The residence time of a particle in the upper atmosphere has been of interest to scientists and astrobiologists. However, only a few comprehensive studies that consider atmospheric conditions and solar radiation in detail have been reported. This study simulates the speed of deposition or levitation of a particle controlled by solar pressure, solar radiation, friction from the air, and gravity. The simulation results showed that the particle motion was significantly affected by gravity, friction from the air, and the photophoretic force caused by solar radiation and thermal gradients inside a particle. As mentioned in previous studies, this study confirmed that a particle could levitate in the mesosphere and stratosphere when it is small enough (≦100 nm for alumina particle; ≦1 μm for water particle). Thus, we conclude that the results of this study are useful for the development of a precise simulation of aerosol dynamics in the stratosphere and mesosphere.

We have developed a technique for coating Ni nanoparticles with an organic solid in gas phase to prevent the oxidation of the particles while keeping them separate. The non-aggregated Ni nanoparticles produced using a rapid cooling system based on a Laval nozzle in a gas phase reactor were coated with trimethylglycine. The fraction of the coating of non-agglomerated particles was greater than 90%. The coating of non-agglomerated Ni nanoparticles was promoted by that the trimethylglycine vapor at 653 K condensed on Ni particles in a temperature range higher than the Curie temperature 627 K of Ni where the magnetism-induced agglomeration of Ni particles could be prevented. It was confirmed that the surfaces of Ni nanoparticles coated with 6 nm thick trimethylglycine did not oxidize even after 34 days of exposure to air.