Carbon aerosol particles, a product of incomplete fossil-fuel burning, are composed of both light and dark components.

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.

Tihomir Novakov and his colleagues were among the first to assert that half of urban particulate concentration is carbon, of which 80 percent is soot. (Portrait photo Roy Kaltschmidt)

“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 1980 Novakov gave the name “black carbon” to particles that are the result of inefficient burning of hydrocarbons.

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.

Indications of long-distance transport

With reliable measuring devices now available to detect black carbon (BC), as well as sampling and analysis techniques to measure the chemical content of atmospheric particles, it became possible to determine how widely BCs spread across the globe from their origins in cities and industrial areas. Novakov, Rosen, and other members of this group, as well as investigators elsewhere, sampled the Arctic atmosphere during the late 70s and early 1980s to determine whether BCs could be found in what was thought to be a pristine environment. Indeed, papers published during these years revealed substantial concentrations of soot throughout the Arctic. In one study, Novakov and colleagues sampled areas of the Alaskan, Canadian, and Norwegian Arctic for BCs—and found them.

Tony Hansen is pictured here at a South Pole research station with an aethalometer (from Greek aethalos, soot), which he and others invented. Used to measure black carbon in the Arctic in the 1980s, it has more recently been used to measure carbon particle concentrations in the Antarctic.

“These results show that the large concentrations of particles found at the Barrow Alaska site are not a local phenomenon but are characteristic of ground-level stations across the western Arctic,” wrote Rosen and Novakov in a 1983 paper. They speculated that “these highly absorbing particles” could have a considerable effect on the Arctic radiation balance and climate.

“If it was in the Arctic, it must be everywhere,” says Novakov today about his thinking at the time. Because BC particles were anthropogenic in origin (from incomplete combustion of fossil fuels), it meant that human activities were having an effect in the farthest reaches of the globe.

“It was obvious that this was evidence of long-range transport,” says Hansen. Even more, the heat-absorbing properties of BC could have a warming effect in that region. The work created a stir in the scientific community, which now had direct proof of the long-range transportation of pollutants—and another question to answer. What effect were BCs having on the climate of the earth?

In 1987, Novakov worked with a group of scientists at New Mexico State University to help them measure the graphitic content of carbon taken from snow samples in New Mexico, Texas, Antarctica, and Greenland. Black carbon’s presence in all of these samples, and in other studies, showed that it had settled out from the atmosphere all over the world.

Black and organic carbon: players in the “indirect effect”

The emergence of this data brought much wider attention to black carbon particles from a scientific community that, by the late 80s to early 90s, had accepted BC presence in the atmosphere as proven. Researchers began to focus their sights on understanding the relative weights of BC’s direct effect, which absorbs and scatters heat and increases global warming, versus its indirect effect, which makes clouds shinier and therefore more reflective of heat, cooling the atmosphere.

This view of aerosol haze near Houston indicates how effective black carbon can be at causing cloud formation. (Photo NASA)

New scientific interest brought research results from groups around the world. The black carbon hypothesis had been proven, but Novakov and colleagues continued to make significant contributions to carbonaceous aerosol science. Novakov turned his attention to the “indirect effect”; he asked whether the other component of carbon aerosols, the organic carbons were present in high enough concentrations in the atmosphere to affect cloud formation. Again, Novakov’s hypothesis went against the conventional wisdom, which was that sulfates, not carbon, were the major player.

In a 1993 paper, Novakov and J. Penner of Lawrence Livermore National Laboratory, demonstrated that organic carbon is an equally effective nucleus for the formation of cloud droplets as sulfate particles are. This finding showed that sulfates, the product of fossil fuel combustion, were not the only significant man-made source of cloud nucleation in the atmosphere, as had been previously thought.

Two papers Novakov published in 1997 with Peter Hobbs and other colleagues at the University of Washington reported measurements of aerosols on the east coast and mid-Atlantic coast of the U.S. Both demonstrated that carbon aerosols contribute more than sulfate to the extinction of solar radiation in the atmosphere in some locations. “I think this work was a turning point,” says Novakov. These papers were cited extensively, and the scientific community had again accepted another hypothesis about carbon particles proposed by Novakov, that they compete with sulfate in climate forcing by aerosols. Now, the scientific community focused more and more on what BCs and OCs were doing to the climate.

