The atmosphere under a microscope

Clusters and aerosol particles are increasingly recognised as central to our understanding of climate and atmospheric issues. Professor Olle Björneholm’s work in developing new sub-molecular perspectives on basic phenomena will allow researchers to look at the fundamental structure of atmospheric systems.

Seen from space the earth is dominated by blue, white and green areas, which are associated with various aqueous systems in the biosphere. The composition of these systems is extremely complex, bringing together condensed water and various dissolved species; studying these systems and their interactions requires sophisticated methods and advanced expertise, an area which forms the primary focus of Professor Olle Björneholm’s research group. “We have developed a number of methods based on synchrotron radiation where we can get information about the microscopic buildup of different materials, which is a very interesting phenomenon in modern technology.

The goal now is to apply these methods to natural systems in order to understand how these processes and environmental systems work in nature, for example in the atmosphere, in water or in soil,” he says. Professor Björneholm is the Principal Investigator of the Clusters and Liquids project, which aims to provide new submolecular perspectives on basic phenomena and processes in the biosphere, work which could help scientists understand the impact of human activity on the environment.

Clusters and aerosol particles

Alongside developing these techniques Professor Björneholm is also applying them in practical examples, and is hopeful that he will soon start to produce aerosol particles in the lab, allowing researchers to study them directly. While these sub-molecular techniques can’t yet be applied to natural substances, they hold the promise of generating new insights into the biosphere. “Naturally occurring water typically contains a lot of molecules and ions; it’s not pure distilled water, there are other things in it.

It could include molecules of acetic acid for instance, which is an acid that is dissolved in vinegar. When we study liquids using sub-molecular perspectives we can see that there is more acetic acid on the outermost surface of the liquid than inside it, in bulk. There are two parts of acetic acid – one is hydrophilic, which means it likes to bind to the water – and one is hydrophobic, which doesn’t like to bind to water. The hydrophilic part sticks in to the water surface, and lots of atoms belong to this part, whereas the other atoms in the molecule stick out of the surface,” explains Professor Björneholm.

This allows scientists to look at the interaction between liquids and surfaces, a key part of Professor Björneholm’s research. “Our techniques are sensitive to the surface, to the outermost molecular layers. We can get information on how the surface of the liquid is different to the bulk of the liquid,” he outlines. The complexity of these systems is such that no experimental technique has yet been developed that is capable of providing a complete picture. Professor Björneholm uses data gathered through several techniques in his research.

“We often collaborate with groups using other techniques and combine information gathered by other authors,” he says. These techniques are being used to look at the surface behaviour of a number of liquids, including water, methanol and ethanol. “If you take a macroscopic amount of water, like you might have in a glass for instance, then the fraction of molecules that are on the surface is extremely small – say one out of several billion molecules are actually at the surface. This doesn’t have a significant effect on the properties of the water,” says Professor Björneholm. “But if you make the water droplets even smaller then the fraction of molecules located at the surface will increase. If we look at a water droplet in the atmosphere in this kind of situation, we find that the surface can actually dominate and determine the properties of the droplet.

This is very interesting for atmospheric scientists, to know that some molecules are ‘hidden’ in the middle of the molecules. In this case I mean hidden in the sense that they don’t participate in surface chemical reactions, or oppositely that they are enhanced at the surface so they are very much available for chemical reactions in the atmosphere.”

This work has real implications for our understanding of how weather is generated, and hence our ability to forecast long-term change. Professor Björneholm’s research group is defining a number of experiments together with atmospheric scientists to look at molecular systems, which could eventually be included in climate models.

“We are modelling liquids and we have also made some ‘real’ atmospheric clusters,” he outlines. One of the key atmospheric systems being looked at is ammonium sulphate, a type of salt. “Atmospheric chemists believe Ammonium sulphate is the most important substance in terms of creating aerosols, and from those aerosols water droplets and clouds are then created,” continues Professor Björneholm. “Sulphur comes from different types of combustion and natural processes, and when it comes up it is oxidised to sulphate. Ammonium itself also comes from natural processes. So these substances occur in the atmosphere and they are very good at binding water to them – they are hygroscopic. This means that they can easily form little water clusters which then grow into droplets, which in turn go on to form what are called cloud condensation nuclei. Water droplets cannot form by themselves in the atmosphere, they need to have something to grow on.”

Surface reactions

A lot of chemical reactions occur in the upper atmosphere, due to factors like ultraviolet radiation from the sun, and the structure of a system is a key factor in determining how it will behave. A molecule on the surface of a liquid can react quickly with surrounding molecules, whereas those in the bulk are unable to do so; including these systems will help scientists improve their understanding of the earth’s radiative balance – essentially incoming radiation compared to outgoing radiation – an important consideration in weather forecasting and climate projections. “A certain amount of energy comes from the sun in the form of solar radiation – light, mostly – of which some hits the earth, while of course an enormous amount misses it.

But of the fraction of solar radiation that hits the earth, a certain amount is reflected and lost again. If you look at pictures of earth from space, a lot of clouds are white and bright, that means they are reflecting more energy,” explains Professor Björneholm. Aerosols also reflect away radiation, and while not as efficiently as clouds, they still have a significant impact. “According to the IPCC the effect of aerosols is the largest unknown factor in terms of predicting the climate,” says Professor Björneholm.

“Research-based estimates suggest they counter the human-induced greenhouse effect by about a third.” Further research is required before aerosols can be fully harnessed to mitigate the impact of the greenhouse effect. Nevertheless, with clusters and aerosol particles recognised as central to our understanding of atmospheric and climate-related issues, this is work which holds enormous relevance in terms of environmental protection. “We can provide a lot of information to atmospheric scientists and definitely contribute to a better understanding of atmospheric change. With a better understanding we have a better chance of doing something about it,” points out Professor Björneholm. While his research group is involved in a collaborative project looking at the role of ammonium sulphates, Professor Björneholm’s future plans centre on developing these techniques further to look at more and more realistic systems. “Of course we can’t simulate the whole atmosphere in the lab, but we can do better than we are doing at the moment. That’s certainly something we are working on,” he says. “Water on nano-surfaces is another hot issue, as well as water splitting – the possibility of splitting water into oxygen and hydrogen on surfaces. If this could be done using solar radiation to produce hydrogen, it could be an energy source.”

For more information, please contact Professor Bjorneholm

Published: Friday, 8th July 2011