Combing the atmosphere: IR gas signatures

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  • Published: Jul 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Combing the atmosphere: IR gas signatures

In-flight spectroscopy

NIST has combined a laser instrument that “combs” the air with a flying multi-copter to scan and map atmospheric gases over kilometer distances.  Credit: NIST

An infrared laser that can comb the air between it and a quadcopter flying perhaps several kilometres away can reveal precisely what gases are in the atmosphere over a city or other region, according to scientists at the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. The mobile, ground-based system could be used to scan and map atmospheric gas plumes over large areas using an "eye-safe" laser instrument to send light that "combs" the air, The quadcopter reflects the light back to the base station instrumentation where the returning laser light is analysed spectroscopically to determine gas signatures in near-real time based on energy absorbed en route by those gases.

At the moment, the system can identify atmospheric water vapour, methane, and carbon dioxide simultaneously. Upgrades to the technology are already planned that could be used to detect specific pollutants or chemical threats and even pinpoint their sources. The "comb and copter" system could be used to scan for gas leaks over oil and gas fields. It might also be used to investigate vehicular or industrial emissions and how they interact with the atmosphere above city roads and above factories, particularly in the atmospheric boundary layers.

Fast-track analysis

Aviation law limits the altitude at which the quadcopter can fly to 120 metres but technically it could be used to retroreflect a distant laser beam at a much greater height under some jurisdictions for atmospheric studies or over sensitive regions of the world. The new setup is faster than NIST's early system which took 200 seconds to determine carbon dioxide levels down to 1 part per million. The new system takes just a minute to obtain that accurate a measurement.

"Now we can do the same sort of atmospheric measurements, with a little higher sensitivity, with a system that we can point to wherever we want," NIST physical chemist Kevin Cossel explains. "The technology and sensitivity are promising."

The laser instrument uses two frequency combs. In the 2014 tests, the dual-comb technique was used to fire a beam at a reflector on a nearby mountain. The new version is not only of higher power, but the telescope optics are improved and a lightweight retroreflector, a specialized 3D mirror, means it can be carried on a quadcopter rather than having to be attached to a mountainside. This opens up the possibility of scanning the atmospheric in different directions and altitudes. The NIST team has also shrunk the kit so that it fits into a volume about the size of a domestic kitchen cooker, or stove, so it is easily transportable by pickup truck.

For all their expertise in lasers and spectroscopy, the NIST scientists turned to unmanned aircraft flight experts on the University of Colorado’s Integrated Remote and In Situ Sensing (IRISS) team. "Flying these things turned out to be challenging," concedes NIST's Nathan Newbury. Quadcopters can wander off course, or worse, crash in unskilled hands, he points out.

It's a gas

The unmanned aircraft used by the team not only reflects the laser light back to the base device but records its specific location, temperature, air pressure and path length so that these parameters can be coupled to the spectroscopic data. The entire system captures gas concentrations and data every 10 seconds. The system will likely complement ground-based sensors and remote, satellite and aircraft based technology.

Currently, the system works in the NIR, near-infrared, band of the electromagnetic spectrum. The NIST team hopes to extend its repertoire to important mid-infrared and so bump up the number of different gases that can be detected and allow scanning for chemical hazards and threats.

Related Links

Optica 2017, 4, 724: "Open-path dual-comb spectroscopy to an airborne retroreflector"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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