“Novakov’s persistence, clarity of thought, and ability to be a maverick has led to real progress in this field,” says Gundel. “He asked interesting questions without being part of the mainstream. The mainstream eventually caught up to him.” As a case in point Novakov has helped organize seven international conferences on carbon particles in the atmosphere since the first one in 1978. The eighth will take place in September 2004 in Vienna, Austria.

A Summation, and the Future

One legacy of Novakov’s group comes from the work of its alumni. Tony Hansen has continued to build aethalometers and to work in the Lab’s Engineering Division, developing state-of-the-art instrumentation for a wide variety of projects from every scientific discipline. Ted Chang continues to work in the Environmental Energy Technologies Division, where his current research deals with improving air pollution control processes and technologies. Hal Rosen now works at IBM.

	Lara Gundel has continued to design devices like this “denuder,” lined with microscopic resin beads, to trap and measure atmospheric pollutants.

Lara Gundel conducts research projects on new methods to accurately measure semivolatile and particulate organic pollutants in ambient air and combustion sources. She won an R&D 100 award in 2000 for developing a fine sorbent coating used in air-sampling devices called diffusion denuders to improve the accuracy of sampling of airborne particles. Ray Dod is retired, but still works with Gundel on studies of organic pollutants. Dick Schmidt continues to make significant contribution to design and development of aerosol instrumentation.

Carbon aerosol research continues at Berkeley Lab, with the involvement of new researchers. Recent papers from the Novakov group focus on determining the history of black carbon concentrations in the atmosphere; to better understand what contribution they have made to climate change over time. The next article in this series will address this research.

Additional information

Following is a bibliography of articles in print.

• Hansen, J. and Nazarenko, L. 2004. “Soot climate-forcing view snow and ice albedos,” Proceedings of the National Academy of Sciences 101:2, 423-428.
• Novakov, T., Chang, S.G., and Harker, A.B. 1974. “Sulfates as pollution particulates: catalytic formation on carbon (soot) particles,” Science 186: 259-261.
• Novakov, T., Shang, S.G. and Dod, R.L. 1977. “Application of ESCA to the analysis of atmospheric particulates,” Contemporary Topics in Analytical and Clinical Chemistry, Vol. 1. Hercules, D.M., Hieftje, G.M., Snyder, L.R. and Evenson, M.A. Plenum Press.
• Novakov, T. 1982. “Soot in the atmosphere,” in Particulate Carbon: Atmospheric Life Cycle, Wolff, G.T. and Klimisch, R.L. eds. New York: Plenum Press.
• Novakov, T. 1987. “The role of analytical chemistry in assessing atmospheric effects of combustion,” in Euroanalysis VI: Reviews on Analytical Chemistry. Paris: Les editions de physique.
• Hansen, A., Rosen, H. and Novakov, T. 1984. “The Aethalometer: an instrument for the real-time measurement of optical absorption by aerosol particles,” The Science of the Total Environment, 36:191-196.
• Rosen, H., Novakov, T. and Bodhaine, B., 1981. “Soot in the Arctic,” Atmospheric Environment, 15:1371-1374.
• Rosen, H., Hansen, A. and Novakov, T. “Role of graphitic particles in radiative transfer in the Arctic haze,” The Science of the Total Environment, 36:103-110.
• Rosen, H. and Novakov, T. “Combustion generated carbon particles in the Arctic atmosphere,” Nature. 306:768-770.
• Chylek, P., Srivasta, V., Cahenzli, L., Pinnick, R., Dod, R., Novakov, T., Cook, T. and Hinds, B, 1987. “Aerosol and graphitic carbon content of snow,” Journal of Geophysical Research, 92: 9801-9809.
• T. Novakov, and J.E. Penner, 1993. “Large contribution of organic aerosols to cloud-condensation-nuclei concentrations,” Nature 365, 823-826.
• D. Hegg, J. Livingston, P. Hobbs, T. Novakov, and P. Russell, 1997. “Chemical apportionment of aerosol column optical depth off the Mid-Atlantic Coast of the United States,” J. Geophys. Res., 102, 25,293-25,303.
• T. Novakov, D. Hegg, and P. Hobbs, 1997. “Airborne measurements of carbonaceous aerosols on the east coast of the United States,” J. Geophys. Res., J. 102, 30,023-30,030.