The role of sodium bicarbonate in the nucleation of noctilucent clouds

It is proposed that a component of meteoric smoke, sodium bicarbonate (NaHC O3), provides particularly effective condensation nuclei for noctilucent cl ouds. This assertion is based on three conditions being met. The first is t hat NaHCO3 is present at sufficient concentration (approximate to 10(4) 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 c oupled with atmospheric modelling. The second condition is that the thermod ynamics 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 fro m 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 depen dence of the free energy on cluster size and the free energy for the sublim ation of H2O to bulk ice. A 1-dimensional model of sodium chemistry was the n employed to show that spontaneous nucleation to form ice particles (n > 1 00) should occur between 84 and 89 km in the high-latitude summer mesospher e. The third condition is that other metallic components of meteoric smoke are less effective condensation nuclei, so that the total number of potenti al nuclei is small relative to the amount of available H2O. Quantum calcula tions indicate that this is probably the case for major constituents such a s Fe(OH)(2), FeO3 and MgCO3.