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( 2 Σ + ) ← X( 2 Σ 1 ) transition at 346.9 nm. The following rate constants were measured (at 298 K): k (CaOH + H → Ca + H 2 O) = (1.04 ± 0.24) × 10 -10 cm 3 molecule -1 s -1 , k (CaOH + O → CaO + OH) < 1 × 10 -11 cm 3 molecule -1 s -1 , and k (CaOH + O 2 → O 2 CaOH, 1 Torr) = (5.9 ± 1.8) × 10 -11 cm 3 molecule -1 s -1 (uncertainty at the 2σ level of confidence). The recycling of CaOH from reaction between O 2 CaOH and O proceeds with an effective rate constant of k eff (O 2 CaOH + O → CaOH + products, 298 K) = 2.8 -1.2 +2.0 × 10 -10 cm 3 molecule -1 s -1 . Master equation modeling of the CaOH + O 2 kinetics is used to extrapolate to mesospheric temperatures and pressures. The results suggest that the formation of O 2 CaOH slows the conversion of CaOH to atomic Ca via reaction with atomic H, and O 2 CaOH is likely to be a long-lived reservoir species on the underside of the Ca layer and a building block of meteoric smoke particles.