The formation of ice in clouds is one of the least well understood aspects of the planet’s climate system and is a major source of uncertainty in our climate projections (Murray et al., 2020). Ice formation in clouds triggers a cascade of processes leading to dramatic changes in cloud coverage, cloud reflectivity and precipitation. However, the special class of aerosol particles which trigger ice formation, known as ice-nucleating particles (INPs), is very poorly understood in terms of their sources, transport, characteristics and distribution around the globe.
A particularly poorly understood group of INPs are those that result from biology from both the terrestrial (O’Sullivan et al., 2018) and marine (Wilson et al., 2015) environments. Hence, a core hypothesis in this project is that life on Earth produces a variety of aerosol particle types which influence weather and climate through the production of ice in clouds. One implication of this is that land use changes through land management and agriculture have dramatically changed the vegetation and soils across large swathes of the planet and may also have changed the supply of biological INP to the atmosphere.
“Life on Earth produces a variety of aerosol particle types which influence weather and climate through the production of ice in clouds”
The overarching goal of this project is to characterise and understand the production and atmospheric processing of biological ice nucleating aerosol types. To achieve this goal you may address the following issues through careful laboratory, field and possibly modelling work (also bear in mind that PhD students have the freedom to define their own research questions):
- Address the seasonal cycle of the ice nucleating characteristics of soil, river water and leaf mould samples. We know that these materials contain copious quantities of ice nucleating entities and are a source of atmospheric INP (O’Sullivan et al., 2015), but little systematic work has been done to reveal how the INP content varies through the year. This may well reveal details of how and why INP are produced. We have to go back to the 1970s to get some intriguing clues(Vali et al., 1976), and we can now use our modern techniques to address this problem. You will make use of the University’s Research Farm which is sited between Leeds and York, and is currently being instrumented as a Critical Zone Observatory (CZO).
- Study the impact of agricultural systems on soil dust INP emissions using field observations and experiments at the University of Leeds Farm. Potential areas of research are outdoor pig farming, tillage systems and agroforestry. Free range pigs have been increasingly in demand by consumers, but through their rooting instinct removes plant cover and loosens soil making it susceptible to aeolian erosion. Ongoing research on reduced tillage in arable land and a long-term experiment in agroforestry can also be potentially used in this project.
- Quantify the impact of atmospheric aging by sunlight and oxidising gases on the ice nucleating ability of biogenic INP. Biogenic material often nucleate ice due to the presence of molecules like proteins or polysaccharides and these molecules may degrade on exposure in the atmosphere. Work in the past has focused on the effect of light on cells suspended in water (where the water will afford some protection). Instead, you will suspend biogenic materials (soil dusts, pollen, fungal material, bacteria) in an aerosol chamber and then quantify its ice nucleating ability before and after exposure. This will give us one of the first estimates of the lifetime of biogenic ice nucleating activity in the atmosphere.
- Take part in a ship-borne field campaign in the Labrador Sea as part of the M-Phase project (PI: Murray; Resolving climate sensitivity associated with shallow mixed phase cloud in the oceanic mid- to high-latitudes). Your focus will be on marine sources of INP in the ocean water.
- Study the wind driven aerosolisation of INP from lichen. We have shown that lichens contain massive quantities of very active INP and since they are often in exposed positions such as on trees they could conceivably produce atmospheric INP. You will build a system to quantify the production of aerosol in our aerosol chamber.
What are ice nucleating particles?
Atmospheric ice-nucleating particles (INP) are aerosol particles with special physical and chemical properties that enable them to induce the formation of ice crystals in clouds below 0°C. In the absence of INP, cloud droplets can supercool to below -33°C. Even a very small concentration of INP can have profound impacts on clouds. For example, only 1 INP per litre of air can cause a cloud to transition from being dominantly liquid to dominantly ice with a massive impact on its radiative and precipitation properties (Murray et al., 2020). However, measuring and identifying INP is especially challenging since total aerosol concentrations are many orders of magnitude greater than INP concentrations.
What INP do in clouds
There are different types of clouds in which ice is important, but the type and mode of action through which INP nucleate ice is distinct in the different cloud regimes. For example, in the lower and mid troposphere at temperatures between 0 and -35oC clouds can exist as supercooled water, ice or a mixture of the two. INP tend to be immersed in supercooled water before they can trigger freezing in this regime. In contrast, in the upper troposphere under cirrus conditions ice can form directly onto aerosol particles well below the supersaturation required to form a liquid cloud. Different populations of aerosol serve as INPs in the different cloud regimes, hence measurements need to be made that distinguish between these different populations.
The gaps in our INP knowledge
Despite decades of research on INP, our understanding of INP sources in the atmosphere, and hence their impact on climate, is in its infancy. Substantial developments are being made by characterizing INP in innovative laboratory and field experiments, and then carrying this new knowledge into atmospheric models. For example, the Leeds group discovered that a specific mineral group in desert dust particles can explain their ice nucleating properties, enabling a global model of these INP to be developed (Atkinson et al., 2013). Similarly, we quantified marine biogenic INP through field measurements in remote environments from research ships and then used our global model to represent the global distribution of these INP (Wilson et al., 2015; Vergara-Temprado et al., 2017).
