2025 News & Events

A new study led by NOAA Research has confirmed a surprisingly high altitude source of groundwater contamination from perchlorates – a class of toxic chlorine-based chemicals known to disrupt thyroid function in humans. The study, led by researchers at NOAA's Chemical Sciences Laboratory (CSL) and published in the journal Proceedings of the National Academies of Science, provides new evidence linking perchlorate formation to tiny airborne particles (aerosols) in the stratosphere, the layer of the atmosphere 7–30 miles above Earth's surface.

Drinking water standards in some U.S. states limit perchlorates, which are used in products such as rocket propellant, explosives, fireworks, batteries and automobile airbags. Although localized perchlorate contamination can arise from industrial sources, past chemical analyses of groundwater and snow samples found that most of the perchlorate deposited to Earth's surface is actually formed in the atmosphere. Measurements of the chlorine atom isotope (³⁶Cl) have implicated the stratosphere in particular, but until now, direct observations supporting this hypothesis had been lacking.

The new study makes use of detailed aerosol chemical measurements collected from high altitude research aircraft in the lower stratosphere. These measurements reveal the presence of perchlorates that, unexpectedly, were concentrated almost entirely in types of aerosol particles that are rare at high alitudues – specifically, those originating from biomass burning and nitrogen-rich sources. These particles make up only a small fraction of the total stratospheric aerosol population, but carry nearly all of its perchlorate load.

"This was a real surprise," said lead author Daniel Murphy of NOAA CSL. "We found that perchlorate avoids the most common stratospheric aerosol particles, which are mostly sulfuric acid. Instead, it clings to particles with more organic content and less acidity – like those from wildfires."

And because the perchlorates are attached to particles, rather than in the gas-phase, they get an expedited ride down to the surface through gravitational settling and precipitation. Once in the ground, perchlorates are astonishingly long-lived, particularly in arid regions where they can persist and build up for thousands of years. Perchlorate concentrations in snow from both Greenland and Antarctica correlate with the increases in stratospheric chlorine from chlorofluorocarbons (CFCs) used as refrigerants from the 1960s to 1990s, highlighting what could be the longest lasting legacy of CFC emissions.

"This study highlights that controls on CFCs by the Montreal Protocol, which were implemented because of Ozone Layer depletion, also prevented some perchlorate groundwater pollution," said David Fahey, Director of CSL and a Co-chair of the Scientific Advisory Panel for the Montreal Protocol.

The new research draws on data collected from three major airborne missions: SABRE (Stratospheric Aerosol processes, Budget and Radiative Effects), DCOTSS (Dynamics and Chemistry of the Summer Stratosphere), and ATom (Atmospheric Tomography Mission), during which an aerosol mass spectrometer called PALMS was used to directly sample the composition of hundreds of thousands of individual aerosol particles from near the surface to the stratosphere and spanning the Southern and Northern Hemispheres during multiple seasons.

The strongest perchlorate signals came from the stratosphere, especially in the mid to high latitudes, with higher levels observed in the southern hemisphere than the northern hemisphere. Despite abundant biomass burning and other less acidic particle types near the surface, the researchers found virtually no perchlorate formation in the lower atmosphere.

One of the most significant findings of the study is that perchlorates are found in non-acidic aerosol particles. It cannot be conclusively determined from this work whether such particles simply collect perchlorate from the air or if they actively cause perchlorate to form on their surfaces; but these new observations, in conjunction with other recent studies, point toward some type of surface-enhanced formation mechanism.

Indirect evidence of this comes from perchlorate spikes previously measured in polar ice cores that correlate with volcanic eruptions that reached the stratosphere. Major pyrocumulus injections of smoke into the stratosphere from intense wildfires, such as occurred during the 2019-2020 Australian bushfires, could also ramp up perchlorate formation.

The estimated amount of perchlorate in the lower stratosphere represents less than 1% of the total reactive chlorine budget, so its abundance is limited only by the efficiency of its formation, not the amount of available chlorine.

Detailed laboratory studies, Murphy notes, would be needed to answer many of the open questions still remaining about the exact chemical mechanism at work.

If it is indeed the case that perchlorates are forming on aerosol surfaces, it would imply that adding particles other than sulfuric acid to the stratosphere, either intentionally or unintentionally, could result in increased production and surface deposition of perchlorates.

This would have implications for the potential side effects of climate interventions like stratospheric aerosol injection (SAI), which proposes injecting additional aerosols into the stratosphere to reflect sunlight and cool the planet. If perchlorate forms more easily on non-acidic particles, as this study suggests, then certain particle types, like calcium carbonate, might provide a favorable surface for perchlorate formation.

"There's a real caution here for things like SAI," said Murphy. "Perchlorate is a reminder that not all of the chemistry of the stratosphere is fully understood, and modifying it could bring unexpected consequences."