title: Internal composition of atmospheric dust particles from focused ion-beam scanning electron microscopy.
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.
authors.affiliation: Materials Measurement Science Division, National Institute of Standards and Technology (NIST) , Gaithersburg, Maryland 20899-8372, United States. joseph.conny@nist.gov
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Conny
mesh_terms: D001272:Atmosphere; D004391:Dust; D008855:Microscopy, Electron, Scanning / Q000379:methods*
chemical_list: D004391:Dust