Alexandra Ponette-Gonzalez, University of North Texas geography professor

Alexandra Ponette-González, a University of North Texas geography professor, prepares to put a glass collection bottle into a collector before a recent rain to measure how much soot has been collected by the tree above. See video of her work at

University of North Texas geography professor Alexandra Ponette-González sees oak trees as big dust collectors.

As it turns out, they are pretty good at it.

Two years ago, Ponette-González began a major research project to figure out just how well some of the city’s trees could filter soot from the air. She received a five-year grant from the National Science Foundation to help pay for the work.

“It’s been a topic in the [scientific] literature and the general conversation for a while,” she said.

But until now, few scientists have set out to measure how much soot trees actually take out of the air and send back in the ground.

The idea is important because doctors know that it’s bad for people and animals to breathe in soot, or more specifically, black carbon. The compound sticks to lung tissue and damages it.

Coal miners, for example, can get deadly black lung disease from breathing black carbon. Most soot in the atmosphere comes from vehicle exhaust, cooking fires and other burning.

Soot also absorbs sunlight and contributes to climate change. If trees prove to be good urban air filters, planting and protecting trees may be an effective strategy to stem the tide.

“We could see a change in decades,” she said.

Ponette-González and several graduate students have worked for the past two years to collect samples and measure what the trees were doing.

Scientists already know that soot is sticky enough to collect and stay on leaves, particularly hairy and waxy ones.

In addition, trees have a large, complicated structure within the branches and canopy that creates wind eddies, helping to stir and capture pollutants from the air.

When it rains, some of the soot washes off the leaf hairs and returns to the soil. The rest of the soot stays stuck in the leaf wax, coming down when the tree sheds its leaves in fall.

Ponette-González and the students gathered samples when it rained and as the trees shed leaves.

Many Denton home and business owners, as well as the city of Denton, UNT and Texas Woman’s University, agreed to host sampling sites. Many other people noticed the scientific equipment parked under trees and out in open fields and called or wrote Ponette-González to ask about it, she said.

When they were done gathering, the team had about 1,500 samples to learn what both post oak and live oak trees could do in filtering black carbon from urban air.

The preliminary results have been promising, particularly with post oak trees, she said.

“Post oak trees are great urban air filters,” she said, adding that didn’t mean that live oak trees weren’t as good.

The team won’t be done processing samples until fall, and then the number crunching starts, she said. That’s when they expect to better understand the filtering by live oak trees, which keeps their leaves year-round.

When the analysis is finished, Ponette-González expects the results will have major implications for geographers, urban planners and more.

For example, she said, soot levels are higher in the winter. Some communities may opt to plant more live oak trees because they are evergreen.

“What goes up must come down,” she said, adding that geographers can help answer an important question about urban trees. “What does the data tell us about the where?”

PEGGY HEINKEL-WOLFE can be reached at 940-566-6881 and via Twitter at @phwolfeDRC.

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