The kinetics and mechanism of an aqueous phase isoprene reaction with hydroxyl radical State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
01 Aug 2011
Received: 09 Mar 2011 – Published in Atmos. Chem. Phys. Discuss.: 11 Mar 2011 Abstract. Aqueous phase chemical processes of organic compounds in
the atmosphere have received increasing attention, partly due to their
potential contribution to the formation of secondary organic aerosol (SOA).
Here, we analyzed the aqueous OH-initiated oxidation of isoprene and its
reaction products including carbonyl compounds and organic acids, regarding
the acidity and temperature as in-cloudy conditions. We also performed a
laboratory simulation to improve our understanding of the kinetics and
mechanisms for the products of aqueous isoprene oxidation that are
significant precursors of SOA; these included methacrolein (MACR), methyl
vinyl ketone (MVK), methyl glyoxal (MG), and glyoxal (GL). We used a novel
chemical titration method to monitor the concentration of isoprene in the
aqueous phase. We used a box model to interpret the mechanistic differences
between aqueous and gas phase OH radical-initiated isoprene oxidations. Our
results were the first demonstration of the rate constant for the reaction
between isoprene and OH radical in water, 1.2 ± 0.4) × 1010 M−1 s−1 at 283 K.
Molar yields were determined based on consumed
isoprene. Of note, the ratio of the yields of MVK (24.1 ± 0.8 %) to
MACR (10.9 ± 1.1%) in the aqueous phase isoprene oxidation was
approximately double that observed for the corresponding gas phase reaction.
We hypothesized that this might be explained by a water-induced enhancement
in the self-reaction of a hydroxy isoprene peroxyl radical
(HOCH2C(CH3)(O2)CH = CH2) produced in the aqueous
reaction. The observed yields for MG and GL were 11.4 ± 0.3 % and
3.8 ± 0.1 %, respectively. Model simulations indicated that several
potential pathways may contribute to the formation of MG and GL. Finally,
oxalic acid increased steadily throughout the course of the study, even
after isoprene was consumed completely. The observed yield of oxalic acid
was 26.2 ± 0.8 % at 6 h. The observed carbon balance accounted for
~50 % of the consumed isoprene. The presence of
high-molecular-weight compounds may have accounted for a large portion of
the missing carbons, but they were not quantified in this study. In summary,
our work has provided experimental evidence that the availably abundant
water could affect the distribution of oxygenated organic compounds produced
in the oxidation of volatile organic compounds.
Revised: 24 Jun 2011 – Accepted: 23 Jul 2011 – Published: 01 Aug 2011
Citation: Huang, D., Zhang, X., Chen, Z. M., Zhao, Y., and Shen, X. L.: The kinetics and mechanism of an aqueous phase isoprene reaction with hydroxyl radical, Atmos. Chem. Phys., 11, 7399-7415, doi:10.5194/acp-11-7399-2011, 2011.