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Prevailing haze

A study tries to explain Boston's air pollution

Students look at a Laptio outside of a BC parking garageFor several weeks during the past summer, Boston College researchers joined scientists from UCLA, Argonne National Laboratory, the Aerodyne Corporation, and the Pacific Northwest National Laboratory to begin developing a comprehensive portrait of the Boston region's air pollution. The research conducted was both microscopic and macroscopic—at the level of chemical reactions on the one hand, and of climate and weather on the other. BC chemistry professor Paul Davidovits was one of the architects of the project, aimed at determining, among other things, the extent to which the area's pollution comes from distant sources—the New York City area and the Ohio River Valley, principally—and the extent to which it is homegrown.

The project is ambitious, says Davidovits, "a bit like unmixing a bowl of chowder." The pollutants from the various sources are distinctive—sulfur compounds arrive on prevailing winds from the Ohio River Valley's coal-burning power plants; industrial compounds and soot drift north from metropolitan New York's factories; and automobile emissions rise from Boston's streets. Although they tend to travel in distinguishable layers in the atmosphere, none of them exist in isolation. They pass over and through the city in a roiling brew of complex chemical reactions, affected by sunlight, temperature, and the weather. At BC, the group from UCLA operated out of a small trailer on the roof of the four-story Beacon Street parking garage, overlooking the football practice fields. For two weeks, sensors mounted on top of the trailer continuously monitored wind speed, humidity, and barometric pressure. Inside, a lunchbox-size device called a differential optical absorption spectrometer analyzed beams of light that the researchers streamed from the trailer and bounced off mirrors mounted on the roofs of two 15-story Brighton apartment buildings, almost a mile away across the Chestnut Hill Reservoir. Atmospheric pollutants absorbed some of the light, altering its spectrum, and by analyzing the changes with the spectrometer, the scientists were able to pinpoint the chemicals that were present in the air at any moment. Boston's lower atmosphere—from ground level to around 500 feet up—was of particular interest to the project's directors because it is where exhaust from automobiles begins to interact with naturally-occurring ozone (O3) to produce secondary pollutants. Mapping those interactions will help make it possible to distinguish between local pollution and pollutants intruding from distant sources.

Meanwhile, inside Davidovits's laboratory on the second floor of the Merkert Chemistry Center, chemical profiling of the street-level breeze was under way, as air from outdoors flowed in a constant stream through an atmospheric mass spectrometer (AMS)—a device invented by Davidovits and colleagues at Aerodyne. Four additional AMS machines—set up in a forest owned by Harvard University about 30 miles westward; in New York's Catskill Mountains; aboard an airborne Gulfstream-I jet; and on a boat in Boston Harbor—also contributed data. At times, a phalanx of bright orange tetroons, weather balloons capable of maintaining a constant altitude, passed over the Boston skyline, taking measurements of atmospheric conditions at different altitudes. The result was a nearly continuous account, geographically and over time, of the air passing through the Boston area.

"There is a huge amount of data," Davidovits said a few days after the measurement phase of the project ended, sweeping his arms as though surveying a grand estate.

"The task now is to unravel it, to coordinate it, to see correlations."

Davidovits and his colleagues are particularly interested in the role that aerosols—minute solid or liquid particles suspended in air—take in air pollution. "It has been recognized in the last few years," said Davidovits, "that aerosols are perhaps the most significant environmental hazard, in terms of causing lung disease, visibility problems, even climate change, by reflecting sunlight." Aerosol particles, Davidovits emphasized, are more than inert bystanders in air pollution; they are active players, their surfaces providing a fertile site for numerous chemical reactions.

Two of Davidovits's graduate students, Haizheng Zhang, from Beijing, China, and Jay Slowik, of Rochester, New York, are studying carbonaceous aerosols—soot, in short. Cars and the coal-burning power plants of the Ohio River Valley (long suspected of being the main source of the East Coast's acid rain) emit thousands of tons of soot each year. Interacting with ultraviolet radiation from the sun and natural ozone, the soot acts as a catalyst for important chemical reactions in the atmosphere—most famously, the reactions that create the major component of acid rain, sulfuric acid, from sulfur dioxide, another major product of coal burning.

Over the summer, while his colleagues measured real-world conditions, Zhang conducted experiments under controlled laboratory conditions to determine the precise relationship between humidity, temperature, and relative concentrations of soot on the formation of pollutant compounds such as sulfuric acid. The aim, he says, is to contribute to the "baseline understanding" of atmospheric chemistry. Slowik, meanwhile, monitored the AMS machines in Harvard Forest and Merkert, and will be comparing his observations with Zhang's standardized measurements. Katie Stainken '04, a chemistry major from Hillsborough, New Jersey, also took part in the pollution project, compiling data from the National Oceanic and Atmospheric Administration on weather patterns in an effort to determine exactly how pollutants from distant sources travel to New England.

For Slowik, the breadth of the project is one of its appeals. Atmospheric pollution research is "an area that's new enough and developing enough that you kind of have to know all sides of the problem," he says.

Complete understanding of the data collected last summer is years away, but the preliminary analysis, says Davidovits, helps confirm an old theory: that southern New England's lower atmospheric pollution comes primarily from the New York metropolitan area, while its pollution at high altitudes—the production point of most acid rain—comes from the coal-fired power plants of the Ohio River Valley.

Davidovits is confident that real breakthroughs will come when the study's finely detailed analysis of chemical processes in the atmosphere are wedded to the knowledge gained about wind and weather patterns in the Boston area. "That's what you hope for in research," he said, "that complete picture."

Tim Heffernan

Photo: Katie Stainken '04 and graduate student Jay Slowik outside the temporary laboratory atop BC's Beacon Street parking garage. Haizheng Zhang, another graduate student, monitors data inside the trailer. By Lee Pellegrini

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