A laboratory study of meteor smoke analogues: Composition, optical properties and growth kinetics

Meteoric smoke forms in the mesosphere from the recondensation of the metallic species and silica produced by meteoric ablation. A photochemical flow reactor was used to generate meteoric smoke mimics using appropriate photolytic precursors of Fe and Si atoms in an excess of oxidant. The following systems were studied: (i) Fe+O3/O2, (ii) Fe+O3/O2+H2O, (iii) Fe+Si/SiO+O3/O2 and (iv) Si/SiO+O3/O2. The resulting nano-particles were captured for imaging by transmission electron microscopy, combined with elemental analysis using X-ray (EDX) and electron energy loss (EELS) techniques. These systems generated particle compositions consistent with: (i) Fe2O3 (hematite), (ii) FeOOH (goethite), (iii) Fe2SiO4 (fayalite) and (iv) SiO2 (silica). Electron diffraction revealed that the Fe-containing particles were entirely amorphous, while the SiO2 particles displayed some degree of crystallinity. The Fe-containing particles formed fractal aggregates with chain-like morphologies, whereas the SiO2 particles were predominantly spherical and compact in appearance. The optical extinction spectra of the Fe-containing particles were measured from 300 nm<λ<650 nm. Excellent agreement was found with the extinction calculated from Mie theory using the refractive indices for the bulk compounds, and assuming that the fractal aggregates are composed of poly-disperse distributions of constituent particles with radii ranging from ∼5 to 100 nm. These sizes were confirmed from measurements of the particle size distributions and microscopic imaging. Finally, the particle growth kinetics of the Fe-containing systems exhibit unexpectedly rapid agglomerative coagulation. This was modelled by assuming an initial period of coalescent particle growth resulting from diffusional (Brownian) coagulation to form primary particles; further growth of these particles is then dominated by long-range magnetic dipole–dipole interactions, leading to the fractal aggregates observed. The atmospheric implications of this work are then discussed.