Measuring and identifying ice-nucleating particles in the atmosphere

A fraction of aerosol particles known as Ice-Nucleating Particles (INPs) has the potential to trigger ice formation in supercooled liquid droplets, dramatically altering the properties of mixed-phase clouds. However, our current knowledge on the way these particles are distributed in the atmosphere is still limited. This thesis aimed to expand our understanding of the sources and concentrations of INPs in the atmosphere at various locations at mid- and high-latitudes. This was done by performing immersion mode INP and aerosol size-resolved composition measurements on board of a research aircraft in three different locations (North-Western Europe, Iceland and the Western North American Arctic).
Aerosol measurements were performed on board of the FAAM BAe-146 research aircraft. Hence, I first characterised the biases of the filter inlet system available on board of this aircraft both theoretically and experimentally, providing recommendations on how to operate the system. I also implemented a methodology to study the size-resolved composition of aerosol samples collected on top of filters using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS). This technique has been applied in parallel with a droplet freezing assay to measure INPs as well as aerosol size-resolved composition in aerosol samples collected on board of the FAAM BAe-146 in different locations.
The first area of study was North-Western Europe. The INP concentration was dominated by the presence of mineral dust at the lower end of the temperature spectrum, while an additional source (likely biological) was the main component at higher temperatures. The second area of study was Iceland, where the INP concentrations were dominated by the presence of local dust. The used methodology allowed me to derive a parameterization of the ice-nucleation ability of airborne Icelandic dust. This parameterization was combined with a global aerosol model which included the Icelandic dust emissions, showing that Icelandic dust significantly contributes to the INP population over the North Atlantic and some areas of the Arctic. The INP concentrations in the Western North American Arctic were the lowest I detected, being compatible with the limit of detection in most cases. The SEM-EDS analysed revealed that mineral dust is more important than sea spray aerosol for the INP population. Overall, across the campaigns, I observed large variability in aerosol concentration, aerosol composition and INP concentration. A part of this variability in the INPs can be explained by the presence of surface area of aerosol and dust in the samples.