The highest clouds on Earth, noctilucent clouds (NLC) named for their night glow, are formed 75-90 km above the ground at the boundary between mesosphere and thermosphere. The extremely low temperatures and pressures make those clouds the only location on our planet where ice is found not only in the common hexagonal but also cubic and amorphous forms.
The formation and evolution of NLC is poorly understood, even though their appearance may be linked to the changes in the global climate. Indeed, it is possible that the first record of NLCs observation was two years after the eruption of the Krakatoa volcano in 1883.
The study of the NLC is limited to satellite observations, where spectroscopic measurements can only resolve short-range interactions and are unable to inform on the long-range structures required to identify ice forms and transitions. Laboratory experiments show that under mesospheric conditions, the first step of ice formation is the homogeneous nucleation of low-density amorphous ice (LDA), followed by crystallisation into a stacking-disordered ice Isd, a combination of hexagonal and cubic forms. LDA is an extremely porous material, observed to pick up and retain volatile molecules from its environment. When located in the mesosphere, these ice structures are subject to strong radiation, collision with incoming energetic particles (electrons, protons, ions), and chemical enrichment from cosmic dust. These conditions make LDA particles effective chambers for complex heterogeneous chemistry.
The processes occurring at a molecular level in the mesosphere can have a dramatic influence on the kinetics of ice particle nucleation and growth, phase transitions, ice particle shape, stability, interactions at the interface, particle coagulation, and sedimentation. Clouds are the barrier between us and space – they regulate Earth’s energy balance, protecting life from solar radiation while retaining infrared heat. NLC form in extreme low temperatures and low particle densities, which makes them very sensitive to any changes either of cosmic, natural or anthropogenic origin. In this project, we combine laboratory experiments and molecular modelling to develop a deep understanding of the fine interactions that influence the growth and stability of mesospheric clouds.
- Can we unravel the nature of the complex ice structures in noctilucent clouds by combining molecular dynamics simulations with spectroscopic measurements?
- How does the uptake of volatile molecules (methane, carbon monoxide and sulphate) affect the ice evolution within noctilucent clouds?
- What is the effect of physicochemical interactions at the ice surfaces on the reactivity of volatile molecules?
Molecules of interest:
In this project, we investigate the interactions between ices formed at extreme low temperatures and molecules whose concentrations may be affected by external factors, such as human-related activities or meteorite showers. We have identified the following compounds as topmost priorities for investigation:
- Carbon compounds:
- methane, CH4, rises from the surface of the planet, with high concentrations recorded over the tropical regions. Furthermore, under UV radiation in the mesosphere, methane will oxidise into water, leading to its increase of 40% over the last century. [Lübken, Geophys. Res. Lett., 2018].
- carbon monoxide, CO, is produced by photolysis of carbon dioxide in the lower thermosphere and has a seasonally-dependent concentration variability in the upper mesosphere, where NLC are formed. [Lee, J. Atmos. Sol.-Terr. Phys, 2018]
- Sulfur compounds:
- sulphur dioxide, SO2, may be brought into the mesosphere in large quantities through ash ejections during volcanic eruptions, or as meteoric smoke particles. Furthermore, sulphur dioxide aerosols are stable in the stratosphere, exchanging into the higher mesosphere
- sulphate and sulphuric acid, H2SO4, formed from sulphur dioxide, found as aerosols near the stratosphere. [Hervig, Geophys. Res. Lett, 2017]
The ice growth will be carried out through vapour deposition in the temperature-controlled Linkam THMS600 stage, which additionally can be fitted with a vacuum chamber.
Samples submerged in liquid nitrogen will be transferred for XRD and FTIR measurements. When formed, the ice forms of interest are known to be stable under liquid nitrogen. Porous ices are known to pick up volatile gasses, but nitrogen has the lowest binding affinity and therefore will not contaminate the sample. Therefore the presence of nitrogen gas, arising from the liquid nitrogen, will allow removal of any other volatiles in the surrounding environment.
If in situ measures are deemed necessary, the Linkam stage can be converted for both XRD and FTIR measurements.
The deionised water bath will be fitted as a source of water vapour, and additional reservoirs with gasses will be added into the sequence.
Classical molecular dynamics (MD) will be performed to sample the ice structures and transitions. Adsorption of the gas molecules will also be accessed to obtain the most statistically probable configurations for further detailed investigations with quantum methods. MD simulations will be performed using GROMACS software on CIRRUS HPC. The analysis will be carried out with GROMACS and in-house scripts.
These statistically representative structures will be transferred for further quantum chemical (QC) studies to obtain information about the spectral activity of the selected molecules at the ice interface. All (IR, Raman, UV/Vis) spectra predictions will be obtained with the Turbomole
The project is interdisciplinary, combining molecular modelling techniques and cryochemistry laboratory experiments. Through the duration of the project, the student will be able to learn and apply numerous experimental and computational techniques. The student will also be offered the chance to acquire skills in software development and the usage of high-performance computing resources. Overall, through the project the student will develop a unique and highly desirable profile in these growing and influential research areas, making the graduate competitive on the job market at both industrial and academic levels.
Training: cryochemistry laboratory techniques (in-house); molecular modelling (e.g. CCP5 summer school, CECAM); high-performance computing and software development (courses offered through ARCHER2).
A motivated student with a degree in Chemistry, Physics, Astrophysics or Geosciences, and interest in atmospheric processes and extreme conditions.
Experience in molecular modelling and/or coding will permit a fast start on the project. Nevertheless, all the necessary training will be provided.