Wildfire smoke causes more air pollution than current atmospheric models can predict. A new study by researchers at King Abdullah University of Science and Technology (KAUST) and the Chinese Academy of Sciences explains why by revealing that, under sunlight, wildfire smoke particles act like tiny chemical factories, producing harmful oxidants such as peroxides, a group of highly reactive pollutants contributing to smog and haze.
The new study helps explain why field measurements consistently detect elevated peroxide levels during wildfire events, even in cities, where the normal 'gas-phase' chemical routes that create them should be blocked by other pollutants such as nitric oxide, a common gas produced by burning fuel.
Professor Chak Chan , study co-author and dean of KAUST's Physical Science and Engineering Division, said the study shows that smoke particles can bypass traditional suppression by nitrogen oxides in polluted environments by generating oxidants internally under sunlight.
"This particle-driven pathway is surprisingly efficient — orders of magnitude faster than what classical pathways can supply," he said.
The team discovered that colored organic molecules in biomass-burning aerosols act as "photosensitizers." When they absorb sunlight, they enter excited states that trigger rapid chains of reactions, producing peroxy radicals and then peroxides inside the particles.
Peroxides are not greenhouse gases, but they do impact atmospheric chemistry in ways that drive haze, secondary particle formation, and respiratory risks. By acting as radical reservoirs, they also influence broader climate and air-quality dynamics.
The findings reveal how wildfire smoke can drive the formation of secondary particulate matter, in addition to being a direct source of particulate matter, a major component of urban air pollution. This has significant implications. Wildfires have quadrupled in size in parts of the western United States since the 1980s, while Mediterranean burn areas have more than doubled in the past two decades. As fires occur more frequently and intensely worldwide, their smoke is increasingly emitting reactive particles that sunlight changes into hidden sources of pollution.
"This overlooked chemistry means that current air-quality and climate models are underestimating oxidant production from wildfires," Chan said. "Updating these models is essential for communities, including here in Saudi Arabia, to better anticipate the health risks and environmental impacts of a warming world."
