Kinetics of O 3 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.

The room-temperature rate constants and product branching fractions of CaO n + ( n = 0-3) + O 3 are measured using a selected ion flow tube apparatus. Ca + + O 3 produces CaO + + O 2 with k = 9 ± 4 × 10 -10 cm 3 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 O 3 , as explored here. A rate constant of 8 ± 3 × 10 -10 cm 3 s -1 is recommended. Both CaO + and CaO 2 + react near the capture rate constant with ozone. The CaO + reaction yields both CaO 2 + + O 2 (0.80 ± 0.15 branching) and Ca + + 2O 2 . Similarly, the CaO 2 + reaction yields both CaO 3 + + O 2 (0.85 ± 0.15 branching) and CaO + + 2O 2 . CaO 3 + + O 3 yield CaO 2 + + 2O 2 at 2 ± 1 × 10 -11 cm 3 s -1 , about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaO n + + O 3 yield CaO n +1 + + O 2 , the expected oxidation. Some fraction of CaO n +1 + is produced with sufficient internal energy to further dissociate to CaO n -1 + + O 2 , yielding the same products as the oxidation of O 3 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.