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1. Spatially separate production of hydrogen oxides and nitric oxide in lightning

   https://doi.org/10.26208/0vnd-tq52  

   Jenkins, J M; Brune, W H (January 2024, Penn State Data Commons)

   Jenkins, Jena M (Ed.)

   The atmosphere’s most important oxidizer, the hydroxyl radical (OH), is generated in abundance by lightning, but the contribution of this electrically generated OH (LOH) to global OH oxidation remains highly uncertain. Part of this uncertainty is due to the abundant nitric oxide (NO) also generated in lightning, which could rapidly remove the LOH before it can oxidize other pollutants in the atmosphere. However, evidence from a previous laboratory study indicated LOH is not immediately consumed by NO, possibly because LOH’s production is spatially separated from the NO production in lightning flashes. This hypothesis of spatially separate OH and NO production is further tested here in a series of laboratory experiments, where the OH decays were measured from spark discharges in air which had increasing amounts of NO added to it. The LOH decayed faster as more NO was added to the air, indicating that the LOH was reacting with the added NO, and not the spark NO. Thus, LOH from lightning flashes is not immediately consumed by the electrically generated NO but is available to oxidize other pollutants in the atmosphere and contribute to global OH oxidation. Subsequent modelling of the laboratory data also supports the spatially separate production of LOH and NO, and further suggests that substantial HONO is also produced by sparks and lightning in the atmosphere. 

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2. Effect of Temperature and Water Droplets on Production of Prodigious Hydrogen Oxides by Electrical Discharges

   https://doi.org/10.1029/2023JD039362  

   Jenkins, J. M.; Brune, W. H. (October 2023, Journal of Geophysical Research: Atmospheres)

   Abstract Recently, electrical discharges have been identified as a potentially significant source of the atmosphere's most important oxidant, the hydroxyl radical. Measurements of hydroxyl, the closely related hydroperoxyl radical, and the nitrogen oxides from sparks and subvisible discharges were made in the laboratory under different environmental and electrical conditions representing those found in the troposphere. However, there were still several conditions not yet investigated that could impact hydroxyl and hydroperoxyl production in electrical discharges. In this study, the production of electrically generated hydroxyl and hydroperoxyl (LHO_x) and nitrogen oxides (LNO_x) was measured under three new conditions not tested previously, including lower pressure, different temperatures, and the presence of cloud droplet‐sized water droplets. In spark discharges, LHO_xwas mostly independent of pressure, increased with increasing temperature, and was unaffected by the water droplets. LNO_xgeneration was independent of temperature from −10 to 40°C and the presence of water droplets, but increased 1.5‐fold with decreasing pressure. LNO_xgeneration was also found to be sensitive to changes in spark intensity and air flow in the laboratory setup. Increasing temperature also made it more likely that a discharge was visible instead of subvisible, but did not impact LHO_xproduction in subvisible discharges. Even under these new conditions, the laboratory results agree with results of LHO_xfrom a field campaign, demonstrating the relevance of the laboratory experiments to the atmosphere. 

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3. Extreme hydroxyl amounts generated by thunderstorm-induced corona on grounded metal objects

   https://doi.org/10.1073/pnas.2201213119  

   Brune, William H.; Jenkins, Jena M.; Olson, Gabrielle A.; McFarland, Patrick J.; Miller, David O.; Mao, Jingqiu; Ren, Xinrong (September 2022, Proceedings of the National Academy of Sciences)

   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. 

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4. pH-dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces

   https://doi.org/10.5194/acp-19-4917-2019  

   Halfacre, John W.; Shepson, Paul B.; Pratt, Kerri A. (January 2019, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The mechanisms of molecular halogen production from frozen saline surfacesremain incompletely understood, limiting our ability to predict atmosphericoxidation and composition in polar regions. In this laboratory study,condensed-phase hydroxyl radicals (OH) were photochemically generated infrozen saltwater solutions that mimicked the ionic composition of oceanwater. These hydroxyl radicals were found to oxidize Cl−, Br−, andI−, leading to the release of Cl2, Br2, I2, and IBr. Atmoderately acidic pH (buffered between 4.5 and 4.8), irradiation of icecontaining OH precursors (either of hydrogen peroxide or nitrite ion)produced elevated amounts of I2. Subsequent addition of O3produced additional I2, as well as small amounts of Br2. At lowerpH (1.7–2.2) and in the presence of an OH precursor, rapid dark conversionof I− to I2 occurred from reactions with hydrogen peroxide ornitrite, followed by substantial photochemical production of Br2 uponirradiation. Exposure to O3 under these low pH conditions alsoincreased production of Br2 and I2; this likely results fromdirect O3 reactions with halides, as well as the production ofgas-phase HOBr and HOI that subsequently diffuse to frozen solution to reactwith Br− and I−. Photochemical production of Cl2 was onlyobserved when the irradiated sample was composed of high-purity NaCl andhydrogen peroxide (acting as the OH precursor) at pH = 1.8. Thoughcondensed-phase OH was shown to produce Cl2 in this study, kineticscalculations suggest that heterogeneous recycling chemistry may be equallyor more important for Cl2 production in the Arctic atmosphere. Thecondensed-phase OH-mediated halogen production mechanisms demonstrated hereare consistent with those proposed from recent Arctic field observations ofmolecular halogen production from snowpacks. These reactions, even if slow,may be important for providing seed halogens to the Arctic atmosphere. Ourresults suggest the observed molecular halogen products are dependent on therelative concentrations of halides at the ice surface, as we only observewhat diffuses to the air–surface interface. 

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5. Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A model

   https://doi.org/10.5194/acp-21-14649-2021  

   Peng, Zhe; Lee-Taylor, Julia; Stark, Harald; Orlando, John J.; Aumont, Bernard; Jimenez, Jose L. (January 2021, Atmospheric Chemistry and Physics)

   Abstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such asremote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, itsevolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicitGenerator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidationof several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading to a similar evolution of OHRVOC per C atom. An upper limit of the total OH consumption during complete oxidation to CO2 is roughly three per C atom. We also explore the trends in radical recycling ratios. We show that differences in the evolution of OHRVOC between the atmosphere and an environmental chamber, and between the atmosphere and an oxidation flow reactor (OFR), can be substantial, with the former being even larger, but these differences are often smaller than between precursors. The Teflon wall losses of oxygenated VOCs in chambers result in large deviations of OHRVOC from atmospheric conditions, especially for the oxidation of larger precursors, where multifunctional species may suffer substantial wall losses, resulting in significant underestimation of OHRVOC. For OFR, the deviations of OHRVOC evolution from the atmospheric case are mainly due to significant OHR contribution from RO2 and lack of efficient organic photolysis. The former can be avoided by lowering the UV lamp setting in OFR, while the latter is shown to be very difficult to avoid. However, the former may significantly offset the slowdown in fragmentation of multifunctional species due to lack of efficient organic photolysis. 

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