TY - JOUR A1 - Novelli, Anna A1 - Kaminski, Martin A1 - Rolletter, Michael A1 - Acir, Ismail-Hakki A1 - Bohn, Birger A1 - Dorn, Hans-Peter A1 - Li, Xin A1 - Lutz, Anna A1 - Nehr, Sascha A1 - Rohrer, Franz A1 - Tillmann, Ralf A1 - Wegener, Robert A1 - Holland, Frank A1 - Hofzumahaus, Andreas A1 - Kiendler-Scharr, Astrid A1 - Wahner, Andreas A1 - Fuchs, Hendrik T1 - Evaluation of OH and HO2 concentrations and their budgets during photooxidation of 2-methyl-3-butene-2-ol (MBO) in the atmospheric simulation chamber SAPHIR JF - Atmospheric Chemistry and Physics N2 - Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by the traditional oxidation mechanisms that largely underestimated the observations. These environments were characterized by large concentrations of biogenic volatile organic compounds (BVOC) and low nitrogen oxide concentration. In isoprene-dominated environments, models developed to simulate atmospheric photochemistry generally underestimated the observed OH radical concentrations. In contrast, HO2 radical concentration showed large discrepancies with model simulations mainly in non-isoprene-dominated forested environments. An abundant BVOC emitted by lodgepole and ponderosa pines is 2-methyl-3-butene-2-ol (MBO), observed in large concentrations for studies where the HO2 concentration was poorly described by model simulations. In this work, the photooxidation of MBO by OH was investigated for NO concentrations lower than 200 pptv in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. Measurements of OH and HO2 radicals, OH reactivity (kOH), MBO, OH precursors, and organic products (acetone and formaldehyde) were used to test our current understanding of the OH-oxidation mechanisms for MBO by comparing measurements with model calculations. All the measured trace gases agreed well with the model results (within 15 %) indicating a well understood mechanism for the MBO oxidation by OH. Therefore, the oxidation of MBO cannot contribute to reconciling the unexplained high OH and HO2 radical concentrations found in previous field studies. KW - Luftreinhaltung Y1 - 2018 U6 - https://doi.org/10.5194/acp-18-11409-2018 SN - 1680-7324 N1 - Code and data availability The data from the experiments in the SAPHIR chamber used in this work are available on the EUROCHAMP data home page (https://data.eurochamp.org/, last access: 1 October 2019, EUROCHAMP, 2019). Supplement The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-3333-2020-supplement. VL - 18 IS - 15 SP - 11409 EP - 11422 PB - Copernicus ER - TY - JOUR A1 - Tan, Zhaofeng A1 - Hantschke, Luisa A1 - Kaminski, Martin A1 - Acir, Ismail-Hakki A1 - Bohn, Birger A1 - Cho, Changmin A1 - Dorn, Hans-Peter A1 - Li, Xin A1 - Novelli, Anna A1 - Nehr, Sascha A1 - Rohrer, Franz A1 - Tillmann, Ralf A1 - Wegener, Robert A1 - Hofzumahaus, Andreas A1 - Kiendler-Scharr, Astrid A1 - Wahner, Andreas A1 - Fuchs, Hendrik T1 - Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets JF - Atmospheric Chemistry and Physics N2 - Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s−1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1. KW - Luftreinhaltung Y1 - 2021 U6 - https://doi.org/10.5194/acp-21-16067-2021 SN - 1680-7324 VL - 21 IS - 20 SP - 16067 EP - 16091 PB - Copernicus GmbH ER -