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Prodigious amounts of the hydroxyl radical (OH) are generated in the laboratory on tree leaves by corona discharges, which also occur on trees during thunderstorms. Production rates of OH and HO₂depend on the applied electric field generating the corona discharge, leaf dryness, and the presence of liquid water on the leaf. However, they are independent of leaf type and corona discharge polarity for a given corona ultraviolet (UV) flux. Production rates of OH, HO₂, and O₃strongly correlate with corona UV flux. Although the contribution of corona‐produced OH to total global OH production is unlikely to be important, corona‐generated OH is likely a few orders of magnitude greater than oxidation by known processes in the vicinity of the affected leaves, potentially influencing atmospheric oxidation and tree and forest ecology.

 

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_xproduction from lightning has never been quantitatively investigated in the laboratory. In this laboratory study, prodigious amounts of HO_xwere 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_xand 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_xwas approximately independent of both pressure and water vapor over the same ranges. These laboratory measurements of excessive HO_xand OH exposure are similar to measurements of electrically generated HO_xdiscovered 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_xcontribution to atmospheric oxidation.

 

Atmospheric electrical discharges are now known to generate unexpectedly large amounts of the atmosphere’s primary oxidant, hydroxyl (OH), in thunderstorm anvils, where electrical discharges are caused by atmospheric charge separation. The question is “Do other electrical discharges also generate large amounts of oxidants?” In this paper, we demonstrate that corona formed on grounded metal objects under thunderstorms produce extreme amounts of OH, hydroperoxyl (HO 2 ), and ozone (O 3 ). Hundreds of parts per trillion to parts per billion of OH and HO 2 were measured during seven thunderstorms that passed over the rooftop site during an air quality study in Houston, TX in summer 2006. A combination of analysis of these field results and laboratory experiments shows that these extreme oxidant amounts were generated by corona on the inlet of the OH-measuring instrument and that corona are easier to generate on lightning rods than on the inlet. In the laboratory, increasing the electric field increased OH, HO 2 , and O 3 , with 14 times more O 3 generated than OH and HO 2 , which were equal. Calculations show that corona on lightning rods can annually generate OH that is 10–100 times ambient amounts within centimeters of the lightning rod and on high-voltage electrical power lines can generate OH that is 500 times ambient a meter away from the corona. Contrary to current thinking, previously unrecognized corona-generated OH, not corona-generated UV radiation, mostly likely initiates premature degradation of high-voltage polymer insulators. 

Lightning increases the atmosphere’s ability to cleanse itself by producing nitric oxide (NO), leading to atmospheric chemistry that forms ozone (O₃) and the atmosphere’s primary oxidant, the hydroxyl radical (OH). Our analysis of a 2012 airborne study of deep convection and chemistry demonstrates that lightning also directly generates the oxidants OH and the hydroperoxyl radical (HO₂). Extreme amounts of OH and HO₂were discovered and linked to visible flashes occurring in front of the aircraft and to subvisible discharges in electrified anvil regions. This enhanced OH and HO₂is orders of magnitude greater than any previous atmospheric observation. Lightning-generated OH in all storms happening at the same time globally can be responsible for a highly uncertain, but substantial, 2 to 16% of global atmospheric OH oxidation.