Scientists used lasers to measure methane emissions from cows more precisely over large areas than previously possible, which could lead to better estimates of how much greenhouse gas livestock produce.

The technique also allowed the researchers to gather readings of several important greenhouse or pollutant gases simultaneously. The team reported the findings March 31 in Science Advances. 

"Part of the goal here was to understand [livestock] methane emissions, as they contribute to the total methane budget for the nation," said Ian Coddington, a project leader at the National Institute of Standards and Technology and a co-author of the study. 

In the U.S., livestock used in agriculture are thought to be the largest source of methane emissions related to human activities, Coddington and his colleagues noted in the paper. But figuring out exactly how much of the gas comes from belching ruminants and their manure has been a challenge. 

Measuring methane from a cow's breath or at one spot on a farm doesn't give a full picture of how the animal's gas production fluctuates over time or how the concentration of gases may vary across the area. In addition, both of these methods are used in feedlots, where cows are closely grouped together, rather than in open pastures, where the animals graze. 

"The cows are so dispersed from one another that all these traditional sensing technologies fail to give you at all an accurate picture of what's going on," said Daniel Herman, a graduate research associate at NIST and first author of the study.

He and his colleagues used tools called optical frequency combs to measure gaseous emissions from an area of 3,700 square meters with nearly 300 cattle outside of Manhattan, Kansas, collecting measurements from late October 2019 to early January 2020. This technique is a form of spectroscopy — which essentially measures how light interacts with matter — that is also being used to detect natural gas leaks in oil and gas operations.

Optical frequency combs provide more precise measurements than traditional methods by emitting evenly spaced pulses of laser light containing frequencies that can range from visible to infrared. These frequencies are a bit like the teeth of a comb, and Herman and his colleagues used combs with more than 175,000 teeth. 

"The fact that it's made up from these discrete teeth [means that] it's like a ruler for frequencies," Herman said. 

The team used optical fibers to transmit light from a laser resting in a trailer to telescopes on the north and south ends of the feedlot. Light launched from the telescopes then hit a mirror 50 meters away, bounced back and was then transmitted back to the trailer. The researchers compared the signal that returned to the one they sent out to determine how much light was being absorbed by gases in the laser's path. 

The combs allowed the researchers to measure the unique signatures from multiple gases at once and cover a larger area than traditional sensors. And the methane emission rates estimated by the combs and traditional sensors placed around the pen agreed with each other to within 6%.

The researchers also observed that methane emissions from the cows varied throughout the day as animals fed and digested. "You could see the methane emissions building and building over the course of the day, and it would increase by nearly two times over the average value sometimes," Herman said. Emissions then fell overnight as the cows slept.

The team was able to measure methane, carbon dioxide, water vapor and ammonia emissions from the pen simultaneously. Ammonia from cattle waste is usually difficult to measure because it tends to stick to the collection tubes of traditional instruments, particularly in humid conditions. But with their laser-based method, the researchers could measure both methane and ammonia concentrations with a precision in the range of tens of parts per billion.

"We were pretty excited that we were able to quantify ammonia fluxes pretty precisely," Herman said. "Ammonia isn't a greenhouse gas, but it's one of the largest sources of aerosol pollution from agriculture."

To account for the impact of weather, the researchers used data from nearby weather stations to model the winds in the area and estimate how much of the emissions were being swept into the path of the laser beam. One source of uncertainty is that these atmospheric models generally become less accurate over larger scales, Herman said.  

The researchers next plan to use their technique to measure the gas plumes emitted from cattle grazing in pastures. 

"In terms of getting these inventories right and knowing ... what we have to do to address climate change, it's really important to understand what's going on in the pasture, and this is a stepping stone to doing that," Coddington said.

In the future, optical frequency combs could be used to monitor how well a new strategy to reduce methane emissions, such as altering the cattle's feed, is working, said Brian Washburn, a physicist at NIST and last author of the paper.

In one recent study, researchers found that supplementing cattle's diet with red seaweed dramatically cuts down on the amount of methane the animals emit.

The study, "Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy," published March 31 in Science Advances, was authored by Daniel I. Herman and Fabrizio R. Giorgetta, National Institute of Standards and Technology and University of Colorado Boulder; Kevin C. Cossel, Eleanor M. Waxman, Gabriel M. Colacion, Nathan R. Newbury, Ian Coddington and Brian R. Washburn, National Institute of Standards and Technology; and Chinthaka Weerasekara, Lindsay C. Hutcherson, Stephen M. Welch, Brett D. DePaola and Eduardo A. Santos, Kansas State University.