A kinetic study of the reactions of MgO with H2O, CO2 and O2: implications for magnesium chemistry in the mesosphere

Abstract

The recombination reactions of MgO with H2O, CO2 and O2 were studied by the pulsed photodissociation at 193.3 nm of magnesium acetyl acetonate [Mg(C5H7O2)2] vapour in the presence of O3, producing MgO in an excess of the reactant and N2 bath gas. MgO was monitored by time-resolved non-resonant LIF by pumping the MgO((B1Σ+–X1Σ+), Δ = 0) transition at 499.4 nm and detecting MgO(B1Σ+–A1Σ+) emission at λ>600 nm. All three recombination reactions were found to be in the fall-off region over the experimental pressure range (2–60 Torr). The data were fitted by RRKM theory combined with ab initio quantum calculations on Mg(OH)2, MgCO3, OMgO2 and MgO3, yielding the following results (180–600 K, 10−6–103 Torr). For MgO + H2O: log10(krec,0/cm6 molecule−2 s−1) = − 32.75 + 7.894log10T − 2.127log102T, krec,∞ = 3.52 × 10−10exp(−334/T) cm3 molecule−1 s−1, Fc = 0.28. For MgO + CO2: log10(krec,0/cm6 molecule−2 s−1) = − 33.70 + 5.827log10T − 1.494log102T, krec,∞ = 6.79 × 10−10exp(−310/T) cm3 molecule−1 s−1, Fc = 0.37. For MgO + O2: log10(krec,0/cm6 molecule−2 s−1) = − 28.05 + 1.423log10T − 0.683log102T; krec,∞ = 1.16 × 10−10exp(−219/T) cm3 molecule−1 s−1, Fc = 0.34 (Fc is the broadening factor). The uncertainty in extrapolating to the mesospheric temperature 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.