000868151 001__ 868151 000868151 005__ 20240712101011.0 000868151 0247_ $$2doi$$a10.5194/acp-20-3333-2020 000868151 0247_ $$2ISSN$$a1680-7316 000868151 0247_ $$2ISSN$$a1680-7324 000868151 0247_ $$2Handle$$a2128/24637 000868151 0247_ $$2altmetric$$aaltmetric:77936945 000868151 0247_ $$2WOS$$aWOS:000521593900003 000868151 037__ $$aFZJ-2019-06725 000868151 082__ $$a550 000868151 1001_ $$0P:(DE-Juel1)166537$$aNovelli, Anna$$b0$$eCorresponding author 000868151 245__ $$aImportance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR 000868151 260__ $$aKatlenburg-Lindau$$bEGU$$c2020 000868151 3367_ $$2DRIVER$$aarticle 000868151 3367_ $$2DataCite$$aOutput Types/Journal article 000868151 3367_ $$0PUB:(DE-HGF)16$$2PUB:(DE-HGF)$$aJournal Article$$bjournal$$mjournal$$s1585830425_13540 000868151 3367_ $$2BibTeX$$aARTICLE 000868151 3367_ $$2ORCID$$aJOURNAL_ARTICLE 000868151 3367_ $$00$$2EndNote$$aJournal Article 000868151 520__ $$aTheoretical, 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. 000868151 536__ $$0G:(DE-HGF)POF3-243$$a243 - Tropospheric trace substances and their transformation processes (POF3-243)$$cPOF3-243$$fPOF III$$x0 000868151 588__ $$aDataset connected to CrossRef 000868151 7001_ $$0P:(DE-Juel1)167140$$aVereecken, Luc$$b1 000868151 7001_ $$0P:(DE-Juel1)2693$$aBohn, Birger$$b2 000868151 7001_ $$0P:(DE-Juel1)16317$$aDorn, Hans-Peter$$b3 000868151 7001_ $$0P:(DE-Juel1)165645$$aGkatzelis, Georgios I.$$b4 000868151 7001_ $$0P:(DE-Juel1)16326$$aHofzumahaus, Andreas$$b5 000868151 7001_ $$0P:(DE-Juel1)16342$$aHolland, Frank$$b6$$ufzj 000868151 7001_ $$0P:(DE-Juel1)171432$$aReimer, David$$b7$$ufzj 000868151 7001_ $$0P:(DE-Juel1)16347$$aRohrer, Franz$$b8$$ufzj 000868151 7001_ $$0P:(DE-Juel1)173788$$aRosanka, Simon$$b9 000868151 7001_ $$0P:(DE-Juel1)167439$$aTaraborrelli, Domenico$$b10 000868151 7001_ $$0P:(DE-Juel1)5344$$aTillmann, Ralf$$b11$$ufzj 000868151 7001_ $$0P:(DE-Juel1)2367$$aWegener, Robert$$b12 000868151 7001_ $$0P:(DE-Juel1)159354$$aYu, Zhujun$$b13 000868151 7001_ $$0P:(DE-Juel1)4528$$aKiendler-Scharr, Astrid$$b14 000868151 7001_ $$0P:(DE-Juel1)16324$$aWahner, Andreas$$b15 000868151 7001_ $$0P:(DE-Juel1)7363$$aFuchs, Hendrik$$b16 000868151 773__ $$0PERI:(DE-600)2069847-1$$a10.5194/acp-20-3333-2020$$gVol. 20, no. 6, p. 3333 - 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