Methane (CH4) is a hydrocarbon naturally present in the atmosphere at a concentration of ppm (parts per million by volume). It was discovered in the 18th century by Alessandro Volta, who was studying the composition of marsh gas. By this time, the usefulness of methane as a fuel for lamps and its potential as an explosive had already been recognized.
There are many ways in which CH4 can be formed from organic matter. There are three main types :
- biogenic methane linked to the decomposition of organic matter in wetlands (swamps, rice paddies, etc.) or by animals (ruminants, termites, etc.). Essentially, two chemical pathways involve fermentative bacteria producing acetate or formate. Under anoxic conditions, methanogenic bacteria generate CH4 through acetic fermentation or hydrogen reduction ofCO2 ;
- thermogenic CH4, produced by the diagenesis and catagenesis of kerogen in source rocks, where it forms natural gas. It is also found in coal mines (firedamp) ;
- pyrogenic CH4, produced by incomplete combustion in natural or man-made biomass fires.
Once emitted into the atmosphere, CH4 is rapidly oxidized. The reaction chain is complex, involving hydroxyl free radicals (-OH) produced by photolysis of ozone in the presence of water vapour (≈ 106 molecules/cm3 of air, whose average lifetime is of the order of a second). The main reaction chain involves nitrogen oxides and produces ozone. On the other hand, if the NO content is low, ozone is consumed by the reaction. In addition to these reactions, there are other secondary CH4 degradation pathways (reaction with atmospheric chlorine and consumption by methanotrophic soil bacteria).
