Hydroxyl radicals - chemical species crucial to the atmosphere's self-cleaning ability - are directly produced by lightning, and in quantities far greater than expected, according to a new study. The analysis also reveals a previously unknown mechanism that may be responsible for a substantial proportion of global atmospheric oxidation. Scientists know that lightning increases the atmosphere's ability to cleanse itself through the production of reactive nitric oxide (NO), which, through subsequent atmospheric chemistry, ultimately leads to the formation of atmospheric oxidants, including ozone (O3) and the hydroxyl radicals (OH). OH is the atmosphere's most important oxidizing species as it controls the concentrations of toxic and climate-relevant gases like carbon monoxide and methane. While previous modeling studies have suggested the possibility that lightning flashes can directly generate these oxidant species, the estimated lifetime of this OH is thought to be too short to notably contribute to atmospheric oxidation. Here, William Brune and colleagues present an analysis of data from a 2012 airborne study of deep convective thunderstorm clouds involving the NASA DC-8 aircraft. They report previously unobserved extreme amounts of atmospheric hydroxyl and hydroperoxyl (HO2) radicals being directly generated by lightning sparks and in other electrified areas of the storm cloud. According to Brune et al., these amounts are several orders of magnitude above any previous atmospheric OH or HO2 measurements. While uncertain by perhaps a factor of ten, the results suggest that lightning-generated OH in all storms happening globally at any given time could be responsible for anywhere between 2 and 16% of the oxidizing capacity of Earth's atmosphere.