Reaction kinetics of CaOH with H and O₂, and O₂CaOH with O: implications for the atmospheric chemistry of meteoric calcium

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 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 2σ 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 modelling of the CaOH + O2 kinetics is used to extrapolate to mesospheric temperatures and pressures. The results suggest that formation of O2CaOH slows down 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.