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April 2007

EOL researchers design new instrument for detecting trace gases

A team of researchers in EOL has created a new instrument by applying an innovative technique for mixing different laser beams. The instrument quantifies formaldehyde and other trace gases in the atmosphere, an important step toward understanding atmospheric chemistry and climate change.

Better known for its liquid form used during embalming and disinfecting, formaldehyde as a gas enters the atmosphere as a byproduct of combustion and industrial emissions, as well as by natural processes. Because it’s 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 billion elsewhere. (Trace gases are generally considered those that comprise less than 1% of the atmosphere.) Still, the gas plays a major role in the formation of ground-level ozone, a dangerous pollutant.

Researchers attempting to measure trace gases, a procedure done with lasers, have confronted multiple difficulties over the years. To be useful during field work, a trace gas sensor must be durable, sensitive, compact, and affordable, a combination of qualities that is hard to obtain.

alan fried group

Clockwise from left: Alan Fried, Dirk Richter, Jim Walega, and Petter Weibring.

Lasers in the mid-infrared region are ideal for detecting the most scarce trace gases, but most of the available laser sources require researchers to cool them in liquid nitrogen. This additional step adds significant weight to airborne payloads, and also means using a laser source and optical system that is not as compact, reliable, and readily available as scientists would prefer.

The new instrument—designed by Alan Fried, Dirk Richter, Jim Walega, and Petter Weibring—leverages a technology called difference frequency generation (DFG) that’s in use at only a handful of labs around the world. It utilizes small, tunable lasers developed for the telecommunications industry that have proved best for detecting trace gases. The researchers mix two of these near-infrared laser beams in a crystal to produce a different frequency, resulting in a narrow beam of light in the desired mid-infrared wavelength.

The process doesn’t require bulky equipment and can be performed with common, inexpensive lasers. The instrument withstands bumpy research flights and operates at room temperature. Furthermore, researchers can adjust DFG lasers to produce an assortment of mid-infrared wavelengths, allowing them to measure not just formaldehyde but other very scant trace gases as well, including an isotope of carbon dioxide that is important for studying the carbon cycle.

The research team deployed the new instrument aboard the C-130 during the MIRAGE field campaign in Mexico in March 2006, making it the first time a DFG laser was operated on an airborne platform anywhere in the world. After working out kinks, they ran it again during two subsequent campaigns. “By the last mission, the technicians were afraid the instrument wasn’t working because none of us were tweaking it anymore,” Dirk says.

Pending funding, the researchers plan to deploy the instrument next year aboard a NASA aircraft for a study of the lowest layer of the atmosphere over the Arctic. Along with detecting formaldehyde, they’ll also try to measure methanol. “The ultimate goal is to develop a multi-species sensor that can be run autonomously on the Gulfstream-V,” Petter says.

In this issue...

Researchers chase pollution plumes across Pacific Ocean during PACDEX

EOL researchers design new instrument for detecting trace gases

Friends of UCAR enhances science education, outreach

Remembering Barry White and Mark Uris

“Center Green Idol” coming to Spring Fling

Delphi Questions

Just One Look


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