Appendix. Elements of atmospheric chemistry
Gaseous air pollution - Technical recommendations

For most elements, we know that oxides are highly stable forms. Consequently, in our very oxygen-rich atmosphere, virtually all elements, regardless of the pollutant in whose form they are emitted, are thermodynamically unstable and therefore tend to evolve towards their most oxidized form, hydrated where appropriate: CO ₂ for carbon, HNO₃ for nitrogen, H₂SO ₄ for sulfur. In the case of halogens, the oxides are not very stable and the terminal form is dihalogen X ₂.

However, the oxidation reactions of molecular species by dioxygen O ₂ are extremely slow and are unobservable on usual time scales. They can, however, occur via highly reactive oxidation intermediates such as radicals, OH ^•...