Abstract
Abstract. Nano-sized meteoric smoke particles (MSPs) with iron-magnesium silicate compositions, formed in the upper mesosphere as a result of meteoric ablation, may remove sulphuric acid from the gas-phase above 40 km and may also affect the composition and behaviour of supercooled H2SO4-H2O droplets in the global stratospheric aerosol (Junge) layer. This study describes a time-resolved spectroscopic analysis of the evolution of the ferric (Fe3+) ion originating from amorphous ferrous (Fe2+)-based silicate powders dissolved in varying Wt % sulphuric acid (30–75 %) solutions over a temperature range of 223–295 K. Complete dissolution of the particles was observed under all conditions. The first-order rate coefficient for dissolution decreases at higher Wt % and lower temperature, which is consistent with the increased solution viscosity limiting diffusion of H2SO4 to the particle surfaces. Dissolution under stratospheric conditions should take less than a week, and is much faster than the dissolution of crystalline Fe2+ compounds. The chemistry climate model UMSLIMCAT (based on the UKMO Unified Model) was then used to study the transport of MSPs through the middle atmosphere. A series of model experiments were performed with different uptake coefficients. Setting the concentration of 1.5 nm radius MSPs at 80 km to 3000 cm−3 (based on rocket-borne charged particle measurements), the model matches the reported Wt % Fe values of 0.5–1.0 in Junge layer sulphate particles, and the MSP optical extinction between 40 and 75 km measured by a satellite-borne spectrometer, if the global meteoric input rate is about 20 tonnes per day. The model indicates that an uptake coefficient ≥0.01 is required to account for the observed two orders of magnitude depletion of H2SO4 vapour above 40 km.
Highlights
The acidic weathering of ferrous (Fe2+)-based silicate minerals such as olivines (Mg2xFe2−2xSiO4) and pyroxenes (MgxFe1−xSiO3), where 0≤x≤1, is an important pathway to the formation of secondary minerals on Earth (Loughnan, 1969; White, 1995)
The data shows the increase in absorbance for peaks at 289 nm and 220 nm from the first spectrum taken
For the most extreme conditions amenable to our laboratory analysis (75 Wt % acid/223 K), corresponding to the midlatitude stratosphere, it is predicted that meteoric smoke particles will fully dissolve in the stratospheric aerosol layer within a week
Summary
The acidic weathering of ferrous (Fe2+)-based silicate minerals such as olivines (Mg2xFe2−2xSiO4) and pyroxenes (MgxFe1−xSiO3), where 0≤x≤1, is an important pathway to the formation of secondary minerals on Earth (Loughnan, 1969; White, 1995). The weathering process is complex and involves dissolution and oxidation steps, but studies indicate that the Fe-rich olivine/pyroxene structures are more soluble at low temperatures and weather faster than Fe-poor or non-Fe minerals (Siever and Woodford, 1979). Amorphous silicate structures are the leading candidates for meteoric smoke particles or MSPs (Hervig et al, 2009; Saunders and Plane, 2011). These are nanoparticles formed by the recondensation of primarily Fe, Mg and Si oxide species resulting from the ablation of meteoroids passing through planetary upper atmospheres (Plane, 2003). The first optical detection (at 1.037 μm) of MSPs between ∼40 and 80 km by Published by Copernicus Publications on behalf of the European Geosciences Union
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