Extracted from Chimot, J., Global mapping of atmospheric composition from space – Retrieving NO2 from OMI, PhD book, Delft University of Technology (TU Delft), The Royal Netherlands Meteorological Institute (KNMI), July 2018. height and tropospheric
A chemical reaction occurs when atoms or molecules collide: i.e. they can merge, break apart or rearrange. A lot of reactions occur because of photons, and thus in presence of sunlight. The reaction rate also depends on the kinetic energy, and thus on temperature and pressure. There is a countless number of chemical reactions determining our atmospheric composition. Altogether, they form a whole complex and coupled system.
As an illustration, perhaps one of the most famous reaction is the photolysis: the Sun light energy breaks a chemical bond in a molecule which usually includes oxygen atoms. For example, the slow dissociation of dioxygen by energetic UV radiation is an important photolysis reaction in the stratosphere:
O2 +hν→2O, λ<240 nm
But the produced oxygen atoms quickly rearrange with dioxygen molecules to form ozone:
This new ozone easily absorbs UV radiation, protecting life on Earth.
O3 +hν→O2 +O, λ<320 nm
The reactions above illustrate the rapid cycling between O and O3 exclusively in presence of O2 and light. But, in presence of an oxygen atom, ozone slowly transforms back to O2:
3O2 +hν↔2O3 +hν
This is only an example of many processes occurring in the stratosphere. They are part of the Chapman mechanism explaining the formation and presence of the ozone layer as a function of the square of the O2 concentration, around 15-30 km height (Chapman, 1930). Above this layer, not enough oxygen molecules are found to produce ozone, while below most UV radiation has been absorbed.
Many more reactions take actually place in our atmosphere.