Toxic chemicals that pollute groundwater are formed up in the stratosphere, surprise findings show

A toxic chemical that pollutes groundwater originates high in Earth's atmosphere, a scientific flight has revealed.

Perchlorates, a group of chlorine-containing chemicals that can disrupt thyroid function, form on particles of smoke and organic material in the stratosphere, between 6 and 31 miles (10 to 50 kilometers) above the planet's surface, according to a new study.

Scientists already knew that perchlorates form high in the atmosphere, because the natural forms of these chemicals show signs of being zinged by cosmic rays from space. However, it wasn't clear exactly how and where they form.

The stratosphere is largely populated by tiny sulfuric acid particles, but the researchers found that perchlorates didn't stick to these common particles. Instead, the team found the toxic chemicals almost exclusively on particles rich in nitrogen and particles from smoke — neither of which frequently make their way so high into the atmosphere.

The question now is whether increasing human pollution of the stratosphere might also increase perchlorates contaminating groundwater, when they eventually fall to Earth's surface.

"We don't know if changing the particles in the stratosphere will cause more perchlorate or not," said Daniel Murphy, the program lead in aerosol properties and processes at the National Oceanic and Atmospheric Administration (NOAA) Chemical Sciences Laboratory. The new study is a call for more research on that question, he told Live Science.

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Synthetic perchlorates are found in explosives, batteries, airbags and rocket propellant. Manufacturing can sometimes lead to groundwater pollution, but most perchlorates are natural. The Environmental Protection Agency (EPA) is set to propose national perchlorate drinking water regulations by November.

Murphy and his colleagues discovered where perchlorates form during a larger project to explore the aerosol particles of the stratosphere. The data mostly came from NASA's WB-57 aircraft, which can fly up to 62,000 feet (19,000 meters). Commercial flights typically stay between 30,000 and 42,000 feet (9,100 to 12,800 m).

"These were the most detailed measurements of the perchlorate," Murphy said. "They had information we'd never had before on what the perchlorate was like for that natural source in the stratosphere."

The researchers, who published their research July 28 in the journal PNAS, compared the perchlorates they detected to previous measurements of perchlorates in rocket fuel and found they were not the same kind. In other words, the stratospheric concentration is not caused by rocket launches but by natural processes in this layer of the atmosphere.

It's currently unclear whether the fact the perchlorates cling only to organic and smoke particles is an intriguing quirk of the chemical or if it has broader implications, Murphy said. If the existence of these particles enables perchlorates to form, adding more of them to the stratosphere could create more of the toxic chemical. This could be a concern because perchlorates that fall to the surface can last a very long time — at least 10,000 years in arid environments, according to 2010 research.

"We know, for example, that wildfires are increasing globally and that might mean that this natural source of perchlorate might increase," Murphy said.

Some geoengineering schemes also propose injecting particles into the stratosphere to reflect sunlight, which could be risky if those particles trigger perchlorate formation.

However, researchers are already set-up to investigate these chemicals. Perchlorate has been found on Mars, which means planetary scientists are already studying it. "People may have built up some lab instrumentation for that reason and they can shift it over," Murphy said. Laboratory studies on perchlorate chemistry can determine whether human activity might inadvertently increase perchlorates at such dizzying heights, he added.