An experimental instrument that combines precision laser spectroscopy and a mobile airborne reflector to map atmospheric gas concentrations could lead to safer methods of detecting hazardous emissions and better modeling of Earth’s atmosphere, its inventors report.
“These measurements are important fundamentally for understanding how gases [mix] and spread in the atmosphere,” said Kevin Cossel, a physical chemist at the National Institute of Standards and Technology (NIST) in Boulder, Colo., and technical lead for the project. “In the future, we also hope to be able to use the system to help detect and quantify hazardous gases,” he added.
To measure gases, the instrument uses a sophisticated type of multifrequency light source, known as a laser comb, to shoot eye-safe lasers through a gas plume at 50,000 slightly different wavelengths simultaneously. The laser sits on an adjustable platform coupled to a telescope. A second component, called a retroreflector, which is a kind of mirror that bounces light back exactly in the direction from which it came, makes it possible to redirect light fired through a portion of the atmosphere straight back to the instrument for analysis.
Until recently, the team had to position the retroreflector by hand on the far side of whatever volume of gas was to be probed, along a line of sight from the laser. Having to set the mirror by hand limited the system to measuring gas concentrations along a single open-air path, Cossel said.
Now, however, the inventors have taken a step that they say makes laser sensing of gas plumes much more versatile. They have mounted the retroreflector on a small remote-controlled drone called a multicopter, which uses helicopter-like propellers to hover and steer. Using a remote control, the scientists can fly the mirror-carrying drone wherever they wish and make it hover to receive a flash of laser light passing through a gas plume at any angle.
With this new mobility, “we’re excited about showing the capability of doing spatial mapping,” said Nathan Newbury, a NIST physicist and the principal investigator for this project. “There’s a lot of interest if we could go vertical to map up to the [atmospheric] boundary layer,” he added.
Ken Davis, professor of atmospheric and climate science at Pennsylvania State University in University Park, expressed his excitement about the possibility of using this system to map “trace gases from sources that are complex in space and time. The mobile reflectors on the copter make that possible.”
Moving Target
The new instrument works because each chemical element and molecule interacts with light in a unique way, absorbing specific wavelengths in a telltale pattern. To determine the composition of a gas, the researchers shine their laser light of known wavelengths (or frequencies) and brightness through the gas and measure how much of it gets absorbed at selected wavelengths. Newbury said that the laser comb’s 50,000 wavelengths cluster around 1.6 micrometers, a near-infrared wavelength well suited to measuring the spectroscopic signatures of water vapor, carbon dioxide, and methane.
In an experiment reported on 20 July in Optica, the multicopter hovered at a single location for a few minutes at a time before moving to a new spot for measurements. Using the telescope to sight the retroreflector, the researchers took gas measurements as far as a kilometer away. The system measured concentrations to a precision of 0.6 part per million for carbon dioxide and 6 parts per billion for methane, on par with previous stationary experiments. The team plans to use the precise GPS receiver on the multicopter to make 3-D maps of atmospheric gas concentrations with high geospatial resolution, according to the research article.
Fine-Toothed Comb
Because the system uses such a “fine-toothed” comb, it can precisely measure the spectroscopic signatures of all of its target molecules simultaneously. That’s “one of the real strengths” of the instrument, Cossel said. The ability to distinguish between gases also means the system can “accurately measure each gas without worrying about interference or confusion from other (potentially unexpected) gases,” he added.
Higher comb resolution allows a more precise measurement of gas concentration, much as a camera with more pixels in its sensor can capture greater detail of a scene or object, Newbury explained. The percentage of light missing from each wavelength in the comb upon its return, along with the air temperature, air pressure, and distance between the comb and the multicopter, reveals the concentration of each gas along the comb’s path.
Because of the system’s high sensitivity and geospatial resolution, the team wrote in Optica, it could be used to map tiny changes in methane emission due to leaks from oil or gas storage facilities, observe changing photochemistry in the air at different altitudes, or scan an area for hazardous gases or chemicals.
Davis explained that “path-integrated trace gas sensors can map out concentrations upwind and downwind of…complex sources and help us to measure their emissions continuously over time. That’s an exciting possibility for future research.”
Room for Improvement
The team hopes to improve the comb and copter system by increasing the multicopter’s flight time and measuring a wider, midinfrared wavelength range that could probe different atmospheric gases. NIST’s Fiber Sources and Applications research group is currently developing a midinfrared laser comb, which Newbury said his team may eventually integrate with a multicopter-mounted retroreflector.
These changes would make the comb and copter system valuable for detecting the emission of toxins and other hazardous substances without the need to send human inspectors into dangerous areas, the researchers said. The Defense Advanced Research Projects Agency in Arlington, Va., has partially funded the project to improve their abilities to detect hazardous materials such as chemical weapons. That’s “the real push for us,” Newbury said.
In the short term, said Cossel, “the first thing that we would like to do is to make the system a bit more user friendly and to start field measurements testing how well the system works.” Toward this end, the NIST scientists have continued to collaborate on the system with colleagues at the University of Colorado Boulder.
“We are working on developing this laser system,” Cossel said, “but a field-ready system is a few years out.”
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern