Indiana University Research & Creative Activity


Volume 31 Number 1
Fall 2008

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Phil  Stevens
Phil Stevens
Photo by Kendall Reeves

Donora in smog
This photo was taken during midday in Donora, Pa., late in October 1948. Between Oct. 26 and 31 of that year, 20 people died and more than 7,000 were sickened as the result of severe air pollution in the small town. The disaster spurred the first efforts toward federal clean-air legislation.
Photo © Pittsburgh Post-Gazette, 2008, all rights reserved. Reprinted with permission.

Better Breathing through Chemistry

by Steve Hinnefeld

A yellow Pac-Man™ video game character races across the bottom of Phil Stevens's Web page, snarfing down dots while being chased by an animated red ghost.

There's a point to this cute reference to 1980s popular culture. Stevens, a professor in the Indiana University School of Public and Environmental Affairs in Bloomington, studies the chemistry of the hydroxyl radical, known as "the Pac-Man of Earth's atmosphere," and the way it gobbles pollutants.

And while the Web graphics may have a retro look, there's nothing low-tech about the atmospheric chemistry that Stevens and his team practice. Capturing and measuring the hydroxyl, or OH, radical in the atmosphere is like catching lightning in a bottle. The substance exists at extremely low concentrations — typically less than one part per trillion. And because it's so reactive, it lasts less than a second.

"It's a really challenging measurement," says Stevens, adding that IU is one of four institutions in the United States with an instrument that can measure OH. "Some people who got into this business long ago are no longer in the business. It's not easy to do."

Why bother? Because gaining more complete knowledge of the reactions of the molecule, made up of single atoms of oxygen and hydrogen, is a key to understanding processes involved in the creation of dangerous pollutants and the process of controlling them.

"The OH radical is one of the holy grails of atmospheric chemistry," Stevens says. "It controls the lifetimes of many important compounds. If you talk about acid rain, it's involved in the formation of sulfuric and nitric acid. If you talk about stratospheric ozone depletion, it controls the lifetime of alternative CFCs (chlorofluorocarbons). It controls the lifetime of methane in the atmosphere, which is an important greenhouse gas. And on the local scale, it initiates the oxidation of volatile organic compounds, which in the presence of nitrogen oxides, leads to ozone and smog formation."

Deciphering the processes behind ozone and smog is the focus of much of the work now being done by Stevens and his students and post-doctoral researchers. The task has taken them to vast and teeming Mexico City, where they were among hundreds of researchers from around the world participating in a systematic study of some of the world's worst air pollution.

They have also taken measurements in forested regions of Indiana and Michigan, where, far from population centers, they are learning how chemicals produced in nature interact with human-made pollutants. Paul Shepson, a professor and head of the chemistry department at Purdue University who has collaborated with Stevens on forest research projects, says Stevens is unsurpassed when it comes to ensuring that tricky measurements produce reliable data that merits the confidence of the scientific community.

As a physical chemist in an IU school focused on policy and environmental affairs, Stevens models the value of interdisciplinary research, Shepson says. "He's attracting students and really being a leader on campus in showing how you can do collaborative work between departments. In that regard, I would call Phil a visionary."

Stevens, who grew up in Connecticut and graduated from Oberlin College in Ohio, got the bug to study atmospheric chemistry when he was choosing a graduate school. He visited the laboratory of Harvard University Professor Jim Anderson, who was using high-altitude balloons and aircraft to learn how pollution was destroying the protective ozone layer high above Earth.

"The students were all excited and enthusiastic," Stevens recalls. "I said, ‘I want to do this.'"

He earned his Ph.D. in chemistry from Harvard. Later, as a post-doc at Penn State, he started building an instrument to measure OH radicals in the lower atmosphere, a specialty that he continues today.

The arc of Stevens' research career points to a fundamental fact about ozone, whose molecules are made up of three oxygen atoms. In simplistic terms, there is good ozone, and there is bad ozone.

The good ozone occurs naturally in Earth's upper atmosphere, the stratosphere, where a layer of ozone filters harmful ultraviolet radiation. Anderson's work at Harvard documented damage done to the upper layer by so-called ozone-depleting substances, primarily chlorofluorocarbons formerly used as refrigerants, aerosol spray propellants, and fire extinguishing agents. The destruction of the ozone layer has been blamed for increases in skin cancer rates and other harmful effects.

In the lower atmosphere, some ozone is necessary for the atmosphere to cleanse itself. But an excess is bad: Ozone is a primary ingredient in photochemical smog. On hot, smoggy days, breathing air with high levels of ozone can irritate the respiratory system, reduce lung function, and bring on asthma attacks. Children, who tend to be active and play outdoors, are especially vulnerable, as are people who work hard or exercise vigorously outdoors and people with respiratory diseases.

"There's a delicate balance in nature, and humans are messing with it," says Shepson, the Purdue chemistry professor.

