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| Last month’s edition of Science Beat recounted the pioneering work of Berkeley Lab’s Tihomir (Tica) Novakov and his group in studying carbon aerosol particles in the atmosphere. Research continues at Berkeley Lab to better understand the history of these particles and to find more accurate ways to measure their mass and light-absorbing effects.
It took decades of persuasion, but researchers in the atmospheric sciences community now accept the significant role played by airborne carbon particles. Evidence suggests that black carbon contributes to the retention of atmospheric heat, a causative factor in climate change, through increased absorption of heat from the sun. In May 2003, Makiko Sato, James Hansen, and seven other researchers affiliated with NASA’s Goddard Institute for Space Studies published a paper with Berkeley Lab’s Tica Novakov in the Proceedings of the National Academy of Sciences, using data from the Aerosol Robotic Network (AERONET) to infer global concentrations of atmospheric black carbon. AERONET’s 250 “sunphotometers” measure the optical depth of the atmosphere around the world. The report attracted considerable media attention by concluding that concentrations of black carbon in the atmosphere are two to four times higher than climatologists had believed, suggesting a much greater effect on climate change. Evidently black carbon increases the amount of the sun’s heat absorbed by the atmosphere, the primary driver of climate change.
Meanwhile, in Geophysical Research Letters, Novakov and coauthors Tom Kirchstetter, Jonathan Sinton, and Jayant Sathaye of Berkeley Lab, along with Hansen, Sato, and V. Ramanathan of the Scripps Institution, documented historical changes in atmospheric black carbon aerosols from 1875 to the present. They looked at the six regions of the world that account for most of today’s black carbon aerosol emissions: China, India, the former Soviet Union, Germany, United Kingdom, and the United States. They used historical records of coal and transportation-fuel burning to estimate how much black carbon might have been emitted from these sources, basing their estimates on today’s power-plant and combustion-engine emissions, plus assumptions about past emission rates. They found that black carbon increased rapidly in the late 1800s, leveled off in the first half of the 1900s, and then began to accelerate over the last 50 years. Industrialization in China and India contributed a substantial fraction of this increase.
Better aerosol samplingClimate scientists now are using both sets of data in their computer models — current global concentrations and historical trends — to better account for black carbon’s effects on climate change. Thomas Kirchstetter is a scientist in the Environmental Energy Technologies Division who works with Novakov. Since 1999 he has been working to hone the accuracy of sampling methods of organic and black carbon, revisiting sampling and measurement processes to see if new technology can improve them. The usual way to measure the mass concentration of carbon particles in the air is to force air through a quartz filter for a period of time, then heat the filter to drive off volatile organic compounds and burn the black carbon. When oxidized over a catalyst, the evolved carbon forms carbon dioxide, a gas that is easy to measure. “The evolved gas analysis system we use for this purpose was developed and refined at Berkeley Lab,” says Kirchstetter. The equipment was built by Richard Schmidt, a critical member of Novakov’s group since its beginnings in the 1970s; says Kirchstetter, “Schmitt fabricates almost all of the instrumentation we use to do research.”
The instrument measures the amount of CO2 released as the temperature rises; the result is a thermogram showing the mass of carbon in the sample, which can be used to distinguish between black and organic carbon. Kirchstetter doesn’t even have to leave the building to sample the Berkeley air, since a stack goes from his lab to the roof of the building, where an air sampler is located. “Measuring carbon aerosols in the air is a tricky business. One problem,” he says, “is that the carbon content on the filter doesn’t always reflect the carbon particle concentration in the atmosphere because of sampling artifacts. Filters adsorb gases, including organic carbon.” A simple method of correcting for adsorbed organic carbon gas is by putting a second filter behind the first in the evolved gas analysis system. Since carbon soot is trapped in the first filter, the second measures only organic carbon gas. The thermogram of the second filter can be subtracted from that of the first to provide a more accurate measure of carbon aerosol concentrations. By correcting for organic carbon, Kirchstetter’s work has led to a more accurate way of measuring carbon in the air. But there are still problems. Different laboratories can measure the same sample of air and get different results because of other artifacts of sampling and analysis. “We are now working with other labs to develop more robust sampling and analytic procedures for black and organic carbon,” Kirchstetter says; Berkeley Lab’s Lara Gundel is also involved in this effort. From rooftop to high-altitude samplingCollaborating 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.
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.
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.
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
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