For the first time, Lawrence Livermore researchers and international collaborators have peered into the makeup of complex airborne particulate matter so small that it can be transported into human lungs -- usually without a trace.
The structure of micron-size particulate matter is important in a wide range of fields from toxicology to climate science (tobacco smoke and oil smoke particles are typically one micron in size).

However, its properties are surprisingly difficult to measure in their native environment: electron microscopy requires the collection of particles on a substrate, visible light scattering provides insufficient resolution, and X-ray studies have, to date, been limited to a collection of particles.

But new research, using intense coherent X-ray pulses from the Linac Coherent Light Source free-electron laser at Stanford, demonstrates a new in situ fractal method for imaging individual sub-micron particles to nanometer resolution in their native environment.

Complex airborne particulate matter (PM) with a diameter less than 2.5 microns can efficiently transport into the human lungs and constitutes the second most important contribution to global warming. Amongst this PM, the structure and composition of carbonaceous soot has been extensively studied:

Pulsed X-ray beams were shot into a jet of aerosolized particles. Since the beam is so small and the particulate matter density is so large, only single particles were hit. The beams were so intense that diffraction from individual particles could be measured for structural analysis. Mass spectrometry on the ejected ion fragments was used to simultaneously probe the composition of single aerosol particles.

"Our results show the extent of internal symmetry of individual soot particles and the surprisingly large variations in their fractal dimensions," said Stefan Hau-Riege, one of the three Lawrence Livermore authors of the paper. "More broadly, our methods can be extended to resolve both static and dynamic structures of general ensembles of disordered particles."

Having a grasp on the general structure has wide implications ranging from solvent accessibilities in proteins, vibrational energy transfer via the hydrodynamic interaction of amino acids, and large-scale production of nanoscale structures via flame synthesis.