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Ann. Geophys., 18, 807-814, 2000
www.ann-geophys.net/18/807/2000/
© European Geosciences Union 2000
The role of sodium bicarbonate in the nucleation of noctilucent clouds
J. M. C. Plane
School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom
Abstract. 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.
Key words: Atmospheric composition and structure
(aerosols and particles; cloud physics and chemistry; middle atmosphere ·
composition and chemistry)
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