Cleaning in a flash Hydroxyl radicals (OH) are the most important oxidizing species in the atmosphere and provide much of its ability to cleanse itself. It is known that nitric oxide production by lightning leads to the formation of OH and other atmospheric oxidants, such as ozone and hydroperoxyl radicals (HO ₂ ), through a variety of chemical reactions. Brune et al. used airborne measurements of OH and HO ₂ to show that lightning also produces them directly and in amounts much greater than expected. They found that this mechanism may be responsible for as much as one-sixth of the oxidizing capacity of Earth's atmosphere. Science , this issue p. 711