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

Abstract

Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR. The chemical structure of myrcene consists of one moiety that is a conjugated π-system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals RO2 formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure-activity relationships (SAR) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bi-molecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21\u2009s−1 and 0.097\u2009s−1 (T\u2009=\u2009298\u2009K) for the 3 isomers resulting from the 3 -OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed to calculate radical production and destruction rates, which are expected to be balanced. Largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx\u2009=\u2009OH+HO2+RO2) production rates were high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced, if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6\u2009×\u200910−11\u2009cm3\u2009s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical terminating organic peroxides. Experiments also allowed to determine the yields of organic oxidation products acetone (yield: 0.45\u2009±\u20090.08) and formaldehyde (yield: 0.35\u2009±\u20090.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber giving a value of 0.13\u2009±\u20090.03. In addition, the reaction rate constant for myrcene\u2009+\u2009OH was determined from the measured myrcene concentration yielding a value of (2.3\u2009±\u20090.3)\u2009×\u200910−10\u2009cm3\u2009s−1.\n