Article pubs.acs.org/JPCA
A New Model for Magnesium Chemistry in the Upper Atmosphere John M. C. Plane* and Charlotte L. Whalley School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K. 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, MgO2, OMgO2, and MgCO3 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 + O2) = (6.2 ± 1.1) × 10−10; k(MgO2 + O → MgO + O2) = (8.4 ± 2.8) × 10−11; k(MgCO3 + O → MgO2 + CO2) ≥ 4.9 × 10−12; and k(MgO + CO → Mg + CO2) = (1.1 ± 0.3) × 10−11 cm3 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 MgCO3 and OMgO2 into longlived reservoir species such as Mg(OH)2. Although no reaction was observed in the laboratory between OMgO2 and O, this is most likely due to the rapid recombination of O2 with the product MgO2 to form the relatively stable O2MgO2. Indeed, one significant finding is the role of O2 in the mesosphere, where it initiates holding cycles by recombining with radical species such as MgO2 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.
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metals is the large ion/neutral ratio. The Mg+/Mg ratio varies from ∼4 to 123,5 compared to Na+/Na and Fe+/Fe ratios of ∼0.27,8 and a Ca+/Ca ratio of ∼2.12 This large ion/neutral ratio is even more striking as Mg+ is not significantly depleted relative to other metals in the MLT, based on their relative abundances in chondritic meteorites.7 Previous work on the mesospheric chemistry of magnesium has shown that atomic Mg is rapidly oxidized by O3 to form MgO,13 which then goes on to react with O2, O3, H2O, or CO214 to form the Mg-containing compounds shown in Figure
INTRODUCTION An estimated 10−50 metric tons (t) of interplanetary dust enters the Earth’s upper atmosphere every day.1 Most of this dust undergoes rapid heating and melting, followed by the evaporation of the constituent elements, a process termed meteoric ablation. Ablation provides a source of metallic species in the mesosphere/lower thermosphere region (MLT) of the atmosphere, at heights between ∼80 and 120 km.2 Magnesium is one of the most abundant meteoritic constituents (9.6% by mass); therefore, meteoric ablation should inject large quantities of Mg and Mg+ into the atmosphere.2 Mg and Mg+ have been observed via their solar-pumped resonance fluorescence transitions at 285 and 280 nm, respectively, by optical spectrometers carried on a rocket3 and on satellites.4−6 Mg+ has also been measured by rocket-born mass spectrometry since the late 1960s.7,8 It is worth pointing out that systematic observations of Mg, sufficient to establish its chemical climatology, have only become available in the last 3 years. Observations by the GOME4 and SCIAMACHY6,9 satellite instruments show that Mg has a column density of ∼2 × 109 cm−2, with Mg+ exhibiting column densities between 3 × 109 cm−2 at its winter minimum and 1.2 × 1010 cm−2 at its summer maximum. The Mg+ layer peaks between 90 and 100 km with a peak ion concentration of (1−5)× 103 cm−3 during daytime, decreasing to 102 cm−3 at night.10 The Mg layer exhibits lower concentrations and a peak altitude at around 88 km.6,11 A striking difference between magnesium and the other meteoric © 2012 American Chemical Society
Figure 1. Schematic diagram showing the role of atomic O in neutral magnesium chemistry in the upper atmosphere.
Special Issue: A. R. Ravishankara Festschrift Received: November 30, 2011 Revised: January 6, 2012 Published: January 9, 2012 6240
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Figure 2. Schematic diagram of the fast flow tube used to study the reactions of neutral magnesium-containing molecules with atomic O. P = photomultiplier tube.
