New Device To Provide More Accurate Emissions Data

The amount of greenhouse gases in specific areas of the Earth’s atmosphere has long been only determined indirectly, by estimating how much gas is emitted from sources like cars and power plants.

Now two U.S. government agencies have developed a unique laser-based device that can accurately measure the amounts of such gases in wide swaths of sky.

The device, developed by the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration (NOAA), eventually could be developed into a portable system that could generate extremely accurate and continuous monitoring of the gases in wide areas of sky.

Their research was published Oct. 29 in the journal Optica.

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The new device is called a laser-frequency comb – a laser-generated apparatus composed of many very precisely defined frequencies that are equally spaced, much like the teeth on a comb. Each “tooth” represents a single color, or frequency, of the gases the comb encounters. The comb can then accurately measure the signatures of these gases.

NIST conducted a demonstration recently in which it had “laser-frequency combs” simultaneously measured the signatures of Earth-warming gases such as carbon dioxide, methane and water vapor along a two-kilometer stretch of land between an NIST laboratory in Boulder, Colo., and a nearby mesa.

The scientists shot a beam of laser light from the roof of the NIST lab to a mirror on the mesa. The beam was then reflected back to a light-spectrum detector in the lab. This way, the researchers could identify the gases absorbed at different frequencies during the beam’s round trip.

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There was one problem, though. Optical frequencies are too high to be measured directly, so the researchers constructed two combs, each with slightly different spacing, or a different frequency, between its teeth. Mixing the light from these two combs brought the optical frequencies low enough that they could be measured directly.

The NIST-NOAA device is a major improvement on current methods of measuring atmospheric contaminants. The same measurements can be made from a satellite equipped with conventional instruments, called spectrometers. But satellites tend to measure atmospheric content globally and rarely focus on specific regions of the Earth.

The current comb system was designed to detect some gases over a distance of two kilometers, but, on paper at least, NIST says it appears that the apparatus could detect even more gases over an even larger area. And its accuracy is maintained by the precision of the spacing between the teeth of the comb.
The laser-frequency comb also isn’t susceptible to signal distortions caused by atmospheric turbulence because it makes its measurements rapidly and repeatedly over the same path. As a result, NIST and NOAA believe it will become an important tool in creating accurate models for greenhouse gas monitoring.

By Andy Tully of Oilprice.com

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