Soot particles are typically only a micron in size. |
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. The research appears in the
June 28 issue of the journal, Nature.
Complex
airborne particulate matter (PM) with a diameter less than 2.5
micrometers 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.
Other
Livermore researchers include Matthias Frank, Mark Hunter, George
Farquar and W. Henry Benner. Other collaborators include: SLAC National
Accelerator Laboratory; Center for Free-Electron Laser Science, DESY;
Max-Planck-Institut fur medizinische Forschung; Max Planck Advanced
Study Group, Center for Free Electron Laser Science (CFEL);
Max-Planck-Institut fur Kernphysik, Saupfercheckweg; PNSensor GmbH,
Otto-Hahn-Ring; Max-Planck-Institut Halbleiterlabor, Otto-Hahn-Ring;
Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstrasse;
Sincrotrone Trieste, Microscopy Section; Advanced Light Source, Lawrence
Berkeley National Laboratory; Laboratory of Molecular Biophysics,
Department of Cell and Molecular Biology, Uppsala University; Cornell
University, Division of Nutritional Sciences; Photon Science, DESY;
National Energy Research Scientific Computing Center (NERSC); University
of Hamburg; and European XFEL GmbH, Albert-Einstein-Ring.
