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May 28, 2004
The Black Carbon Story, part 2

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.

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