The ice-nucleating activity of fertile soils and crop pathogens

Atmospheric ice formation plays a key role in controlling the radiative properties and lifetimes of supercooled clouds. Ice-nucleating particles, which initiate the heterogeneous nucleation of ice in supercooled clouds, are rare in the atmosphere and crucial for the formation of ice in clouds. Therefore, our understanding of the sources and atmospheric concentrations of ice-nucleating particles is critical for our understanding of the impact of clouds on the climate. Biological aerosol particles have been identified as potentially important ice nucleators, particularly at temperatures above -10°C. However, our current understanding of the relative contribution of biological material to regional and global ice-nucleating particle populations remains poor. In particular, crop agriculture contributes up to 25% of global dust emissions, creating a potentially important source of biological ice-nucleating particles which is yet to be fully understood. This thesis investigated different agricultural sources of ice-nucleating particles with the overall goal of determining the extent to which agriculture influences regional and global ice-nucleating particle populations. Agricultural soil samples from the UK and Canada were extracted to examine the ice-nucleating activity of the submicron entities within the soil. This analysis revealed that the ice-nucleating activity of agricultural soils from different locations varied significantly but this variation was not attributable to concentrations of surfactants within the soil. The ice-nucleating activity of two common fungal crop pathogens was also analysed to determine the relative influence of crop agriculture on the regional ice-nucleating particle populations, which indicated that the ice-nucleating activity of these spores was stable when stored. Finally, the size-resolved ice-nucleating particle concentrations of agricultural soil samples were analysed using a lab-based aerosol chamber technique which showed that the ice-nucleating activity of agricultural soils is evenly distributed across its size distribution. Through these techniques, we have been able to unpick some of the complexities in understanding agricultural sources of ice-nucleating particles by highlighting the potential importance of biological, macromolecular substances to the ice-nucleating activity of these soils.