Developed by a team from the US National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, the system uses an eye-safe laser instrument to send light that “combs” the air to an airborne multi-copter and analyses the colours of light absorbed along the way to identify gas signatures in near-real time.
The so-called “comb and copter” system may be useful to scan for leaks in oil and gas fields, study the mixing of vehicle emissions and other gases in the boundary between the earth’s surface and the next layer of the atmosphere, or, with planned upgrades, detect pollutants or chemical threats and their sources. The work is described in Optica.
Researchers used the comb light to measure carbon dioxide, methane and water vapour along a 2km path between a telescope on the roof of a NIST Boulder lab and a retroreflector mounted on a small, unmanned aircraft.
The multi-copter hovered in selected spots to measure gases along a horizontal path and at various altitudes of up to 120m. Higher-altitude flights are technically feasible but currently limited by unmanned aerial vehicle flight rules.
The new system measured carbon dioxide levels of one part per million in 60 seconds, compared to 200 seconds in a NIST study from 2014.
“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,” said Kevin Cossel, a physical chemist at NIST. “The technology and sensitivity are promising.”
The laser instrument uses two frequency combs - measurement tools made up of thousands of precise frequencies - to identify gases based on the amounts of specific colours of light absorbed. The 2014 test showed that the dual-comb technique can precisely, reproducibly sense trace gases in the atmosphere. In that work, the comb light sent by the telescope was returned from a mirror mounted on a nearby mountain.
The latest version of the instrument features several upgrades, including more power, an improved telescope and a lightweight retroreflector. NIST researchers also made the comb system more compact and transportable.
The multi-copter used in the experiment was fitted with a retroreflector as well as instruments for measuring location, temperature, air pressure and path length. The telescope system must track the motion of the retroreflector as the multi-copter moves and hovers. The entire system retrieves gas concentrations every 10 seconds.
The comb and copter system is expected to complement conventional technology. Mobile ground-based point sensors can make regional maps but must be driven in a vehicle or flown on a plane. Satellite instruments can sense atmospheric gases remotely with global coverage but sample specific regions on Earth infrequently and with only coarse spatial resolution.
In the near future, researchers plan to use the flying comb system to study mixing in the earth’s boundary layer, which is said to be a major source of uncertainty in atmospheric models, and to scan for emissions from oil and gas facilities, which can lead to the formation of ozone.
The “comb and copter” system currently detects gas signatures in the near-infrared band of the electromagnetic spectrum. NIST researchers hope to extend that coverage to the mid-infrared, which would increase the number of detectable gases and enable applications such as scanning for chemical hazards and threats.
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