Kinetic Study of the Reactions of CaO with H 2 O, CO 2 , O 2 , and O 3 : Implications for Calcium Chemistry in the Mesosphere

CaO(X 1 Σ + ) plays a central role in the atmospheric chemistry of meteor-ablated calcium. A series of CaO reactions was studied by the pulsed photodissociation at 193.3 nm of calcium acetyl acetonate [Ca(C 5 H 7 O 2 ) 2 ] vapor, producing CaO in an excess of reactant and N 2 bath gas. CaO was monitored by time-resolved nonresonant LIF, by pumping the CaO(B 1 Π − X 1 Σ + ) transition at 385.9 nm and detecting B 1 Π − A 1 Σ + emission at λ > 693 nm. The recombination reactions of CaO with H 2 O, CO 2, and O 2 were found to be in the falloff region over the experimental pressure range (2−12 Torr). The data were fitted by RRKM theory combined with ab initio quantum calculations on Ca(OH) 2, CaCO 3 and CaO 3, yielding the following results (180−600 K and 0−10 3 Torr). For CaO + H 2 O, log ( k rec,0 /cm 6 molecule -2 s -1 ) = −23.39 + 1.41 log T − 0.751 log 2 T, k rec, ∞ = 7.02 × 10 -10 exp(−38.4/ T ) cm 3 molecule -1 s -1, F c = 0.31. For CaO + CO 2: log ( k rec,0 /cm 6 molecule -2 s -1 ) = −36.14 + 9.24 log T − 2.19 log 2 T, k rec, ∞ = 7.97 × 10 -10 exp(−190/ T ) cm 3 molecule -1 s -1, F c = 0.36. For CaO + O 2: log ( k rec,0 /cm 6 molecule -2 s -1 ) = −42.19 + 13.15 log T − 2.87 log 2 T; k rec, ∞ = 9.90 × 10 -10 exp(−195/ T ) cm 3 molecule -1 s -1, F c = 0.43 ( F c is the broadening factor). The uncertainty in extrapolating to the mesospheric temperature range (120−250 K) is determined using a Monte Carlo procedure. The reaction between CaO and O 3 is fast with a small T dependence: k (204−318 K) = (5.70 +2.01 - 1.43 ) × 10 -10 exp[(−2.22 ± 0.62) kJ mol -1 / RT ] cm 3 molecule -1 s -1, where the quoted uncertainties are at the 95% confidence level. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.