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Calcium Chemistry in the Mesosphere/Lower Thermosphere (MLT) |
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In the upper mesosphere/lower thermosphere (MLT) thin layers of metal atoms are present between 80 and 105 km due to the ablation of
interplanetary dust particles (meteoroids) entering the Earth's atmosphere. These metal atoms have been observed by ground-based
lidar and the calcium layer has been found to have some unexpected properties1. Ca is extremely depleted compared to Na
(by a factor of 100-300) which is surprising since these elements have similar abundances in most meteoritic minerals. Also, the Ca
abundance peaks in the summer which is opposite to the layers of Na and Fe, and the abundance ratio of Ca+ ions to Ca is
about 2, whereas the Na+/Na and Fe+/Fe ratios are between 0.1 and 0.2. Furthermore, calcium, along with the
other meteoric metals, is of interest since metals in the atmosphere are involved in various phenomena such as sporadic E layers,
sporadic metal layers and meteor smoke and it has been suggested that metals are also connected to noctilucent clouds and the Junge
layer in the stratosphere2. Since only Ca and Ca+ can be observed directly in the atmosphere using lidar, it is necessary to use laboratory studies and modelling to understand the chemistry controlling the Ca layer. The two experimental techniques used to measure the relevant rate coefficients in our laboratory are pulsed laser photolysis / laser induced fluorescence (PLP/LIF) and a fast flow tube. |
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Calcium Ion Chemistry |
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PLP/LIF Apparatus4 In this technique Ca+ is produced from a precursor, Ca(TMHD)2, using a pulsed ArF excimer laser operating at 193 nm. The Ca+ ions are probed at 393.4 nm using a frequency-tripled Nd-YAG pumped dye laser. The reaction takes place in a chamber, as shown in the diagram below, which has two pairs of orthogonal, horizontal side-arms and a vertical sidearm on the top. Ca(TMHD)2 is heated in one of the side-arms, then the vapour is entrained in He bath gas and carried to the reaction chamber. The reaction mixture also enters the chamber via several of the side-arms. A photomultiplier tube attached to the vertical side-arm measures the LIF signal, from which the pseudo-first order rate coefficient is determined. The following reactions have been measured using this technique4,5.
Ca+ + N2O → CaO+ + N2 |
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Fast flow tube4 In this technique Ca+ is produced by the pulsed ablation of calcite by an Nd-YAG laser. The Ca+ ions flow down the tube entrained in a He bath gas and the reactants are added further downstream. Either the loss of Ca+ or production of product ions is monitored with a quadrupole mass spectrometer. This technique has the advantage that successive reactants can be added along the tube, allowing a wider range of reactions to be studied than in the PLP/LIF apparatus. The following ligand-switching reactions have been studied so far:
Ca.CO2+ + H2O → Ca.H2O+ + CO2 For these reactions the rate coefficients are determined using a computer model of the flow tube, which accounts for gas-phase chemistry and diffusion and loss to the walls. The reactions of CaO+ with CO and O and the reaction between CaO2+ and O have also been studied using this method. |
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Calcium Neutral Chemistry |
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| Ca atoms are produced by the vaporisation of Ca pellets in a high temperature oven and are detected using LIF at the downstream end of the flow tube at 422.7 nm. CaO is also monitored by off-resonance LIF (pumped at 385.9 nm, detected at (λ > 693 nm). The first reactions to be studied using this method are:
CaO + O → Ca + O2 |
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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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