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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 Oct 2019

Submitted as: research article | 21 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Importance of isomerization reactions for the OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

Anna Novelli1, Luc Vereecken1, Hans-Peter Dorn1, Andreas Hofzumahaus1, Frank Holland1, Zhujun Yu1,a, Simon Rosanka1, David Reimer1, Georgios I. Gkatzelis1,b,c, Birger Bohn1, Domenico Taraborrelli1, Franz Rohrer1, Ralf Tillmann1, Robert Wegener1, Astrid Kiendler-Scharr1, Andreas Wahner1, and Hendrik Fuchs1 Anna Novelli et al.
  • 1Forschungszentrum Jülich, Institutefor Energy and Climate Research: Troposphere (IEK-8), 52425 Jülich, Germany
  • anow at: Institute of Mass Spectrometry and Atmospheric Environment, Jinan University, Guangzhou 510632, China
  • bnow at: NOAA Earth Systems Research Laboratory, Boulder, Colorado 80305, USA
  • cnow at: Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado 80309, USA

Abstract. Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions. 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 the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41 °C) 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 one, 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 parts per billion (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 photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde 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 on the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (< 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions.

Anna Novelli et al.
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Anna Novelli et al.
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Short summary
Experimental evidences that the regeneration efficiency of the OH radical is maintained globally at values higher than 0.5 for a wide range of NO concentrations as a result of the isomerization of peroxy radicals in the OH-oxidation of isoprene are provided from a simulation chamber study. In addition, current available models were tested and suggestions on how to improve their ability to reproduce the measured radical and oxygenated volatile organic compound concentrations are provided.
Experimental evidences that the regeneration efficiency of the OH radical is maintained globally...