Hydroxyl radical ($\mathrm{OH}$)

The hydroxyl radical is $\mathrm{HO}\cdot$, an oxygen-centered species with an unpaired electron. It differs from the hydroxide ion $\mathrm{O}{\mathrm{H}}^{-}$ despite containing the same two elements.

It reacts rapidly with many molecules through pathways such as hydrogen abstraction and addition to multiple bonds. Its atmospheric role follows from those kinetics: even a low abundance can strongly influence how long other reactive trace gases persist.