"Laboratory studies on the photochemistry of metallic layer reservoirs and nucleation of mesospheric smoke particles"

Viernes 25 de Abril

Aula 215, 12:00

Laboratory studies on the photochemistry of metallic layer reservoirs and nucleation of mesospheric smoke particles.

Juan Carlos Gomez Martin

School of Chemistry, University of Leeds

Ablation produces layers of neutral metal atoms (Fe, Mg, Na), which peak between 85 and 95 km in the terrestrial atmosphere. Below 85 km the metals become oxidized to a range of oxides, hydroxides and carbonates. These compounds then polymerise with silicates to form nanometre-sized meteoric smoke particles (MSP), which constitute a permanent sink for meteoric metals and are involved in a range of phenomena in the middle atmosphere, such as the formation of noctilucent clouds (NLC) or the charge balance in the D region.

Lidar observations have recently revealed the production of Fe during daytime between 70 and 80 km. This indicates the presence of Fe reservoirs which could be photolysed and/or react with daytime-enhanced atomic H. The reaction kinetics and photochemistry of species such as FeOH or FeO₃, and the analogous Mg and Na species, have not been studied experimentally.

An important uncertainty in NLC research is the nature of their condensation nuclei. Ab initio calculations suggest that the smallest MSP ice nuclei could be the metal silicate molecules FeSiO₃ and MgSiO₃, which should form rapidly and be stable in the mesosphere, and have large dipole moments enabling stable binding with H₂O molecules. Additionally, quantification of observations made by rocket-borne MSP detectors requires measured electron attachment rates and photo-detachment cross-sections.

In order to verify experimentally all these processes and to obtain the physical parameters required for quantifying field observations and carry out MSP modelling, we have constructed a new instrument comprising the following sub-systems:

• a ToF-MS equipped with a detector enabling observation of positive and negative ions of up to 1200 amu, and a positive post-accelerated detector capable of detecting positive ions of up to 5000 amu;
• a range of ionisation sources including electron impact, laser VUV single photon ionisation (~10.5 eV) and UV-VIS laser multi-photon resonance ionisation.
• a temperature-controlled (90-400 K) flow tube reactor coupled to the detection system at the downstream end via on-axis sampling pinhole;
• a laser ablation metal source at the upstream side of the flow tube.

In this talk the experimental results obtained using this set-up for sodium oxides, hydroxides and carbonates and their atmospheric implications for the Earth and other planets will be discussed.