Some people see trees for the forest; others see the forest for the trees. But researchers at the University of Utah saw the forest, and the trees, for something else entirely. Hanging from the branches of pine trees near their campus, they saw thousands of tiny air quality sensors.
Those sensors, more commonly called pine needles, are constantly gathering microscopic pieces of dirt, metal, and other compounds floating around in the air. That makes them ideal gauges of air pollution levels, said Peter Lippert, a geoscientist at the University of Utah. Building on previous work using trees to track particulate matter pollution, his lab found they can track levels of air pollution by looking at how metal-coated pine needles respond to magnetic fields. This method could enable measurements of air pollution that are more detailed and nuanced than ever before, said Lippert.
Tracking Pollution in the Air
Air pollution can be especially bad in Salt Lake City, where the university is located. Temperature inversions, where a lid of cold air traps warmer air beneath it, can trap smog in the mountain valley the city sits in, causing air quality to plummet. On the worst days, the city stews under a noxious haze of stale air.
Especially worrying are tiny particles of iron and other metals that mix with airborne dust and soot. They come from windblown dust and fossil fuel burning, as well as local sources like car brakes. These metallic nanoparticles can make their way into our bodies when inhaled and even inside our brains. Recent research suggests they may play a role in Alzheimer’s and other neurodegenerative diseases.
Still, data on how much of this metallic particulate matter pollution is floating around are lacking.
“We don’t have a good understanding of, particularly for trace metals and elements, what the distribution is in urban environments,” said Gwen O’Sullivan, the chair of the Department of Environmental Science at Mount Royal University in Canada. But data show that people with lower socioeconomic status are more likely to live near sources of particulate matter pollution like factories or highways.
“Your proximity to that roadway will really have an implication for your health,” O’Sullivan said. New measurements using ubiquitous pine needles could enable data to be gathered at a scale of tens of meters and could help create fine-scale maps of pollution.
Air Quality Sensors All Around Us
The technique works by applying a magnetic field to pine needles in the lab. With a normal pine needle, nothing much will happen. But one that’s covered in a fine layer of iron particles will respond with a magnetic field of its own because of the ferromagnetic nature of the metal grains.
Lippert said that in early tests, the method has proven to be a reliable way of measuring levels of metal particulate matter pollution. In one experiment, published in 2020 in the journal GeoHealth, he and his coauthors sampled needles from four locations in Salt Lake City and found levels of air pollution were higher near roads. For their latest research, which will be presented at AGU’s Fall Meeting 2021 on 15 December, they 3D printed thousands of fake pine needles to compare with the real ones. The real ones won out, said Lippert, likely because they’re covered in wax, which helps grab onto the tiny particles of metal.
There’s still work needed to better understand how the pine needles hold on to their dusting of pollution. For example, Lippert said researchers don’t know how long particulate pollution stays on the pine needles and how the particles might be changed by chemicals in the plants. But the technique is already enabling powerful insights into air pollution. Using magnetism, he said, scientists are able to get a good idea of what size the particles are and perhaps even what kinds of compounds are in the air—both things with big consequences for the people breathing those particles in.
Lippert envisions expanding the project to a network of crowdsourced science—in this case, a network of volunteers who would gather pine needles near them to send in for analysis. Although there are already air sensors that make these kinds of measurements, maintained by groups like the EPA, they’re few and far between.
“EPA sensors are good, and we need those, but we need a much more dense network of information to design better, healthier cities,” Lippert said.
—Nathaniel Scharping (@nathanielscharp), Science Writer