Inside a Virginia Tech laboratory, Yangyang Liu and Peter Vikesland watched microscopic particles reveal a secret that could reshape how scientists understand air pollution—and why the air we breathe changes as it travels through the sky.
For decades, researchers imagined airborne pollution particles as chemically uniform droplets, the same composition all the way through. But the Virginia Tech team discovered something far more complex: these particles behave like M&M candies, with drastically different chemistry on the inside than on the outside. The interior may be acidic, but the outer surface becomes strongly alkaline thanks to fatty compounds—oils released during cooking or produced in pollution reactions—that coat the particle and create tiny electric fields. This shell fundamentally changes how the particle behaves once it's released into the air.
"Most people imagine a droplet as being the same all the way through, like a drop of water," Liu explained in their recently published findings. "But we discovered that these airborne particles behave more like an M&M candy. The inside and the outside have a very different chemistry."
The discovery matters because most important pollution reactions happen at the particle's surface, where it touches the air. If the surface behaves differently than the interior—and it does—these particles can transform far faster than scientists have historically expected. This reshapes our understanding of three critical questions: what we actually breathe when cooking smoke, wildfire smoke, or urban pollution fills the air; how pollution travels and how long it lingers in the atmosphere as it spreads; and how weather and climate models should account for these particles, which help form clouds and influence how sunlight moves through the atmosphere.
The implications ripple outward to pollution prediction models that governments and health agencies rely on to forecast air quality and health impacts. If particle surfaces behave differently from their interiors, those models need updating to reflect reality.
To make their discovery, Liu and Vikesland didn't collect air samples directly from the field. Instead, they created controlled laboratory simulations, generating tiny aerosol droplets coated with fatty acids and using confocal Raman microscopy to study the chemistry happening at the droplet surface. These controlled conditions allowed them to measure the extremely small electric fields at play and observe the complex reactions that form a highly alkaline outer shell. It's meticulous work—the kind that reveals hidden layers in the systems we move through every day without noticing.
Their findings, published in the Proceedings of the National Academy of Sciences, challenge a long-held assumption and point toward a more dynamic picture of how pollution evolves once released into the atmosphere. By revealing how much chemical activity happens on particle surfaces, the research underscores a simple but urgent truth: the air we breathe is far more active than we thought, and understanding that activity is essential if we want to predict where pollution goes and how it affects the people breathing it in. As cities work to improve air quality and scientists refine climate models, this fundamental shift in understanding could prove invaluable.