clouds. In the other parts of this thesis, we found that iodine nucleation and breakup of larger particles are potential formation pathways for Aitken mode particles over the pack ice. However, detailed chemical composition measurements of cloud nucleating particles would be needed to determine whether these formation mechanisms are important for Arctic cloud formation.