|Carbonaceous Aerosols and Climate
How Researchers Proved That Black Carbon is a Significant Force in the Atmosphere
|Contact: Allan Chen, [email protected]|
They can absorb light, or scatter it. They are present in the atmosphere because of the incomplete combustion of fossil fuels. Now they are thought to have a significant impact on global warming. But until just 10 or 15 years ago, the scientific community did not accept that carbonaceous aerosol particles were common in the atmosphere. That they accept it now is because of the work of a research group led by Tihomir Novakov at Lawrence Berkeley National Laboratory, which has been studying these particles since the 1970s.
Carbon aerosol particles are composed of light-scattering organic carbon, OC, and light-absorbing black carbon, BC. They are important to the atmosphere because they can block solar radiation and scatter visible light, and because they are as common as sulfates, which are a well-known particle component of the atmosphere from the burning of fossil fuels.
Carbon and sulfate aerosols, or particles, can affect the climate in two ways. The "direct effect" is the scattering and absorption of solar radiation by aerosols. Both sulfate and carbon (particularly OC) aerosols scatter light back to space thus acting to reduce the warming caused by "greenhouse" gases. BC heats the atmosphere by absorbing the sunlight. However, BC also results in surface cooling because it blocks the light from reaching the surface. There is also an "indirect effect," in which aerosols affect the reflectivity of clouds, making them "shinier." This also has the opposite effect of reducing global warming by reflecting the sun's heat back into space. Climatologists are now trying to understand the sum of these effects on global climate change.
In a recent paper in the Proceedings of the National Academy of Sciences, eminent climate researcher James Hansen of NASA's Goddard Institute for Space Sciences argues that black carbon on the surface of Earth's ice caps absorbs heat from the sun, accelerates the melting of the ice caps, and increases global warming. In that same paper, Hansen called Berkeley Lab's Novakov "the godfather of black carbon studies."
A Voice in the Desert
Novakov's atmospheric aerosol research group, part of what is now Berkeley Lab's Environmental Energy Technologies Division, began studying carbon aerosols in the 1970s; it was one of the Division's earliest research areas. In 1978, there were 18 staff scientists and research associates in the group, publishing annual reports of their research. Many worked at the Lab until they retired; some still do, including Shih-Ger (Ted) Chang, who is now developing methods of improving chemical processes to reduce power plant emissions.
In 1974 Novakov, with Chang and A. B. Harker, published a paper in Science which made the claim that carbon constitutes 50 percent of the total particulate concentration in urban atmospheres, and that as much as 80 percent of the particulate carbon is in the form of soot, i.e., black carbon.
"We were a voice in the desert," says Novakov. "It was an unconventional view and it was a long time before the scientific community agreed with us, even though it made sense to some." The conventional view at the time was that sulfate and OC aerosols were produced primarily by photochemical smog reactions in the atmosphere. Many researchers in the United States thought that black carbon was insignificant in the atmosphere, gone since the Industrial Revolution because of the diminishing number of coal fires used to generate heat in homes and factories. But evidence from Novakov's group was building that would change this thinking.
"What Novakov did is ask why air pollution is black?" says Lara Gundel, a scientist in his group in the 70s. "If air pollution was formed photochemically, then what was the black component?" Gundel, who continues to do research into organic and particle air pollution at Berkeley Lab today, adds, "In the 1970s, the air pollution community was all about smog, and how ozone contributes to its formation. There was not much interest in these particles."
To prove their assertion, Novakov, Ray Dod, Dick Schmidt and other colleagues began to develop new measurement approaches. A 1977 paper, for example, reviewed the use of electron spectroscopy for chemical analysis, or ESCA, to make the first attempt to chemically characterize particulate carbon. ESCA uses electrons from x-rays to identify chemical composition by measuring their spectra under x-ray bombardment. By comparing the spectrum of carbon at room temperature with that of a heated sample, which drives off the volatile organic carbon, the researchers determined that most of black carbon in air samples was inorganic soot, not organic carbon.
On a Solid Footing
By the 1980s, the scientific community had begun to take the group's "black carbon hypothesis" more seriously. In 1980 General Motors sponsored a symposium in Warren, Michigan, titled "Particulate Carbon: Atmospheric Life Cycle," during which Novakov delivered an address on soot in the atmosphere. He first used the term "black carbon" in this paper, which reported on his group's work to quantify soot in various U.S. cities' atmospheres, and on the increasing weight of evidence that black carbon was a substantial part of the atmosphere's particle burden.
In the 1980s Novakov frequently referred to black carbon as "produced solely by the incomplete combustion of fuels," asserting that they were as important in contributing to both local and regional air pollution, such as the Arctic haze, as other pollutants like sulfates. "I tried to emphasize that they were interesting because they were a measure of inefficient combustion," he says.
However, the group still needed better measurements of BCs and their persistence in the atmosphere over time. It was in the early 80s that Hal Rosen and Tony Hansen, physicists in Novakov's group (Rosen is now at IBM, and Hansen is in Berkeley Lab's Engineering Division) began applying optical methods for characterizing and measuring BC. Rosen used Raman spectroscopy to unambiguously demonstrate that BC is composed of graphitic-like carbon. Rosen put together a simple device that measured the absorption of light by black carbon deposited on a filter with air passing through it.
"I deduced that the rate at which the filter became black with carbon was proportional to the amount of carbon in the air," says Hansen. This suggested that a real-time measurement device was possible. Hansen, Rosen, Novakov, and others developed what they called an "aethalometer" to make this measurement, and described it in a 1984 paper. Aethalos is a Greek word meaning "blackened with soot." Lara Gundel's work was essential in making the aethalometer a quantitative device for measuring BC concentrations.
Hansen built a number of these devices. One of the first was used for a study of haze in the Arctic atmosphere that began in 1983. Hansen continued to work for the Lab, but he also began to build aethalometers for researchers around the world, as a private consultant. One of Hansen's still-functioning instruments, built for the Canadian Arctic's Alert research station, has been measuring BCs for 15 years.