If you look up "smog" in a dictionary from more than 50 years ago, the definition is likely to be "a combination of smoke and fog." Stevens, who has won several teaching awards from IU, likes to show his students a dramatic photograph taken in Donora, Pa., in October 1948, when coal smoke and industrial fumes trapped in fog turned mid-day as dark as night. Twenty people died and about 7,000 became ill in the Donora Disaster, spurring the first efforts for federal clean-air legislation.

In the 1950s, however, scientists began describing photochemical smog, formed when sunlight struck mixtures of pollutants associated first with automobile traffic. This new kind of smog was the choking haze that hung over traffic-congested cities.

"We used to think of it as a local, urban problem, Los Angeles and maybe New York," Stevens says. "But nowadays, if you look at areas in the U.S. … ."

As he speaks, Stevens calls up a map of the United States on his computer, showing ozone levels. There are zones of bad air around cities in the West, of course. But ozone levels are at least moderately high in a broad swath across the Midwest, East, and Southeast. On a recent summer day, the worst air in the nation was in Atlanta and in Greenwich and Bridgeport, Conn., according to the Environmental Protection Agency's Air Quality Index. The primary culprit: ozone.

In Indiana, air-quality monitors installed in rural Greene and Jackson counties have found pollution levels that were out of compliance with federal standards.

Ground-level or "bad" ozone is produced when volatile organic compounds (VOCs) react first with the hydroxyl radical. The products of these reactions then react with nitrogen oxides, and, in the presence of sunlight, form ozone. Nitrogen oxides come from combustion processes, including the operation of motor vehicles, power plants, and factories. VOCs come from motor vehicle exhaust, solvents, gasoline vapors, industrial chemicals, and other anthropogenic, or human-caused, sources. But they also occur naturally in the form of isoprene, terpenes, and other substances produced by trees. That means forested regions can be vulnerable to ozone pollution if they are downwind from sources of nitrogen oxides.

"I tend to call it the Reagan effect — you know, trees cause pollution," Stevens says, referring to a widely mocked statement made by Ronald Reagan when he was campaigning for president in 1980. "An urban area tends to be dominated by the anthropogenic component. But as the plume moves out from the city, it's still chemically evolving. And trees naturally emit a lot of VOCs."

The tree-produced volatile organic compounds aren't a problem unless they combine with man-made nitrogen oxides (NOx). "I tell my students that Reagan was half right," Stevens adds. "He should have listened to Nancy and just said NO(x)."

Today's college students, born around the end of Reagan's second term as president, don't always catch the reference to the former first lady's "just say no " anti-drug campaign. But they do get the point that ozone formation involves complex chemistry, which makes it harder to control than direct forms of air pollution—even after decades of federal regulatory efforts.

"We don't have all that chemistry totally nailed down," Stevens says.

Efforts to improve that understanding involve conducting experiments on how the precursors to ozone interact with each other and performing detailed measurements of chemicals in the atmosphere, followed by high-powered computer analysis to determine if the concentrations change under differing conditions. Stevens and his cohorts conduct instantaneous measurements of hydroxyl and other radicals with a highly sensitive device that makes use of laser-induced fluorescence techniques. The instrument is used for measurements in the field and in the laboratory.

"You basically pull air into a little sampling cell, and then you fire a laser at it," Stevens explains. "With this particular wavelength of laser, you can ‘excite' the OH, and it will fluoresce as it relaxes." The OH has a unique spectroscopic "signature" that makes it possible to identify and measure at miniscule concentrations, Stevens says, but adds, "the instrument has to be operating at peak efficiency all the time. It's challenging."

In 2006, Stevens and some members of his team took the instrument to Mexico City as part of a project called MILAGRO — an acronym for Megacity Initiative: Local and Global Research Observations (but also the Spanish word for miracle). More than 20 million people live in the Mexico City metropolitan area. MILAGRO was the first international effort to fully analyze the impact of a "megacity" on air quality.

"These megacities are global sources of pollutants that can end up getting transported around the world," Stevens says. "There's evidence of pollution from China reaching California. And half the time, pollution from Mexico City travels north, into the Gulf of Mexico and into the U.S."

Data from that project are still being analyzed. This year, Stevens's team spent a month collecting and analyzing air samples in southern Indiana's Morgan-Monroe State Forest, then moved to the University of Michigan Biological Station, in northern Michigan, 15 miles from the Mackinac Bridge.

Shepson, the Purdue chemist whose research team also worked at the Michigan station, points out that understanding the role forests play in atmospheric chemistry is important to addressing global climate change. "We really don't know how forests are going to behave in the future. This is one of the big uncertainties in climate models," he says. "What we're trying to do is understand how forests interact with the atmosphere and how changing environmental conditions can influence the health of forests."

Stevens says that developing a more complete picture of atmospheric chemistry in both urban and rural locales will help scientists understand how to guard against not only climate change, but also smog, ozone depletion, and acid rain, interconnected threats to the very atmosphere that makes life on Earth sustainable.

"It's a resource we all share," Stevens says. "There's one atmosphere. And it's no longer, ‘If you don't like the smog, move out of L.A.' It's in our back yard."

Steve Hinnefeld is a media specialist in IU's Office of University Communications in Bloomington.