Carbonaceous Aerosols and Climate Change:
How Researchers Proved Black Carbon is a Significant Force in the Atmosphere
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 effect 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 this idea now is thanks to the work of a research group, led by Tihomir Novakov at Lawrence Berkeley National Laboratory (Berkeley Lab), which has been studying these particles since the 1970s.
Carbon aerosol (CA) particles are composed of light-scattering organic carbon (OC) and light-absorbing black carbon (BC). These particles 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 well-known particles found in the atmosphere as a result of the burning of fossil fuels.
Both carbon and sulfate particles can affect climate in ways that either increase or reduce the effects of global warming. Sulfate and carbon (particularly OC) aerosols scatter light back to space, which reduces the warming caused by greenhouse gases. BC alone has a dual effect: it absorbing sunlight, which heats the atmosphere, but also blocks light from reaching Earth's surface, which has a cooling effect. An additional, indirect effect of aerosols is to increase the reflectivity of clouds (i.e., make cloud surfaces "shinier"). This change in reflectivity reflects the sun's heat back into space, which reduces global warming. Climatologists are now trying to understand the sum of all of these effects on global climate change.
A Voice in the Desert
Novakov, who has been called the "godfather of black carbon studies" by eminent National Aeronautics and Space Administration climate researcher James Hansen, heads the atmospheric aerosol research group in what is now Berkeley Lab's Environmental Energy Technologies Division (EETD). This research group began studying carbon aerosols during the 1970s; their work was one of the division's earliest research areas. In 1974, Novakov, with aerosol research group member S.G. Chang as well as A.B. Harker, published a paper in Science claiming that carbon constitutes 50 percent of the total particulate concentration in urban atmospheres and that as much as 80 percent of particulate carbon is in the form of soot or BC.
"We were a voice in the desert," says Novakov. "It was an unconventional view, and it was 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 BC was insignificant in the atmosphere, no longer present since the Industrial Revolution had ended and the number of coal fires used to generate heat in homes and factories was dwindling. But evidence from Novakov's group would change this thinking.
"What [Novakov] did," says Lara Gundel, a scientist in his group during the 1970s, "is ask 'why air pollution is black?' If air pollution was formed photochemically, then what was the black component?" Gundel, who continues to research 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, reviews the use of electron spectroscopy for chemical analysis (ESCA) methods for making the first attempt to characterize the chemical components of particulate carbon. By comparing the spectrum of carbon at room temperature with that of carbon that had been heated (heating drives off volatile organic carbon), they determined that most of the black carbon in air samples was soot: inorganic, not organic carbon. (ESCA identifies chemical composition by measuring the spectra of electrons under x-ray bombardment).
On a Solid Footing
By the 1980s, the scientific community had begun to take the group's "black carbon hypothesis" more seriously. General Motors sponsored a symposium in 1980 in Warren, Michigan (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 as he 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.
During the 1980s, Novakov frequently described black carbon as "produced solely by the incomplete combustion of fuels," asserting black carbon particles were as important in contributing to both local and regional air pollution (e.g., 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 BC particles and their persistence in the atmosphere over time. During the early 80s, Hal Rosen and Tony Hansen, who were then physicists in Novakov's group began applying optical methods for characterizing and measuring BC. Rosen used Raman spectroscopy to unambiguously demonstrate that BC is composed of graphitic 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. From this initial model, Hansen, Rosen, Novakov, and others developed a real-time measurement device, which they called an "aethalometer" and described the device in a 1984 paper. ("Aethalon" is a Greek word meaning "to blacken with soot.") Gundel's work was essential in making the aethalometer a quantitative device for measuring BC concentrations.
Hansen built a number of aethalometers. One of the first was used for a study, which began in 1983, of haze in the Arctic atmosphere. Hansen continued to work for Berkeley Lab and also began building aethalometers for researchers around the world as a private consultant. One of Hansen's instruments, built for the Canadian Arctic's Alert research station, is still functioning and has been measuring BC particles for 15 years.
