A new technique using a pair of laser-based instruments could one day help scientists more accurately detect concentrations of the greenhouse gases considered responsible for climate change.
Researchers at the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration are refining a method using two frequency combs to indirectly measure the amount of carbon dioxide, methane and water vapor over a substantial distance.
A single frequency comb emits a laser beam that contains many different wavelengths or frequencies of light, with each "tooth" of the comb representing a different frequency. The concentration of the different gases is measured by how much light is absorbed. The more of a gas that is present, the higher the amount of absorption.
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To make the frequency comb better able to identify gas absorption signatures, the laser beam contained light frequencies too high to be directly detected. In order to see what light had been absorbed at what wavelength, the researchers needed a way to spread out the frequencies.
To do this, the researchers used a second frequency comb with slightly differently spaced "teeth."
The researchers brought the laptop-sized sensors to the roof of the NIST laboratory in Boulder, Colo. They directed the laser beams toward a mirror on a mesa, which reflected the beams back to a detector in the lab.
The difference in spacing between the combs corresponded with certain radio frequencies that were measurable and could be correlated to specific greenhouse gases. The demonstration was the first time that the technique had been shown to work over long distances outdoors.
Researchers have been using frequency comb technology to study greenhouse gases. However, these are the first portable sensors that are able to detect multiple gases over a relatively long distance.
'Combing' the atmosphere with lasers
Today, scientists measure the concentrations of greenhouse gases in the air by taking samples using single-point sensors. In order to estimate how much CO2 is around a power plant, or how much methane is being emitted from a feedlot, researchers would have to take multiple measurements from around the desired area with ground-based sensors and extrapolate from those measurements what the overall amount of CO2 or methane might be within the given area.
Another way to measure gases is by taking point measurements from an airplane, which also makes the experiment more expensive.
Neither of these methods is ideal for scientists.
"What people worry about is representation error," said Nathan Newbury, a physicist at NIST who is involved in the study. "Did this sample accurately represent the area?"
Since typical ground sensors have a range in meters, variables like wind or exhaust from a passing car can alter measurements enough to give an inaccurate representation of greenhouse gas concentrations.
The dual laser frequency comb method would help reduce that uncertainty, because researchers could continuously measure the amounts of CO2 and methane over a substantial distance.
In the demonstration experiment, the researchers collected data continuously over three days. This allowed them to average the results over the entire path length, correcting for some variability.
It will take another year or two before the technology is ready for use outside of the laboratory and still longer to make it effective over greater distances and a broader spectrum of gases. The researchers also hope to measure gases over multiple paths at once.
For now, Newbury said, the main goal is to help agencies like NOAA develop a more accurate representation of greenhouse gas concentrations over a broader area.
"There is a huge problem of doing the modeling accurately. The more data we can give, that will help them have more confidence in the model," he said.
A pressing need to monitor the oil and gas industry
Eventually, the instrument would be part of the global greenhouse gas reference network maintained by NOAA. But scientists and regulators already feel a great need for it to monitor the oil and gas industry, said Pieter Tans, a scientist in the Global Monitoring Division at NOAA's Earth System Research Laboratory and a co-author of the study.
The industry is the second-highest emitter of methane, a greenhouse gas that is 86 times as potent as carbon dioxide on a 20-year time scale. Companies report their emissions to U.S. EPA every year, but scientists have recently found that they are likely lowballing their numbers.
"The companies who declare their emissions to EPA, they have an incentive to not overstate it, to say the least," Tans said. "So it is no surprise that when we do our measurements [of methane] for an entire [oil and gas] field, that our numbers always tend to be higher than the emissions inventories."
The instrument would help scientists check out the accuracy of inventories like EPA's by tracking greenhouse gases continually over a region.
Such continuous measurements have not been possible so far. Instead, scientists have flown aircraft and driven mobile vans loaded with methane sniffers around oil and gas basins, but their measurements have been mere snapshots in time.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500
