Basic Atmospheric Chemistry: A Quantum Chemical Study on Hydration of Mesospheric NaOH

Environmental Context.A natural global layer of sodium atoms exists in the mesosphere, 80–95 km above sea level, where it originates—along with lithium, iron, and calcium—from ablation of meteors. Sodium, as its hydroxide, readily associates with free water to form NaOH·(H2O)n clusters. The clusters strongly emit IR radiation and may therefore affect the upper atmosphere’s temperature profile; the clusters are also likely to be a source of nuclei for noctilucent clouds. The same NaOH-based processes may also occur at lower altitudes in the troposphere where water is more abundant than carbon dioxide. Abstract.The sequential association of water molecules with NaOH, a key upper-atmosphere metal-containing molecule, is investigated using quantum chemical calculations. The first several H2O–NaOH·(H2O)n–1 bond strengths are sizeable (respectively 82, 70, 56, 42, 42, and 36 kJ mol–1 according to calculations), suggesting that the termolecular association reactions of NaOH·(H2O)n–1 with H2O may well be efficient upper-atmospheric processes. Such reactions would provide an alternative or additional pathway to the production of hydrated sodium bicarbonate, which has been implicated in the nucleation of noctilucent clouds. The NaOH·(H2O)n complexes are also characterized by very large IR intensities across the 3–5 μm wavelength range, suggesting that they may contribute disproportionately to the IR emission profile of the upper atmosphere.