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February 18, 2005
Aerosols Overstay Their Welcome
ALS-obtained images will help scientists refine climate models

A team of scientists has determined that carbon-containing aerosols, like obnoxious relatives who linger a bit too long after the holidays, remain in the atmosphere much longer than previously thought — meaning they have more time to wreak havoc on the Earth's climate.

The results, based on high-resolution images obtained at Berkeley Lab's Advanced Light Source (ALS), will help climate scientists refine the computer models used to predict climate change. The images reveal that aerosol particles oxidize more slowly than earlier estimates indicated, which buys them more time to waft about in the atmosphere.

A wide-field satellite image of eastern Asia shows large amounts of aerosol in the air. (Image NASA)

"These longer-lived particles will increase the carbon-containing aerosol burden on climate models by 70 percent," says Mary Gilles, a researcher in the Chemical Sciences Division, who conducted the work with Satish Myneni of the Earth Sciences Division. Lynn Russell of San Diego's Scripps Institute of Oceanography led the research, which was appeared in Science magazine and was presented at the annual meeting of the American Geophysical Union in December, 2004.

"Our work suggests the climate models need more work, which we've known all along," adds Gilles. "But this study shows us how we can improve them."

Most people equate aerosol with hairspray and household cleaning products, but a large portion of the microscopic particles floating in the air originate from the incomplete burning of coal and oil, and from dust storms. Once in the atmosphere, these tiny carbon-containing particles can have a huge impact. Lighter-colored organic carbon particles cool regions of the planet by scattering sunlight back into space. Other aerosol particles composed of black carbon, or soot, warm the atmosphere by absorbing sunlight and heating the surrounding air.

Impacts like these are why scientists are striving to learn how long carbon-containing aerosols remain in the atmosphere. And one way to gauge an aerosol's ability to stay aloft is to determine its oxidation rate. Because oxidized aerosols absorb moisture and subsequently form clouds and fall as rain, the faster an aerosol particle oxidizes, the less time it spends in the atmosphere, and the less impact it has on the climate.

Hoping to learn more about these oxidation rates, the team examined four sets of carbon-containing aerosol particles that had undergone vastly different journeys. One sample of particles, collected by an airplane over the Sea of Japan, had drifted with the winds for 30 hours after originating from the factories and fields of China. Another sample, collected over the Caribbean Sea, contained aerosol particles that emanated from a dust storm that swept across Africa five days earlier. Two more samples of mostly industrial combustion emissions were collected near Princeton, New Jersey about 10 hours after the particles entered the atmosphere. One sample was collected on a foggy day, the other on a clear day.

  (Left to right) Satish Myneni, Mary Gilles, and Lynn Russell at the Advanced Light Source beam line where much of their work on carbon aerosols was conducted. (Photo Roy Kaltschmidt)

The scientists then used scanning transmission x-ray microscopy at the ALS to analyze particles from each of the four samples. The spectral images, with a spatial resolution of 35 nanometers (one nanometer is one billionth of a meter), gave the team a clear view of the chemical bonds that characterize each aerosol particle, such as the ratio of oxidized carbon to normal carbon in each particle as a function of its size.

They found that in some samples, smaller particles had higher ratios of oxidized carbon to carbon. They also found that drier particles mainly oxidize on the surface, while wetter particles oxidize throughout. After studying the telltale signatures of these chemical reactions, the team found that all the particles — whether they're from an African dust storm, Asian factory, or New Jersey smokestack — oxidize at a rate of between 13 and 24 percent per day. These conversion rates are a factor of three slower than the 60 percent per day typically used in current climate models.

"The bottom line is that the rate of oxidation is slower than is currently used in models," says Gilles. "And oxidation has a strong effect on their lifetime in the atmosphere, which has a strong effect on the global climate."

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