Concentrations of radioactive particles injected into the high layer of the atmosphere by activities like nuclear testing are larger than expected. This discovery is reported in a paper published in Nature Communications this week. The work also provides evidence that volcanic eruptions can redistribute the particles from higher to lower atmospheric layers, bringing them closer to Earth.

Nuclear weapon tests, the burn-up of the satellites and accidents in nuclear power plants have injected radioactive material into the high layers of the atmosphere - the stratosphere - over the past 50 years. While deposition of particles onto water droplets or solid surfaces quickly removes most of the radioactive debris from the lower atmospheric regions, radioactive particles remain in the stratosphere much longer as such deposition is not possible. However, researchers believed that the concentration of radioactive plutonium in the stratosphere is negligible. Jose Corcho Alvarado and colleagues now present data from high altitude aerosol collections performed regularly since 1970, which paint a different picture. They establish that the concentration of radioactive plutonium and caesium in the stratosphere is much higher than previously assumed and find that the mean residence time of those particles is 2.5-5 years.

Using the 2010 Eyjafjallajokull eruption as an example, the authors go on to show that volcanic eruptions could potentially cause redistribution from the higher to the lower layers of the atmosphere. However, the elevated concentrations of particles in the lower atmospheric layers are not likely to be worrying for human health at this stage as they are very small.