Kinetic study of the reactions of CaO with H2O, CO2, O-2, and O-3: Implications for calcium chemistry in the mesosphere

CaO(X(1)Sigma (+)) plays a central role in the atmospheric chemistry of met eor-ablated calcium. A series of CaO reactions was studied by the pulsed ph otodissociation at 193.3 nm of calcium acetyl acetonate [Ca(C5H7O2)(2)] vap or, producing CaO in an excess of reactant and N-2 bath gas. CaO was monito red by time-resolved nonresonant LIF, by pumping the CaO(B(1)Pi - X(1)Sigma (+)) transition at 385.9 nm and detecting B(1)Pi - A(1)Sigma (+) emission at lambda > 693 nm. The recombination reactions of CaO with H2O, CO2, and O -2 were found to be in the falloff region over the experimental pressure ra nge (2-12 Torr). The data were fitted by RRKM theory combined with ab initi o quantum calculations on Ca(OH)(2), CaCO3 and CaO3, yielding the following results (180-600 K and 0-10(3) Torr). For CaO + H2O, log (k(rec,0)/cm(6) m olecule(-2) s(-1)) = -23.39 + 1.41 log T - 0.751 log(2) T, k(rec,infinity), = 7.02 x 10(-10) exp(-38.4/T) cm(3) molecule(-1) s(-1), F-c = 0.31. For Ca O + CO2: log k(rec,0)/cm(6) molecule(-2) s(-1)) = -36.14 + 9.24 log T - 2.1 9 log(2) T, k(rec,infinity) = 7.97 x 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(-1) s(-1)) = -42.19 + 13.15 log T - 2.87 log(2) T; k(rec,infinity) = 9.90 x 10-(10) ex p(-195/T) cm(3) molecule(-1) s(-1), F-c = 0.43 (F-c is the broadening facto r). The uncertainty in extrapolating to the mesospheric temperature range ( 120-250 K) is determined using a Monte Carlo procedure. The reaction betwee n CaO and O-3 is fast with a small T dependence: k(204-318 K) = (5.70 12.11 -1.43) x 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. Finall y, the implications of these results for calcium chemistry in the mesospher e are discussed.