Microphysics of ice particles at the polar summer mesopause (Microlce)
Final Report Abstract
The summer polar mesopause at about 87 km altitude is a unique and fascinating region of the Earth’s atmosphere. A variety of geophysical phenomena occur in this altitude region, which are linked to the extremely low temperatures. Ice particles forming at these low temperatures become visible as so called noctilucent clouds (NLC). Despite an observational record of decades, the basic formation of these ice particles remained a long-standing open question. In this project, a novel laboratory experiment was developed to study the formation process of ice particles on meteoric smoke particle (MSP) analogues. The laboratory results are incorporated in a microphysical model for accompanying modeling studies. The newly developed laboratory experiment is worldwide unique in the sense that it provides realistic mesospheric conditions and is thus an ideal setup to determine decisive parameters for the nucleation of mesospheric ice particles. The desorption energy Fdes was found to be 0.43 eV - 0.45 eV (depending on the MSP composition), which is a factor of 2.5 larger than previously assumed. The critical saturations Scrit which mark the onset of nucleation were experimentally determined and fit well to the Scrit values predicted by classical nucleation theory (CNT) with a constant contact parameter of m=0.91 for silica particles and m=0.98 for iron oxide particles. It was surprising to find this good agreement with CNT without the necessity for a further temperature or size dependent fit parameter since the initial assumptions of the CNT are not fulfilled. Before including the above mentioned parameters in a microphysical model, the sensitivity of the modeled NLC characteristics on the nucleation rate was investigated. It was found that nucleation rates larger than currently assumed lead to too small and too many ice particles compared to observations. This is exactly the situation when the experimentally determined parameters are incorporated into the model. More realistic NLCs, i.e. fewer and larger particles, are only obtained when the nucleation rate is significantly lower. One possible and previously disregarded process that leads to a reduction of the nucleation rates is the heating of the MSPs by solar and terrestrial radiation. Calculating the balance of relevant power sources and sinks showed that the MSP equilibrium temperature can be up to 15 K higher than the temperature of the surrounding atmosphere. To consider this effect in the simulations, an extension to the CNT was developed which explicitly takes into account the temperature of the ice nuclei. As a consequence of warmer MSPs, the nucleation rate is reduced as well as the fraction of the nucleating MSP population, which results in lower ice number densities. After performing NLC simulations for a variety of possible MSP compositions (and thus different equilibrium temperatures), it was found that strongly absorbing MSP materials such as (magnesium-) wuestit and hematite lead to the most realistic NLCs. Based on these results, the laboratory setup was modified to also heat the MSPs and the theoretically derived results could be confirmed in the sense that higher saturations are required for nucleation to occur. Additionally, this setup now allows for the measurement of the refractive index of MSP analogues, which will be a valuable result for the interpretation of remote sensing measurements.
Publications
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(2014) “On the heterogeneous nucleation of mesospheric ice on meteoric smoke particles: Microphysical modeling“ J. Atmos. Sol. Terr. Phy., 118, 180 – 189
Asmus, H.; Wilms, H.; Strelnikov, B. & Rapp, M.
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(2015) “A Linear Trap for Studying the Interaction of Nanoparticles with Supersaturated Vapors“ Aerosol Sci. Technol., 49, 682-690
Duft, D.; Nachbar, M.; Eritt, M. & Leisner, T.
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(2016) “Laboratory measurements of heterogeneous CO2 ice nucleation on nanoparticles under conditions relevant to the Martian mesosphere” J. Geophys. Res. Planets, 121, 753-769
Nachbar, M.; Duft, D.; Mangan, T. P.; Gomez Martin, J. C.; Plane, J. M. C. & Leisner, T.
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(2016) “Nucleation of mesospheric ice particles: Sensitivities and limits” J. Geophys. Res., 121, 2621-2644
Wilms, H.; Rapp, M. & Kirsch, A.
