Overview

Project Highlights:

  • Entirely new way of exploring how aerosol particles begin to nucleate from their molecular constituents
  • Will reveal the type and evolution of the network of physical & chemical interactions that lead to a sustainable incipient aerosol particle
  • Will provide baseline data essential for refining numerical models of aerosol formation rates

 

Overview

Aerosol particles are important constituents of the Earth’s atmosphere. They play a vital role in the lower atmosphere by affecting the Earth’s radiation balance through the scattering of solar radiation. They also have other highly significant consequences, including detrimental effects on human health. Secondary aerosol particles form by condensation of molecules in the atmosphere. However, the mechanism of this process is very poorly understood and is one of the fundamental unknowns in atmospheric chemistry.

Sulfuric acid has been identified as a critical component but neither it, nor its combination with water, is sufficient to explain the rate of new particle nucleation. At least one additional component is necessary and small organic bases, such as methylamine and dimethylamine, have recently been suggested as prime suspects. To understand how small organic amines might combine with sulfuric acid to initiate the growth of aerosols, it is critical to know how these molecules interact with each other at the molecular level and how this interaction develops into a sustainable intermolecular network, as found in an aerosol particle. This is the target of this studentship. The molecular ingredients will be brought together, molecule-by-molecule, and their interactions will be probed using infrared spectroscopy. The focus will be on the interaction between sulfuric acid molecules and the small amines likely to be significant in the atmosphere, methylamine and dimethylamine. Water molecules can also be added to establish the importance of ternary nucleation.

We will use a novel approach in which molecules are combined inside nanodroplets of liquid helium. As well as providing a nanoscale trap, the helium acts as the transmitter of spectroscopic signals: helium atoms evaporate every time spectroscopic absorption takes place by the molecules inside the nanodroplet, and this loss of helium from the droplet provides our signal. We are international leaders in this technique but it has not previously been used to address problems in environmental science. The student will deliver new information on the fundamental mechanism of aerosol formation. These data are critical in fixing parameters in numerical models of aerosol formation rates, which will in turn feed into global climate models.

Figure 1: Experimental arrangement showing helium droplets source and spectroscopic apparatus.

Methodology

Superfluid helium nanodroplets provide an inert, cold and gentle environment in which to trap and assemble molecules into larger structures. This makes it possible to follow an incipient nucleation event, molecule-by-molecule. Infrared spectra will be recorded via a signal depletion technique which is well established in our laboratory. This technique combines IR absorption with mass spectrometry and enables us to use mass-selective detection scheme to extract size-specific information. The spectra will be interpreted using quantum-based simulation techniques.

The principal experimental challenge is to add a gas delivery and pickup cell system to our existing apparatus which can cope with the corrosive effects of sulfuric acid vapour. This will require some modification of the gas delivery lines, the pickup cell design, and the independent pumping of the sulfuric acid pickup cell to minimise leaks into the main vacuum system.

Training and Skills

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

The student will acquire an array of experimental and computational skills from this specific project. Experimentally, they will be trained to work with high vacuum systems, cryogenic techniques, mass spectrometry, lasers and laser optics, and various aspects of data acquisition. The student will also learn aspects of equipment design, since it will be necessary to make modification to the apparatus, as detailed in the previous section.

Alongside the experimental work the student will be trained to operate the high level molecular simulation software necessary to interpret the spectroscopic information they will gather.  

Timeline

Year 1: Laser safety training; training in use of helium nanodroplet experiments; training in Gaussian software and other ab initio quantum chemical simulation packages, as appropriate; design, construction and fitting of corrosion-resistant pick-up cell.

Year 2: IR study of pure sulfuric acid clusters + ab initio predictions; IR studies of pure methylamine and dimethylamine in helium nanodroplets; data analysis and write initial publications; preliminary studies of mixtures of sulfuric acid/amine molecules.

Year 3: Extensive IR studies of clusters of sulfuric acid combined with amines, with and without water; data analysis + publications; completion of any outstanding experiments; communication of findings to leading scientists in the aerosol science field.

Year 4: putting final touches to data analysis; write thesis and undergo examination

Partners and collaboration (including CASE)

We have received strong support from Andreas Kürten of the Institute for Atmospheric and Environmental Sciences, Goethe-University, Frankfurt, to proceed with these studies. Andreas’s work is directly concerned with the nucleation of new aerosol particles and involves both laboratory studies (e.g. cloud chamber measurements) and field work. You will see high profile examples of that work illustrated in the publication list below. If this studentship is awarded a formal collaboration between our group and the Kürten group will begin.

Further Details

Contact: Professor Andrew M. Ellis, andrew.ellis@le.ac.uk

https://www2.le.ac.uk/departments/chemistry/people/academic-staff/andrew_m_ellis