NIST’s Comb Systems Measure All Primary Greenhouse Gases in the Air

National Institute of Standards and Technology (NIST) researchers upgraded their laser frequency-comb instrument to simultaneously measure nitrous oxide, carbon dioxide, water vapor, and the major air pollutants ozone and carbon monoxide. The NIST work specifically involves a shift from the spectrum of light analyzed in the near-infrared to the mid-infrared to enable the identification of more and different gases.

NIST’s older, near-infrared comb systems cannot identify nitrous oxide, ozone, or carbon monoxide; combined with an earlier version of the system that measures methane, NIST’s dual-comb technology can now measure all four primary greenhouse gases. The development could help in understanding and monitoring emissions of the heat-trapping gasses that are implicated in climate change, as well as assess urban air quality.

NIST researchers used a laser frequency-comb instrument (illustration at lower right) to simultaneously measure three airborne greenhouse gases — nitrous oxide, carbon dioxide, and water vapor — plus the major air pollutants ozone and carbon monoxide over two round-trip paths (arrows) from a NIST building in Boulder, Colo., to a reflector on a balcony of another building and another reflector on a nearby hill. Courtesy of N. Hanacek/NIST.

The NIST instruments identify gas signatures by precisely measuring the amount of light absorbed at each color in the laser spectrum as specially prepared beams trace a path through the air. The comb systems can measure a larger number of gases than conventional sensors (those that sample air at specific locations), and offer greater precision and longer range than similar techniques that use other sources of light.

Current applications include the detection of leaks from oil and gas installations, as well as the measurement of emissions from livestock.

Researchers demonstrated the new system over round-trip paths with lengths of 600 m (approximately 1970 ft) and 2 km (approximately 1.25 miles). The light from two frequency combs was combined in optical fiber and transmitted from a telescope located at the top of a NIST building in Boulder, Colo. One beam was sent to a reflector on a balcony of another building. A second beam was sent to a reflector on a hill. The comb light bounced off the reflector and returned to the original location for analysis to identify the gases in the air.

A specially engineered crystal material, periodically poled lithium niobate, that converts light between two colors is the key component of the researchers' frequency combs. The system used in the experiment split the near-infrared light from one comb into two branches. It used fiber and amplifiers to broaden and shift the spectrum of each branch differently and boost power, and then recombined the branches in the crystal. This produced mid-infrared light at a lower frequency that was the difference between the original colors in the two branches.

The system was precise enough to capture variations in atmospheric levels of all the measured gasses, and agreed with results from a conventional point sensor for carbon monoxide and nitrous oxide. The detection of multiple gases at once allows measurement of correlations between the multiple gases; for example, measured ratios of carbon dioxide to nitrous oxide agreed with other studies of emissions from traffic. In addition, the ratio of excess carbon monoxide versus carbon dioxide agreed with similar urban studies but was only about one-third the levels predicted by the U.S. National Emissions Inventory (NEI). These levels provide a measure of how efficiently fuel combusts in emissions sources such as cars.

The NIST measurements echoed other studies to suggest that there is less carbon monoxide in the air than the NEI predicts, and put the first hard numbers on the reference levels or “inventories” of pollutants in the Boulder-Denver area.

“The comparison with the NEI shows how hard it is to create inventories, especially that cover large areas, and that it is critical to have data to feed back to the inventories,” lead author Kevin Cossel said. “This isn’t something that will directly impact most people on a day-to-day basis — the inventory is just trying to replicate what is actually happening. However, for understanding and predicting air quality and pollution impacts, modelers do rely on the inventories, so it is critical that the inventories be correct.”

Researchers plan to extend the reach of the new instrument to longer distances, as already demonstrated for the near-infrared system. They also plan to boost detection sensitivity by increasing the light power and other tweaks, to enable detection of additional gases.

Increased understanding of air quality, specifically the interplay of factors influencing ozone formation, is one of the improvements that the work may spawn.

The work was funded by the Defense Advanced Research Projects Agency and the NIST Special Programs Office.

The research was published in Laser & Photonics Reviews (www.doi.org/10.1002/lpor.202000583).