Internal Composition of Atmospheric Dust Particles
from Focused Ion-Beam Scanning Electron Microscopy
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Abstract
Use of focused ion-beam scanning
electron microscopy (FIB-SEM)
to investigate the internal composition of atmospheric particles is
demonstrated for assessing particle optical properties. In the FIB-SEM
instrument equipped with an X-ray detector, a gallium-ion beam mills
the particle, while the electron beam images the slice faces and energy-dispersive
X-ray spectroscopy provides element maps of the particle. Differences
in assessments of optical behavior based on FIB-SEM and conventional
SEM were shown for five selected urban dust particles. The benefit
of FIB-SEM for accurately determining the depth and size of optically
important phases within particles was shown. FIB-SEM revealed that
iron oxide grains left undetected by conventional SEM could potentially
shift the single-scattering albedo of the particle from negative to
positive radiative forcing. Analysis of a coke-like particle showed
that 73% of the light-scattering inclusion went undetected with conventional
SEM, causing the bulk absorption coefficient to vary by as much as
25%. Optical property calculations for particles as volume-equivalent
spheres and as spheroids that approximated actual particle shapes
revealed that the largest effect between conventional SEM and FIB-SEM
analyses was on backscattering efficiency, in some cases varying several-fold