Mineral and biological ice-nucleating particles above the South East of the British Isles†

A small fraction of aerosol particles known as Ice-Nucleating Particles (INPs) have the potential to trigger ice formation in cloud droplets at higher temperatures than homogeneous freezing. INPs can strongly reduce the water content and albedo of shallow mixed-phase clouds and also influence the development of convective clouds. Therefore, it is important to understand which aerosol types serve as INPs and how effectively they nucleate ice. Using a combination of INP measurements and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), we quantify both the INP concentrations over a range of activation temperatures and the size-resolved composition. We show that the INP population of aerosol samples collected from an aircraft over the UK during July of 2017 is consistent with ice-nucleation on mineral dust below about −20 °C, but some other INP type must account for ice-nucleation at higher temperatures. Biological aerosol particles above ∼2 μm were detected based on visual detection of their morphological features in all the analysed samples at concentrations of at least 10 to 100 L−1 in the boundary layer. We suggest that given the presence of biological material, it could substantially contribute to the enhanced ice-nucleation ability of the samples at above −20 °C. Organic material attached to mineral dust could be responsible for at least part of this enhancement. These results are consistent with a growing body of data which suggests mineral dust alone cannot explain the INP population in the mid-latitude terrestrial atmosphere and that biological ice nucleating particles are most likely important for cloud glaciation.
Mineral dust is responsible for the ice-nucleating properties of aerosol samples collected over the British Isles below ∼−20 °C, while an additional source, potentially biogenic, must explain the ice-nucleating properties at higher temperatures.