New Study will Enable More Accurate Climate Forecasts to be Made

Global climate models may be overstating the warming properties of black carbon particles, according to recent field tests.

"Our results don't change the fact that things are going to warm-just to what degree, literally," says Christopher Cappa, a civil and environmental engineering professor at the University of California, Davis, and a lead co-author of the study published in the journal Science. "Our findings should result in more accurate predictions."

A component of soot, black carbon is created through combustion-when fuels such as oil, coal, and wood are burned. Because black carbon absorbs sunlight, it can warm the atmosphere.

Reducing black carbon in the atmosphere has been a target for near-term climate mitigation. Earlier studies have suggested black carbon's warming impacts are second only to carbon dioxide.

Unlike greenhouse gases, which can live in the atmosphere for centuries, black carbon particles have lifespans of only one to two weeks, making it more difficult to quantify their impacts on a global scale through direct measurements. So scientists have had to rely more heavily on mathematical models to understand black carbon particles' impacts on climate change.

Climate models regarding black carbon have been based on theories and laboratory experiments showing that as other chemicals are condensed onto black carbon particles, the warming properties of these particles increase.

In the new study, an international team of researchers left the lab and went into the field, where they used direct measurements to establish to what extent laboratory experiments translated to the real atmosphere.

The researchers found that the chemical and physical changes that occurred in the field had less impact on black carbon's warming ability than lab experiments and models had forecast.

Atmospheric changes to the black carbon particles increased light absorption by about 6 percent, rather than the 100 percent increase suggested by previous studies.

"This study has implications for thinking about the direct impact of black carbon on Earth's energy budget," says Cappa.

In their new study, Cappa and colleagues collected data during two field studies. For one, Cappa boarded the research vessel Atlantis and sailed along the California coast from San Diego to San Francisco.

The team measured the light absorption by black carbon particles, the extent to which black carbon particles mixed with other chemical components in the urban atmosphere, and how black carbon particles changed over time.

During the second, measurements were made at a ground site in Sacramento.

The research could have implications for regulatory efforts to reduce the emissions of soot from fossil fuel combustion.

Cappa says further measurements on a variety of sources and in more locations are needed to quantify this on a global scale.

The National Oceanic and Atmospheric Administration and the US Environmental Protection Agency funded the work. Timothy Onasch, a principal scientist at Aerodyne Research Inc. in Billerica, Massachusetts, and an associate research professor of chemistry at Boston College, is also lead co-author of the study.

By. Kat Kerlin