Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets

Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation Atmospheric PHotochemistry In Large Reaction Chamber). chemical structure myrcene consists one moiety that is conjugated ? system (similar to isoprene) and another triple-substituted olefinic unit 2-methyl-2-butene). Hydrogen shift reactions organic peroxy radicals (RO2) formed reaction isoprene with OH are known be importance for regeneration OH. Structure–activity relationships (SARs) suggest similar hydrogen like may apply isoprenyl part RO2 during oxidation myrcene. addition, SAR predicts further isomerization would competitive bimolecular typical forested environments low concentrations nitric oxide. Assuming can rapidly interconvert addition elimination O2 isoprene, bulk rate constants 0.21 0.097 s?1 (T=298 K) three isomers resulting from 3?-OH 1-OH respectively, derived SAR. Measurements trace gases experiments allowed us calculate radical production destruction rates, which expected balanced. largest discrepancies between rates were found RO2. Additional loss due could explain observed discrepancies. uncertainty total (ROx=OH+HO2+RO2) high yield ozonolysis. However, results indicate only balanced if constant hydroperoxy (HO2) lower (0.9 1.6×10-11 cm3 s?1) than predicted Another explanation significant fraction products (yield: 0.3 0.6) these include HO2 instead radical-terminating peroxides. Experiments also determine yields acetone 0.45±0.08) formaldehyde 0.35±0.08). Acetone produced different pathways, so compounds reflect branching ratios initial Yields determined consistent nitrate gas-phase budget analysis reactive oxidized nitrogen chamber, giving value 0.13±0.03. + measured concentration, yielding (2.3±0.3)×10-10 s?1.