Abstract
Abstract. Accurate determination of the optical properties of analogues for meteoric smoke particles (MSPs), which are thought to be composed of iron-rich oxides or silicates, is important for their observation and characterization in the atmosphere. In this study, a photochemical aerosol flow system (PAFS) has been used to measure the optical extinction of iron oxide MSP analogues in the wavelength range 325–675 nm. The particles were made photochemically and agglomerate into fractal-like particles with sizes on the order of 100 nm. Analysis using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) suggested the particles were most likely maghemite-like (γ-Fe2O3) in composition, though a magnetite-like composition could not be completely ruled out. Assuming a maghemite-like composition, the optical extinction coefficients measured using the PAFS were combined with maghemite absorption coefficients measured using a complementary experimental system (the MICE-TRAPS) to derive complex refractive indices that reproduce both the measured absorption and extinction.
Highlights
The ablation of cosmic material in the mesosphere leads to the formation of nanometre-sized meteoric smoke particles (MSPs) (Plane et al, 2015)
Wavelength-dependent complex refractive indices (RIs) have been derived for iron oxide meteoric smoke analogues generated under atmospherically relevant conditions using two different experimental systems
Analysis of particles collected from both experiments suggested a maghemite-like composition to be most likely, for the particles produced in the photochemical aerosol flow system (PAFS), a magnetite-like composition could not be definitively ruled out
Summary
The ablation of cosmic material in the mesosphere leads to the formation of nanometre-sized meteoric smoke particles (MSPs) (Plane et al, 2015). In terms of the composition, at present only two types of investigations exist: rocket-borne instruments (e.g. Faraday cup detectors and electric work-function studies) and remote sensing (e.g. optical spectroscopy). One example is the ECOMA (Existence and Charge state Of Meteoric smoke particles in the middle Atmosphere) project (Rapp et al, 2010). This work constrained the MSP size and work function, with electronic structure calculations inferring a likely MSP composition of Fe and Mg hydroxide clusters with low silica content (Rapp et al, 2012)
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