Modelling of aerosol and cloud processes in the summertime high Arctic

Warming in the Arctic region is significantly faster than the global average rate, and drives further climate changes. Clouds play a major role in the energy budget of the Arctic and are a key uncertainty in projections of Arctic climate change. The behaviour of aerosol particles and their interactions with clouds is particularly uncertain. Measurements of Arctic clouds have revealed a high sensitivity to perturbations in aerosol concentrations, but climate models show large differences in projections of the Arctic aerosol budget depending on parameterisations used for key processes, such as sea spray aerosol emissions, new particle formation, and wet removal. In this thesis, we use novel measurements of aerosol size distributions from the Arctic Ocean to evaluate Arctic aerosol and cloud processes in regional and global models. We examine primary marine organic carbon, biomass burning, wet removal and new particle formation. Our results show that there is a seasonal transition at the end of summer in the high Arctic from a free-tropospheric source of particles to a boundary layer source of particles. The transition is driven by declining photochemical rates at the end of summer, and coincides with the onset of iodic acid emissions which drive new particle formation in the high Arctic boundary layer during the sea ice freeze period. We show that simulating
this transition improves model-observation agreement in the concentration of particles smaller than 100 nm diameter, something that could not be achieved by modifying the primary carbon emissions or wet removal in the model. However, we also reveal biases in the simulation of Arctic clouds in the regional model. Our results show that aerosol processes cannot be considered in isolation, but rather that to model the impact of the changing Arctic aerosol budget on the regional climate system, accurate simulations of cloud behaviour are required.