The Cloudy Nature of Aerosols : Portrayed by the modelled and observed atmosphere

For decades, the environmental sciences have engaged in measuring and modelling natural systems to increase our understanding and prediction of climate change. This thesis advances these efforts by analyzing and applying in-situ instruments, remote sensing, and modelling approaches. The main focus lies in utilizing a state-of-the-art global Earth System Model (ESM) EC-Earth3, which is used to simulate the behavior of climate systems. ESMs are able to capture key feedback mechanisms and forcings by e.g. clouds, aerosols, and atmospheric chemistry. However, due to the computational demands of ESMs, parameterizations are often simplified to balance accuracy with efficiency. The primary methodology of this thesis utilizes the EC-Earth3 model to test a novel approach to new particle formation (NPF) and to analyze the resulting impact on the aerosol population and climate effects. The updated model includes a more comprehensive module that simulates NPF through both neutral and ion-induced nucleation involving sulfuric acid and ammonia (H2SO4-NH3), introducing new processes and input that were not previously accounted for in the model. Running the updated model configuration revealed significant changes in simulated aerosol distributions compared to the previous model version. This thesis further identifies the key aspects of different NPF mechanisms in EC-Earth3 at both global and regional scales. In addition, I investigate the model aerosol-cloud interactions (ACI) by analyzing the impact of aerosols on cloud properties, with a focus on aerosol precursors from biogenic volatile organic compounds (BVOCs) and their relationship to temperature. The analysis reveals significant differences in these processes between the EC-Earth3 model when compared with three other ESMs. I also utilize datasets from satellite observations to further study ACI. A study using various satellite aerosol and cloud sensors data revealed that downwelling stratospheric aerosols from volcanic eruptions can significantly reduce cirrus cloud fractions and alter cloud properties in mid-latitude regions of the northern hemisphere.