Accurate measurements of greenhouse gases in the atmosphere are vital to our understanding of how pollution influences climate change. But some methods for detecting and identifying gas molecules remotely perform less reliably when the air is cloudy or smoky — even a piece of dust can skew results — especially under the conditions in which monitoring gases is most needed, such as in smokestacks.
To tackle this issue, Gerard Wysocki, assistant professor of electrical engineering, has developed a new technology called chirped laser dispersion spectroscopy (CLaDS) that is robust enough to be used in smoky or foggy environments, and can be used at longer distances, making it less likely to be damaged in industrial settings. CLaDS has already been deployed in Baltimore, where Wysocki's research team — including postdoctoral research associate Michal Nikodem, and graduate students Clinton Smith and Genevieve Plant, all in Princeton’s electrical engineering department — measured the concentration of the potent atmospheric greenhouse gas nitrous oxide.
CLaDS involves firing a laser composed of two colors of light through a gas sample. The laser light interacts with various molecules in the sample and is then analyzed by the CLaDS detector. The composition of the gas is determined by how the propagation of laser light is affected as it travels through the sample to the detector.
With CLaDS, Wysocki and his team do not simply measure the intensity of the received light, but rather its propagation velocity, which is altered by the target molecules and detected as a phase of the returning wave. Although phase measurements are widely used, Wysocki enhanced the phase signal with "chirping," or quickly modulating the frequency of the laser light, similar to how sound waves are used in sonar.