More plutonium and cesium remain in the stratosphere than in the troposphere (which sits directly above the earth), though the amount is still very miniscule, measured in microbecquerels (1 microbecquerel = 0.000001 becquerel).
First, from BBC News (1/7/2014; emphasis is mine):
Nuclear weapon test debris 'persists' in atmosphere
By Rebecca Morelle
Radioactive particles from nuclear tests that took place decades ago persist in the upper atmosphere, a study suggests.
Previously, scientists believed that nuclear debris found high above the Earth would now be negligible.
However this research shows that plutonium and caesium isotopes are still present at surprisingly high concentrations.
The work is published in the journal Nature Communications.
Lead author Dr Jose Corcho Alvarado, from the Institute of Radiation Physics at Lausanne University Hospital in Switzerland, said: "Most of the radioactive particles are removed in the first few years after the explosion, but a fraction remains in the stratosphere for a few decades or even hundreds or thousands of years."
However, he said the levels were not high enough to pose a risk to human health.
At the height of the Cold War, when the nuclear arms race was in full swing, weapons were being developed and tested around the world.
But more than 50 years on, their radioactive legacy remains.
While nuclear explosions initially throw material up into the air, scientists had thought that the radioactive particles would remain for a limited time.
In the troposphere (the lower layer of the atmosphere that sits directly above the Earth), the isotopes are removed fairly quickly, as they are "washed out" by attaching to rain or snow or are drawn down by gravity.
However, in the stratosphere (the layer that sits from about 10-50km above the Earth), the Swiss team believes that some particles become trapped.
"The concentrations we measured were in the order of about 1,000 to 1,500 levels higher in the stratosphere than in the troposphere," said Dr Jose Corcho Alvarado.
While the tests were carried out over Switzerland, the team said they expected similar levels would be found at the same latitude elsewhere around the world.
The scientists also found that this material can be moved around in the atmosphere by natural events such as volcanic eruptions.
For example, in 2010 after Iceland's Eyjafjallajökull volcano erupted, plutonium levels in the lower atmosphere increased.
While scientists say the long-term effects are not clear, the lead author Dr Jose Corcho Alvarado said: "It is important to say that this is not dangerous for the population."
He added that the nuclear debris could be tracked to find out more about how particles in the atmosphere move around.
The paper at the Nature Communications (peer-reviewed) (emphasis is mine):
Anthropogenic radionuclides in atmospheric air over Switzerland during the last few decades
The atmospheric nuclear testing in the 1950s and early 1960s and the burn-up of the SNAP-9A satellite led to large injections of radionuclides into the stratosphere. It is generally accepted that current levels of plutonium and caesium radionuclides in the stratosphere are negligible. Here we show that those radionuclides are present in the stratosphere at higher levels than in the troposphere. The lower content in the troposphere reveals that dry and wet deposition efficiently removes radionuclides within a period of a few weeks to months. Since the stratosphere is thermally stratified and separated from the troposphere by the tropopause, radioactive aerosols remain longer. We estimate a mean residence time for plutonium and caesium radionuclides in the stratosphere of 2.5–5 years. Our results also reveal that strong volcanic eruptions like Eyjafjallajökull in 2010 have an important role in redistributing anthropogenic radionuclides from the stratosphere to the troposphere.
The diagrams presented in the paper showing the activity concentration of plutonium (239+240) and cesium (137) in the atmosphere. The unit is microbecquerel/cubic meter (μBq/m3):