Measuring atmospheric gas with LIDAR

Atmospheric gas monitoring covers a wide range of issues, depending on the chemical species observed. These include climatology, meteorology, the environment, public health, industrial safety, defense, energy, and even economics and finance! However, the atmospheric medium - transparent to the naked eye, materially elusive and extended in all three dimensions of space - is not an easy one to study. Of course, many instruments are capable of measuring the concentration of a wide variety of chemical species in situ. But although extremely useful, such measurements are by definition punctual, very limited in space. This is why it is particularly interesting to develop tools for characterizing the atmospheric environment from a distance.

LIDAR (Light Detection and Ranging) is a technique with remarkable potential. This is a remote sensing technique similar to RADAR (RAdio Detection And Ranging), but where the emitter - usually a laser - operates in the optical frequency range. As the emitted wavelength is very small (< 20 µm), it is capable of interacting with gas molecules and dust particles suspended in the atmosphere. By capturing the very small fraction of light backscattered by atmospheric molecules and aerosols, it is possible to access a wide range of atmospheric properties, including the concentration of gases present. What's more, since laser-matter interaction is continuous as the laser beam propagates through the atmosphere, the lidar technique enables measurements not only at a distance, but also with spatial resolution along the line of sight. Lidar can therefore deliver much more information than in situ instrumentation.

The two main lidar techniques for remotely measuring the concentration profile of a gas are Raman lidar and DIAL (Differential Absorption Lidar) lidar. This article provides an introduction to these two techniques. The physical basis of Raman spectroscopy and absorption spectroscopy is outlined, as well as how they can be used in lidar systems. Examples of operational systems and gas concentration measurements are also discussed for illustrative purposes. Finally, a discussion is proposed, aimed at highlighting the respective advantages and disadvantages of these two methods.

At the end of the article, readers will find a glossary and table of acronyms, notations and symbols.