Portable UV spectrometer can detect air pollutants across 2.5 km with high precision

Birgitta Schultze-Bernhardt and her team at the Institute of Experimental Physics at Graz University of Technology (TU Graz) have developed a new type of UV dual-comb spectrometer that detects gaseous air pollutants with unrivaled accuracy and sensitivity. Using ultraviolet double laser light, the device measures the concentration of harmful gases such as formaldehyde within half a second.

Thanks to its compact design and a measuring range of up to two and a half kilometers, the spectrometer is not only suitable for laboratory analyses, but also for mobile measurements in cities, industrial areas and agricultural regions.

The work is published in the journal PhotoniX.

Fingerprint of pollutants

As a starting point for its measurement, the device generates two laser pulses in the ultraviolet spectral range within fractions of a second. When this UV light hits gas molecules, it excites them electronically and causes them to rotate and vibrate—physicists refer to this as rovibronic transitions.

These transitions are different for every gaseous substance and swallow up part of the laser light in a unique way. “Every air pollutant therefore has its own fingerprint, which our UV dual-comb spectrometer recognizes,” says Schultze-Bernhardt.

Schultze-Bernhardt and her team developed the first version of their spectrometer a good two years ago. At the time, it was the first of its kind in the world, but large laboratory set-ups were necessary for the measurements. The new version has been shrunk to the size of a cardboard removal box. One reason for this is that one laser source instead of two now generates the double laser pulse.

“This also allows us to dispense with the complex electronic stabilization of the system,” explains Schultze-Bernhardt.

Resolution of 1 GHz

The new spectrometer can detect the frequencies of UV light with a resolution of 1 GHz and thus significantly outperforms all conventional UV spectrometers. This enabled the researchers to gain new, fundamental insights into the UV light absorption of the air pollutant formaldehyde:

“We measured absorption patterns of formaldehyde that had never been observed experimentally before, as the resolution of any previous devices was too imprecise,” says Schultze-Bernhardt.

57-year-old rotational constants corrected

The measurements in Graz have shown that the rotational constants of formaldehyde, which have been available in physics databases and textbooks since the 1960s, are incorrect.

“In collaboration with the Harvard-Smithsonian Center for Astrophysics, Atomic and Molecular Physics in the U.S., we have corrected the values of this fundamental, molecule-specific parameter by up to 15%,” says Schultze-Bernhardt.

Collaboration with Rolf Breinbauer from the Institute of Organic Chemistry at TU Graz, who produced high-purity formaldehyde for the investigations in a two-stage process, also contributed to this progress in basic research.

Practical application in environmental protection

Beyond basic research, the spectrometer has the potential to make the measurement of air pollutants and gas leaks in cities and industrial areas much more precise and easier.

“In principle, our device can accurately detect any semi-transparent, gaseous substance. And we are currently working on determining the concentration of several pollutants with a single measurement,” says Schultze-Bernhardt. The experimental physicist is also currently developing a UV spectrometer that can also be used by laypersons to monitor air quality, for example in companies or environmental authorities.

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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