Journal article

Understanding the aqueous phase ozonolysis of isoprene: Distinct product distribution and mechanism from the gas phase reaction

Wang H, Huang D, Zhang X, Zhao Y, Chen Z ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 15 (2012) pp. 7187-7198

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Abstract

The aqueous phase reaction of volatile organic compounds (VOCs) has
not been considered in most analyses of atmospheric chemical processes.
However, some experimental evidence has shown that, compared to the
corresponding gas phase reaction, the aqueous chemical processes
of VOCs in the bulk solutions and surfaces of ambient wet particles
(cloud, fog, and wet aerosols) may potentially contribute to the
products and formation of secondary organic aerosol (SOA). In the
present study, we performed a laboratory experiment of the aqueous
ozonolysis of isoprene at different pHs (3?7) and temperatures (4?25
°C). We detected three important kinds of products, including carbonyl
compounds, peroxide compounds, and organic acids. Our results showed
that the molar yields of these products were nearly independent of
the investigated pHs and temperatures, those were (1) carbonyls:
56.7 ± 3.7 % formaldehyde, 42.8 ± 2.5 % methacrolein (MAC), and 57.7
± 3.4 % methyl vinyl ketone (MVK); (2) peroxides: 53.4 ± 4.1 % hydrogen
peroxide (H2O2) and 15.1 ± 3.1 % hydroxylmethyl hydroperoxide (HMHP);
and (3) organic acids: undetectable (limit). Based on the amounts of products formed and the isoprene
consumed, the total carbon yield was estimated to be 94.8 ± 4.1 %.
This implied that most of the products in the reaction system were
detected. The combined yields of both MAC + MVK and H2O2 + HMHP in
the aqueous isoprene ozonolysis were much higher than those observed
in the corresponding gas phase reaction. We suggest that these unexpected
high yields of carbonyls and peroxides are related to the greater
capability of condensed water, compared to water vapor, to stabilize
energy-rich Criegee radicals. This aqueous ozonolysis of isoprene
(and possibly other biogenic VOCs) could potentially occur on the
surfaces of ambient wet particles and plants. Moreover, the high-yield
carbonyl and peroxide products might provide a considerable source
of aqueous phase oxidants and SOA precursors.

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Authors

  • H. L. Wang

  • D. Huang

  • X. Zhang

  • Y. Zhao

  • Z. M. Chen

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