Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 14415-14431, 2017
https://doi.org/10.5194/acp-17-14415-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
05 Dec 2017
Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols
Man Mei Chim1, Chiu Tung Cheng1,a, James F. Davies2, Thomas Berkemeier3, Manabu Shiraiwa4, Andreas Zuend5, and Man Nin Chan1,6 1Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, China
2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
3School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
4Department of Chemistry, School of Physical Sciences, University of California, Irvine, California, USA
5Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Québec, Canada
6The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China
apresent address: Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan
Abstract. Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxyl functionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.

Citation: Chim, M. M., Cheng, C. T., Davies, J. F., Berkemeier, T., Shiraiwa, M., Zuend, A., and Chan, M. N.: Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols, Atmos. Chem. Phys., 17, 14415-14431, https://doi.org/10.5194/acp-17-14415-2017, 2017.
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In this work, we report that methyl-substituted succinic acid present at or near the surface of aqueous organic droplets can be efficiently oxidized by gas-phase OH radicals. The alkoxy radical chemistry appears to be an important reaction pathway. In addition, our model simulations reveal the relative importance of functionalization and fragmentation processes, alongside volatilization, in the evolution of the particle-phase reaction, which is largely dependent on the extent of oxidation.
In this work, we report that methyl-substituted succinic acid present at or near the surface of...
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