Extreme oxidant amounts produced by lightning in storm clouds

Abstract

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 HO2 were discovered and linked to visible flashes occurring in front of the aircraft and to subvisible discharges in electrified anvil regions. This enhanced OH and HO2 is orders of magnitude more 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 2-16% of global atmospheric OH oxidation.

Electrical Discharges Produce Prodigious Amounts of Hydroxyl and Hydroperoxyl Radicals

“…During the DC3 field campaign enhanced HO x was measured, usually with little corresponding enhanced LNO x or electrical activity detected in the volume of air that the aircraft flew through (Brune et al, 2021). These experiments confirm the observation that LHO x can be generated in weak, subvisible discharges, with little to no detectable NO x present.…”

“…However, previous studies have shown that the differences in LNO x production between different discharge types is mostly due to differences in current and how long the discharge lasts (Cooray et al, 2009 et al, 2011), and information on both of those variables has been provided in this work. Additionally, both AC and DC pulses can interact with atmospheric gases in the same way (Raizer, 1997), making the AC discharges used in this study suitable for reproducing the discharges encountered during the DC3 field campaign, which were typically the weaker anvil discharges and not the strong high-current lightning (Brune et al, 2021). As a test, we used DC pulses from a Marx generator also to make sparks and found that comparable values for LNO x and LHO x were produced for weak currents from both devices.…”

“…These experiments confirm the observation that LHO x can be generated in weak, subvisible discharges, with little to no detectable NO x present. The behavior of LHO x , LO 3 , and LNO x shown in Figure 5 (Brune et al, 2021).…”

“…The OH lifetime due to HONO formation alone should be a few milliseconds, but the observed decay gives an OH lifetime of hundreds of milliseconds. Using the same Framework for 0-D Atmospheric Modeling with the Master Chemical Mechanism, v3.3.1 used by Brune et al (2021) for analysis of the DC3 observations, we modeled the OH decay measured in these experiments. The model was initialized in multiple ways, with the same amount of initial HO x and NO x as observed in the experiments, the same HO x but less NO x , and the same HO x and no NO x .…”

“…Evidence that LHO x is being produced in the atmosphere was discovered during the Deep Convective Clouds and Chemistry (DC3) field campaign, which occurred over the central United States in 2012 (Barth et al, 2015). During DC3, the NASA DC-8 aircraft, outfitted to measure many chemical constituents including HO x , NO x , and O 3 , flew through thunderstorm outflow anvils, some of which were electrified according to measurements by lightning mapping arrays (Brune et al, 2021). Within these electrified regions, OH and HO 2 were often several hundred parts per trillion (pptv), orders of magnitude larger than typical background values of a few tens of pptv for HO 2 and tenths of pptv for OH.…”