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Tuning into trace gases

New laser-based sensor detects scarce compounds aloft



by Bob Henson

Alan Fried

Clockwise from left: Alan Fried, Dirk Richter, Jim Walega, and Petter Weibring. (Photo by Carlye Calvin.)

There’s a new instrument helping NCAR researchers in their quest to quantify formaldehyde and other trace gases. Unlike some of its predecessors, this instrument operates at room temperature and travels well, even on a bumpy flight aboard a research aircraft. The new tool, a product of NCAR’s Technology Development Facility (TDF), promises to contribute to studies of atmospheric chemistry, climate change, and a wide range of other areas.

If you install new carpet in your living room, there may be plenty of formaldehyde floating around. In the free atmosphere, it’s a different story. Formaldehyde (H2CO) enters the atmosphere as a byproduct of combustion and industrial processes and is produced in the atmosphere by the breakdown of hydrocarbons. Because it’s so quickly oxidized, formaldehyde is one of the most scant of trace gases, typically making up 1 to 50 parts per billion in polluted urban areas and only about 10 to 100 parts per trillion elsewhere. Yet formaldehyde is a major player in the formation of ozone, a noxious and dangerous pollutant near ground level.

Trace gases are generally considered those that make up less than 1% of the atmosphere; for instance, carbon dioxide represents about 380 parts per million. Even this is plentiful compared to “ultra” trace gases, which constitute still-tinier proportions of free air measured in parts per billion or trillion. Researchers attempting to measure ultra-trace-gas constituents have met with a host of difficulties over the years. To be useful in field work, trace gas sensors must be durable, sensitive, compact, selective, and affordable, none of which are easy qualities to attain.

Of the lasers used to detect trace gases, small tunable lasers developed for telecommunications are among the best. The catch is that the gases that are most scarce tend to absorb most strongly in the mid-infrared wavelength region, but most of the available laser sources operate at shorter wavelengths. “To measure trace gases, you really have to get into the strong mid-infrared absorption,” says TDF director Alan Fried. “We have lasers that have gotten us there, but they have to be cooled by liquid nitrogen.” This process adds a lot of weight to airborne payloads, and the laser source and associated optical system are not as compact, reliable, and readily available as scientists would prefer.

The new NCAR sensor relies on a technology called difference frequency generation (DFG). It’s being used at only a handful of labs around the world, mostly for fundamental lab studies rather than field work. Dirk Richter became familiar with DFG while completing his doctorate at Rice University under Frank Tittel. After Richter joined NCAR in 2000, he and Fried began paving the way for the first use of DFG in atmospheric science. They were joined on the project by Petter Weibring, who came to NCAR as a postdoctoral researcher in 2003 with a decade-plus of lidar and instrumentation experience, and by associate scientist Jim Walega, who has worked with instrumentation for field campaigns for more than two decades.

A useful blend

Like a painter who mixes two common dyes to simulate a rare third color, the DFG process takes two near-IR laser beams and mixes them in a crystal to produce the difference frequency. The result is a narrow beam of light in the desired mid-IR wavelength, generated without the bulky setup normally needed. “The lasers we use are common, off-the-shelf types,” says Fried.

Another advantage of the tunable DFG lasers is that they can be adjusted to produce a variety of mid-IR wavelengths. “You can have unique mixing schemes,” says Richter. This opens the door for measuring other extremely scant gases. One example is 13CO2, an isotope of carbon dioxide found in a small fraction of carbon dioxide molecules, most of which are 12CO2. Because 13CO2 is taken up by vegetation more slowly than standard 12CO2, “measuring the 13CO2-to-12CO2 ratio is an important new tool for studying dynamics of the carbon cycle,” says Fried.

The acid test for the DFG device was whether it would hold up under the rigors of airborne research. Early last year, the TDF group took the new instrument on its maiden voyage aboard the NSF/NCAR C-130 during the Megacities Impacts on Regional and Global Environment (MIRAGE) study in Mexico. Despite a rough start, says Walega, the team made modifications and adjustments and the kinks were quickly worked out.

Further improvements were made between the next two campaigns (a NASA follow-up to MIRAGE and NOAA’s Texas Air Quality Study of 2006), so that, in the end, the instrument worked semi-autonomously. By the last mission, says Richter, “the technicians were afraid the instrument wasn’t working because none of us were tweaking it anymore.” He adds that these three campaigns marked the first time in the world a DFG system was operated on an airborne platform.

If funding comes through, plans are in the works to deploy the new ­instrument in 2008 aboard NASA’s DC-8 aircraft for the agency’s Arctic Research of the Composition of the Troposphere from Aircraft and Satellites program. Along with detecting formaldehyde, the TDF team may also try to measure methanol. “The ultimate goal is to develop a multi-species sensor that can be run autonomously on the NSF/NCAR Gulfstream-V aircraft,” says Weibring.

schematic

Above is a schematic of the difference frequency generation technique as applied at NCAR. An input signal with a wavelength of 1.5 microns (red, at left) and a 1-micron “pump” signal (blue, at left) are fed through a lithium niobate crystal. The output includes the original input and pump signals, as well as an “idler” signal of 3.5 microns—the so-called difference frequency generated in the crystal. Normally it would take a far more cumbersome operation to produce the 3.5-micron signal.

The 22 January edition of the journal Optical Express includes a report written by Richter, Weibring, and Fried. They’re joined on the paper by four colleagues from NTT Photonics Laboratories, the Japanese maker of a new type of lithium niobate crystal used to route and combine the laser beams. “They read our papers and felt we were one of the best groups to push the technology and see how far we could go,” says Richter. A common ingredient in cell phones, these new lithium niobate crystals enabled the NCAR researchers to make the DFG process roughly a hundred times more efficient by squeezing the incoming light into a fiber-like structure only six microns wide. As Richter notes, “It’s all very tiny stuff.”
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