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, CaO(2), CaO(3), CaCO(3) 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 + O(2)) = (3.1) x 10(-10) at 300 K and (1.3) x 10(-10) at 203 K; k(CaO(2) + O --> CaO + O(2)) = (2.2) x 10(-11) at 300 K and (1.6) x 10(-11) at 203 K; k(CaO(2) + H --> products, 298 K) = (1.2 +/- 0.6) x 10(-11); k(CaCO(3) + O --> CaO(2) + CO(2), 300 K) < or = 1.0 x 10(-12); k(CaCO(3) + H--> CaOH + CO(2), 298 K) > or = 2.8 x 10(-12) and < or = 3.6 x 10(-11); k(CaO(3) + H--> CaOH + O(2), 298 K) > or = 1.7 x 10(-11); k(Ca(OH)(2) + H --> CaOH + H(2)O, 298 K) > or = 1.1 x 10(-11); k(CaOH + H --> Ca + H(2)O, 298 K) > or = 1.1x 10(-11) cm(3) molecule(-1) s(-1). The kinetics of the reactions of Ca and CaO with NO(2) and N(2)O were also studied, yielding k(Ca + NO(2) --> CaO + NO) = (2.6 +/- 0.3) x 10(-10) at 300 K and (2.0 +/- 0.3) x 10(-10) at 203 K; k(CaO + NO(2) --> CaO(2) + NO) = (8.1 +/- 2.0) x 10(-10) at 300 K and (2.9 +/- 1.0) x 10(-10) at 202 K; k(CaO + N(2)O --> CaO(2) + N(2)) = (4.2 +/- 1.7) x 10(-11) at 300 K and (2.2 +/- 1.2) x 10(-12) at 206 K; k(CaO + H(2) --> Ca + H(2)O, 300 K) = (3.4 +/- 1.3) x 10(-10) cm(3) 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.