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July 14, 2004
Carbon in the Air: Making Better Measurements to Help Understand Climate Effects

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.

Climate forcing is a measure of the additional power absorbed by Earth's atmosphere, expressed here as instantaneous watts per square meter. Two different computer models using AERONET data collected by "sunphotometers" show forcing caused by black carbon and organic carbon from human sources.

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.

Estimated fossil-fuel black carbon emissions since 1875, in gigatons (billions of tons) per year.

Better aerosol sampling

Climate 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 evolved gas analysis system has direct access to ambient air through a stack leading to a sampler on the roof (inset). (Photos Ted Gartner)

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.

Carbon in the Air, part 2