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July 14, 2004
Carbon in the Air, part 2

From rooftop to high-altitude sampling

Collaborating with other institutions is not hard when you have a wide network of contacts, and Tom Kirchstetter has participated in a variety of air sampling experiments in recent years.

Project SAFARI studied linkages among natural and human-caused emissions into the atmosphere, included grass fires, in southern Africa. (Globe image SeaWiFS)

In the summer of 2000, Kirchstetter joined Peter Hobbs of the University of Washington and a multinational group of scientists in the South African Regional Science Initiative (SAFARI); they spent five weeks in research aircraft measuring carbon aerosols. "We often saw smoke plumes from burning savannah," Kirchstetter says. "Some were prescribed burns, some were natural fires." SAFARI provided a wealth of data about particles from burning biomass in southern Africa.

INDOEX, the Indian Ocean Experiment, is another recent source of data. In collaboration with colleagues at the University of Puerto Rico, Kirchstetter analyzed samples taken from the Indian Ocean, where pollutants drift in from the Indian subcontinent. The Asian land mass is a source of regional-scale brown clouds from burning of forests, diesel fuel and coal, which have much larger climate effects regionally than globally.

One such cloud drifts south from Asia over the Indian Ocean, a brown haze implicated in regional climate effects like intensified periods of drought and rainfall, as demonstrated by climate model simulations run by Surabi Menon, Kirchstetter's colleague in EETD's Atmospheric Sciences Department.

Kirchstetter is one of a number of scientists working with V. Ramanathan of the Scripps Institute in analyzing samples for the Atmospheric Brown Cloud project, which was established to examine the climate effects of these phenomena. And as part of the Mega Cities project developed by Mario Molina of MIT, Kirchstetter will analyze air samples collected over large metropolitan areas, beginning with Mexico City, to understand how aerosols and other pollutants in brown clouds affect large cities.

Organic carbon – contributing to climate change?

"Most climate models include black carbon as the only light-absorbing aerosol species," says Kirchstetter. "Organic carbon is assumed to be purely scattering, not absorbing." Thus organic carbon particles are not considered a contributor to global warming.

But, says Kirchstetter, "there is some indication that you can produce nonblack carbon particles that are light-absorbing." In particular, differences between smoke from biomass burning and from diesel fuel combustion may have implications for climate change.

"In addition to the mass concentration of these particles," says Kirchstetter, "we're studying their optical properties, so they can be represented realistically in climate models."

Recently, Kirchstetter has found evidence that organic carbon from biomass burning (e.g., wood smoke) behaves differently from that produced by diesel fuel burning. Using a specially designed instrument built by Dick Schmidt, he and Novakov are measuring the relationship between a range of wavelengths of light and their absorption by organic carbon from both wood smoke and diesel combustion.

The device incorporates elements of the aethalometer developed at Berkeley Lab (described in last month's edition of Science Beat, in the first part of this series) and commercial optical spectrometers. A filter sample is placed between a group of light-emitting diodes, each emitting a precise color, and a detector, which produces a plot of how much light the carbon in the filter absorbs at different wavelengths. Heating the filter removes the carbon aerosol, and a repeat measurement gauges how the carbon affected the light absorption.

MULTI, the multiple wavelength light transmission instrument, measures how light of different wavelengths emitted by LEDs is absorbed by carbon particles trapped in a filter (inset). (Photos Ted Gartner)

Kirchstetter has made numerous measurements with this device, some with samples of ambient Berkeley air collected from his lab's roof sampler, some from the SAFARI project — mostly biomass smoke — and some drawn from the nearby Caldecott Tunnel, a heavily traveled stretch of Highway 24 used by drivers in the San Francisco Bay Area; carbon aerosols in these samples are mainly from diesel smoke.

The way aerosol light absorption varies as a function of wavelength is called "spectral dependence." Kirchstetter and Novakov found that SAFARI samples from biomass burning, which contained a lot of organic carbon, exhibited a spectral dependency different from that of diesel particles from the Caldecott Tunnel, which contained a lot of black carbon — suggesting that there was material other than black carbon in the biomass-burning samples that was absorbing some of the sun's heat. They suspected organic carbon.

Investigating further, they extracted organic carbon from their samples using a strong solvent. As expected, this made the spectral dependencies of the SAFARI samples look more like those of ambient air and tunnel samples. Their conclusion: "Biomass smoke samples actually have an organic component that absorbs some light. . . . More generally, under certain combustion conditions, emitted organic carbon particles may contribute to light absorption."

Thus organic carbon may be having an effect on climate change not accounted for by current computer models — a new and fruitful area for additional research. Kirchstetter is now working with EETD's Doug Black to combine the multiple-wavelength light transmission instrument and the evolved gas analysis method. They plan to develop a field version of both instruments, one to measure mass concentration of carbon aerosols and the other to measure its light-absorbing effect.

Traffic in the Caldecott Tunnel and brush fires in Africa can both cause haze, but the mechanisms appear to be different. (Globe image SeaWiFS)

Kirchstetter and Black will soon begin a study of "coated" particles, black carbon particles that have mixed with other chemicals in the atmosphere and acquired another layer of material. As yet the effect of mixing or coating is poorly understood, but mixed particles may have an effect on climate change different from the sum of their components.

This summer Kirchstetter and his colleagues Rob Harley from the University of California Berkeley and Tony Strawa of NASA Ames Research Center will also be making new measurements of pollutant emissions in the Caldecott Tunnel. Kirchstetter's original samples were taken when gasoline in California still contained the additive MTBE, since removed. New measurements may show whether the change in gasoline formulation has changed automotive pollutant emissions for better or worse.

Additional information

Additional references

  • "Laboratory and field investigation of the adsorption of gaseous organic compounds onto quartz filters," by T. Kirchstetter, C. Corrigan and T. Novakov Atmospheric Environment 2001, vol 35, pp 1663-1671
  • "Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon," by T. Kirchstetter, T. Novakov and P. Hobbs, in press