Car and Coal Emissions Interact

A mix of pollutants that's unique to urban areas may be stimulating the formation of noxious aerosols in the atmosphere.

The aromatic acids produced when automobile exhaust photochemically oxidizes appear to help atmospheric sulfuric acid form the seed particles that become aerosols.

Aerosols are an environmental scourge implicated in numerous human health problems, including lung disease. It’s well known that sulfuric acid—formed from coal burning in power plants—is one of the primary components of newly forming aerosol particles, but how the particles nucleate so fast has been a long-standing mystery.

Now, Texas A&M University atmospheric sciences professor Renyi Zhang and colleagues, including Nobel Laureate and chemistry professor Mario J. Molina at Massachusetts Institute of Technology, have discovered that adding toluic or benzoic acid to sulfuric acid under atmospheric conditions increases the rate of aerosol formation 10-fold [Science, 304, 1487 (2004)].

The authors also performed quantum mechanical calculations that show the aromatic acids form stable complexes with sulfuric acid, which are characterized by very strong hydrogen bonds. “We believe that’s the main reason why the complex favors particle formation,” Zhang says.

Researchers have studied the possibility that other compounds, including ammonia and iodides, may also interact with sulfuric acid to boost aerosol production. Organic acids are “another possibility, which seems to me likely to be a significant contributor, given the ubiquitous presence of organics in air,” says Barbara J. Finlayson-Pitts, chemistry professor at the University of California, Irvine.

University of Helsinki aerosol and environmental physics and chemistry professor Markku Kulmala is more skeptical, noting that the aromatic-acid-induced increase in aerosol production rate is fairly small and that other compounds may be more important in helping aerosols nucleate.

But Zhang says this new mechanism is significant because it helps explain why aerosols are so prevalent in urban areas. “It provides a direct link between organic compounds produced by automobiles and atmospheric sulfuric acid,” he says.