Electron microscope gives visiting chemist unique look at airborne particles

April 28, 1995

By Diane LaMacchia, DMLaMacchia@LBL.gov

Analytical chemist Lynn McInnes visited LBL recently from the University of Colorado to study the chemical composition of sea salt and sulfate aerosols.

To examine samples of the airborne particles, she used the Analytical Electron Microscope (AEM) at LBL's National Center for Electron Microscopy. A national user facility that is open to all qualified researchers, NCEM offers unique capabilities not available elsewhere. "At the facilities here you can actually do chemical analysis on an individual aerosol particle," McInnes says.

She and collaborators from the National Oceanographic and Atmospheric Administration (NOAA), the University of Seattle, and the University of Colorado's Cooperative Institute for Research in Environmental Sciences are analyzing two types of particulate matter: remote background aerosols, which contain natural compounds such as sea salt and sulfuric acid, and aerosols that contain anthropogenic pollutants such as soot.

In this day and age, it is not easy to find the former.

"There are only a few places left in the world where you can get them," McInnes says. "The South Pacific is one such place. In the Atlantic, you are more likely to pick up pollutants coming from the American and European continents."

The aerosol samples she brought to LBL were collected over the Pacific Ocean on a 1993 NOAA cruise of the Surveyor from Punta Arenas, Chile, to Seattle, Wash. She also brought samples from land-based monitoring stations at Cheeka Peak, Wash., and Niwot Ridge, Colo. Samples from Cheeka Peak, a coastal station, were collected when the wind was coming off the water, mainly bringing a marine source of particles to the land. Samples from the inland station, Niwot Ridge (elev. 9,500 ft.), contained some pollution from Denver.

Aerosol particles are known to scatter incoming solar radiation and serve as nuclei for cloud formation. Studies of remote aerosols help establish a background set of measurements with which to compare polluted situations in an effort to learn how pollutants affect the amount of sunlight that gets to the ground. Like previous studies on the effects of carbon dioxide in the atmosphere, these studies will add to our knowledge of how human behavior may precipitate climactic change.

McInnes is particularly interested in sulfate particles--their concentration, size, longevity, and physical properties. Sulfate is one of the major pollutants contributed to the atmosphere by human activities, but there are also natural sources of sulfate. The ocean releases gaseous dimethyl-sulfide (DMS) as a by-product of phytoplankton, the plant material of the ocean. Once in the atmosphere, DMS can change form to become sulfate particles. The gases can also form sulfate on sea salt particles. McInnes is studying the processes by which the pollutant material attaches to the natural aerosol and how the combined aerosols result in changes to cloud coverage.

Typically in the past, McInnes says, people used bulk techniques in measuring aerosol properties and chemical composition. In that situation, the major components by mass--often the larger particles--dominate the findings. But McInnes came to LBL to study the small particles that dominate by number.

She says the AEM was a good tool for her study, because it can isolate one particle and look at its spectra to provide elemental information. Its special detectors analyze which elements the particle has reacted with, from carbon on up. Because the particles are so small, electrons pass right through them, generating images on a screen below.

"This is a really good facility," McInnes says, "because not only does it have these microscopes, but it has dark room facilities to print up the images, computer facilities to do automated image analysis, and personnel who are qualified in each of those areas. They make sure you've collected the information you want."

By studying the size distribution and chemistry of the aerosol samples, McInnes hopes to understand something about their physical properties and behavior. For example, will particles grow into cloud droplets when they are exposed to water vapor? How does gaseous sulfur become sulfate, and how much sulfate is out there now? How long will the particles stay in the atmosphere?

At bottom lies a larger question, McInnes says. "Are we putting all of this material into the atmosphere to stay there forever, or will it get removed?" The answer could be an important one for understanding climate change.

NCEM Fellowship program unites young scientists, state-of-the-art equipment

Analytical chemist Lynn McInnes and other young investigators are able to use the unique facilities at LBL's National Center for Electron Microscopy thanks to a special fellowship program.

"The NCEM Visiting Scientist Program was designed to help those who couldn't otherwise have access to such facilities," says NCEM Acting Head Michael O'Keefe. (NCEM Head Uli Dahmen is on sabbatical). "We invite applications from young scientists who are just starting out in their field, or are starting up a lab and don't have their equipment yet, but want to get started."

The fellowships are up to three month's duration and carry a stipend to defray travel and living expenses.

McInnes, a research assistant from the University of Washington, was one of three participants in last year's program. Another, an assistant professor from NC State University, worked on polymers during his visit, while an assistant professor from Rice University studied structural transitions, plasticity, and controlled fracture initiation in a variety of ceramics. There are nine applicants this year, O'Keefe says.

The finalists are selected based on recommendations of the NCEM Steering Committee, a group composed of researchers from LBL, other national labs, universities, and industry that meets annually.