|
Figure 4 shows Transmission electron
microscopy (TEM) images of micron-size particle aggregates produced from
three distinct chemical systems of relevance to the Earth's upper
atmosphere. Clearly, despite having different compositions (as identified
from x-ray analysis), the particles share certain features i.e. the
aggregates are comprised of large numbers of smaller particles and they
have extended, fractal-like morphologies [Saunders and Plane, 2006a]. These
observations hint at a likely common particle growth mechanism for such
Fe-containing aerosol systems (see modelling
section).
Figure 4 TEM images of fractal particle aggregates of laboratory
generated meteoric smoke mimics with compositions identified as (i) Fe2O3,
(ii) FeOOH and (iii) Fe2SiO4 - Figure taken with
permission from Saunders and Plane, 2006a.
For each system, the particles were generated by the photo-oxidation of
precursor gases in a laminar flow reactor; iron pentacarbonyl [Fe(CO)5]
for the production of gas-phase Fe atoms and tetraethyl orthosilicate
[TEOS] for the production of Si atoms and / or SiO molecules. Subsequent
gas-phase oxidation and polymerisation reactions then lead to condensable
species which undergo homogeneous nucleation to the aerosol phase (see
Figure 5).
Figure 5 Possible gas-phase reaction pathways leading to the formation
of meteoric smoke mimics with end compositions as identified in TEM
analysis. Proposed polymerisation reactions leading to structural re-arrangement
in the solid-phase to form the minerals hematite (Fe2O3),
goethite (FeOOH) and fayalite (Fe2SiO4) are indicated
by dashed arrows - Figure taken with permission from Saunders and Plane,
2006a.
Particle sampling onto TEM grids allowed for subsequent imaging and
analysis as described above. Time-resolved particle size distributions were
measured using a scanning mobility particle sizer (SMPS) instrument. Also,
generated particles were passed into a multi-pass (White) absorption cell
and the particle light extinction spectra recorded [Saunders and Plane,
2006c].
|
