Aerosol Origins

ATMOSPHERIC CHEMISTRY models have long underestimated the actual levels of organic aerosols in the air, in some cases by a factor of 10. Now, researchers at Carnegie Mellon University believe that the gap in the models involves oxidation of hydrocarbon emissions into potentially hazardous secondary organic aerosols (Science 2007, 315, 1259).

Fine particulate matter in the atmosphere is composed of sulfates, primary organic aerosols such as hydrocarbons from exhaust pipes, and secondary organic aerosols (SOAs), which are more highly oxidized species.

Identifying a missing source of SOAs is more than just bookkeeping, comments Paul J. Ziemann, an atmospheric chemist at the University of California, Riverside. "The important point is that secondary organic aerosols are highly oxidized and hydrophilic. Consequently, SOA-containing particles are able to take up water more readily and to act as nuclei for cloud formation."

This capacity means that SOAs affect climate cooling. To boot, many believe that highly oxidized organic compounds are likely to be dangerous, Ziemann says. "SOAs are thought to be a prime candidate for causing at least some of the health effects" that led to current EPA air pollution standards for fine particulate matter.

Previously studied SOAs include those that stem from oxidation of monoterpenes from trees or toluene from motor exhaust. But these highly volatile SOAs don't exist long enough to form much particulate matter in the atmosphere, notes Frank N. Keutsch, an atmospheric chemist at the University of Wisconsin, Madison.

Allen L. Robinson and colleagues identified an unrecognized source of SOAs that now seems so obvious, he says, that "it's surprising we didn't figure it out before. Instead of condensing immediately into primary organic aerosols, a large fraction of the hydrocarbons churning out of cars are being photo-oxidized into secondary organic aerosols." Thus the emissions stay in the gas phase long enough to be oxidized by the sun.

These oxidation products then condense into particulate matter and provide a supply of SOAs that models have been missing. Experiments in smog chambers support the conclusion, Robinson says, as do comparisons of the predictions of revised models with actual organic aerosol measurements. The results suggest that the proportion of SOAs among organic aerosols may increase from 60% to 90% in some areas.

The results "may have an impact on vehicle emissions testing, if regulators listen to what Robinson and coworkers are saying," Ziemann notes.