Research Magazine 2010

A Breath of Fresh Air


One of the first scientific facts that schoolchildren learn is that human respiration involves the intake of oxygen and the exhalation of carbon dioxide. Scientists have long understood that respiration works in the reverse for vegetation: growing plants take in carbon dioxide—carbon, for short—and release oxygen. Just how much carbon a plant will absorb is one question that researchers from the Department of Geography & Environment are tackling, thanks to a two-year exploratory grant from the National Science Foundation and the U.S. Forest Service known as Urban Long-Term Research Area, or ULTRA-Ex.

“What we’re doing is basically a giant carbon footprint analysis of metropolitan Boston,” says Associate Professor Nathan Phillips, one of the leads on the project. Except that in addition to measuring CO2 released by humans, Phillips and his team are “also linking that to a whole other part of Boston that is exchanging carbon as well, and that’s the natural systems.”

Why carbon? Professor Suchi Gopal, another member of the ULTRA team, explains that they “made carbon the currency because all biological systems live and breathe; carbon is like cash that flows all over the system.”

Graduate student Jared Newell, left, and Associate Professor of Geography & Environment Nathan Phillips are part of a team using the Picarro sensor, seen here, to measure airborne carbon dioxide levels above BU’s Stone Science Building and around Boston.

Graduate student Jared Newell, left, and Associate Professor of Geography & Environment Nathan Phillips are part of a team using the Picarro sensor, seen here, to measure airborne carbon dioxide levels above BU’s Stone Science Building and around Boston.

Photo courtesy of Nathan Phillips

Because carbon dioxide is not regulated by the EPA, few studies have been done so far to characterize the gas in urban, suburban, or rural environments. So before ULTRA researchers can solve the problem of offsetting carbon emissions, they need to learn more about the sources of carbon, as well as the sinks—natural or artificial systems that absorb the gas—in and around Boston. To this end, they are using a Picarro sensor installed on the roof of a campus building to take measurements of airborne carbon dioxide levels ten times per second.

Last spring, Assistant Professor Lucy Hutyra teamed up with Phillips to take the Picarro on the road. After placing it in the trunk of Hutyra’s car, they drove from Boston to Worcester, taking measurements of the carbon gas levels every five seconds using a sensor protruding through the car’s sunroof. Preliminary findings suggest that carbon emissions increase dramatically in urban areas, but some of the data they collected was surprising. “We went to places that we thought would be significant sources for carbon,” says Hutyra. “And some of them were what we expected and some of them weren’t.”

Phillips is also interested in learning which species of plants are most efficient in carbon uptake. “The urban environment is like an experimental treatment,” he says. “Are plants more or less healthy? Are they growing faster or slower?”

Certain species of vegetation may hold the key to the carbon question. “There is data that suggests that some invasive species grow like gangbusters in the urban environment, and that native vegetation doesn’t do as well,” says Phillips. One of the most common invasive species is Ailanthus altissima, an East Asian tree that thrives in urban environments all along the East Coast, leading Phillips to wonder, “If those ailanthus trees were not growing in the parking lot, what would be there? Would it be a nice native red oak tree, or are invasive species occupying niches in the urban environment that might be too degraded for native species to occupy?”

Hardy species of vegetation like the ailanthus may be the answer to increasing the amount of carbon sinks in metropolitan areas like Boston. The more plants that are growing, the more carbon is absorbed. Satellite imagery can show researchers how much vegetation exists in a certain area, but it cannot tell them how much carbon is stored in each plant. A ground-level census of the vegetation in and around Boston—combined with the data that Picarro has collected about the levels of gasses in the air—could lead BU researchers toward finding ways of offsetting the high levels of carbon found in urban and suburban areas.

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Nathan Phillips talks about BU’s rooftop real estate.

Another aspect of ULTRA is studying the effects of human behaviors on carbon emissions in Boston. Led by Robert Kaufmann, professor and chair of the Department of Geography & Environment, this project is focusing on when and why people are using certain appliances and is also tracking commuting habits to see how residents might be nudged in sustainable directions.

In 2012, BU researchers will compete for the next level of ULTRA, which would provide funding for a decade. The team is confident that their findings to date will pique interest in future research. “Once we have the core information, we can start to play what if games,” says Hutyra. “What if we increase the canopy cover? What if we required new housing developments to set aside green space? What would that mean in terms of the carbon impact? But before we can really do that, we have to complete our understanding of where we are today.”