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EXPERIMENTAL SECTION Reactions 1−5 were studied using a fast flow tube, which has been used previously to study the reactions of neutral Fe- and Ca-containing species with atomic O.15,16 The stainless steel flow tube, shown schematically in Figure 2, has an internal diameter of 37.5 mm and consists of sections of tube, cross pieces, and nipple sections connected by conflat flanges sealed with copper gaskets. The tube has a total length of 1130 mm from the upstream entry point of the carrier gas (designated as I1 in Figure 2) to the downstream LIF detection cell. Magnesium atoms were produced continuously by heating magnesium pellets (Aldrich, 99.5%) to a temperature between 700 and 800 K. The pellets were located in an aluminum oxide crucible placed inside of a tungsten basket heater, positioned 1120 mm upstream of the LIF cell. The Mg atoms were entrained in the main carrier flow of N2, which entered the tube upstream of the crucible. All reactant flows were injected via side ports (I2 and I3) downstream of the crucible. The gas flow exited the tube through a throttle valve to a booster pump backed by a rotary pump, providing a volume displacement rate of 110 L s−1. Typically, a total gas flow rate of 3200 sccm was used at a pressure of 1.2 Torr, creating a flow velocity of 33.1 m s−1. The Reynolds number was always below 80, ensuring laminar flow within the tube. Mg was detected by LIF at 285.2 nm (Mg(31P1−31S0)) using a frequency-doubled Nd:YAGpumped dye laser (pulse rate, 10 Hz; pulse energy, 10 mJ), frequency doubled with a BBO crystal. Atomic O was produced by the microwave discharge of N2 to generate N atoms, which were then titrated with NO (N + NO → O + N2). As shown in Figure 2, the microwave cavity was mounted on a side arm, which consisted of a 150 mm quartz tube that projected 10 mm into the flow tube, in order to decrease the time taken for the atoms to become well mixed in the flow and to avoid large initial wall losses. The O atom flow entered 750 mm downstream of the magnesium source. The [O] was determined by titration with NO2, where the NO + O recombination generates electronically excited NO2*.17 The resulting chemiluminescence from NO2* was measured by a photomultiplier tube with an interference filter at 590 ± 5 nm, situated 260 mm downstream of the microwave discharge. The first-order loss of O atoms to the flow tube walls was determined by measuring the [O] at different flow velocities at constant pressure (and hence different flow times in the flow tube). The change in relative [O] was monitored via the addition of NO just upstream of the photomultiplier tube and measurement of the relative intensity of the resulting NO2*
1. These compounds can potentially undergo the following significantly exothermic reactions with atomic O
MgO + O → Mg + O2 ΔH0K = − 245 kJ mol−1
(1)
MgO2 + O → MgO + O2 ΔH0K = − 161 kJ mol−1
(2)
OMgO2 + O → MgO2 + O2 ΔH0K = − 125 kJ mol−1
(3)
MgCO3 + O → MgO2 + CO2 ΔH0K = − 127 kJ mol−1
(4)
where the standard reaction enthalpies at 0 K are determined from electronic structure calculations (see below). Because the atomic O mixing ratio above 85 km in the MLT can approach 1%,1 this sequence of reactions could recycle these compounds back to Mg very efficiently. These reactions are therefore key to determining the effectiveness of these Mg-containing compounds as reservoirs (or permanent sinks) for magnesium in the MLT. In this paper, we describe a kinetic study of reactions 1−4, which do not appear to have been studied previously. The rate coefficients were measured using a fast flow tube with detection of Mg by laser-induced fluorescence (LIF) and atomic O by chemiluminescence (following titration with NO). In order to establish the technique, the reaction
MgO + CO → Mg + CO2 ΔH0K = − 272 kJ mol−1
(5)
was also studied as CO is a useful surrogate for atomic O in reaction 1. The experimental results were then interpreted using electronic stucture calculations and rate theory, which was also used to explore other possible reaction pathways for species such as OMgO2 and MgCO3 in the atmosphere. Finally, these results were incorporated into a new atmospheric model of magnesium and compared with the satellite observations described above. 6241
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chemiluminescence. The [O] was ∼1013 atom cm−3 at the point of injection. Materials. N2 (99.999%, BOC Gases), O2 (99.999%. BOC Gases), CO2 (99.995%, BOC Gases), and CO (99.999%, BOC Gases) were used without further purification. O3 was produced by flowing O2 through a commercial ozonizer. The resulting 5− 8% O3 in O2 mixture was collected on silica gel held at 156 K by an ethanol slush bath, and the O2 was pumped off at the same temperature to produce a 10−20% O3/O2 mixture.
