Optical frequency combs are essentially high-tech “rulers” for measuring different colors of light; they’re useful for making better atomic clocks and hunting for exoplanets, among other things. Now scientists at the National Institute of Standards and Technology (NIST), collaborating with researchers at Kansas State University (KSU), have introduced the “agricomb,” an optical frequency comb that measures the gassy emissions from cow burps—the first use of frequency combs in an agricultural setting. The tool could one day help boost agricultural yields and enable the design of cleaner farms, according to a recent paper published in the journal Science Advances.
According to the authors, the so-called “digestive processes” of livestock account for the largest US source of methane and ammonia emissions. (The former is a major greenhouse gas, while ammonia is an atmospheric pollutant.) A single cow belches about 220 pounds of methane every year.
That’s one reason why there are calls in some quarters to drastically cut down on consumption of beef. However, some scientists—notably Frank M. Mitloehner of the University of California, Davis—have pointed out that cows and other ruminants nonetheless currently account for just 4 percent of all greenhouse gases produced in the US, thanks to better breeding, genetics, and nutrition, among other advances.
Being able to make extremely precise measurements of methane and ammonia emissions could help improve matters even more. But that can be a challenge, according to the NIST/KSU authors, because management practices can vary widely from farm to farm. And cattle in grazing systems are unevenly distributed, which can limit the usefulness of conventional optical sensors. Ammonia concentrations are especially difficult to measure precisely with conventional sensors. That’s where the new optical frequency “agricomb” can help.
Optical frequency combs are created with lasers, which emit continuous, closely spaced, brief pulses of light of many different colors. Over time, the properties of that light are converted to frequency numbers to create something that looks like a comb. Each “tooth” in the comb is a different color (frequency) of light, depending on how fast the light wave oscillates. Faster oscillations yield larger frequency numbers, so blue light waves will oscillate faster than red waves, for instance, with yellow and green waves falling somewhere in between. The teeth serve as a ruler to measure different colors of emitted light.
NIST scientists have pioneered several advances in optical frequency combs over the last two decades—including a custom-made “astrocomb” introduced in 2018 to precisely measure the frequencies of starlight. The astrocomb is a key tool in the ongoing hunt for exoplanets, since one way to search for planets orbiting distant stars is to look for tiny periodic variations, or wobbles, in the apparent colors of starlight over time.
But these are challenging measurements when it comes to stars in the so-called “Goldilocks zone,” where conditions are neither too hot nor too cold for life. NIST’s astrocomb has some 5,000 “teeth” that serve as specific color calibration points. So it can calibrate and track the colors in an M dwarf star’s spectral fingerprint—this type makes up 70 percent of stars in our galaxy—and detect any of those telltale variations.
The new agricomb is a two-comb system; the different spacings of teeth in the two combs further enhance the precision of the measurements. It identifies the targeted trace gases based on the exact shades and amounts of infrared light that are absorbed by the atmosphere when the light from the comb is sent back and forth across a designated open-air area.
The researchers set up their portable system inside a trailer parked next to a feedlot holding about 300 cows in pens. The cows consumed a mix of hay and corn silage. Using the agricomb, the NIST scientists measured the parts-per-million concentrations of methane and ammonia across 100 meters (about 109 yards) upwind and downwind from the cow pens—both from the cows’ burps and from manure on the ground.
Finally, the NIST/KSU team compared those measurements to measurements from a commercial sensor set up to sample the air at multiple points along the edges of the feedlot. The NIST agricomb’s methane measurements were comparable to those of the commercial sensor. The agricomb was also better at capturing emissions in downward plumes—and therefore better for determining gas sources—as well as being able to measure many different gases at the same time.
“For the future our plan is to work with KSU to do a pasture measurement, where the cattle eat native grasses,” said co-author and NIST physicist Brian Washburn. “The different feed, plus microbial activity in grassland soils that consumes methane, may mean less atmospheric methane production in the pasture than in the feedlot. The cattle spend about 75 percent of their life in the pasture, so this measurement would be more representative of the net methane production. This would also be a harder measurement, since it would take place over a larger area, about 500 meters by 500 meters, with fewer animals, about 40 head.”