Science & aerosols
How advances in spectroscopy may change climate science
When you look at the spectrum of sunlight, you see a broad rainbow of colours over which are superimposed many fine dark bands. These correspond with the particular wavelengths of light that are absorbed when electrons transition from one energy level to another within the atoms and molecules of the sun’s atmosphere. The fact that these lines are quite sharp is actually one of the easiest ways to verify that we do indeed live in a quantum universe, in which an atom’s electrons are only allowed to occupy certain energy levels and not those in between.
Information gathered from this kind of spectroscopy has been of enormous importance in understanding both the fundamental structure of matter and how chemical reactions work at a nuts and bolts level. However, in spite of its usefulness, absorption spectroscopy does have its limitations. It’s especially hard to do on very diffuse and lightly absorbing materials such as the low pressure gasses of relevance to atmospheric physics.
Since the late 1970s, scientists have also been using a slightly different approach to spectroscopy, illuminating a gas sample with a tuneable laser and studying the electrons that are liberated from atoms and molecules as the wavelength of the light is changed – so called photoelectron spectroscopy. This technique is particularly effective when used with negative ions where it’s called photodetachment spectroscopy. In this case the extra electron becomes detached from the atom or molecule when the laser light hits, creating a neutral and a free electron. Because the behaviour of the electron is inherently linked to the atom or molecule it was originally sited on, measurements made on such an electron can yield information not only about the original ion but also the remaining neutral too.
One of the advantages of this technique is that it’s sensitive. With conventional absorption spectroscopy if you hit a gas cell with a million photons and three are absorbed the change in intensity from 100% to 99.9997% is very hard to see. However, using photodetachment spectroscopy, the three resulting electrons are, with modern detectors, relatively easy to see.
Photodetachment spectroscopy is also great because whilst it enables you to see the same information as conventional absorption spectroscopy, it also probes additional electron transitions that can occur as these excited electrons fall between finely spaced upper energy levels in atoms and molecules – something completely invisible in a conventional absorption spectrum.
To extract all the information from such an interaction between light and matter, one would ideally like to know not only the number of electrons released when the laser hits, but also their energy and direction. Early photodetachment spectroscopy systems could only achieve that by essentially swinging the detector assembly around the sample. This was very time consuming and also could lead to contamination of the results caused by mechanical errors and residual electric and magnetic fields associated with the large metallic detector assembly physically moving about.
However, a team of scientists at the Australian National University have recently developed a photodetachment spectroscopy system that is by far the most sensitive, accurate and efficient in the world. The key to success was the design of a special electrostatic lens and detector assembly.
The electrostatic lens is a series of charged cylinders that produce an electric field with a smooth and accurate curvature. Electrons passing through this field are bent in the same way photons of light hitting a curved glass lens are brought to a focus. “Our system builds on the work of many others in the field,” Dr Steve Cavanagh says, “though we’re really proud of the fact that at this point in time, ours is by far the most effective system of its kind in the world.”
The ANU system is also innovative in that it requires no moving parts and unlike many early systems, does not require the gas/laser interaction zone to be microscopic. “We can allow the laser spot to reach a couple of millimetres without compromising our resolution,” Dr Steve Gibson says, “and that’s really important when you’re dealing with low pressure gasses and very low concentrations of the species you’re trying to study.”
But what is it that drives scientists to study the photodetachment process in the first place? “What we’re looking at here is essentially the foundations of the chemical reaction process that drives everything.” Dr Cavanagh explains, “In the process of a chemical reaction there’s often a transitional phase between the reactants and end products, and that can last for as little as a femtosecond. Photodetachment spectroscopy is one of the only ways you can actually study this and if we understand this better, we understand all of chemistry better.”
As with any new scientific tool, like a big new telescope, one of the decisions to make is where do you point it? What’s the most important thing to study at this point in time? In the case of the ANU photodetachment spectrometer, one of the answers is atmospheric chemistry and it’s effect on climate.
It’s been known for some time that negative-ions play a pivotal role in the formation of aerosols within the upper atmosphere. These microscopic clusters of molecules can, depending on their nature, reflect or absorb solar radiation. They also serve as nucleation sites for water vapour droplets creating clouds, which of course also have the ability to reflect sunlight away from the Earth.
“Aerosols, play an enormous role in regulating the amount of solar radiation hitting the surface of the planet.” Dr Gibson says. “And the formation, reaction and dissociation of these aerosols is turning out to be way more complicated than anyone had imagined. Unless you can really understand this process, you’re totally in the dark when it comes to accurate climate modelling.”
One compound of particular interest is sulphur trioxide SO3. SO3 and water combine to create sulphuric acid, the basis for acid rain. Volcanos and human activity, via industry and transport, inject vast amounts of SO2 into the atmosphere which can further oxidise to produce SO3 and of course there’s no shortage of water up there. However, this reaction turns out to be not so straight-forward as was once thought. There is a potential barrier to overcome before a single SO3 and H2O molecule can combine. And over most of the Earth’s atmosphere, there simply isn’t enough free energy to overcome this barrier. However, in the presence of two water molecules, the barrier is lower and with three, lower still. “Given enough water molecules, the barrier effectively disappears. “Dr Cavanagh says, “So we see a very efficient conversion of Sulphur trioxide into sulphuric acid.”
Apart from acid rain, these sulphuric acid molecules are important as they tend to form little clusters with other molecules such as water and ozone which form the basis of aerosols which can serve as nucleation sites for water droplets that will eventually build clouds.
“One of the things that this work has highlighted is the relative stability of negative ion species,” Dr Cavanagh says “Compounds that would be hopelessly unstable as neutrals can often persist as negative ions for long enough for chemical reactions to take place.”
This explains a relatively recent discovery that high concentrations of highly reactive sulphur compounds such as SO3 have been found in the exhaust of jet engines when one would theoretically expect the extreme conditions to dissociate them. “What we think is happening is that the presence of negative ions is substantially increasing the lifetime and hence reactability of many of these compounds,” Dr Cavanagh explains, “And that’s quite worrying because if they’re produced at ground level, they will mostly dissociate by the time they reach the upper atmosphere. But what we’re doing, particularly with modern aircraft, is injecting them directly into the upper atmosphere at altitudes of 10 to 15 kilometres. If aerosols create clouds and clouds reflect sunlight it may not all be bad news, but in effect we’re engineering our own atmosphere before we understand the science properly. And that’s quite concerning given the consequences of humankind’s latest efforts at atmospheric engineering with CO2.”