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. Theoretical, laboratory, and chamber studies have shownfast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene,largely due to unimolecular reactions which were previously thought not tobe important under atmospheric conditions. Based on early fieldmeasurements, nearly complete regeneration was hypothesized for a wide rangeof tropospheric conditions, including areas such as the rainforest whereslow regeneration of OH radicals is expected due to low concentrations ofnitric oxide (NO). In this work the OH regeneration in isopreneoxidation is directly quantified for the first time through experimentscovering a wide range of atmospherically relevant NO levels (between 0.15and 2 ppbv – parts per billion by volume) in the atmospheric simulationchamber SAPHIR. These conditions cover remote areas partially influenced byanthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting ina surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3higher than explainable in the absence of unimolecular reactions. Themeasured radical concentrations were compared to model calculations, and thebest agreement was observed when at least 50 % of the total loss ofisoprene peroxy radicals conformers (weighted by their abundance) occurs viaisomerization 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 1,6α-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 peroxyradicals (di-HPCARP-RO2), depending on their relative yield. Theagreement between all measured and modelled trace gases (hydroxyl,hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum ofmethyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearlyindependent of the adopted yield of HPALD and di-HPCARP-RO2 as bothdegrade relatively fast (&lt;1 h), forming the OH radical and CO amongother 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 unpollutedisoprene-dominated regions.

atmospheric chemistry and physics

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