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

Abstract

Sodium superoxide (NaO2) is believed to be a major sink for meteor‐ablated Na in the upper atmosphere. However, the rate constant for the reaction between NaO2 and O is not known, and its determination is the subject of this study. NaO2 was produced by the reaction between Na vapor, an excess of O2, and the carrier gas N2 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 [O2], from which it was deduced that k(NaO2 + O) = (2.2 ± 1.0) × 10−11 cm3 molecule−1 s−1 at 300 K (3σ uncertainty). As a prelude to this experiment, the recombination reaction between O2 and Na was studied by the conventional flow tube technique, yielding k(Na + O2 + N2) = (3.14 ± 0.20) ×10−30 (300/T)−(1.52±0.27) cm6 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 NaHCO3 rather than NaO2 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 NaHCO3 + H → Na + H2CO3, 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.

Chemical Kinetics of the NaO (A 2Σ+) + O(3P) Reaction

“…Our estimate that the Na + O + He reaction should be modestly faster than the Na + O 2 + He reaction is supported by a reported estimate of a 290 K rate for the Na + OH + He rate constant of (4.1 ( 0.9) × 10 -30 cm 6 s -1 (OH has an electron affinity of 1.83 eV), based on a measured value at 635 K extrapolated with a Tröe unimolecular reaction formulation to lower temperatures. 26 The estimate for the reaction rate coefficient for the NaO 3 + O reaction was taken from ref 12.…”

“…The evaluation of DeMore et al 23 recommends a temperature independent value for k 2 of 3.7 × 10 -10 cm 3 s -1 , while a temperature dependent rate coefficient which corresponds to a 300 K value of 2.7 × 10 -10 cm 3 s -1 has been adopted by Plane and coworkers for atmospheric modeling. 12,13 Under conditions where NaO(A 2 Σ + ) dominates, these values should be increased in future models.…”

“…More recent analyses based on atmospheric kinetic models plus measured Na layer intensities and Na D-line emissions by Plane and co-workers suggest that an f 2 ∼ 0.1 is sufficient. 12,13 Unfortunately, the only laboratory measurement of f 2 , determined by Plane and Husain 10 in a relatively high pressure, slow flow heat pipe experiment, indicated that f 2 e 0.01. Since Plane and Husain used a slow flow reactor, with estimated NaO residence times before reaction of order 1 s, it is certain that they were predominantly studying NaO in its ground X 2 Π state.…”

Development of the mesospheric Na layer at 69° N during the Geminids meteor shower 2010

“…Another sink is the uptake of NaHCO 3 and Na + on meteoric smoke particles, likely highly efficient, where Na species adsorb on silica (SiO 2 ) surfaces (Plane, 2004). Models for the mesospheric Na layer have been developed by Helmer and Plane (1993) and McNeil et al (1995). Their results indicate that more than 90 % of the total Na at 95 km occurs as atomic Na.…”

“…The major reservoir for Na below 90 km is believed to be the molecule NaHCO 3 , which has a relatively long lifetime in the mesosphere (Plane, 2000). Above 95 km, Na + is the major reservoir species for atomic Na (Helmer and Plane, 1993). Another sink is the uptake of NaHCO 3 and Na + on meteoric smoke particles, likely highly efficient, where Na species adsorb on silica (SiO 2 ) surfaces (Plane, 2004).…”