Incoherent Detection of Airborne Pollutants

Hank Hogan

Researchers and regulators have a new tool when it comes to detecting airborne pollutants and pinpointing their source, thanks to a group from University College Cork in Ireland. The group’s members have demonstrated an incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) method that has a detection limit of a few parts per billion by volume for the trace atmospheric gases HONO and NO₂.

The technique could be applied to other trace gases and pollutants. Team leader Albert A. Ruth also noted that the new approach works in confined areas. “IBBCEAS can be employed locally, including indoor environments.”

A schematic diagram shows an experimental incoherent broadband cavity-enhanced absorption spectroscope that detects HONO and NO₂ at a level of parts per billion by volume. Incoherent broadband ultraviolet light from an LED is focused and filtered before entering a gas chamber. The light bounces between high-reflectivity dielectric mirrors, creating a long absorbing path length. When the light exits, it is imaged onto a CCD for absorption measurements. Sample gas enters via the inlet and leaves by the outlet. L = lens; F = filter; M₁, M₂ = dielectric mirrors; P = pressure gauge; M = metallic mirror. Reprinted with permission of Environmental Science and Technology.

The method is particularly well suited for monitoring nearby sources. Thus, pollutants could be measured inside industrial plants and factories, at airports, and near landfills or major traffic junctions.

Ruth characterized the technique as a twist on old-fashioned absorption spectroscopy. Incoherent broadband light, the type produced by arc lamps and LEDs, enters a sample-filled cavity with highly reflective mirrors on each end. The light bounces back and forth multiple times before exiting, with the signal captured by a detector capable of differentiating between the wavelengths comprising the light.

The increase in the absorbing path length makes the technique work. Using a broadband source enables the detection of many species at the same time, provided each has a distinguishable absorption. The setup can be compact and the dynamic range can be large, allowing both weak and strong absorbers to be measured simultaneously.

On the downside, the method requires calibration to yield the absolute concentration of the absorbing species. It also does not have the spectral resolution possible with a laser.

In their demonstration, the researchers extended the technique to the ultraviolet, a region where many atmospheric trace gases absorb. They used mirrors from Layertec GmbH of Mellingen, Germany, in two chambers. In the smaller one, the mirrors were 115 cm apart; in the larger, they were separated by 4.5 m.

For a light source, they used an ultraviolet LED from Latronics GmbH of Aachen, Germany, with a peak output at 365 nm. They placed a bandpass filter from Semrock Inc. of Rochester, N.Y., between the chamber and the LED to eliminate light from outside the region of interest. Their detector was a spectrograph equipped with a CCD from Andor Technology plc of Belfast, UK.

After calibrating the system, they introduced a mixture of HONO/NO₂into the chambers and measured the absorption from 366 to 378 nm. They determined the detection limit for the smaller chamber to be roughly four and 14 parts per billion by volume for HONO and NO₂, respectively, for a 20-s acquisition time. For the larger chamber and a 10-min acquisition time, the limits were 30 times lower.

These results were comparable to those of other gas absorption spectroscopy techniques with low spatial resolution. They were not as good as those achieved with wet chemical methods. However, wet methods usually require more sample preparation and time and often are designed only for a particular compound. In addition, IBBCEAS can handle a variety of samples. “It can not only be applied to the gas phase, but also to liquids, films and surfaces,” Ruth said.

Plans call for using the technique and instrument in field applications, including a study of bromine oxide sources in the Canadian arctic this spring. The group is looking for partners to help commercialize the technology.

Environmental Science and Technology Feb. 1, 2008, pp. 890-895.