The role of sodium bicarbonate in the nucleation of noctilucent clouds

J. M. C. Plane (1)

1 School of Environmental Sciences [Norwich]

It is proposed that a component of meteoric smoke, sodium bicarbonate (NaHCO₃), provides particularly effective condensation nuclei for noctilucent clouds. This assertion is based on three conditions being met. The first is that NaHCO₃ is present at sufficient concentration (±10⁴ cm^-3) in the upper mesosphere between 80 and 90 km. It is demonstrated that there is strong evidence for this based on recent laboratory measurements coupled with atmospheric modelling. The second condition is that the thermodynamics of NaHCO₃(H₂O)_n cluster formation allow spontaneous nucleation to occur under mesospheric conditions at temperatures below 140 K. The Gibbs free energy changes for forming clusters with n = 1 and 2 were computed from quantum calculations using hybrid density functional/Hartree-Fock (B3LYP) theory and a large basis set with added polarization and diffuse functions. The results were then extrapolated to higher n using an established dependence of the free energy on cluster size and the free energy for the sublimation of H₂O to bulk ice. A 1-dimensional model of sodium chemistry was then employed to show that spontaneous nucleation to form ice particles (n >100) should occur between 84 and 89 km in the high-latitude summer mesosphere. The third condition is that other metallic components of meteoric smoke are less effective condensation nuclei, so that the total number of potential nuclei is small relative to the amount of available H₂O. Quantum calculations indicate that this is probably the case for major constituents such as Fe(OH)₂, FeO₃ and MgCO₃.

Domaines: Océan, Atmosphère; Sciences de la Terre

J. M. C. Plane. The role of sodium bicarbonate in the nucleation of noctilucent clouds. Annales Geophysicae, 2000, 18 (7), pp.807-814. ⟨hal-00316709⟩