The sources and activity of ice-nucleating particles in the high latitudes

Ice-nucleating particles play an important role in the climate system by influencing cloud radiative properties, cloud lifetime and precipitation. Understanding the sources and concentration of INPs in the atmosphere is therefore of crucial importance to accurately model clouds in the atmosphere. This is particularly pronounced in the mid to high latitudes where mixed-phase clouds, which are highly sensitive to the presence of INPs, are common and play a key role in modulating the effects of climate change through cloud-climate feedbacks. However, the sources and characteristics of INPs in the high latitudes, and their spatial and temporal variation in the atmosphere, are still not well known. This thesis applies a range of new and existing techniques to advance our understanding of the sources and concentrations of INPs, and their impact on cloud microphysical properties. In Chapter 2, I use in-situ aerosol sampling and laboratory analysis to characterise the ice-nucleating ability of glacial dust emitted from the Copper River, Alaska. The results show that this dust nucleates ice at temperatures relevant for mixed-phase clouds and is considerably more active than low-latitude desert dust due to the presence of a biogenic component that enhances the ice-nucleating activity. I then use particle dispersion modelling to show that dust can reach regions of the atmosphere where it could trigger cloud glaciation in concentrations where it would dominate over low-latitude INP sources. In Chapter 3, I test and apply a method to retrieve ice crystal number concentration from remote sensing observations to data from Summit Station, Greenland, leading to the first observations of ice crystal number concentration in clouds over the Greenland Ice Sheet. I combine this data with an existing dataset of INP concentrations to identify ice-formation mechanisms in these clouds and show that there is a secondary ice production process active between -10 and -18◦C. Finally, in Chapter 4, I use aircraft sampling and laboratory analysis to determine INP concentrations in the northeast Atlantic. I combine this with back trajectory analysis to determine the potential sources of sampled INPs and show that the sources of INPs can vary over small spatial scales and that INP concentrations in clouds can be enhanced by contact with land.