The hydroxyl radical, ^•OH, is the neutral form of the hydroxide ion (OH⁻). Hydroxyl radicals are highly reactive (easily becoming hydroxy groups) and consequently short-lived; however, they form an important part of radical chemistry.^[2] Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H₂O₂ (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

Hydroxyl radicals are created in the atmosphere by two principal chemical reactions:

• During daylight hours, a photochemical reaction occurs in the atmosphere where different wavelengths of light interact with water and terpenes (secreted from plants) in the air to produce simpler by-products known as Reactive Oxygen Species (ROS). One of the main types of ROS is the hydroxyl radical.
• In addition, during the entire 24-hour cycle, OH is formed through the reaction between terpenes and ozone.

From: Wikipedia