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Evergreen needles act as air quality monitors



Evergreen needles act as air quality monitors

Peter Lippert (left) and Grant Rea-Downing study artificial pines tested as passive air quality monitors. Photo from September 201

9. Credit: Paul Gabrielsen / University of Utah

Every tree, even an evergreen one, can be an air quality monitor. That’s the conclusion of researchers at the University of Utah who measured the magnetism of particles on needles of evergreen trees on the U-campus. This measurement, they found, was correlated with overall air quality, suggesting that the analysis of needles – a relatively simple and inexpensive process – could provide a high-resolution image of air quality year-round.


“Wherever you have a tree, you have a data point,” says Grant Rea-Downing, a doctoral student in geology and geophysics. “A tree does not cost $ 250 to implement. We will be able to map particle distributions with a very high resolution at very low cost.”

The results are published in GeoHealth.

How magnetic particles end up on leaves

Rea-Downing and his colleagues – Associate Professor Pete Lippert and fellow students Courtney Wagner and Brendon Quirk – are all geoscientists at the Department of Geology and Geophysics, whose regular research is on a very different scale than pine needles.

“Day by day,” says Lippert, “we move mountains and close ocean basins by using the magnetism of rocks to find out the geography of previous continents.”

In a course entitled “The Magnetic Earth”, Lippert introduced Rea-Downing, Wagner and Quirk to papers from British scientists who measured the magnet of deciduous leaves to assess air quality. “I knew the students kind of would be blown away by what the study showed and what the consequences of the results were,” Lippert says.

Particles in the air come from many sources, including naturally blown dust, brake dust and the by-product of solid or fossil fuel combustion.

“It’s things in the air,” says Lippert, “and it has to come out someday.”

When it falls out of the air, some of it, of course, falls on tree leaves and evergreen needles. Some of the particles contain enough iron to be detected using the kind of high-precision magnetometers that Lippert uses in his geological work. The ferrous particles in the air may be too small to see, but magnetism, he says, is a way of seeing the unseen.

The newspapers made an impression on Rea-Downing, who saw Salt Lake City’s air quality in sharp contrast to the normally clean air in his original coastal California. He could easily apply the method in Lippert’s research laboratory.

“That kind of hill to climb to do this was actually pretty flat,” he says. “We have trees outside, we have seasonally poor air quality, and we have a fully equipped paleomagnetic lab, which means I literally just had to go outside and pull some leaves off some trees and keep them in a magnetometer.”

“We are not the first to explore the magnetism of pine trees to monitor air quality,” says Lippert, “but no one had tried this to study winter inversions in the basins of the American West.”

With financial support from the U’s Global Change and Sustainability Center, the researchers went to work.

Evergreen needles act as air quality monitors

Scanning of electron micrographs of evergreen needle surfaces. Images from each location are displayed under both non-inversion (left) and inversion (right). Credit: University of Utah

Sylvan guard post

The team selected four Austrian pine trees on the U-campus to try. Three of the trees were in a line perpendicular to North Campus Drive, a widely used campus artery, with each tree successively further away from the roadway. The fourth was near the EU building, away from traffic. They collected pine needles twice: once in June 2017 after a summer with relatively good air and again in December 2017 during some of the worst air quality of the winter.

With his dust mask with particle filtration on, Wagner collected the December samples in what she described as a “freezing fog”, as a temperature inversion throughout the valley had led to a thick yellowish mist and frost on pine needles. Back in the lab, the team carefully cut needles into short segments using ceramic scissors to avoid metal contamination and insert them into the magnetometers.

One of their experiments revealed that the magnetization of December needles was almost three times higher than June needles. Another magnetic experiment performed at super low temperatures suggested that the iron-bearing particles deposited during the inversion were extremely small (some as small as 1/5000 the width of a human hair) and found that they are composed of magnetite, an iron mineral which, as the name suggests, is naturally magnetic. The team also examined needles under an electron microscope and confirmed that December needles were significantly soiled. The concentration, size and composition of the particles are all linked to other studies with the health risks of air pollution.

They also looked at the elements of the particles. The amounts of iron in the dust correlated with the amounts of other elements such as titanium, vanadium and zirconium, “and a number of other things associated with brake dust or the burning of fossil fuels,” says Lippert.

Other elements in the particles were associated with catalysts, he says, using chemical catalysts to detoxify exhaust. “And these concentrations, no surprise, are highest near the roadside.”

Comparison of the trees at different distances from the roadway showed a decrease in the concentration of magnetic particles over a distance of 50 to 150 feet. This may be due to the distance from the cars, the researchers say, but possibly also to the height, as the traction of trees went up a small hill.

Artificial guy

Now, the team has teamed up with atmospheric scientist Gannet Hallar and chemical engineer Kerry Kelly to explore other issues that the study raised. They developed a new kind of passive air monitor – a 3D-printed, artificial pine branch with needles to catch particles. The artificial needles are installed next to natural needles and can serve as an experimental platform to more clearly understand how and when particles settle on evergreen needles, results they can compare directly with measurements of particle distributions measured by equipment in Hallars and Kelly’s Laboratories.

“If we get heavy rain, we can go and collect before and after that rain and see if that signal just flushes away every time you have a rain event,” Rea-Downing says. “Or do the biological needles actually absorb material and actually hold the signal longer than the synthetic needles?”

With each tree as a potential data point, pine needle analysis could provide a more comprehensive insight into what, when, and why air pollution in urban areas shows variation in air quality on a scale of ten feet. The analysis is straightforward and cheap, says Lippert.

“We already have a lot of trees out in the countryside,” Lippert says. “They’re pretty low. So this democratizes our ability to monitor air pollution across the valley. This can be easily exported to any community. It allows us to do more with less, or that’s our hope.”


Pine needles from old Christmas trees could be turned into paint and food sweeteners in the future


More information:
Grant Rea – Downing et al., Evergreen needle magnetization as a proxy for particulate pollution in urban environments, GeoHealth (2020). DOI: 10.1029 / 2020GH000286

Provided by the University of Utah

Citation: Evergreen needles act as air quality monitors (2020, September 16) retrieved September 16, 2020 from https://phys.org/news/2020-09-evergreen-needles-air-quality.html

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