Reactions on the surface of airborne particles may explain why they grow faster than traditional models predict (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1120593109). Atmospheric aerosols affect precipitation and climate, and they are also associated with adverse health effects. Whereas primary aerosols are emitted directly into the air, secondary aerosols form in the atmosphere. Traditionally, scientists believed that secondary organic aerosols grew through a mechanism in which volatile organic compounds reacted with atmospheric oxidants, making the compounds less volatile and able to condense onto particles. Now, a group led by Barbara D’Anna of Claude Bernard University, in Lyon, France, sees evidence for aerosol growth through light-induced heterogeneous reactions on the particles’ surfaces. Using a laboratory flow-tube reactor with volatile terpenes and seed particles containing humic acid, a component of soil formed through biodegradation of organic matter, they found that the particles appeared to trap and oxidize the terpenes, probably through radical-producing photoreactions of humic acid. The experimental particle growth rates match those observed in field studies, suggesting that the heterogeneous reaction mechanism should be included in atmospheric modeling, the researchers say.