But black carbon particles still seen as important climate factor
By Summit Voice
SUMMIT COUNTY — Soot may not play as big a role in atmospheric warming as previously believed, according to a new study based on field observations in sometimes smoggy Southern California.
Some previous efforts at modeling the warming impacts of soot suggested that the tiny atmospheric black carbon particles could increase heat absorption by as much as a factor of two, but the new research shows black carbon absorption enhancements of just 6 percent, suggesting that climate models may be overestimating warming by black carbon.
“These findings do require us to reduce our projections about the amount of heating soot produces, at least under some experimental conditions,” said Boston College Professor of Chemistry Paul Davidovits, an authority on airborne particles, known as aerosols.
But the findings don’t point to soot as being a harmless climate forcer,” said Davidovits. “Soot remains an important climate heating agent, as well as a health problem that has been well documented.”
“The team’s field measurements in California showed the enhancement of absorption was very small — approximately six percent instead of by a factor of two,” Davidovits said. “In one respect, it shows that nature is much more complicated than our initial laboratory experiments and modeling indicated. Now we will try to unravel and understand that complexity.”
The results suggest that tackling soot as a way to make inroads against global warming may not be as effective as believed. Still, scientists agree that black carbon in the atmosphere has a significant effect on global and regional climate, with earlier studies ranking the warming effects of black carbon particles second only to carbon dioxide gas.
Unlike carbon dioxide and other greenhouse gasses, which can survive in the atmosphere for decades and centuries, black carbon has a relatively short life span of approximately one to two weeks. Black carbon is part of a group of pollution sources known as Short-Lived Climate Forcers, including methane gas and ozone, which are produced on earth.
During their lifetime, black carbon particles are coated with airborne chemicals, which sophisticated laboratory tests have shown can act like lenses capable of increasing the ability of the particles to absorb sunlight and heat the atmosphere. That raised a critical question as to whether targeting black carbon emissions in an effort to reduce climate change could yield relatively quick results on a regional or global level.
Led by principal investigators Christopher D. Cappa, a professor of engineering at the University of California, Davis, and Timothy B. Onasch, principal scientist at Aerodyne and an associate research professor of chemistry at Boston College, the team analyzed air samples in real-time near the California cities of Los Angeles, San Francisco and Sacramento.
Complex measurements helped determine the chemical, physical and optical properties of the black carbon particles, Onasch said. The new findings set the stage for further studies around the world under different atmospheric conditions in order to better understand how chemical coatings from a range of emission sources affect the absorptive properties of black carbon,” he added.
“When you put a soot particle into the atmosphere, we known it contains an elemental carbon component and we know what it’s absorption will be based on mass and size,” said Onasch. “But black carbon particles in the air are constantly changing. They collect inorganic and organic materials, they grow, change shapes, and change composition. These changes affect the absorption or warming capability of the black carbon. So the question remains: to what extent exactly?”