Chemical engineer keeps her head in the clouds for research

By Steven Schultz

Princeton NJ -- Chemical engineer Lynn Russell had just agreed to have her picture taken on the roof of Fine Hall, where she is installing pollution sampling equipment, and had one last question. "Who will be in charge of the weather?" she joked.

In Russell's line of research, the question is not necessarily a joke. She is studying the possibility that air pollution is affecting cloud formation and rainfall on the East Coast. Preliminary evidence suggests that pollutant particles -- ranging from tiny clusters of molecules to bits of soot -- may be inhibiting rainfall, although the connection remains uncertain.

	Lynn Russell (right), assistant professor of chemical engineering, and students in her lab are installing pollution-monitoring equipment on the roof of Fine Hall to study solid particles suspended in the air. From left, graduate students Anita Adhitya and Elizabeth Koch examine a particle-collecting device with Russell.

As lawns turn brown and crops fail for lack of rain, the possibility seems to require a serious look. Russell recently received a highly competitive $450,000 grant from the James McDonnell Foundation to probe the physical mechanisms that might connect precipitation patterns to the atmospheric chemistry of pollution.

In the background material accompanying her proposal for the grant, Russell asked: "... what if our fireplaces and air conditioners meant that we could no longer grow Jersey tomatoes or even a few grassy lawns ...?" People, she concludes, may be more in charge of the weather than they believe.

Close-to-home impact

Russell's work is part of an emerging area of climate research that recognizes the importance of solid particles suspended in the air, as opposed to just atmospheric gasses. And she is one of the few people whose work connects what had traditionally been distinct areas of research: the complex chemistry of particles and the bigger-scale processes of weather and climate, said Venkatachalam Ramaswamy, a leading atmospheric scientist at Princeton's Geophysical Fluid Dynamics Lab and an occasional collaborator with Russell. "The links between these various processes are where our big gaps in knowledge are, and this is where Lynn is adding significant input," said Ramaswamy.

The effects of atmospheric particles are hard to overestimate. Subtle changes in their abundance and chemistry can alter how much sunlight gets reflected back into space from the clouds and the atmosphere, which in turn could have dramatic cooling or warming effects for the planet, said Russell.

Even more noticeable to the average person, however, may be particles' effect on weather. Russell believes that understanding that connection could add an important dimension to the debate over global warming, which tends to focus on the long-term, global consequences of greenhouse gas emissions. If particle pollution is indeed changing rainfall patterns, the effects will be felt much more quickly than those of global warming, and the social, economic and ecological consequences could be just as significant, she said.

"The consequences of global warming are longer term than most of us think about in our daily decisions," said Russell. "From a policy point of view, the implications may be shorter term and closer to home to have an impact."

Ironically, cutting back on particle-based smog could exacerbate greenhouse warming, because it would remove a protective layer that is reflecting sunlight back into space. "In the short term, you pay a price in the warming trend," Russell said, noting that some measures to cut particle emissions would, after decades, reduce greenhouse gas levels as well.

Little drops, little rain

Several lines of evidence suggest a connection between airborne particles, called aerosols, and weather. In 1998, researchers studied weather and pollution on Sable Island, Nova Scotia, which is downwind of New York City, and observed a weekly pattern in which pollution levels fell and rainfall increased during the weekends compared to the weekdays. Other studies have shown that the haze of pollution can reduce evaporation, making less water available for cloud formation.

Russell's own work, which spans a range of problems in atmospheric chemistry, also has stoked scientific interest in pollution and weather. Aerosol particles find their way into the atmosphere from many natural and human-influenced sources, from sea salt and desert dust to carbon-based emissions from burning fuels. Russell has developed innovative ways of distinguishing one kind of particle from another and measuring their size and concentration.

Among her findings is the basic observation that human-caused particles are often abundant. In some places, 80 percent of the particles in the air are organic, or carbon-based, aerosols, which come mainly from burning fuel. She also found that these particles absorb water differently than natural inorganic salts suspended in the air, ultimately taking up less water. If pollution causes the same amount of water in a cloud to be distributed among 100 particles instead of 10, it might reduce the odds of forming droplets big enough to fall out as rain, Russell speculated.

"Particles need to get big to rain out, so it takes a long time for these small particles to get big enough," she said.

Rush of adrenalin and science

Russell began studying air pollution as a graduate student at the California Institute of Technology, where she researched the physics and chemistry of salts and other aerosols from the ocean. After receiving her Ph.D. in 1995, she participated in airplane-based studies of aerosols at the National Center for Atmospheric Research in Boulder, Colo. She came to Princeton in 1997.

One of the challenges of her research is finding ways to collect and analyze data about tiny particles that are spread over great distances. "You can imagine it as though you are trying to make controlled observations in a cloud just as you would in a little reactor you have in the lab," she said.

Over the years Russell has experimented with single and twin-engine aircraft, research ships and other vehicles for gathering data, but relies mostly on giant ex-military aircraft stuffed with scientific instruments and flown as part of collaborations involving dozens of scientists. Visiting polluted skies around the world, from Asia to the California coast, Russell has developed an expertise in gathering high quality data under difficult circumstances.

"Doing research in flight, in the middle of nowhere, in a limited amount of time is a rush of both adrenalin and science," she said. Crammed on a plane for hours, Russell spends a lot of time talking with colleagues "and crawling over them and stepping on them. As a consequence you learn a lot and think of new ideas," she said.

The monitoring station Russell and her students are installing on Fine Hall will provide a source of long-term data that cannot be obtained from field trips.

Immense benefit

More than just field research, Russell's strength is in combining her measurements with laboratory-based experiments and computer simulations of atmospheric processes. "I find it an immense benefit to work with someone who not only takes observations, but who really understands both the benefits and the limitations of the observations," said Ramaswamy.

In a recent paper, Ramaswamy and Russell analyzed data from a study that compared normal clouds to clouds in the exhaust plume of a ship. "Ships make plumes of a lot of particles," said Ramaswamy, "and when those particles interacted with the cloud droplets, we could see that they were modifying the cloud structure. Then combining those observations with computer modeling, we were able to actually quantify how much of the cloud was modified and what was the extent of the modification. It was very satisfying work for me."

For Russell, the payoff comes in fitting together each new piece of the huge and complex puzzle of our atmosphere. "I am sure it will be some time before we can understand how we are changing the weather, but it's certainly a challenging problem," Russell said.

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