Indications of Long-Distance Transport
Once reliable measuring devices were available to detect BC and sampling and analysis techniques permitted scientists to measure the chemical content of atmospheric particles, it became possible to determine how widely BC particles were spreading across the globe from their origins in cities and industrial areas. During the late 1970s and early 1980s Novakov, Rosen, other members of their group, and investigators elsewhere sampled the Arctic atmosphere to determine whether BC particles could be found in what was thought to be a pristine environment. Papers published during these years reveal substantial concentrations of soot found throughout the Arctic. In one study, Novakov and colleagues found BC particles in the Alaskan, Canadian, and Norwegian Arctic regions.
In a 1983 paper, Rosen and Novakov wrote, "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." They speculated that "these highly absorbing particles" could have a considerable effect on the Arctic radiation balance and climate. Novakov says he concluded at the time, "If it was in the Arctic, it must be everywhere."
Finding BC particles, which are anthropogenic (produced by human activity, i.e., the incomplete combustion of fossil fuels), in the Arctic signified 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.
Of even greater concern was that the heat-absorbing properties of BC could have a warming effect in the Arctic. The work created a stir in the scientific community, which now had direct proof of the long-range transport of pollutants and also now had another question to answer: what effect were BC particles having on Earth's climate?
In 1987, Novakov worked with a group of scientists at New Mexico State University to 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 its detection in other studies, showed that it had settled out from the atmosphere all over the world. (In a recent paper in the Proceedings of the National Academy of Sciences, 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, accelerating the melting of these caps as well as increasing global warming.)
The Effect of Black and Organic Carbon on Cloud Formation
The emergence the data gathered by Novakov and his fellow researchers brought increasing attention to black carbon particles from a scientific community that, by the late 1980s and early 90s, had accepted the presence of BC in the atmosphere as proven. Researchers began to focus on understanding the relative contributions of BC's absorption and scattering of light, which increases global warming, and its intensification of the heat reflection of clouds, and blocking sunlight, which cools the atmosphere.
Novakov turned his attention to the effect of BC on cloud formation and asked whether the other component of carbon aerosols, the OC particles, were present in the atmosphere in high enough concentrations to affect cloud formation. Again, Novakov's hypothesis went against the conventional wisdom, which was that sulfates, not carbon, were the major force in cloud formation.
In a 1993 paper, Novakov, along with J. Penner of Lawrence Livermore National Laboratory, demonstrated that organic carbon is as effective a nucleus as sulfate particles for the formation of cloud droplets. This finding showed that sulfates were not the only significant man-made source of cloud nucleation in the atmosphere, as had been previously thought.
Novakov published two papers in 1997, with Peter Hobbs and other colleagues at the University of Washington, reporting measurements of aerosols on the east coast of the U.S. Both studies demonstrated that, in some locations, carbon aerosols contribute more than sulfates to the extinction of solar radiation in the atmosphere. "I think this work was a turning point," says Novakov. These papers were cited extensively, and the scientific community accepted another Novakov hypothesis about carbon particles: that these particles compete with sulfate in creating climate change. The scientific community began focusing attention on what BC and OC particles were doing to 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." Novakov's impact has been felt not only in his own research but also in his role in organizing seven international conferences on carbon particles in the atmosphere, starting in 1978. The eighth conference 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. S. G. Chang continues to work in EETD, developing methods of improving air pollution control processes and technologies to reduce power plant emissions. Hal Rosen now works at IBM. Lara Gundel researches new methods to accurately measure semi-volatile 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 organic pollutant studies. Dick Schmidt continues to contribute to the design and development of aerosol instrumentation at Berkeley Lab. Others from the early days of Novakov's group (in 1978 there were 18 staff scientists and research associates in the group) either still work at the Lab or did so until they retired.
Carbon aerosol research continues at Berkeley Lab with the involvement of new researchers. Recent papers from the Novakov group focus on the history of BC concentrations in the atmosphere to determine the contribution that BC particles have made to climate change over time. The next article in this series will address this research.
Hansen, J. and Nazarenko, L. 2004. "Soot climate forcing via 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. eds. New York: 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. 1984. "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. 1983. "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.
Novakov, T. and Penner, J.E. 1993. "Large contribution of organic aerosols to cloud-condensation-nuclei concentrations". Nature 365, 823- 826.
Hegg, D., Livingston, J., Hobbs, P., Novakov, T. and Russell, P. 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.
Novakov, T., Hegg, D. and Hobbs, P. 1997. "Airborne measurements of carbonaceous aerosols on the east coast of the United States", J. Geophys. Res. 102, 30,023-30,030.