A previously unrecognized category of chemical reactions appears to play a major role in aerosol formation in the atmosphere. In laboratory studies, researchers at the University of North Carolina at Chapel Hill found that when inert, seed aerosols coated with an inorganic acid, such as H₂SO₄, were introduced into an atmosphere containing aliphatic aldehydes, organic aerosol formation increased by as much as fivefold over experiments with acid-free aerosols. These findings should improve predictive models for aerosol formation, hazy conditions, and climate change.

Secondary organic aerosols have been linked to health effects, blamed for reduced visibility in scenic areas, and are believed to generate a net cooling effect in the troposphere. The U.S. EPA regulates aerosols under the national ambient air quality standards and is under pressure to toughen those standards to include finer particles.

Researchers already know that terpenes from natural vegetation and aromatic hydrocarbons from anthropogenic sources are precursors to organic aerosols. Myoseon Jang and colleagues in Richard Kamens’s laboratory recognized that photooxidation reactions of volatile organic compounds yield large amounts of multifunctional organic carbonyls. Using Teflon film reaction chambers, they were able to demonstrate that particles coated with 2–5% H₂SO₄—a value characteristic of diesel soot—underwent acid-catalyzed heterogeneous reactions with these volatile carbonyls to form organic aerosols. In fact, they report a rich chemistry of new reactions involving species such as di- and tricarbonyls, multifunctional conjugated carbonyls, and alcohols. Increasing relative humidity also affected the reactions by retarding aerosol growth.

The researchers say that standard analytical methods used to study atmospheric reactions tend to miss the aldehyde role, because the reaction products can easily convert back to their original starting material during the sample preparation. They found greater success with infrared spectroscopy techniques.

This new class of reactions is not limited to urban areas. The researchers show that there is sufficient acid in undeveloped locations to promote the carbonyl reactions. Infrared spectra collected in the relatively isolated Smoky Mountains, an area famous for its natural haze, showed evidence of the acid-catalyzed reactions. (Science 2002, 298, 814–817)