“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.” CFEL is a joint venture of Deutsches Elektronen-Synchrotron DESY, the German Max Planck Society and the University of Hamburg.
Aerosol particles like soot play important roles in a wide range of fields from toxicology to climate science. Despite their importance, their properties are surprisingly 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.
Using the world’s most powerful X-ray laser LCLS at the U.S. SLAC National Accelerator Laboratory in Stanford (California), the team captured images of single 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 scientific journal Nature. Free-electron lasers like LCLS or the European XFEL currently being built in Hamburg consist of particle accelerators that send unbound (free) electrons on a tight slalom course where they emit X-ray light.
The study focused on particles less than 2.5 micrometres in diameter. This is the size range of 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 team recorded patterns from 174 individual soot particles and 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.” Also, the structure of the airborne soot seems to be 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.