Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets
Authors: Tan Zhaofeng , Hantschke Luisa , Kaminski Martin , Acir Ismail-Hakki , Bohn Birger , Cho Changmin , Dorn Hans-Peter , Li Xin , Novelli Anna , Nehr Sascha , Rohrer Franz , Tillmann Ralf , Wegener Robert , Hofzumahaus Andreas , Kiendler-Scharr Astrid , Wahner Andreas , Fuchs Hendrik 
Published: October 2021

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.