Chemical Sensing of Trace Gases and Particulate Matter with Optical Cavities

Reliable concentration assessment of the major atmospheric oxidants (hydroxyl free radicals OH, nitrate radicals NO 3 , etc), and their precursors (nitrous acid HONO, nitrogen dioxide NO 2 , and formaldehyde H 2 CO) is essential for understanding the budget of the atmospheric oxidation capacity and then predicting chemical processes that affect regional air quality and global climate change. Real-time in situ monitoring of these key atmospheric species is challenging because of their high reactivity, short lifetimes on the order of minutes or less and ultralow concentrations in the range of pptv (parts per trillion by volume). In gas sensing by Beer-Lambert-based absorption spectroscopy, the absorption intensity follows an exponential law with the optical absorption length, the use of long path absorption schemes is thus the essential way to improve the spectroscopic measurements sensitivity. Optical cavities bridge the gap between sensitivity and spatial scale, delivering long optical pathlengths in a small physical footprint. High finesse optical cavity is usually formed with high reflectivity dielectric mirrors to achieve long optical path lengths of up to a few tens of kilometres for high-sensitivity spectroscopy applications [1].