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Magnesium Chemistry in the Mesosphere/Lower Thermosphere (MLT) |
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Introduction Each day approximately 120 tonnes of extra terrestrial material is deposited into the Earth’s upper atmosphere.1 This material undergoes frictional heating when it collides with atmospheric molecules causing metallic species to ablate from the meteoroids surface. The ablated neutral atoms undergo charge transfer with the dominant ionic species in the upper mesosphere/lower thermosphere (MLT), those being O2+ and NO+, producing singly-charged metallic species. These neutral and ionic metal atoms then form metallic layers in the MLT at altitudes between 75 and 110 km. There are many metallic species in the MLT, with the two most abundant being Fe+ and Mg+. The lidar (laser radar) technique has been used extensively to observe these metallic layers, with studies being done on Na, K, Li, Fe, Ca and Ca+.2 However neither Mg nor Mg+ can be observed using lidar as their resonance transitions are in the UV region, below 300 nm, which is strongly absorbed by stratospheric O3. This has led to the study of magnesium relying on rocket-borne mass spectrometers,3, 4 and more recently satellite observations.3, 5, 6 These observations have provided an insight into magnesium in the MLT and highlighted a few interesting characteristics. A striking difference between magnesium and other meteoric metals is the large ratio between Mg+ and Mg. This ratio has been observed to be as low as 3.37 and as high as 228 and 30.9 Magnesium seems to be the only metal displaying such a large ion/neutral ratio. Na+/Na and Fe+/Fe display ratios of ~ 0.23, 4 and Ca+/Ca has a ratio of ~ 2.10 This is even more striking as Mg+ is not significantly depleted relative to other metals in the MLT. The Na+/Mg+ ratio is ~ 0.1 (cosmic abundance 0.05) and the Fe+/Mg+ ratio is ~ 1 (cosmic abundance 0.8).3 Therefore the depletion of Mg is unlikely to be caused by differential ablation. One explanation is that there are substantial differences between the atmospheric chemistries of Mg, Na and Fe. For example the very large Mg+/Mg ratio, compared to other metals, could indicate that Mg+ ions have a long lifetime or that Mg is efficiently removed into a permanent sink below 90 km. Below is a proposed reaction scheme for atmospheric magnesium. |
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Nonetheless observations alone cannot provide the answers for this peculiar behaviour and this is where laboratory
work is needed. Current work uses a pulsed-laser photolysis / laser-induced fluorescence (PLP/LIF) system to
measure the rate coefficients for the reactions below.
Mg+ + N2O → MgO+ + N2 |
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PLP/LIF apparatus
Mg+ is produced from a precursor, Mg(AcAc)2, using a pulsed ArF excimer laser operating at 193 nm. The Mg+ ions are probed at 279.6 nm using a frequency-doubled nitrogen-pumped dye laser. The reaction takes place in a chamber, as shown in the diagram below. The apparatus has two pairs of orthogonal, horizontal side-arms and a vertical sidearm on the top. Mg(AcAc)2 is heated in one of the side-arms, then the vapour is entrained in He bath gas and carried to the reaction chamber where it is photolysed by the excimer laser. The reaction mixture also enters the chamber via several of the side-arms. A photomultiplier tube situated on top of the vertical side-arm measures the LIF signal and from these decays a pseudo first-order rate coefficient can be obtained. |
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References
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For information related to this project Go to > Mesosphere > Meteor Smoke > Ablation |
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