Small Nd:YAG Laser Enables in Situ Studies of Airborne Particulates
Aaron J. Hand
Many man-made and natural activities emit airborne particulates, filling our atmosphere with pollution that can damage human respiratory systems, reduce visibility and even alter the local climate through interactions with solar radiation. Scientists have developed ways to measure the elemental composition of aerosols, but large, unwieldy instruments discourage their use in the field.
Researchers at Oak Ridge National Laboratory in Tennessee (managed by Lockheed Martin Energy Research Corp., also in Oak Ridge) have developed a measurement system that combines technologies adopted from aerosol science and laser-induced plasma spectroscopy. Using a small, Q-switched Nd:YAG laser from New Wave Research, they were able to keep their instrument portable -- the size of carry-on luggage and weighing about 50 lbs.
Elemental composition is typically measured by pumping aerosol particles from a source -- perhaps a couple hundred feet away -- and analyzing the filtered particles with x-ray fluorescence or instrumental neutron activation techniques, according to Mengdawn Cheng, a researcher at the laboratory's Envi-ronmental Sciences Div.
However, these instruments are not portable. "By not analyzing the elemental composition at the source, we have learned that many biases and analytical error can result," Cheng said, noting that future emission measurements will rely on in situ analysis with portable instruments.
Using New Wave's Tempest Nd:YAG to excite the atoms, researchers detected the chromium component of an aerosol at 425, 427 and 429 nm. Note the sharp signal.
The instrument developed by Cheng and his colleagues uses New Wave's Tempest to excite the atoms that make up the aerosol. It is a small laser (13 × 7 × 3.5 in.) but provides enough energy to activate the atoms. The Tempest offers an energy output of up to 200 mJ per pulse at 1064 nm. The researchers have used this wavelength and the second and fourth harmonics of the wavelength for this application.
The design of the laser is also user-friendly, enabling researchers to switch from one wavelength to another easily in the field. This is important, Cheng noted, because his group uses different wavelengths and energy levels depending on the application. To measure aerosol chromium, for example, they have used 50 to 120 mJ per pulse at 532 nm, and 15 to 30 mJ per pulse at 266 nm.
New Wave's laser has other benefits. "The warm-up time is relatively short, and beam quality is stable -- for a small laser like this, that is really good," Cheng said.
Despite the features that the laser offers, he said, the instrument would not achieve the analytical performance needed without the use of aerosol beam focusing technology. "Such a technology allows aerosol particles to be concentrated on the fly and to be focused into the laser focal point to be excited," he said.
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