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 (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.

Particle Formation from Photooxidation of αpinene, Limonene, and Myrcene

“…For example, myrcene has a number of allylic hydrogen atoms that are amenable to H-shift: Vereecken and Noziere report H-shift rates of 2–8 s –1 that would apply to the myriad myrcene-(OH)­O 2 species. See Tan et al who present a detailed accounting of these species and potential H-shift rates based on the Vereecken and Noziere study. Our typical [NO] is 4 × 10 10 cm –3 , and the pseudo-first-order rate coefficient for myrcene-(OH)­O 2 + NO is 0.4 s –1 ; thus, a dominant role for H-shift reactions that produce HOPs is likely for myrcene in our experiment.…”

Comparison of isoprene chemical mechanisms at atmospheric night-time conditions in chamber experiments: Evidence of hydroperoxy aldehydes and epoxy products from NO3 oxidation

“…OH reactivity (k OH , the inverse of the chemical lifetime of the OH radical) was determined by a laser flash photolysis instrument, in which the time resolved decay of artificially produced OH radicals is observed (Fuchs et al, 2017). If, as in this work, the OH-reactivity from inorganic compounds is known, the contribution from organic compounds can be derived and compared to values based on the measurements of single compounds (Tan et al, 2021;Hantschke et al, 2021). In general, differences between measured and calculated OH reactivity can be used to determine if the detection of organic products that are reactive towards OH are complete.…”

Handheld Crop Pest Sensor Using Binary Catalyst-Loaded Nano-SnO2 Particles for Oxidative Signal Amplification

“…This peroxy isomer has been captured for further oxidation into aldehyde or acetone, which in turn further oxidizes to CO 2 . Thus, the multiple oxidation sites as well as the repeated oxidation of the products to decompose into the highly oxidized form of carbon, viz., CO 2 , led to the significant response to minimum concentration . This hypothesis of the aldehyde/acetone and CO 2 formation was confirmed through FTIR and gas chromatograph analysis.…”

“…Thus, the multiple oxidation sites as well as the repeated oxidation of the products to decompose into the highly oxidized form of carbon, viz., CO 2 , led to the significant response to minimum concentration. 39 This hypothesis of the aldehyde/acetone and CO 2 formation was confirmed through FTIR and gas chromatograph analysis. The FTIR analysis of the volatile products eluted from the Carbotrap X adsorbent placed at the outlet of the sensor chamber in the presence of the sensor and in its absence is shown in Figure 6e.…”

“…Here, a sensor supported by a wide-band-gap n-type semiconductor, i.e., SnO 2 (3.6 eV), with the ability to work at a low working temperature for high selectivity to sense βmyrcene, has been developed. The β-Myrcene is one of the signature molecules having several oxidation sites, 39 which provides the feasibility to detect it at ppb scale. Thus, real-time trace β-myrcene detection in the headspace of a tomato plant infested with T. absoluta has been demonstrated.…”