A new model for magnesium chemistry in the upper atmosphere

摘要

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Mg-containing species, which are important for the chemistry of meteor-ablated magnesium in the upper mesosphere/lower thermosphere region. The study is motivated by the very recent observation of the global atomic Mg layer around 90 km, using satellite-born UV-visible spectroscopy. In the laboratory, Mg atoms were produced thermally in the upstream section of a fast flow tube and then converted to the molecular species MgO, MgO _2, OMgO _2, and MgCO _3 by the addition of appropriate reagents. Atomic O was added further downstream, and Mg was detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined at 300 K: k(MgO + O → Mg + O _2) = (6.2 ± 1.1) × 10 ~(-10); k(MgO _2 + O → MgO + O _2) = (8.4 ± 2.8) × 10 ~(-11); k(MgCO _3 + O → MgO _2 + CO _2) ≥ 4.9 × 10 ~(-12); and k(MgO + CO → Mg + CO _2) = (1.1 ± 0.3) × 10 ~(-11) cm ~3 molecule ~(-1) s ~(-1). Electronic structure calculations of the relevant potential energy surfaces combined with RRKM theory were performed to interpret the experimental results and also to explore the likely reaction pathways that convert MgCO _3 and OMgO _2 into long-lived reservoir species such as Mg(OH) _2. Although no reaction was observed in the laboratory between OMgO _2 and O, this is most likely due to the rapid recombination of O _2 with the product MgO _2 to form the relatively stable O _2MgO _2. Indeed, one significant finding is the role of O _2 in the mesosphere, where it initiates holding cycles by recombining with radical species such as MgO _2 and MgOH. A new atmospheric model was then constructed which combines these results together with recent work on magnesium ion-molecule chemistry. The model is able to reproduce satisfactorily some of the key features of the Mg and Mg ~+ layers, including the heights of the layers, the seasonal variations of their column abundances, and the unusually large Mg ~+/Mg ratio.