Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

Autor: Chim, MM, Cheng, CT, Davies, JF, Berkemeier, T, Shiraiwa, M, Zuend, A, Chan, MN
Rok vydání: 2017
Předmět:
Zdroj: ATMOSPHERIC CHEMISTRY AND PHYSICS, vol 17, iss 23
Atmospheric Chemistry and Physics, vol 17, iss 23
Chim, MM; Cheng, CT; Davies, JF; Berkemeier, T; Shiraiwa, M; Zuend, A; et al.(2017). Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols. ATMOSPHERIC CHEMISTRY AND PHYSICS, 17(23), 14415-14431. doi: 10.5194/acp-17-14415-2017. UC Irvine: Retrieved from: http://www.escholarship.org/uc/item/04s6329t
DOI: 10.5194/acp-17-14415-2017.
Popis: 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. Achange in aerosol composition affects the hygroscopicity and leads to aconcomitant 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, amodel compound for small branched dicarboxylic acids found in atmospheric aerosols, at ahigh relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by asoft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with ahigh-resolution mass spectrometer reveal two major products: afive carbon atom (C5) hydroxyl functionalization product (C5H8O5) and aC4 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, akinetic model of oxidation (atwo-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 anet loss of organic species and associated particle-phase water, and thus asmaller droplet size. Overall, fragmentation and volatilization processes play alarger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.
Databáze: OpenAIRE