2026 News & Events

Scientists at NOAA's Chemical Sciences Laboratory (CSL) have uncovered a previously overlooked, but widespread, class of organic-rich, ultrafine particles high up in the Earth's atmosphere.

These tiny particles – most of which are less than 0.11 micrometers in diameter (roughly 100 times smaller than a speck of dust) – were found to be surprisingly abundant in the lowest parts of the stratosphere, nearly 7 miles above the surface.

While too small to be picked up by most satellite sensors and balloon-borne instruments, these particles account for as much as 90% of the surface area available for atmospheric chemical reactions and condensation of trace gases.

"These particles have been mostly invisible to us until now," said Ming Lyu, lead author of the new study published in Science and a researcher with the University of Colorado CIRES affiliated with NOAA CSL. "Most instruments and satellites miss them because they are just too small, but they are really abundant and so, as a whole, they can have a big impact."

Aerosol surfaces provide the 'reaction platform' for a variety of microphysical and chemical reactions in the atmosphere. The amount of particle surface area determines how quickly certain chemical reactions can occur, including those that impact stratospheric ozone.

Stratospheric aerosols also play an important role in global surface climate by either reflecting sunlight or absorbing terrestrial radiation. The direction and magnitude of this effect is highly dependent on size, composition, and abundance.

Tropospheric Origins, Stratospheric Impact

The measurements were made during NOAA's SABRE (Stratospheric Processes, Budget, and Radiative Effects) mission in February of 2023. Researchers loaded NASA's WB-57 high-altitude aircraft with a suite of highly specialized instruments, many developed by NOAA CSL, to make detailed measurements of particle concentrations, size distributions, and chemical makeup, along with numerous trace gases, in the far northern stratosphere.

The newly recognized mode of ultrafine particles was found to be associated with higher levels of nitrous oxide (N₂O), which is a common marker of recent air movement up from the surface (troposphere) since N₂O is emitted only at ground-level from sources such as agriculture, industry, and energy production.

Using particle mass spectrometry, the researchers found that the population of very small aerosols in the lower stratosphere are rich in organics, with about 50% of their mass consisting of organics from surface sources, thus confirming their tropospheric origins. Once formed in the upper troposphere from surface emissions, these organic-rich nanoparticles are carried up into the stratosphere along with N₂O and other pollutants through tropical updrafts, convective storms, and the gradual uplifting of air in the tropics.

A Blind Spot in Atmospheric Monitoring

The new findings highlight a critical gap in current atmospheric monitoring technology. Most satellite instruments are geared toward detecting particles larger than 0.2 µm in diameter, while balloon-borne instruments are limited to sizes greater than 0.15–0.3 µm. As a result, this abundance of organic nanoparticles had gone largely unnoticed and underappreciated.

These new measurements were only achievable thanks to advanced custom-built instrumentation capable of making detailed measurements of particles down to 0.003 µm (3 nm) in diameter. New observations such as these are also a critical test for computer models. In this case, the authors found that a chemistry-climate model could not replicate the observed 'bimodal' size distribution of particles in the lower stratosphere that arises when these tiny nanoparticles mix and combine with the larger, existing stratospheric sulfate particles.

While the model did predict the presence of smaller particles, their formation via new particle formation in the lower stratosphere was inconsistent with in situ observations during SABRE, and the modeled particles were often much larger than observed and contained hardly any organics.

"The model treats all small particles as essentially sulfate – only, but we're seeing a large contribution from organics," said Lyu. "This mismatch could lead to errors in how we simulate particle growth, air chemistry, and aerosol radiative impacts."

Implications for Climate Intervention

The findings are particularly relevant for the potential viability of stratospheric aerosol injection (SAI), a form of climate intervention that involves injecting either particles or gases that would create particles (e.g., sulfur dioxide) into the stratosphere to reflect sunlight and cool the planet.

Many SAI strategies propose deployment in tropical or subtropical regions of the lower stratosphere – precisely where these small organic particles dominate. Not only do these nanoparticles provide an unrecognized expanse of surfaces that drive chemical reactions, they also serve as a 'condensation sink,' soaking up chemical gases. By doing so, they can grow large enough to begin reflecting sunlight.

"If you're thinking about injecting sulfur dioxide vapor or other condensable gases into the stratosphere, these small background particles are going to be the first ones that the new material sticks to," explained co-author Charles Brock. "That changes everything in terms of how you design and predict the effects."

NOAA does not conduct SAI experiments in the atmosphere, so scientists use computer models to try and simulate the effects of SAI on weather and climate patterns.To more accurately simulate stratospheric processes, balloon-borne instruments must be improved to detect particles below 0.1 micrometers, and models must better capture the size, composition, and behavior of organic-rich aerosols.

"Understanding these small particles is critical for predicting how the stratosphere would respond to any sort of perturbation, whether natural, like a volcano, or human-caused," said Lyu. The full dataset from the SABRE mission is publicly available, and the researchers hope it will spur improvements in both modeling and instrumentation.