Measurement and characterisation of atmospheric ice-nucleating particles

Ice-nucleating particles (INPs), a small fraction of the overall atmospheric burden, are responsible for heterogeneous ice nucleation in the atmosphere. INPs have the potential to catalyse ice formation in super-cooled cloud droplets, thus are able to influence cloud properties and radiative effects. Despite the importance of INPs, our knowledge of their spatial and temporal variation in the atmosphere is poor, limiting the predictive capacity of climate models. This thesis aimed to improve the knowledge surrounding atmospheric INPs through their measurements and characterization, but also through the development of instrumentation by which to do so.

The Portable Ice Nucleation Experiment (PINE) chamber was developed in order to meet the technical needs required to answer important questions about the abundance of INPs in our atmosphere. PINE is a portable INP counter capable of making measurements autonomously, with a high-time resolution across the entire temperature regime of mixed-phase clouds. Upon completion of the first PINE prototype, PINE was deployed to the Hyytiälä forestry research station with the goal of validating it in a field environment, and also quantifying the INP concentrations in a boreal environment.

A series of field and laboratory studies were carried out in order to improve our knowledge of the distribution and properties of INPs in the atmosphere. Combustion aerosol generated during Bonfire Night celebrations in the U.K. was shown to be an ineffective ice nucleator, unable to compete with ambient INPs. Viruses were shown to potentially play a role in atmospheric ice nucleation in marine environments, whilst the INPs responsible for greater than expected concentrations in a boreal forest environment were characterized through wet-dispersion experimental techniques.

Overall, this thesis serves to further our understanding of INP concentrations in the atmosphere through space and time, and elucidate the nature of the aerosol particles responsible. Further to that, the techniques and instrument development described here will aid in furthering this understanding through robust experimentation.