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The CERN CLOUD experiment: Measurements and modelling to understand aerosol effects on climate

Prof Ken Carslaw (SEE), Dr Kirsty Pringle (SEE), Dr Hamish Gordon (SEE)

Contact email: k.s.carslaw@leeds.ac.uk

Aerosol particles play a major role in Earth’s climate system by scattering and absorbing solar radiation and by influencing the behaviour of clouds. We now have fairly advanced computer models of aerosols on a global scale, which enable estimates to be made of their effect on climate. Nevertheless, the climatic effect of changes in aerosols (the so-called aerosol radiative forcing) remains the largest source of uncertainty in simulating historical climate change over the industrial period (IPCC, 2013). To improve these models we need to develop more advanced treatments of the chemical and physical processes.

This PhD will obtain new aerosol measurements from the unique CERN CLOUD chamber experiment and use them to improve the aerosol processes in a global climate. Model simulations will then be performed to understand the impact on climate change. 

The CERN CLOUD experiment was established to make fundamental measurements of poorly understood aerosol processes under extremely well controlled laboratory conditions. Since beginning operation in 2009, the CLOUD experiment has made a string of fundamental discoveries related to the formation of new aerosol particles in the atmosphere. These new processes have been implemented in our global aerosol model and led to important new understanding about how aerosols behave and how they influence climate (Riccobono et al., 2014; Gordon et al., 2016). The project has an ambitious plan for future experiments that will help us to understand how human and natural processes influence aerosols, clouds and climate.

The CLOUD experiment recreates the atmosphere inside a large chamber and measures processes ranging from the nucleation and growth of aerosols to their activation into cloud droplets. A comprehensive array of about forty advanced instruments, including particle counters, gas analysers, and mass spectrometers, are used to analyse the gases and aerosols. CLOUD is the most advanced facility in the world for such research, and has so far supported over twenty international PhD students.

CLOUD experiments are performed as intensive ‘campaigns’ in which about fifty scientists work intensively for a few weeks – much like other CERN experiments. Campaigns are followed by a lengthy period of data analysis, interpretation and, ultimately, computer modelling to explore the effects of new discoveries in atmosphere. The Leeds climate modelling group works very closely with the experimental team, and students have always participated in the campaigns so that they can gain a deep understanding of the measured data.  

For more information about the experiment, please watch the CERN videos (links below).


Figure 1: The CLOUD chamber at CERN.

Your research will involve the development, testing and analysis of some leading-edge models of the atmosphere and climate. We developed the aerosol model GLOMAP (google it), which is also now implemented in the UK’s climate and Earth system model (HadGEM and UKESM). We also run very high resolution models that can resolve individual clouds (Figure 2), which will allow the aerosol processes to be studied in great detail. A good example of a recent modelling study based on CLOUD data is Gordon et al. (2016). In that paper we showed that the discovery of new particle formation solely from gases emitted by trees and vegetation significantly affects climate.


Figure 2: Simulated cloud field over the Pacific Ocean using the Met Office Unified Model at 1 km resolution. The shading shows the outgoing longwave radiation.

Objectives

The direction of the PhD will depend on the research plan at CERN as well as the interests and experience of the student. The CLOUD research programme is defined on an annual basis. There have been ten previous intensive campaigns, which have explored new particle formation and growth throughout the atmosphere, the role of galactic cosmic rays in new particle formation, the influence of biogenic organic compounds, and the chemical processes inside cloud droplets (see key publications below).

Experiments in future will be equally wide-ranging and will always be targeted towards understanding real processes in the atmosphere and building more realistic models. One major challenge in future will be to explore aerosol processes in the atmosphere at much smaller scales than in the past, such as how aerosols are formed in complex cloudy environments. We also aim to understand how air pollutants influence aerosol formation, especially when mixed with natural gases emitted by the biosphere.  The PhD project will involve:

  1. Active participation in CLOUD campaigns, gaining experience of advanced instrumentation, data analysis and interpretation.

  2. Working with experimental and analysis teams to develop physical and chemical mechanisms suitable for inclusion in global climate models.

  3. Developing and running sophisticated atmospheric models to understand how processes discovered in the experiment influence the atmosphere and climate. There is the opportunity to use results from model simulations to feed back into the design of future CLOUD experiments.

Training and supervision

The principal supervisor, Ken Carslaw, is a Professor of atmospheric science and currently Director of the Institute for Climate and Atmospheric Science. He has published over 160 papers on aerosols, clouds and climate and won the Royal Society Wolfson Merit Award. The Carslaw group developed the Global Model of Aerosol Processes (GLOMAP) (originally through a PhD studentship), which will be used in this PhD project. GLOMAP-based research has supported twelve highly successful students at Leeds, with three going on to academic positions in the UK and winning prestigious career prizes such as the Leverhulme Prize and the George Walker Award. We put great emphasis on career development and excellent training of PhD students, with GLOMAP students at Leeds winning a School publication prize every year for the last 5 years.

You will join the vibrant Atmospheric Composition research group of 6 Academic Staff and one of 50 PhD students across the Institute for Climate and Atmospheric Science (ICAS) with extensive programmes in observations, modelling and lab studies. You will be embedded within a highly diverse aerosol group, with research interests spanning uncertainty quantification, global nucleation, volcanic impacts, ice nucleation, dust, natural aerosol processes and aerosol-cloud interaction.

The co-supervisors are Dr Kirsty Pringle (a member of the Institute’s Centre of Excellence for Modelling the Atmosphere and Climate) and Dr Hamish Gordon (a postdoctoral scientist, formerly a CERN research fellow). Dr Pringle has extensive experience of supporting PhD students in learning to run atmospheric models. Dr Gordon has long experience of working in CLOUD, including experimental work and modelling.

Student profile

The PhD is well suited to students with a degree in chemistry or physics who want to pursue research at the frontier of physical-chemical science that is highly relevant to air quality and climate. It would also be suitable for students with a maths or computing-based degree who are keen to apply their knowledge in an exciting field of research. With such a background the PhD might focus more on the simulation of complex atmospheric processes, while still benefiting from close interaction with the research teams at CERN.

Further information and publications

VIDEO: The CERN experiment: How it works: https://cds.cern.ch/record/2154271

VIDEO: Discovery of pure biogenic nucleation: https://cds.cern.ch/record/2155296

Gordon et al. (2016) Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation, Proceedings of the National Academy of Sciences.

Troestl et al. (2016) The role of low-volatility organic compounds in initial particle growth in the atmosphere, Nature, 533, doi: 10.1038/nature18271.

Kirkby et al. (2016) Ion-induced nucleation of pure biogenic particles, Nature, 533, pp.521-526. doi: 10.1038/nature17953

Riccobono et al. (2014) Oxidation Products of Biogenic Emissions Contribute to Nucleation of

Atmospheric Particles, Science, 344, doi: 10.1126/science.1243527

Almeida et al. (2013) Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere, Nature, 502, doi: 10.1038/nature12663

Kirkby et al. (2011) Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation, Nature, 476, pp.429-435. doi: 10.1038/nature10343

Related undergraduate subjects:

  • Chemistry
  • Computing
  • Physics