A New Model for Magnesium Chemistry in the Upper Atmosphere

Abstract

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(2)MgO(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.

Global investigation of the Mg atom and ion layers using SCIAMACHY/Envisat observations between 70 and 150 km altitude and WACCM-Mg model results

“…Here we develop a global model of meteoric magnesium in the atmosphere by combining three components: a treatment of the injection of meteoric constituents into the atmosphere (Feng et al, 2013;Marsh et al, 2013a;Plane et al, 2014), a description of the neutral and ion-molecule chemistry of magnesium in the mesosphere and lower thermosphere Plane and Whalley, 2012) and WACCM. The Mg meteoroid injection function (MIF) has a similar seasonal variability as those of Fe and Na.…”

“…This is reduced back to Mg via MgOH by reaction with H atoms (R12a). The subsequent reaction with H atoms (R12b) is most likely faster than R12a, and so the reaction rate is not explicitly listed in Table 3 ( Plane and Whalley, 2012). The polymerization of MgO 2 H 2 to form meteoric smoke is parametrized by a dimerization rate coefficient (reaction R13 in Table 3).…”

“…In a dynamical equilibrium of meteoroid input and the abovementioned loss reactions, the neutral Mg layer is formed between 85 and 90 km altitude and the Mg + layer is formed slightly above the Mg layer (see, e.g. Plane and Helmer, 1995;McNeil et al, 1998 andWhalley, 2012).…”

“…WACCM-Mg combines different models: a Meteor Input Function model is coupled to a meteor differential ablation model, which provides the metal input in the MLT. The metal release into the upper atmosphere is then used as one input to simulations using the Whole Atmosphere Community Climate Model (WACCM) (Marsh et al, 2013b) including a new chemistry model of Mg and Mg + (Plane and Whalley, 2012). WACCM is a general circulation model that incorporates interactive chemistry for both neutral and ion species.…”

In situ observations of meteor smoke particles (MSP) during the Geminids 2010: constraints on MSP size, work function and composition

“…However, in the upper mesosphere there is a large excess of H 2 O over these meteoric constituents (by a factor of ∼10 4 ). Reactions involving H 2 O and H atoms (produced by H 2 O photolysis) are, therefore, likely to convert metal oxides into hydroxides such as FeOH (Plane and Whalley, 2012;Self and Plane, 2003). Furthermore, theory indicates that SiO 2 should hydrolyse to form OSi(OH) 2 and Si(OH) 4 , and that these hydrated forms are unreactive towards Fe and Mg compounds.…”