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Quantifying the reduced reactivity of glassy or ultra-viscous atmospheric aerosol

Prof. Ben Murray (SEE), Dr Johan Mattsson (Physics)

Contact email: b.j.murray@leeds.ac.uk

The composition, evolution and lifetime of atmospheric aerosol particles is of key importance to their impact on our planet’s climate, the chemistry of our atmospheric chemistry and human health. In the past it was typically assumed that most aerosol particles were liquid droplets, but over the last few years it has become increasingly apparent that many organic containing aerosol are in fact ultra-viscous or even glassy and we are still trying to understand the implications of this.

In Leeds we have led the field in understanding the role of aerosol viscosity in cold-cloud formation (Murray et al. Nature Geoscience, 2010) and the interaction of organic aerosol with water vapour (e.g. Price et al., Chemical Science, 2015). In these studies we have shown that at low humidity and low temperature (representative of the atmosphere) water diffusion slows dramatically and also that when these aerosol become so viscous that they behave mechanically like a solid that they can trigger ice formation in clouds.  But, there is still very little information on the reactivity of organic aerosol when in a very viscous or even a glassy state. In principle the reaction rates of organic aerosol should slow right down when aerosol become very viscous which would increase their lifetime in the atmosphere.

There are two regions of the atmosphere where we think that this is important, the troposphere and the stratosphere. As mentioned above, glassy or ultra-viscous aerosol are thought to be important for the formation of ice clouds in the upper troposphere, but organic aerosol originating from the troposphere also seem to persist in the stratosphere (e.g. Murphy et al. QJRMS, 2014). The heterogeneous chemistry of stratospheric aerosol and their ability to nucleate crystals of acid-hydrates as well as water ice might be very important for our understanding of stratospheric ozone.  The stratosphere is very dry (and relatively cold), so organic aerosol are likely to exist in a glassy state, we hypothesise that because they exist in a glassy state, their reactivity is reduced because reactive gases like ozone cannot penetrate into them and their lifetime is therefore extended.

Objectives

In this project, you will experimentally quantify the rate at which organic material reacts with ozone under conditions pertinent to the upper troposphere and lower stratosphere. This will be achieved by:

  1. Learning to use our Raman microscope system which is coupled to a temperature and humidity controlled stage. 
  2. Identifing a model organic solution (or solutions) containing representative organic molecules as well as sulphates which would be representative of aerosol in the upper troposphere and lower stratosphere
  3. Using our already established technique for quantifying diffusion to determine water diffusion in these solutions as a function of RH and T using our Raman isotope tracer technique (Price et al., 2014; Price et al. 2015).
  4. Exposing model aqueous organic solutions to well defined concentrations of ozone.  This will involve generating ozone in a quantitative manner and introducing it to the flow cell while maintaining RH and T.  The Raman microscope will be used to probe the reaction of ozone with the organic solution.  We anticipate the rate of reaction dramatically slowing under conditions where diffusion is very slow, i.e. at low T and RH.

Figure 1. The relative humidity and temperature controlled Raman microscope system.

Figure 2. The cell containing the disk of solution which is probed using the Raman laser system. This solution disk is placed within the RH and T controlled stage.

Potential for high impact outcome

The properties of atmospheric aerosol and their role in cloud formation and atmospheric chemistry are of first order importance. In Leeds we are in a unique and strong position of working at the interface of disciplines.  In this case, the long-standing collaboration between Johan Mattsson in Physics and Ben Murray in SEE allows us to develop a fundamental physical understanding which we can then apply to improve the understanding of the atmosphere. This interdisciplinary approach has worked well in the past and we have a strong record of producing high impact science, with papers in Nature, Nature Geoscience and PNAS. In this project, it is anticipated the student will produce several first-author papers in good journals with potential for a high impact paper.

Training

The student will work under the supervision of Prof. Murray in SEE and Dr Mattsson in Physics and Astronomy and builds on a long standing and successful collaboration. As well as becoming an expert in atmospheric aerosol research, this project provides a high level of specialist scientific training in: (i) the use of complex instrumentation, ii) interpretation and analysis of large amounts of data and iii) use of tools such as IDL or Python coding languages. In addition, there is also potential for training in the use of (and using) the extensive suite of analytical instrumentation in the Cohen Labs. 

Co-supervision will involve regular meetings between all partners and the student will take part in weekly group meetings with Murray’s research team. The student will become part of the Institute for Climate and Atmospheric Science in which they will interact with a broad range of scientists interested in complementary areas, from global aerosol modelling to aircraft measurements of atmospheric composition and polar stratospheric cloud research to the impact of aerosol on clouds.

The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include courses on using programming languages such as Python, through to managing your degree and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

The student should have a strong interest in global environmental problems, a strong background in a quantitative science (chemistry, maths, physics, engineering, environmental sciences) and a flair for experimental work.

References

  • Price, H. C., Mattsson, J., Zhang, Y., Bertram, A. K., Davies, J. F., Grayson, J. W., Martin, S. T., O'Sullivan, D., Reid, J. P., Rickards, A. M. J., and Murray, B. J.: Water diffusion in atmospherically relevant [small alpha]-pinene secondary organic material, Chemical Science, 6, 4876-4883, 2015.
  • Murphy, D. M., Froyd, K. D., Schwarz, J. P., and Wilson, J. C.: Observations of the chemical composition of stratospheric aerosol particles, Q. J. R. Meteorol. Soc., 140, 1269-1278, 2014.
  • Price, H. C., Murray, B. J., Mattsson, J., O'Sullivan, D., Wilson, T. W., Baustian, K. J., and Benning, L. G.: Quantifying water diffusion in high-viscosity and glassy aqueous solutions using a Raman isotope tracer method, Atmos. Chem. Phys., 14, 3817-3830, 2014.
  • Murray, B. J., Wilson, T. W., Dobbie, S., Cui, Z. Q., Al-Jumur, S., Mohler, O., Schnaiter, M., Wagner, R., Benz, S., Niemand, M., Saathoff, H., Ebert, V., Wagner, S., and Karcher, B.: Heterogeneous nucleation of ice particles on glassy aerosols under cirrus conditions, Nature Geosci., 3, 233-237, 2010.

Related undergraduate subjects:

  • Chemistry
  • Engineering
  • Environmental science
  • Mathematics
  • Physics