7.4.5 The Hydroxyl Radical

The hydroxyl radical (OH) is the primary cleansing agent of the lower atmosphere, providing the dominant sink for many greenhouse gases (e.g., CH₄, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons) and pollutants (e.g., CO, non-methane hydrocarbons). Steady-state lifetimes of these trace gases are determined by the morphology of their atmospheric distribution, the kinetics of their reaction with OH and the OH distribution. Local abundance of OH is controlled mainly by local abundances of NO_x, CO, CH₄ and higher hydrocarbons, ozone, water vapour, as well as the intensity of solar UV radiation at wavelengths shorter than 0.310 µm. New laboratory and field work also shows significant formation of O(¹D) from ozone photolysis in the wavelength range between 0.310 µm and 0.350 µm (Matsumi et al., 2002; Hofzumahaus et al., 2004). The primary source of tropospheric OH is a pair of reactions starting with the photodissociation of ozone by solar UV radiation.

Additionally, in the remote, and in particular upper, troposphere, photodissociation of oxygenated volatile organic chemicals such as peroxides, acetone and other ketones, alcohols, and aldehydes may be the dominant sources of OH radical (e.g., Müller and Brasseur, 1999; Collins et al., 1999; Jaeglé et al., 2001; Tie et al., 2003; Singh et al., 2004). Over continents, measurements in the lower troposphere suggest that processing of unsaturated hydrocarbons or photolysis of carbonyls can also sustain a large pool of radicals (e.g., Handisides et al., 2003; Heard et al., 2004). Furthermore, the net formation of OH by photolysis of nitrous acid (HONO) was found to be the dominant OH radical source in urban atmospheres (e.g., Ren et al., 2003) and in a forest canopy (Kleffmann et al., 2005). The hydroxyl radical reacts with many atmospheric trace gases, in most cases as the first and rate-determining step of a reaction chain that leads to more or less complete oxidation of the compound. These chains often lead to formation of HO₂, which then reacts with ozone or NO to recycle back to OH. Tropospheric OH and HO₂ are lost through radical-radical reactions leading to the formation of peroxides or with NO₂ to form nitric acid (HNO₃). Sources and sinks of OH involve most of the fast photochemistry of the troposphere.