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Table 1. Flow Tube Wall Loss Rates at 300 K for Mg (measured) and Mg-Containing Molecules (estimated using the listed dipole moments and polarizabilities to calculate corresponding diffusion coefficients)
RESULTS
Kinetic Analysis in the Flow Tube. The Mg-containing species (MgX) were produced in situ from atomic Mg by addition of appropriate reactants downstream of the Mg source. Atomic O was added further downstream in order to convert MgX back to Mg, either directly or via an intermediate such as MgO2 or MgO. Therefore, experiments were monitored by measuring the relative intensity of Mg in the presence and absence of atomic O. As the gas-phase chemistry in the flow tube is complex, the rate coefficients of interest had to be determined using a model that described all of the processes that occurred between the Mg source and the LIF detection point, including diffusion and loss to the walls of Mg, MgX, and O and also any additional reactions occurring between reactants and products. The model has been described in detail in Woodcock et al.18 and Broadley and Plane.16 The first-order loss rates of Mg and O due to deposition on the flow tube walls were measured by the relative changes in [Mg] and [O] as a function of flow time (which was varied by changing the total flow rate at a fixed pressure). The measured O loss rate varied day-to-day, most likely due to a buildup of surface coatings on the flow tube walls, and therefore was measured prior to each experimental run. The average O loss rate was 207 ± 24 s−1, where the stated error is the standard deviation of the measurements. The error of an individual measurement of the wall loss rate was typically 11%. If loss of Mg to the flow tube walls is assumed to occur with an uptake efficiency close to unity, then the diffusion coefficient of Mg in N2, DMg−N2, can be calculated using the following expression, which describes the diffusion out of a cylinder of radius, r19
D Mg−N = kDiffMgP 2
r2 5.81
species
dipole moment debye
Mg MgO MgO2 OMgO2 MgCO3
0 6.2d 3.1e 0.4e 11.6e
polarizability 10−24 cm3
ionization potential eV
diffusion coefficienta Torr cm2 s−1
wall loss rateb s−1
10.6c 7.7e 5.2e 5.5e 6.3e
7.65 8.0f 8.05e 10.4e 9.10e
136 117 119 127 89
96 83 75 90 63
a
Calculated using the standard formalism in Maitland et al.20 Measured in the present system for Mg and scaled for the Mgcontaining species (see text). cMeasured.34 dMeasured.35 eCalculated at the B3LYP/6-311+g(2d,p) level of theory.25 fMeasured.46 b
than a 5% change in the pertinent rate coefficient retrieved from fitting the model to the experimental data. Rate coefficients were obtained for reactions 1−5 by integrating the pertinent coupled ordinary differential equations using a fourth-order Runge−Kutta numerical scheme.21 The rate coefficient of interest, kx, was obtained by minimizing the sum over N data points of the squared difference between the measured relative Mgimeas (obtained by averaging the LIF signal from ∼103 laser shots) and the modeled Mgimodel
⎛ Mg meas − Mg model ⎞2 i ⎟ χ 2 = ∑ ⎜⎜ i ⎟ SD i ⎠ i ⎝ N
(II)
where SDi is the standard deviation of Mgimeas when averaging the Mg LIF signal of each data point. χ2 was calculated over a range of kx values in order to locate its minimum and thus the optimized value of kx. The uncertainty in kx was set equal to the standard deviation of the difference between the value of kx obtained by carrying out a fit to each experimental point separately and the value obtained from the global fit using eq II. This uncertainty was then combined in quadrature with the systematic error arising from the rate coefficients and diffusion coefficients in the model. The systematic error was estimated by using a Monte Carlo procedure that calculated the standard deviation of kx from 104 model fits in which these model parameters were randomly sampled within their 1σ uncertainties. The systematic errors that arose from calibrations of the pressure, temperature, and mass flow rates were minor (
