January 7, 2014
Particles of radioactive plutonium from nuclear testing have remained high in the stratosphere for more than 50 years, and volcanic eruptions such as Iceland’s Eyjafjallajökull in 2010 can bring those particles into the lower atmosphere, researchers report January 7 in Nature Communications. They caution, however, that the concentrations of particles in the lower atmosphere are small and do not threaten human health.
Between 1945 and 1998, nations around the world tested nuclear weapons underground, underwater and high in the atmosphere. The atmospheric tests, conducted in the 1950s, 1960s and 1970s—along with the burn-up of a plutonium-powered SNAP-9A satellite in 1964—created radioactive debris that became attached to particles in the air, called aerosols. In the troposphere—the lowest part of the atmosphere extending from the ground to about 17 to 20 kilometers up—these particles washed out within weeks to months. But a combination of factors, such as the barrier-like tropopause, keep the particles in the stratosphere (the next layer up, extending to about 50 kilometers) for longer. But how long?
Studies done in the 1960s and 1970s, in which aerosols were sampled with aircraft and balloons, showed that most radioactive particles lingered in the stratosphere for about one to four years. Larger particles, those in the range of one to 10 micrometers settled even faster, last
only weeks to months in the stratosphere. (The particles didn’t just disappear; they moved down into the troposphere during interruptions in the tropopause that allow mixing between the troposphere and stratosphere, events that happen most often in spring.) Because nuclear tests were conducted so long ago, all these radioactive stratospheric particles should mostly be gone by now, scientists had concluded.
The 2010 eruption of Eyjafjallajökull started scientists to think that those conclusions might be wrong. When the volcano erupted, researchers in Switzerland began taking aerosol samples from the troposphere, and they found elevated concentrations of radioactive particles. Levels of plutonium and cesium (another byproduct of nuclear testing) were up to three orders of magnitude higher than levels found in aerosols at ground level. These measurements contradicted the earlier aerosol studies, which had found low levels throughout the troposphere; something was up.