Upper troposphere hydroxyl radical

Velthoven, Observations of upper tropospheric sulfur dioxide- and acetone-pollution Potential implications for hydroxyl radical and aerosol formation, Geopkys. Res. Lett., 24,57-60, 1997. 

In the troposphere ozone is produced by photodissociation (equation 3) not of 02 but of N02, initiated with wavelengths <410 nm, followed by recombination (equation 2) with 02. In the upper troposphere and lower stratosphere the photolysis of ozone (equation 4) yields excited-state oxygen atoms which react with water to produce hydroxyl radicals (equation 5). These are crucial to the removal of organic compounds from the troposphere, since they readily abstract hydrogen atoms (equation 6) to yield organic radicals which subsequently undergo further oxidative degradation. 

Hydroxyl radical levels in the upper troposphere vary from about 0.01 to 0.1 ppt. In the lower stratosphere OH depends on solar zenith angle and ranges up to about 1 ppt. As noted above, OH is essentially independent of NO in the lower stratosphere, whereas in the upper troposphere OH decreases as NO decreases. This fundamentally different behavior of OH with respect to changes m NO characterizes the troposphere/stratosphere transition. 

Arnold, F., G. Knop, and H. Ziereis. 1986. Acetone measurements in the upper troposphere and lower stratosphere — implications for hydroxyl radical abundances. Nature 321 505-507. 

While ozone is a good molecule high in the stratosphere, its reviews in the lower troposphere are decidedly mixed. It is the source of minuscule concentrations of hydroxyl radical (OH, 10 molecules/ cm ). Hydroxyl is so extraordinarily active that it reacts with hydrocarbons (methane, benzene, etc) and most atmospheric compounds. Some extremely stable molecules, including chlorofluorocarbons like CFCI3 and CF2CI2 and N2O (nitrous oxide from fertihzers, a source of stratospheric NO), do not react with OH and migrate to the upper stratosphere (see chapter 8 for the consequences). 

---