Dataset Information

Reaction Kinetics of CaOH with H and O2 and O2CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium.

ABSTRACT: The ablation of cosmic dust particles entering the Earth's upper atmosphere produces a layer of Ca atoms around 90 km. Here, we present a set of kinetic experiments designed to understand the nature of the Ca molecular reservoirs on the underside of the layer. CaOH was produced by laser ablation of a Ca target in the fast flow tube and detected by non-resonant laser-induced fluorescence, probing the D(2?+) ? X(2?1) transition at 346.9 nm. The following rate constants were measured (at 298 K): k(CaOH + H ? Ca + H2O) = (1.04 ± 0.24) × 10-10 cm3 molecule-1 s-1, k(CaOH + O ? CaO + OH) < 1 × 10-11 cm3 molecule-1 s-1, and k(CaOH + O2 ? O2CaOH, 1 Torr) = (5.9 ± 1.8) × 10-11 cm3 molecule-1 s-1 (uncertainty at the 2? level of confidence). The recycling of CaOH from reaction between O2CaOH and O proceeds with an effective rate constant of keff(O2CaOH + O ? CaOH + products, 298 K) = 2.8-1.2+2.0 × 10-10 cm3 molecule-1 s-1. Master equation modeling of the CaOH + O2 kinetics is used to extrapolate to mesospheric temperatures and pressures. The results suggest that the formation of O2CaOH slows the conversion of CaOH to atomic Ca via reaction with atomic H, and O2CaOH is likely to be a long-lived reservoir species on the underside of the Ca layer and a building block of meteoric smoke particles.

SUBMITTER: Gomez Martin JC

PROVIDER: S-EPMC5611678 | biostudies-literature | 2017 Sep

REPOSITORIES: biostudies-literature

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Reaction Kinetics of CaOH with H and O2 and O2CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium.

Gomez Martin Juan Carlos JC Plane John M C JMC

ACS earth & space chemistry 20170714 7

The ablation of cosmic dust particles entering the Earth's upper atmosphere produces a layer of Ca atoms around 90 km. Here, we present a set of kinetic experiments designed to understand the nature of the Ca molecular reservoirs on the underside of the layer. CaOH was produced by laser ablation of a Ca target in the fast flow tube and detected by non-resonant laser-induced fluorescence, probing the D(2Σ+) ← X(2Σ1) transition at 346.9 nm. The following ...[more]

PMID: 28959798

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Project description:The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH3O2) using the conversion of CH3O2 into methoxy radicals (CH3O) by excess NO, followed by the detection of CH3O, has been used to study the kinetics of the self-reaction of CH3O2. Fourier transform infrared (FTIR) spectroscopy has been employed to determine the products methanol and formaldehyde of the self-reaction. The kinetics and product studies were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) in the temperature range 268-344 K at 1000 mbar of air. The product measurements were used to determine the branching ratio of the reaction channel forming methoxy radicals, rCH3O. A value of 0.34 ± 0.05 (errors at 2σ level) was determined for rCH3O at 295 K. The temperature dependence of rCH3O can be parametrized as rCH3O = 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}. An overall rate coefficient of the self-reaction of (2.0 ± 0.9) × 10-13 cm3 molecule-1 s-1 at 295 K was obtained by the kinetic analysis of the observed second-order decays of CH3O2. The temperature dependence of the overall rate coefficient can be characterized by koverall = (9.1 ± 5.3) × 10-14 × exp((252 ± 174)/T) cm3 molecule-1 s-1. The found values of koverall in the range 268-344 K are ∼40% lower than the values calculated using the recommendations of the Jet Propulsion Laboratory and IUPAC, which are based on the previous studies, all of them utilizing time-resolved UV-absorption spectroscopy to monitor CH3O2. A modeling study using a complex chemical mechanism to describe the reaction system showed that unaccounted secondary chemistry involving Cl species increased the values of koverall in the previous studies using flash photolysis to initiate the chemistry. The overestimation of the koverall values by the kinetic studies using molecular modulation to generate CH3O2 can be rationalized by a combination of underestimated optical absorbance of CH3O2 and unaccounted CH3O2 losses to the walls of the reaction cells employed.

Dataset Information

Reaction Kinetics of CaOH with H and O2 and O2CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium.

Publications

Reaction Kinetics of CaOH with H and O<sub>2</sub> and O<sub>2</sub>CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium.

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