Wildfire Smoke Particles Cling Together Far Longer Than Models Predict, Purdue Study Finds

Organic molecules inside wildfire smoke, pollution and urban haze can "hold on" to one another in ways that dramatically slow how quickly they evaporate from the atmosphere, according to a new study led by Purdue University chemistry professor Alexander Laskin, published in the Proceedings of the National Academy of Sciences. Existing atmospheric models estimate evaporation based on how volatile an individual chemical is on its own, without accounting for the hundreds of other molecules surrounding it in a real mixture.

Using a novel mass spectrometry technique developed in Laskin's lab, the team tracked more than 1,500 individual chemical species across 33 complex mixtures representative of real biomass-burning smoke and urban haze. The surrounding molecular neighborhood, rather than a chemical's own properties, often determined how easily it escaped. "Spray perfume onto a glass plate and the scent disappears quickly. Spray the same perfume onto a thick wool sweater and the smell lingers for days because the fabric traps the fragrance molecules," Laskin said. "An aerosol mixture acts like a wool sweater, holding molecules much more strongly than if they were alone."

This "matrix effect" slowed evaporation by three to five orders of magnitude, meaning current models that assume smoke disperses quickly likely underestimate how long it actually stays in the atmosphere. Working with the Weizmann Institute of Science to apply explainable machine learning and the Department of Energy's Pacific Northwest National Laboratory to run atmospheric models, the team showed the effect significantly increases predicted smoke persistence, transport and cloud formation.

The researchers now aim to build machine learning models that account for interactions among thousands of molecules simultaneously, rather than treating each compound independently, to improve air-quality forecasts, assess public health risks, and guide environmental mitigation policy.