Electrical Discharges Produce Prodigious Amounts of Hydroxyl and Hydroperoxyl Radicals

Reaction with the hydroxyl radical (OH) is often the first step in the removal of many atmospheric pollutants. The nitrogen oxides (NO_x) generated by lightning can increase the amount of HO_x (HO_x = OH + HO₂) present in the atmosphere, but direct HO_x production from lightning has never been quantitatively investigated in the laboratory. In this laboratory study, prodigious amounts of HO_x were generated by both visible and subvisible electrical discharges over ranges of pressure and water vapor mixing ratios relevant to the troposphere. Also measured were NO, total nitrogen oxides (NO_x), ozone (O₃), and OH exposure, which is the integral of the hydroxyl radical concentration over time since the discharge. HO_x and OH exposure were approximately independent of pressure from 360 to 970 hPa and increased only slightly as water vapor increased from 1,000 to 8,000 parts per million volume (ppmv), while NO_x was approximately independent of both pressure and water vapor over the same ranges. These laboratory measurements of excessive HO_x and OH exposure are similar to measurements of electrically generated HO_x discovered in electrified anvil clouds during a 2012 airborne study, thus demonstrating the relevance of these laboratory results to the atmosphere and the importance of understanding the electrically generated HO_x contribution to atmospheric oxidation.