THE POSSIBLE RESURRECTION OF THE CHAPMAN MECHANISM FOR ATMOSPHERIC SODIUM CHEMILUMINESCENCE AND RUMINATIONS ON NAO REACTION DYNAMICS

A critical reappraisal and kinetic modeling is presented of a sodium/N 2O system. The analysis provides a compelling argument that previously reported kinetic decay rates may refer not to the reaction of O with NaO but rather to that of O with Na2O. The system shows pronounced pro pensity for a significant portion of the NaO to be quantitatively conv erted to Na2O on a time scale commensurate with that of the Na/N2O tit ration reaction. The O atom in the system does appear to originate fro m the photolysis of a residual level of NaO. The observed Na(2P) chemi luminescence, used to track the O atom decay rates, can be consistent with the O + NaO reaction as previously surmised. It is unlikely that the alternate O + Na2 and O + Na2O chemiluminescent channels can gener ate the observed intensity levels. This reanalysis, which provides for the observed first order dependences on N2O(Na2O) and O atom concentr ations has significant implications for the Chapman atmospheric mechan ism of the sodium airglow. Its conclusions resurrect the viability of the original scheme which requires efficient branching of the O + NaO reaction to Na(2p). Recent suggestions invoking the participation of N aO(A2 SIGMA + ) require the latter to have a metastable nature with re spect to its radiative and collisional quenching (N2,O2) channels, for which there is no current evidence. An additional evaluation of the r ate constant measurements for the fast reactions of NaO with NO or CO indicates that these most probably are kinetically complex and involve long-lived transition states. Their rate constants are predicted to h ave small negative activation energies and pressure dependences. In th e case of NaO with CO this may explain its low Na(2P) chemiluminescent efficiency. For the NaO + O reaction, a rate constant of about 4 x 10 (-11) cm3 molecule-1s-1 is predicted at room temperature. This is simi lar to that used in earlier atmospheric models. Its magnitude circumve nts the consequences of the reaction's large entropy decrease which ot herwise implies too large a cross-section for the reverse reaction. A smaller value also is more likely to be consistent with a normal short -lived collisional transition state, which will allow for a more signi ficant Na(2P) quantum efficiency. (C) 1993 John Wiley & Sons, Inc.