CIRES and NOAA researchers have discovered a widespread class of ultrafine aerosol particles in Earth's stratosphere, a finding that reveals a significant gap in scientific understanding of a key region of the atmosphere. 

Researchers discovered the particles during a 2023 field campaign. The tiny aerosols, roughly 100 times smaller than a dust particle, are surprisingly abundant in the lowest parts of the stratosphere, accounting for as much as 90 percent of the total aerosol surface area.  

The discovery of these “nanoparticles” highlights a critical shortcoming in current atmospheric monitoring methods, according to the researchers. Most satellite and balloon instruments can’t detect particles this small, so these nanoparticles have gone largely unnoticed.

“These particles have been mostly invisible to us until now,” said Ming Lyu, a CIRES researcher working in NOAA’s Chemical Sciences Laboratory and lead author of a new study detailing the findings. “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 [on the stratosphere].”

Improved instruments lead to discovery

Aerosols are tiny solid particles or liquid droplets suspended in the air. Their surfaces provide the "reaction platform" for a variety of physical and chemical reactions in Earth’s atmosphere. The amount of particle surface area determines how quickly certain chemical reactions can take place, including those that impact the ozone layer.  

Aerosols are abundant in Earth’s stratosphere, which spans from about 6 to 31 miles (10 to 50 kilometers) above Earth's surface and contains the protective ozone layer. Stratospheric aerosols play an important role in regulating Earth’s climate by reflecting and absorbing heat and sunlight. 

Most of what scientists know about stratospheric aerosols comes from satellite sensors and balloon-borne instruments. However, these instruments usually can’t detect particles smaller than 0.2 microns in diameter. 

But NOAA and CIRES scientists built custom instruments that can detect particles as small as 3 nanometers in diameter (about 100 times smaller than the smallest particles detected by traditional sensors). They installed these instruments on NASA’s WB-57 high-altitude aircraft and collected observations from the Arctic stratosphere during the February 2023 SABRE mission. SABRE flights take detailed measurements of stratospheric particle concentrations, size distributions, chemical makeup, and numerous trace gases. 

The 2023 SABRE flights allowed researchers to observe these ultrafine particles, which are smaller than 0.2 microns in diameter, for the first time. The researchers found that about half of these nanoparticles were organic chemicals lofted to the stratosphere from Earth’s surface. 

Implications for climate intervention 

The new findings are particularly relevant for assessing the viability of stratospheric aerosol injection (SAI). This proposed climate intervention method involves injecting sulfur dioxide into the stratosphere to reflect sunlight and cool the planet.

“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,” said Charles Brock, co-author of the new study. “That changes everything in terms of how you design and predict the effects.” 

According to the researchers, balloon-borne instruments must be improved to detect particles smaller than 0.1 microns in diameter if scientists want to more accurately simulate stratospheric processes. Scientists will also have to revise their atmospheric models to better capture the size, composition, and behavior of organic-rich aerosols, the authors said.

“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,” Lyu said.