Eye-safe tool opens new doors to aerosol mapping
by Bob Henson
When Shane Mayor was completing his doctorate in the late 1990s at the University of Wisconsin-Madison, he used a lidar (a laser-based radar) to study boundary-layer turbulence. The device wasn't exactly childproof. Like many research lidars, this one operated at a wavelength that could produce serious eye problems were it to shine directly in someone's face. "It was not the kind of instrument that got you invited to field projects," says Mayor.
Such optical risks, and the challenge of working around them, have kept lidars from achieving as much research versatility as they otherwise might have. Now Mayor is spearheading a new lidar, developed at NCAR, that's innocuous to the human eye but powerful in detecting tiny yet important aerosols.
The Raman-shifted Eye-safe Aerosol Lidar, or REAL, got its first workout in a field project at the Pentagon this spring (see sidebar). It's now stationed at the east end of NCAR's Foothills Lab campus, where its developers are working on upgrades.
Shane Mayor with REAL. (Photo by Carlye Calvin.)
REAL emerged from several years of collaboration among Mayor, optical and systems engineer Scott Spuler, and associate scientist Bruce Morley. Mayor launched the project in 2001 as a postdoctoral researcher in NCAR's Advanced Study Program. It was championed by ASP director William "Al" Cooper, a microphysicist. "Al was very supportive of my taking on a hardware project," says Mayor. An initial grant of $50,000 came from NCAR director Tim Killeen's opportunity fund in 2001. Further support came from NSF through ASP and NCAR's Atmospheric Technology Division.
"I'd spent years analyzing lidar data, but I'd never actually built one," recalls Mayor. Realizing the value of having an optical specialist on hand, he began the search that landed Spuler in 2002. With his experience in customized, high-end optics, Spuler "transformed the way we design lidars," Mayor says.
On the right wavelength
NCAR already had nearly a decade of experience operating a scanning aerosol backscatter lidar (SABL). It's been one of the most popular instruments among scientists who use NCAR's deployment pool. However, SABL is not eye-safe, which prohibits its use near airports or in urban areas, among other settings. "That's one reason why it was a high priority to create a completely eye-safe aerosol backscatter lidar," says Mayor.
The challenge in detecting tiny aerosols, on the order of a micron in width, is that wavelengths between about 0.4 and 1.4 microns focus directly onto the retina, thus posing a hazard when concentrated in an intense beam. At longer wavelengths, the detectors that pick up the lidar's returning signals become less effective. Some Doppler lidars operate in the eye-safe wavelengths of 2 and 10 microns by making use of less-than-ideal detectors through a technique known as heterodyning. Still, says Mayor, the aerosol information that results is noisy.
Mayor wanted a lidar in the sweet-spot range near 1.5 microns. At that wavelength, the beam could safely carry substantial power, yet it would be invisible and would suffer relatively little due to scattering from molecules or naturally occurring light. According to Spuler, 1.5 microns "is also the wavelength used in optical telecommunications, so we can take advantage of optical components designed for that industry."
However, no lasers existed that could provide sufficient pulse energy for lidars at that wavelength. So Mayor's team combined traditional techniques, such as shifting the light from a common laser wavelength, with newer techniques such as injection seeding, where one laser beam is injected into another to amplify the initial signal.
Along with obtaining the desired wavelength and pulse energy, the group wanted to train its new lidar's return signal on tiny and extremely responsive photodetectors, small enough to fit on the end of a fiber. With Spuler's prowess at tracing optical paths, the team devised a way to concentrate the returning lidar beam as it enters a 40-centimeter (16-inch) diameter telescope and lands on a photodetector only 200 microns (0.008 in) in diameter.
Bruce Morley examines data from REAL. (Photo by Carlye Calvin.)
The high-end optics allow aerosols to be tracked by the scanning lidar beam every tenth of a second at distances separated by as little as 3 meters (10 feet). The resulting image-which looks much like a common radar display, only in miniature-captures phenomena rarely observed by researchers. "A radar sees reflections from big targets like raindrops and bugs," says Spuler. "We're seeing reflections from particles about 10,000 times smaller."
At the Pentagon, REAL's display lit up with signals from the myriad of aerosol plumes common in the center of a busy, densely populated urban area. At its NCAR home, REAL watches the daily ebb and flow of pollutants and routinely captures trails of exhaust from freight trains passing within a few kilometers of the site. "Our system can tell the shape of these plumes and where they're headed," Mayor says. The aerosols reveal such meteorological processes as boundary layer growth, atmospheric waves, and the speed and direction of the wind.
Near-term goals for REAL include extracting small-scale wind data and adding a depolarization feature to help distinguish aerosol shapes. With a patent application in the works, Mayor is hoping for interest from the private sector. "We feel this system fulfills a commercial need for detecting aerosol plumes in clear air."
For now, Mayor says, "one of our top priorities is making this available to the university community." Although REAL is not yet part of the NSF-sponsored deployment pool at NCAR, Mayor hopes that it will be, and he encourages university scientists to inquire about potential use. "We hope this is significant for the lidar community."
|Forecasting for the Pentagon
A breakthrough blend of high-tech instruments and weather forecasting models took shape at the Pentagon during April and May. Coordinated by scientists at NCAR, the tests scanned for potential airborne toxins near the Pentagon and predicted their motion and impact on the building. Partners from academia and the public and private sector joined NCAR in the effort, sponsored by the Defense Advanced Research Projects Agency (DARPA).
"Knowing how to properly respond to an attack or a toxic industrial incident requires the best modeling tools and sensors available today, and these must all work in a coordinated fashion in real time," says NCAR project leader Scott Swerdlin.
Understanding air circulation around the Pentagon is a unique challenge, says Swerdlin. The air circulations are very complex because of the building's size and unusual geometry. Temperature inversions, especially at night, could allow an airborne hazard to spread below rooftop height, which adds to the complexity of a monitoring system.
To tackle the problem, NCAR and partners employed a set of nested computer models—each with a different strength—that predicted weather conditions from the entire Washington region inward to the Pentagon itself, drawing on data from Doppler radar and lidar. At its finest scale, the system charted air flow every 2 meters (7 feet) immediately around the Pentagon. Future versions of the system will blend the nested models, allowing them to share information every 15 minutes.
"The weather modeling system tested here was one of the most complex ever constructed," says NCAR's Thomas Warner, lead scientist on the project.
Along with radar, lidar, and surface stations, the monitoring equipment included:
- A 7-meter (23-foot) instrumented balloon tethered above the Pentagon. Deployed by the University of Colorado, the setup included sensors studded along the balloon's tethering wire. As the balloon rose and fell, the sensors sampled air flow and turbulence.
- Periodic releases of sulfur hexafluoride (SF6). This inert, invisible, nontoxic gas helped scientists verify the accuracy of the computer models and sensors that track dispersal of airborne material. The releases were coordinated by NOAA with assistance from the U. S. Army's Dugway Proving Ground.
"It was a very challenging exercise," says Swerdlin. "The field study went without a hitch, and we collected a great dataset that will help us to develop a more effective operational system.
"We called on a lot of experienced players and advanced weather forecasting systems in order to precisely time the sulfur hexafluoride releases. As a result of that study, particularly the indoor infiltration test, we now know better ways of altering the heating, ventilation, and air conditioning controls to protect Pentagon occupants from terrorist attacks or toxic industrial chemical incidents."
Raman-shifted Eye-safe Aerosol Lidar
Scanning Aerosol Backscatter Lidar