Soot's Climate-Altering Properties Change Within Hours of Entering Atmosphere
Billions upon billions of soot particles enter Earth’s atmosphere each second, totaling about 5.8 million metric tons a year — posing a climate-warming impact previously estimated at almost one-third that of carbon dioxide.
Now, researchers say the climate-altering properties of these particles can change within just hours of becoming airborne, rather than days as previously assumed.
A study led by researchers at New Jersey Institute of Technology (NJIT) has revealed the surprising speed at which soot particles gather chemicals and water vapor after being released into the atmosphere.
Researchers say this rapid transformation of airborne soot — known as “atmospheric aging” — could mean its impact on weather, climate and air quality occurs more quickly, and in ways not fully captured by current atmospheric models up until now.
The findings have been highlighted on the cover of Environmental Science & Technology.
“Soot is a unique aerosol that absorbs sunlight extremely well but barely scatters it, which makes it a potent climate agent from the moment it’s emitted,” said Alexei Khalizov, professor of chemistry at NJIT and senior author of the study funded by the National Science Foundation. “What’s surprised us is just how quickly soot changes after entering the air, dramatically altering its ability to warm or cool the atmosphere. Our results suggest that forecasting soot’s climate impact is far more complex than previously realized.”
Until recently, Khalizov says, much remained unknown about how quickly soot nanoparticles change shape and chemistry once airborne, and how these changes affect their ability to trap or reflect solar energy — known as radiative forcing.
According to Khalizov, soot particles rapidly acquire chemical coatings through capillary condensation, a process where tiny crevices on irregular surfaces of soot particles draw in chemical vapors. As humidity increases, the chemicals collected on soot particles help them absorb water, now through capillary condensation of water vapor. This water uptake transforms the particles’ shape and behavior. These hydrated particles can also promote cloud formation, reflecting sunlight and cooling the atmosphere.
“Until now, models treated soot particles as simple spheres, but in reality, soot particles are aggregates — clumps of many smaller particles. The lace-like shape lets soot collect chemicals much faster than previously thought,” Khalizov explained. “That means soot’s climate properties evolve quicker, affecting both its warming and cooling effects, and also its lifetime.”
At NJIT’s Aerosol and Atmospheric Chemistry Lab, the team used a custom-built aerosol system to study how soot particles change after entering the atmosphere, focusing on particles about 240 nanometers wide — the typical size of atmospheric soot. The group exposed these particles to trace gases like sulfuric acid and oxidation products of volatile organic compounds (VOCs) under varying humidity levels to mimic real atmospheric chemical and moisture conditions.
The team tracked key changes tied to atmospheric aging in real time, measuring aspects such as particle size, mass and shape using advanced instruments. Samples were then collected and examined under scanning electron microscopes, revealing how the particles evolved in high resolution.
To complement their experiments, the researchers collaborated with NJIT’s Laboratory for Materials Interfaces led by Professor Gennady Gor to develop a novel computer model to simulate how chemical vapors condense on soot, forming liquid-like coatings that boost the particles’ ability to attract moisture and form clouds — key factors in their climate impact.
Encouraged by the model’s success in describing laboratory results, they then extended their simulations to real atmospheric conditions in collaboration with Professor Nicole Riemer at the University of Illinois Urbana-Champaign.
The results showed that soot particles begin forming coatings and changing shape within tens of minutes, with nearly 80% of particles becoming processed after several hours. For comparison, in simulations where soot was treated as spheres, only 20% became processed — and it took much longer.
This rapid transformation makes the particles more compact and increases their sunlight absorption, intensifying their warming effect, according to Khalizov.
“Initially, these fluffy particles, after mixing with other chemicals, change shape into denser clumps and become more likely to absorb sunlight and convert it to heat, producing more warming,” he explained. “At the same time, they reflect more light and form clouds, leading to cooling. These two competing effects make it more challenging to predict the overall effect of soot while its particles are suspended in the air.”
Khalizov said the study’s insights into soot’s rapid atmospheric aging could lead to more accurate forecasts of its environmental effects, by helping models better represent how soot particles change and influence climate and air quality over time.
The team now plans to explore how these changes affect soot’s lifetime in the atmosphere and its broader effects on weather patterns and public health.
“Our study looked at soot aging in a remote environment. The next big questions involve figuring out the role of this new aging mechanism in a polluted urban environment and testing it within a large-scale climate model,” Khalizov noted. “Addressing these will be key to managing soot’s climate footprint more effectively.”