A study of the reaction NaO₂+ O → NaO+ O₂: Implications for the chemistry of sodium in the upper atmosphere

Sodium superoxide (NaO 2 ) is believed to be a major sink for meteor‐ablated Na in the upper atmosphere. However, the rate constant for the reaction between NaO 2 and O is not known, and its determination is the subject of this study. NaO 2 was produced by the reaction between Na vapor, an excess of O 2 , and the carrier gas N 2 in the upstream section of a fast flow tube reactor. Excess atomic O was then added, so that a steady state developed in which the sodium was partitioned between its atomic form and a variety of oxides. The steady state fraction of atomic Na, monitored by laser‐induced fluorescence, was then observed as a function of [O] and [O 2 ], from which it was deduced that k (NaO 2 + O) = (2.2 ± 1.0) × 10 −11 cm 3 molecule −1 s −1 at 300 K (3σ uncertainty). As a prelude to this experiment, the recombination reaction between O 2 and Na was studied by the conventional flow tube technique, yielding k (Na + O 2 + N 2 ) = (3.14 ± 0.20) ×10 −30 (300/ T ) −(1.52±0.27) cm 6 molecule −2 s −1 . These experimental results were then incorporated into a one‐dimensional model of sodium in the upper atmosphere between 65 and 110 km. It is shown that formation of NaHCO 3 rather than NaO 2 is the dominant removal process for atomic Na below 90 km. This new model, whose only important assumption is the rate constant for the reaction NaHCO 3 + H → Na + H 2 CO 3 , predicts a seasonal variation of the atomic Na layer in excellent agreement with recent lidar observations at 40° and 69°N and also demonstrates that temperature fluctuations produced by gravity waves should induce significant chemical responses in the Na layer below 88 km.