A kinetic study of the reactions of MgO with H2O, CO2O2: implications for magnesium chemistry in the mesosphere

The recombination reactions of MgO with H2O, CO2 and O-2 were studied by th e pulsed photodissociation at 193.3 nm of magnesium acetyl acetonate [Mg(C5 H7O2)(2)] vapour in the presence of O-3, producing MgO in an excess of the reactant and N-2 bath gas. MgO was monitored by time-resolved non-resonant LIF by pumping the MgO((B(1)Sigma (+)-X(1)Sigma (+)), Deltav = 0) transitio n at 499.4 nm and detecting MgO(B(1)Sigma (+)-A(1)Sigma (+)) emission at la mbda >600 nm. All three recombination reactions were found to be in the fal l-off region over the experimental pressure range (2-60 Torr). The data wer e fitted by RRKM theory combined with ab initio quantum calculations on Mg( OH)(2), MgCO3, OMgO2 and MgO3, and yielding the following results (180-600 K, 10(-6)-10(3) Torr). For MgO + H2O: log(10)(k(rec, 0)/cm(6) molecule(-2) s(-1)) = -32.75 + 7.894log(10) T - 2.127log(10)(2) T, k(rec,infinity) = 3.5 2 x 10(-10)exp(-334/T)cm(3) molecule(-1) s(-1), F-c = 0.28. For MgO + CO2 : log(10) (k(rec,0) /cm(6) molecule(-2) s(-1)) -33.70 + 5.827log(10) T - 1.4 94log(10)(2) k(rec,infinity) = 6.79x 10(-10)exp(-310/T) cm(3) molecule(-1) s(-1), F-c = 0.37. For MgO + O-2 : log(10) (k(rec,0) /cm(6) molecule(-2) s( -1)) = 28.05 + 1.423log(10) T - 0.683log(10)(2) T;k(rec,infinity) = 1.16 x 10(-10)exp(-219/T) cm(3) molecule(-1) s(-1), F-c = 0.34 (F-c is the broaden ing factor). The uncertainty in extrapolating to the mesospheric temperatur e range (120-250 K) was determined using a Monte Carlo procedure. Finally, the implications of these results for magnesium chemistry in the mesosphere are discussed.