Abstract
Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). 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 (SARs) 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 bimolecular 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 and 0.097 s−1 (T=298 K) for the three 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 us to calculate radical production and destruction rates, which are expected to be balanced. The 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=OH+HO2+RO2) production rates was 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 1.6×10-11 cm3 s−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 us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). 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±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.
Highlights
Monoterpenes are emitted from vegetation accounting for approximately 160 Tg of the 1000 Tg of biogenic volatile organic compounds (VOCs) that are released into the atmosphere per year (Guenther et al, 2012; Sindelarova et al, 2014)
In order to estimate effective bulk myrcene RO2 radicals (MyO2) isomerization reaction rates, the distribution of MyO2 isomers is estimated by using reaction rate constants for the oxygen addition and elimination reactions recommended for isoprene (Novelli et al, 2020). This results in a total bulk MyO2 loss rate of approximately 0.21 and 0.097 s−1 (T = 298 K) for the three isomers resulting from the 3 -OH and 1-OH addition, respectively
The chemical mechanism of the oxidation of myrcene by OH has not been investigated in detail so far
Summary
Monoterpenes are emitted from vegetation accounting for approximately 160 Tg of the 1000 Tg of biogenic volatile organic compounds (VOCs) that are released into the atmosphere per year (Guenther et al, 2012; Sindelarova et al, 2014). Few studies have been conducted to investigate the oxidation of myrcene (Deng et al, 2018; Atkinson, 1997; Reissell et al, 2002; Kim et al, 2011) In these studies, acetone, formaldehyde and 4-vinyl-pentenal have been identified as major oxidation products from the reaction of myrcene with OH, but yields determined in these studies vary. The reaction rate constant of myrcene + OH was determined to be (2.1 ± 0.2) × 10−10 (Atkinson et al, 1986) and (3.4+−11..50) × 10−10 (Hites and Turner, 2009) at room temperature, with a discrepancy of up to 60 %. These results demonstrate the photo-oxidation of myrcene requires further investigation
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