Radioactivity soundings of the upper atmosphere in Finland 1962-2020

Radioactivity soundings have been performed in Finland since the early 1960s to measure radiation and radioactivity levels in the atmosphere up to the height of 40 km. A sonde package based on a Geiger-Müller (GM) tube is carried up to the stratosphere by a balloon filled with hydrogen or helium. En route the GM tube count rate values are sent to the ground station with a radio transmitter. Vaisala Corporation developed a radioactivity sounding system in the early 1960s. Radioactivity soundings were performed at the company's aerological test station in Helsinki as a part of development and quality assurance activities. At least once, in October 1962, these radioactivity soundings revealed the presence of abnormal radioactivity in the stratosphere over Finland due to atmospheric nuclear tests. The Finnish Defence Forces acquired the Vaisala's radioactivity sounding system in 1963. This system was placed at the Finnish Meteorological Institute's (FMI) meteorological observatory of Jokioinen. The staff of the observatory operated the system. Initially radioactivity soundings were performed once a week but later sparser and sparser so that in 1980 only one sounding, in November after the so far last atmospheric nuclear test, was performed. Only a couple of publications have been produced from the artificial radioactivity observations in the upper atmosphere, perhaps due to the sensitive nature of the subject. After 1980 no more radioactivity soundings were performed in Finland during the 1980s, not even during the 1986 Chernobyl accident. In the aftermath of the Chernobyl accident Vaisala developed a new generation of radioactivity sondes that are incorporated into the company's meteorological sounding systems. The FMI has performed these radioactivity soundings since the early 1990s at its sounding stations at Jokioinen, Tikkakoski and Sodankylä. Usually one or two soundings are performed per year, often during nuclear accident preparedness exercises. The radioactivity soundings described above have brought information on cosmic radiation even if the main motivation was the surveillance of artificial radioactivity in the upper atmosphere. However, balloon soundings dedicated to cosmic radiation research have been made by the University of Oulu. These sounding activities were coordinated by SPARMO organization (Solar Particles and Radiation Monitoring Organization), later with the name SBARMO (Scientific Ballooning and Radiation Monitoring Organization). Altogether 114 soundings were performed from 1965 to 1979. In addition to scientific results these activities helped the then new university to network with the international scientific community. Balloon-borne radiation sondes have shown to be a flexible and a cost-efficient method to obtain data on the radiation environment of the upper atmosphere. The information about the vertical distribution of a radioactive plume provided by soundings is essential for a reliable atmospheric dispersion estimation. This, in turn, helps to plan and execute protective measures, e.g. stable iodine prophylaxis. On the other hand, the altitude information about observed airborne radioactivity of unknown origin benefits the inverse modeling to find out the possible source areas of the release. This can be useful for example in detecting clandestine nuclear activities.