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Climate models may overstate black carbon’s influence

August 30, 2012

Resarch vessel Atlantis

Courtesy photo of the research vessel Atlantis that UC Davis professor Chris Cappa and a team of international scientists boarded in 2010 to collect data about black carbon, or soot, in the urban atmosphere.

Global climate models may be overstating the warming properties of black carbon particles, according to new research led by the University of California, Davis. The study will be published online Aug. 31 in the journal Science.

“Our results don’t change the fact that things are going to warm — just to what degree, literally,” said Christopher Cappa, a UC Davis civil and environmental engineering professor. “Our findings should result in more accurate predictions.”

Cappa is the study’s lead co-author, along with Timothy Onasch, a principal scientist at Aerodyne Research Inc. in Billerica, Mass., and an associate research professor of chemistry at Boston College.

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,” said Cappa.

In their new study, Cappa and his colleagues collected data during two field studies in 2010: CalNex 2010 and the Carbonaceous Aerosols and Radiative Effects Study (CARES). For CalNex, 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 CARES, 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 said that further measurements on a variety of sources and in more locations are needed to quantify this on a global scale.

Funding for the study came from the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency.

About UC Davis

UC Davis is a global community of individuals united to better humanity and our natural world while seeking solutions to some of our most pressing challenges. Located near the California state capital, UC Davis has more than 34,000 students, and the full-time equivalent of 4,100 faculty and other academics and 17,400 staff. The campus has an annual research budget of over $750 million, a comprehensive health system and about two dozen specialized research centers. The university offers interdisciplinary graduate study and 99 undergraduate majors in four colleges and six professional schools.

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