Why measure atmospheric gas concentration?
Measuring atmospheric gas with LIDAR

The atmosphere is a medium whose chemical properties largely determine the conditions of human life on Earth. The study of these properties is therefore a vital necessity, but paradoxically, the dry atmosphere is composed overwhelmingly of two species whose concentration varies little, and which do not give rise to great monitoring needs, except in specific industrial cases; these are nitrogen (N ₂ , 78%) and oxygen (O ₂ , 21%). But then, the devil is in the detail, and in the end, it's the so-called "minority" atmospheric species that require the greatest measurement care.

• First and foremost, water vapor (H ₂ O), whose concentration typically ranges from 0 to 4% (with peaks of up to 6% in extreme cases), is...