Field study finds soot particles absorb significantly less sunlight than predicted by models; climate models may be overestimating warming by BC
1 September 2012
Although viewed as a potential target in the global effort to reduce climate change, atmospheric black carbon particles absorb significantly less sunlight than scientists have predicted, according to a new study by an international team of researchers, published in the journal Science.
Mathematical models and laboratory experiments used to study airborne soot particles led to projections that the absorption-boosting chemicals that coat black carbon could yield an increase in absorption by as much as a factor of two. But direct measurements in two California regions found black carbon absorption enhancements of just 6%, suggesting that climate models may be overestimating warming by black carbon, the researchers report.
Atmospheric black carbon (BC) warms Earth’s climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (Eabs) and mixing state are reported for two California regions. The observed Eabs is small–6% on average at 532 nm–and increases weakly with photochemical aging. The Eabs is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial Eabs for BC are possible.—Cappa et al.
The results highlight the early challenges in a nascent sector of climate science and could have implications for regulatory efforts to reduce the production of black carbon, or soot, by curbing the burning of fossil fuels. 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.
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. 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.—Boston College Professor of Chemistry Paul Davidovits
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 (SLCFs), including methane gas and ozone, which are produced on earth.
During their lifetime, black carbon particles are coated with airborne chemicals, which laboratory tests have shown can act like lenses capable of increasing the ability of the particles to absorb sunlight and heat the atmosphere. That has 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. (Earlier post.)
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 near the California cities of Los Angeles, San Francisco and Sacramento.
Researchers tested air samples using a combination of real-time techniques, including aerosol mass spectrometry and photoacoustic spectroscopy. These techniques are capable of making measurements to determine the chemical, physical and optical properties of the black carbon particles, said Onasch, whose Billerica, MA-based company has developed the aerosol mass spectrometer instruments.
Onasch said the recent 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.
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. 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?—Timothy Onasch
Christopher D. Cappa, Timothy B. Onasch, Paola Massoli, Douglas R. Worsnop, Timothy S. Bates, Eben S. Cross, Paul Davidovits, Jani Hakala, Katherine L. Hayden, B. Tom Jobson, Katheryn R. Kolesar, Daniel A. Lack, Brian M. Lerner, Shao-Meng Li, Daniel Mellon, Ibraheem Nuaaman, Jason S. Olfert, Tuukka Petäjä, Patricia K. Quinn, Chen Song, R. Subramanian, Eric J. Williams, and Rahul A. Zaveri (2012) Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon. Science 337 (6098), 1078-1081. doi: 10.1126/science.1223447
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