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New Rice University Sensor Sniffs Greenhouse Gases

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0121_LASER-1A particularly difficult argument to have with those who do not have a background in environmental topics is one that involves the actual effects of greenhouse gases. You can drone on all day about the science and math involved, but without direct visual evidence, it is sometimes difficult to convince others.

In general, we simply do not know what we are breathing, because it all seems the same. This leads to an intellectual and physical disconnect because it is hard to believe or pay attention to something that you cannot taste, see, or feel.

Rice University researchers are looking to demystify GHG’s by creating a highly sensitive, portable sensor that test the air quality for the most damaging of these gases. The brainchild of Frank Tittel, this device uses a thumbnail-sized quantum cascade laser (QCL) as well as turning forks (which cost less than a dime) to detect very small amounts of nitrous oxide and methane.

The QCL works by emitting light from the mid- to far-infrared portion of the spectrum, allowing for a much better detection of gases than common lasers that operate in “near-infrared.” The technique used is known as “quartz-enhanced photoacoustic absorption spectroscopy,” (QEPAS) and has the potential to produce accurate results in devices as small as a common smartphone.

This is an important endeavor for Tettel, who understands the stakes involved in observing these gases and finding solutions to our GHG issues:

“Human activities such as agriculture, fossil fuel combustion, wastewater management and industrial processes are increasing the amount of nitrous oxide in the atmosphere. The warming impact of methane and nitrous oxide is more than 20 and 300 times, respectively, greater compared with the most prevalent greenhouse gas, carbon dioxide, over a 100-year period. For these reasons, methane and nitrous oxide detection is crucial to environmental considerations.”

What enable the technique to actually work is a tiny quartz tuning fork, which vibrates at a specific frequency when stimulated. “The ones we use are made for digital watches and are very cheap,” said Rice postdoctoral researcher and co-lead author Wei Ren. “The fundamental theory behind this is the photoacoustic effect.”

The laser beam is focused between the two prongs of the quartz tuning fork. When light at a specific wavelength is absorbed by the gas of interest, localized heating of the molecules leads to a temperature and pressure increase in the gas.

The laser beam is focused between the two prongs of the quartz tuning fork. When light at a specific wavelength is absorbed by the gas of interest, localized heating of the molecules leads to a temperature and pressure increase in the gas.

“If the incident light intensity is modulated, then the temperature and pressure will be as well,” Ren said. “This generates an acoustic wave with the same frequency as the light modulation, and that excites the quartz tuning fork. He continued by saying, “The tuning fork is a piezoelectric element, so when the wave causes it to vibrate, it produces a voltage we can detect. That signal is proportional to the gas concentration.”

Ren mentioned that the unit can detect methane our NO2 almost immediately (1 second). “This was a milestone for trace-gas sensing, now we’re trying to minimize the size of the whole system.”

They expect to see a smaller QEPAS device in use later this year, as Rice and the University of Houston take part in a pollutants survey in the city.

Source: Analyst

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