Fluorescence Detects Atmospheric NO2

Paula M. Powell

The atmospheric content of NO₂ can serve as an indicator of air pollution, ozone depletion and even a change in climate. So it's no surprise that the Environmental Protection Agency requires states to monitor atmospheric levels of the gas. The problem is that most commercial air-monitoring systems do not indicate true levels. Instead, the result is often a measure of a mix of NO, NO₂ and other components. They also can be limited in their capability to accurately monitor daytime concentrations during the summer.

A portable, compact, laser-induced fluorescence-based system for detecting NO₂ atmospheric content mounts the optical system along with the electronics used to control the laser, vacuum pumps, computers and all other calibration equipment in a standard instrument rack 150 cm high and 55 cm deep.

To resolve these issues, researchers are exploring a variety of monitoring techniques, including laser-induced fluorescence.

Scientists at the University of California, Berkeley, have developed and field-tested a compact system for laser-induced fluorescence-based detection of atmospheric NO₂ with sensitivity of 145 parts per trillion by volume min^-1 and a signal-to-noise ratio of 2. According to project researcher Patricia A. Cleary, this is more than adequate for field-based atmospheric monitoring of the gas. The 118-kg, 0.5-m³ device is also lower in cost than other detectors because it combines a simplified optical setup with a commercial CW, external-cavity, tunable, 640-nm diode laser.

The Berkeley system has lower sensitivity than research-grade detection instruments because of its low laser power (16 mW) and a smaller absorption cross section of NO₂ at 640 nm, which both depend on the commercial availability of tunable diode lasers at certain powers and wavelengths. To work around these issues, Ronald C. Cohen, who is involved in the project, reports that the scientists use continuous supersonic expansion to increase the gas population in the rotational field excited by the laser. Supersonic expansion is a common laboratory technique for producing cold gas phase samples, which the researchers were able to adapt to a field detector, in part through the use of low-cost portable pump technology.

Cohen said that recent adaptations to the detection system enable it to convert other chemicals, such as nitric acid and organic nitrates, to NO₂, effectively expanding the range of species that can be observed. The researchers plan further experiments this spring in association with the National Institute of Standards and Technology, headquartered in Gaithersburg, Md. The instrument will then see field-testing as part of an experiment to study the effects of atmospheric nitrogen oxides on Lake Tahoe.