Atmospheric Nanoscience

Particles in the nanometer size range (about 1000 to 100000000 atoms) are, due to their large fraction of surface atoms, expected to show specific properties that do occur neither in atoms nor in bulk compounds. Recently, techniques have been established which can probe the specifications of small particles. The development of our novel TRAPS apparatus expands the accessible size range of free, levitated nanoparticles to 3 – 30 nm diameter.

In the atmosphere, nanoparticles have fundamental importance for chemical and physical processes. For instance they occur in higher regions like the Mesopause. The Mesospause stretches from about 80 km to 90km height. It separates the Mesosphere from the Thermosphere and is distinguiseh by the minimum temperature found all over the Earth’ atmosphere. The temperature in the mesopause can fall below 130 K in the polar north summer. In this time, the temperature drops below the water vapor frost point temperature (below 120°C at this pressure). If condensation nuclei (e.g. meteoric dust particles) are present, ice clouds can form in the region of supersaturation. Since the light scattering efficiency of these clouds is only weak they can be best seen after sunset when the sun still illuminates the therefor so called noctilucent clouds (NLC).

Though noctilucent clouds have been observed already in the 19th century, their origin was for a long time under discussion. The density of water molecules in the Mesopause is so low that it was hard to believe that these clouds could be formed by homogeneous ice nucleation of water vapor. In the 1960s sounding rocket flight measurements delivered a first proof. It is generally accepted today that aerosol dust particles in a size range smaller than 10 nm, formed in the contrails of ablating meteors, act as condensation nuclei for mesospheric ice cristals that can reach sizes up to 80 nm in diameter. Nowadays activities in the Mesopause are routinely observed with suitable radar techniques, lidar from ground stations or satellites, sounding rocket flights and falling spheres.

In order to study the micro-physical processes by laboratory experiments, we have developed a setup that allows to accumulate and prepare free size selected nano-particles in an atmosphere of interest (gas, pressure, temperature, humidity) to study chemical and optical processes under realistic ambient conditions with state of the art observation techniques. In our apparatus, we plan to investigate the material properties of mesospheric particles, ice-formation, photon induced processes as well as catalytic reactions on the particle surface. Our novel setup with its experimental possibilities is so far unique in the world.