Nucleation of Atmospheric Aerosols
Professor Andrew Ellis (Department of Chemistry)
Dr Stephen Ball (Department of Chemistry)
- 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.
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.
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-bymolecule. 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.
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