Structure Of Floating Soot Particles Seen For The First Time


“For the first time we can actually see the structure of individual aerosol particles floating in air, their ‘native habitat’,” said DESY scientist Henry Chapman from the Center for Free-Electron Laser Science (CFEL) in Hamburg. “This will have important implications for various fields from climate modelling to human health.”

The CFEL is a joint venture of Deutsches Elektronen-Synchrotron DESY, the German Max Planck Society and the University of Hamburg.

Soot, along with other aerosol particles, plays an important part in a variety of different scientific fields, from climate science to medical science. Until now, though, their properties have been very difficult to measure. “Visible light doesn’t provide the necessary resolution, X-ray sources are usually not bright enough to image single particles, and for electron microscopy particles have to be collected onto a substrate, which potentially alters their structure and encourages agglomeration.”

In order to finally see their structure, researchers used the world’s most powerful X-ray laser, LCLS, at the U.S. SLAC National Accelerator Laboratory in Stanford (California). Using this, the researchers caught images of individual soot particles floating through the laser beam. “We now have a richer imaging tool to explore the connections between their toxicity and internal structure,” said SLAC’s Duane Loh, lead author of the study appearing in this week’s issue of the journal Nature.

LCLS is a free-electron laser like the European XFEL that is currently being built in Hamburg, they consist of “particle accelerators that send unbound (free) electrons on a tight slalom course where they emit X-ray light.”

The researchers focused on particles less than 2.5 micrometres in diameter, that’s the size range of the particles that “efficiently transport into the human lungs and constitute the second most important contribution to global warming.”

“Microscopic soot particles were generated with electric sparks from a graphite block and fed with a carrier gas of argon and nitrogen into a device called an aerodynamic lens, that produces a thin beam of air with entrained soot particles. This aerosol beam intercepted the pulsed laser beam. Whenever an X-ray laser pulse hit a soot particle, it produced a characteristic diffraction pattern that was recorded by a detector. From this pattern, the scientists were able to reconstruct the soot particle’s structure.”

“The structure of soot determines how it scatters light, which is an important part of understanding how the energy of the sun is absorbed by the Earth’s atmosphere. This is a key factor in models of the Earth’s climate,” explained co-author Andrew Martin from DESY. “There also are many links between airborne particles around two micrometres in size and adverse health effects. Using the free-electron laser we are now able to measure the shape and composition of individual airborne particles. This may lead to a better understanding of how these particles interfere with the function of cells in the lungs.”

The researchers recorded patterns from 174 different soot particles and then measured their compactness, using a property called fractal dimension.

“We’ve seen that the fractal dimension is higher than what was thought,” said Chapman. “This means that soot in the air is compact, which has implications for the modelling of climate effects.”

Chapman also notes that the structure of the airborne soot is surprisingly variable. “There is quite some variation in the fractal dimension, which implies that a lot of rearrangement is going on in the air,” explains Chapman.

One of the main long-term goals of the research is to capture images of airborne particles as their size, shape and chemical make-up changes in response to their environment, said Michael Bogan from SLAC, who led the research. “Scientists can now imagine being able to watch the evolution of soot formation in combustion engines from their molecular building blocks, or maybe even view the first steps of ice crystal formation in clouds.”

In every-day settings soot is almost never pure. In order to learn about the effects of mixing with other aerosols, “the researchers added salt spray to the soot particles, resulting in larger particles with soot attached to the tiny salt crystals. Such composite particles might form in coastal cities and are expected to have a much larger climate effect than soot alone. Composite aerosols are more difficult to analyse, but the new technique could clearly discern between soot, salt and mixtures of both. As the aerosol particles are vaporized by the intense X-ray laser pulse, the researchers could use mass spectroscopy to examine the composition of each individual particle imaged.”

“Even though the aerosol particles are destroyed by the X-ray laser pulse, the pulse is so short that it out-runs this destruction. Therefore the diffraction patterns are of high quality and represent the undamaged object. The novel X-ray technique can find wide application to study all sorts of aerosols and can also be extended to resolve the static and dynamic morphology of general ensembles of disordered particles, the researchers state.”

“We are now able to study the structure of soot by measuring individual particles in a large ensemble,” explains Martin. “Biological samples, like cells and large proteins, have a similar size to the soot particles we studied and also lack a fixed, reproducible structure. In the future it may be possible to extend these techniques beyond aerosols, to study the structural variations in biological systems.”

Source: Helmholtz Association of German Research Centers
Image Credits: Duane Loh & Andy Freeberg, SLAC National Accelerator Laboratory; Mike Bogan/SLAC

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