THE atmospheric sodium nightglow and luminescence of meteor trails, both of which occur at mesospheric altitudes of 85-95 km, are emitted by sodium atoms in the excited 2P state. The Chapman mechanism1-8 attributes these to the reaction between NaO and oxygen atoms, requiring an unusually high rate of formation of excited Na*(2P) relative to ground-state Na(2S) atoms from the NaO + O reaction. But laboratory studies of the kinetics9 show a very low Na*(2P) formation rate (branching ratio f< 0.01). NaO is itself formed by the reaction of sodium atoms with ozone. Molecular-beam experiments10 and photoelectron spectroscopy11 have shown recently that this reaction yields largely excited-state (2Σ+) NaO rather than the ground-state (2Π) species studied in the NaO + O kinetics experiments9, thereby suggesting a resolution of the apparent discrepancy with the Chapman mechanism. By extending the symmetry correlation between reactant and product electronic states considered by Bates and Ohja6, we show here that reaction of excited-state NaO with oxygen atoms does indeed yield branching ratios consistent with the Chapman mechanism. We infer that the NaO + O potential-energy surfaces leading to excited Na*(2P) atoms involve doublet rather than quartet spin configurations, and the branching ratio f is close to zero for ground-state NaO but ~2/3 for excited-state NaO. If confirmed experimentally, this finding may enable the sodium nightglow to be used as a quantitative measure of mesospheric ozone concentrations5,7,8.