Kinetics of O₃ with Ca⁺ and Its Higher Oxides CaO_n⁺ (n = 1–3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere

Abstract

The room-temperature rate constants and product branching fractions of CaO n + (n = 0–3) + O3 are measured using a selected ion flow tube apparatus. Ca+ + O3 produces CaO+ + O2 with k = 9 ± 4 × 10–10 cm3 s–1, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values. Most likely, those values were underestimated due to the reformation of Ca+ from the sequential chemistry of higher calcium oxide cations with O3, as explored here. A rate constant of 8 ± 3 × 10–10 cm3 s–1 is recommended. Both CaO+ and CaO2 + react near the capture rate constant with ozone. The CaO+ reaction yields both CaO2 + + O2 (0.80 ± 0.15 branching) and Ca+ + 2O2. Similarly, the CaO2 + reaction yields both CaO3 + + O2 (0.85 ± 0.15 branching) and CaO+ + 2O2. CaO3 + + O3 yield CaO2 + + 2O2 at 2 ± 1 × 10–11 cm3 s–1, about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaO n + + O3 yield CaO n+1 + + O2, the expected oxidation. Some fraction of CaO n+1 + is produced with sufficient internal energy to further dissociate to CaO n–1 + + O2, yielding the same products as the oxidation of O3 by CaO n +. Mesospheric Ca and Ca+ concentrations are modeled as functions of day, latitude, and altitude using the Whole Atmosphere Community Climate Model (WACCM); incorporating the updated rate constants improves agreement with concentrations derived from lidar measurements.