Biological INP may be extremely important
Biological ice nucleating materials are some of the most effective ice nucleating materials we know of, but their distribution in the atmosphere is very poorly defined and their sources are also poorly defined (Murray et al., 2012). For instance, certain fungus and bacteria have evolved the capacity to produce proteins which nucleate ice at just below the melting point of ice. Other materials like pollen and spores have also been shown to nucleate ice. In addition, fragments of these biological species are also known to nucleate ice and soils contain a mishmash of ice nucleating materials which can become airborne (O’Sullivan et al., 2014; O’Sullivan et al., 2015), but have a poorly defined role in cloud glaciation and climate.
Research environment in Leeds
You will join the vibrant Ice Nucleation group in the Institute for Climate and Atmospheric Science (ICAS). ICAS covers climate, air pollution, meteorology and climate impacts, with extensive programmes in observations, modelling and lab studies. Atmospheric science at Leeds is ranked 9th in the Centre for World University Rankings (http://cwur.org/2017) and 13th in the Academic Ranking of World Universities out of 400 (http://www.shanghairanking.com). Wider interdisciplinary experience is guaranteed through our new cross-campus Priestley Centre (http://climate.leeds.ac.uk). Peer exchange and learning occurs through frequent institute and group seminars, discussion meetings and paper review groups.
We also have formal partnerships with both the UK Met Office and also the Karlsruhe Institute of Technology. The KIT-ICAS partnership has led to exchange of students and staff and many joint publications.
The supervisors have an outstanding track record of PhD student supervision, with students having won School of Earth and Environment PhD publication prize (out of 200 students), the Piers Sellers Priestly prize as well as several national and international prizes.
Short accessible pieces on atmospheric ice nucleation:
BBC Brainwaves: https://www.bbc.co.uk/sounds/play/b09kf9y1 go to 3:17 to hear Ben Murray talking about atmospheric ice formation.
Murray, Cracking the problem of ice nucleation, Science, 2017
Koop and Mahowald, The seeds of ice in clouds, Nature, 2013
References (more in depth reading)
Atkinson, J. D., Murray, B. J., Woodhouse, M. T., Whale, T. F., Baustian, K. J., Carslaw, K. S., Dobbie, S., O’Sullivan, D., and Malkin, T. L.: The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds, Nature, 498, 355-358, 10.1038/nature12278, 2013.
Murray, B. J., O’Sullivan, D., Atkinson, J. D., and Webb, M. E.: Ice nucleation by particles immersed in supercooled cloud droplets, Chem. Soc. Rev., 41, 6519-6554, 10.1039/c2cs35200a, 2012.
Murray, B. J., Carslaw, K. S., and Field, P. R.: Opinion: Cloud-phase climate feedback and the importance of ice-nucleating particles, Atmos. Chem. Phys. Discuss., 2020, 1-23, 10.5194/acp-2020-852, 2020.
O’Sullivan, D., Murray, B. J., Malkin, T. L., Whale, T. F., Umo, N. S., Atkinson, J. D., Price, H. C., Baustian, K. J., Browse, J., and Webb, M. E.: Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components, Atmos. Chem. Phys., 14, 1853-1867, 10.5194/acp-14-1853-2014, 2014.
O’Sullivan, D., Murray, B. J., Ross, J. F., Whale, T. F., Price, H. C., Atkinson, J. D., Umo, N. S., and Webb, M. E.: The relevance of nanoscale biological fragments for ice nucleation in clouds, Scientific Reports, 5, 10.1038/srep08082, 2015.
O’Sullivan, D., Adams, M. P., Tarn, M. D., Harrison, A. D., Vergara-Temprado, J., Porter, G. C. E., Holden, M. A., Sanchez-Marroquin, A., Carotenuto, F., Whale, T. F., McQuaid, J. B., Walshaw, R., Hedges, D. H. P., Burke, I. T., Cui, Z., and Murray, B. J.: Contributions of biogenic material to the atmospheric ice-nucleating particle population in North Western Europe, Scientific Reports, 8, 13821, 10.1038/s41598-018-31981-7, 2018.
Vali, G., Christensen, M., Fresh, R. W., Galyan, E. L., Maki, L. R., and Schnell, R. C.: BIOGENIC ICE NUCLEI .2. BACTERIAL SOURCES, J. Atmos. Sci., 33, 1565-1570, 10.1175/1520-0469(1976)033<1565:binpib>2.0.co;2, 1976.
Vergara-Temprado, J., Murray, B. J., Wilson, T. W., O’Sullivan, D., Browse, J., Pringle, K. J., Ardon-Dryer, K., Bertram, A. K., Burrows, S. M., Ceburnis, D., DeMott, P. J., Mason, R. H., O’Dowd, C. D., Rinaldi, M., and Carslaw, K. S.: Contribution of feldspar and marine organic aerosols to global ice nucleating particle concentrations, Atmos. Chem. Phys., 17, 3637-3658, 10.5194/acp-17-3637-2017, 2017.
Wilson, T. W., Ladino, L. A., Alpert, P. A., Breckels, M. N., Brooks, I. M., Browse, J., Burrows, S. M., Carslaw, K. S., Huffman, J. A., Judd, C., Kilthau, W. P., Mason, R. H., McFiggans, G., Miller, L. A., Najera, J. J., Polishchuk, E., Rae, S., Schiller, C. L., Si, M., Temprado, J. V., Whale, T. F., Wong, J. P. S., Wurl, O., Yakobi-Hancock, J. D., Abbatt, J. P. D., Aller, J. Y., Bertram, A. K., Knopf, D. A., and Murray, B. J.: A marine biogenic source of atmospheric ice-nucleating particles, Nature, 525, 234-238, 10.1038/nature14986, 2015.