A kinetic study of reactions of calcium-containing molecules with O and H atoms: implications for calcium chemistry in the upper atmosphere

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Ca-containing species, many of which are important for describing the chemistry of meteor-ablated calcium in the Earth's upper atmosphere. Ca atoms were produced thermally in the upstream section of a fast flow tube, and then converted to the molecular species CaO, CaO2, CaO3, CaCO3 or Ca(OH)2 by the addition of appropriate reagents. Atomic O or H was added further downstream, and both Ca and CaO were detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined: k(CaO + O → Ca + O2) = (3.1+2.0−1.5) × 10−10 at 300 K and (1.3+3.4−0.6) × 10−10 at 203 K; k(CaO2 + O → CaO + O2) = (2.2+7.0−1.4) × 10−11 at 300 K and (1.6+2.9−0.7) × 10−11 at 203 K; k(CaO2 + H → products, 298 K) = (1.2 ± 0.6) × 10−11; k(CaCO3 + O → CaO2 + CO2, 300 K) ≤ 1.0 × 10−12; k(CaCO3 + H→ CaOH + CO2, 298 K) ≥ 2.8 × 10−12 and ≤3.6 × 10−11; k(CaO3 + H→ CaOH + O2, 298 K) ≥ 1.7 × 10−11; k(Ca(OH)2 + H → CaOH + H2O, 298 K) ≥ 1.1 × 10−11; k(CaOH + H → Ca + H2O, 298 K) ≥ 1.1× 10−11 cm3 molecule−1 s−1. The kinetics of the reactions of Ca and CaO with NO2 and N2O were also studied, yielding k(Ca + NO2 → CaO + NO) = (2.6 ± 0.3) × 10−10 at 300 K and (2.0 ± 0.3) × 10−10 at 203 K; k(CaO + NO2 → CaO2 + NO) = (8.1 ± 2.0) × 10−10 at 300 K and (2.9 ± 1.0) × 10−10 at 202 K; k(CaO + N2O → CaO2 + N2) = (4.2 ± 1.7) × 10−11 at 300 K and (2.2 ± 1.2) × 10−12 at 206 K; k(CaO + H2 → Ca + H2O, 300 K) = (3.4 ± 1.3) × 10−10 cm3 molecule−1 s−1. Electronic structure calculations of the relevant potential energy surfaces were performed to interpret the experimental results, and the atmospheric implications of these measurements are then discussed.