Why do some particles have the superpower of ice nucleation?


   Faculty of Environment

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  Prof Benjamin Murray, Dr Thomas Whale  No more applications being accepted  Competition Funded PhD Project (Students Worldwide)

About the Project

Clouds in the Earth’s atmosphere, and thereby the planet’s climate, are strongly affected by the presence of atmospheric particles that have the special ability to nucleate ice. The vast majority of aerosol particles in the atmosphere do not nucleate ice, but a rare class of particles have the superpower of ice nucleation. However, our understanding of why these particles nucleate ice is very limited and this prevents us from describing ice nucleation theoretically and means we cannot correctly predict which particle types will nucleate ice and which will not. 

In this project you will work towards improving our understanding of what makes a particle nucleate ice. To achieve this you will: 

1. Investigate the role of minor chemical contaminants in promoting ice nucleation in various materials – there is evidence that suggests the inclusion of elements like lead, strontium and iodine promote nucleation in otherwise inert materials. 

2. Study the repeat freezing of droplets containing aerosol particles sampled from the natural atmosphere to see if nucleation occurs at the air-water interface or in the bulk of the suspension. 

3. Examine cluster formation in macromolecules from pollen influences ice nucleation. 

How you will tackle these objectives

Role of impurities in promoting ice nucleation: Our strategy here is to use soot and secondary organic aerosol particles, materials which are inert in terms of its ice nucleating ability, and add impurities to it in a controlled manner. Our hypothesis is that impurities such as lead (Pb) and iodine (I) compounds will enhance the ice nucleating ability of these aerosol particles. This is based on observations of enhanced Pb within ice crystal residues in the atmosphere and iodine has been found to enhance nucleation in organic aerosol. This is based on observations of enhanced Pb within ice crystal residues in the atmosphere and iodine has been found to enhance nucleation in organic aerosol. To do this you will generate aerosol in our aerosol chamber, characterise its size distribution and then sample the aerosol into a suite of ice nucleating instruments. These instruments will include our new on-line expansion system (PINE, Mohler et al. 2021). Students in the Ice Nucleation group recently published a paper using this system to study the contrail ice forming ability of oil droplets (Ponsonby et al., 2023). 

Location of nucleation, interface or bulk: We will make use of a new technique described by Bieber and Borduas-Dedekind, 2023 making use of our cryomicroscope equipped with a high speed camera (Holden et al., 2019). In this clever experiment a disk of aqueos suspension is created between two glass slides and then cooled until ice formation is initiated. The high-speed camera is used to pinpoint where the crystal growth started and in doing so identify if the molecules that nucleate ice reside at the interface. Bieber and Borduas-Dedekind (2023) observed that ice-nucleating proteins from bacteria tend to reside at the air-water interface and we will find out if similar molecules are common in atmospheric aerosol.  

Cluster formation: We know that aggregation of ice nucleating macromolecules leads to nucleation at much higher temperatures than the individual molecules can achieve. But, we do not understand the conditions under which these aggregates or clusters of macromolecules form and also how fragile they are once they do form. You will make use of the droplet freezing instruments to examine this problem with well defined macromolecules from pollen by adding chemicals that promote or inhibit aggregation as well as breaking aggregates using sonication or filtration. This may relate back to the first objective, where traces of certain materials may promote aggregate formation.  

Geology (18) Physics (29)

Funding Notes

There are two funded places (one home and one international) across all projects advertised in the EPSRC DTP. This means that not all projects will be recruited to- only two candidates will be successful and are selected on the basis of merit.

The award will cover the full fees (home or international) plus an annual maintenance stipend matching UKRI standard (£18,622 in 2023/24).

Where will I study?