During the winter months in altitudes between 15 and 25 km polar stratospheric clouds (PSCs) are always observed above Antarctica and often over the northern polar region. PSCs form at low temperatures through condensation of water and nitric acid on sulfuric acid aerosols that are present in the stratosphere. Relatively inactive chlorine?containing molecules are chemically changed on surfaces of PSC particles. After the return of sun in the Spring activated chlorine will produce large ozone losses leading to the formation of an ozone hole. Despite the importance of PSCs for the ozone chemistry in the polar stratosphere, hardly any chemical analysis is available although composition and phase (liquid or solid) are important for the chlorine activation process. The fragile particles are very sensitive to changes in temperatures and thus a transfer into an instrument is difficult.
On 25 January 2000, instruments of European and American scientists were launched onboard a balloon gondola from Kiruna/Sweden to perform a comprehensive investigation of PSCs. In 20?23 km altitude clouds were repeatedly encountered. The main instrument on the gondola was a particle composition analyzer developed at the Max-Planck-Institute for Nuclear Physics, Heidelberg, that transfers the particles without alterations to a mass spectrometer for molecular analysis. Days before the balloon flight a cold region had developed above northern Scandinavia in which PSCs were formed.
During the 3-hour flight, nitric acid trihydrate crystals (NAT crystals) were identified for the first time. These particles contain three molecules of water and one molecule of nitric acid and can exist in the atmosphere up to 7° above the ice point. Besides NAT particles, liquid aerosols were also identified and analyzed. Surprisingly, those were found at the lowest atmospheric temperatures encountered during the flight. Together with the other particle experiments from the USA, Italy, France, and Denmark details of physical and chemical particle parameters were measured never obtained before. It was found that the complexity of PSC particle formation has actually increased. It is presently not clear which processes will change liquid into solid (crystalline) particles and many small into a few large ones.
At the present time, two gondolas are again waiting in Kiruna for polar stratospheric clouds. Instruments on the gondola have been further improved and are extensively calibrated. The research conducted in past and present experiments was supported by the European Union and for our American partners by the National Science Foundation.
Figure: Balloon gondola before the flight in Kiruna, Sweden, on 25. Jan. 2 000 with instruments from 5 countries participating in a comprehensive investigation of polar stratospheric clouds (PSCs). The central instrument is a particle analyzer developed at the Max-Planck-Institute for Nuclear Physics, Heidelberg.