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@ARTICLE{Novelli:868151,
      author       = {Novelli, Anna and Vereecken, Luc and Bohn, Birger and Dorn,
                      Hans-Peter and Gkatzelis, Georgios I. and Hofzumahaus,
                      Andreas and Holland, Frank and Reimer, David and Rohrer,
                      Franz and Rosanka, Simon and Taraborrelli, Domenico and
                      Tillmann, Ralf and Wegener, Robert and Yu, Zhujun and
                      Kiendler-Scharr, Astrid and Wahner, Andreas and Fuchs,
                      Hendrik},
      title        = {{I}mportance of isomerization reactions for {OH} radical
                      regeneration from the photo-oxidation of isoprene
                      investigated in the atmospheric simulation chamber {SAPHIR}},
      journal      = {Atmospheric chemistry and physics},
      volume       = {20},
      number       = {6},
      issn         = {1680-7316},
      address      = {Katlenburg-Lindau},
      publisher    = {EGU},
      reportid     = {FZJ-2019-06725},
      pages        = {1-32},
      year         = {2020},
      abstract     = {Theoretical, laboratory, and chamber studies have shown
                      fast regeneration of the hydroxyl radical (OH) in the
                      photochemistry of isoprene, largely due to unimolecular
                      reactions which were previously thought not to be important
                      under atmospheric conditions. Based on early field
                      measurements, nearly complete regeneration was hypothesized
                      for a wide range of tropospheric conditions, including areas
                      such as the rainforest where slow regeneration of OH
                      radicals is expected due to low concentrations of nitric
                      oxide (NO). In this work the OH regeneration in isoprene
                      oxidation is directly quantified for the first time through
                      experiments covering a wide range of atmospherically
                      relevant NO levels (between 0.15 and 2 ppbv – parts per
                      billion by volume) in the atmospheric simulation chamber
                      SAPHIR. These conditions cover remote areas partially
                      influenced by anthropogenic NO emissions, giving a
                      regeneration efficiency of OH close to 1, and areas like the
                      Amazonian rainforest with very low NO, resulting in a
                      surprisingly high regeneration efficiency of 0.5, i.e. a
                      factor of 2 to 3 higher than explainable in the absence of
                      unimolecular reactions. The measured radical concentrations
                      were compared to model calculations, and the best agreement
                      was observed when at least $50 \%$ of the total loss of
                      isoprene peroxy radicals conformers (weighted by their
                      abundance) occurs via isomerization reactions for NO lower
                      than 0.2 ppbv. For these levels of NO, up to $50 \%$ of
                      the OH radicals are regenerated from the products of the 1,6
                      α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals
                      through the photolysis of an unsaturated hydroperoxy
                      aldehyde (HPALD) and/or through the fast aldehydic hydrogen
                      shift (rate constant ∼10 s−1 at 300 K) in
                      di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2),
                      depending on their relative yield. The agreement between all
                      measured and modelled trace gases (hydroxyl, hydroperoxy,
                      and organic peroxy radicals, carbon monoxide, and the sum of
                      methyl vinyl ketone, methacrolein, and hydroxyl
                      hydroperoxides) is nearly independent of the adopted yield
                      of HPALD and di-HPCARP-RO2 as both degrade relatively fast
                      (<1 h), forming the OH radical and CO among other
                      products. Taking into consideration this and earlier
                      isoprene studies, considerable uncertainties remain on the
                      distribution of oxygenated products, which affect radical
                      levels and organic aerosol downwind of unpolluted
                      isoprene-dominated regions.},
      cin          = {IEK-8},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-8-20101013},
      pnm          = {243 - Tropospheric trace substances and their
                      transformation processes (POF3-243)},
      pid          = {G:(DE-HGF)POF3-243},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000521593900003},
      doi          = {10.5194/acp-20-3333-2020},
      url          = {https://juser.fz-juelich.de/record/868151},
}