The heterogeneous reaction of hydroxyl radicals with sub-micron squalane particles: a model system for understanding the oxidative aging of ambient aerosols

Smith, J. D.; Kroll, J. H.; Cappa, C. D.; L., D.; Liu, C. L.; Ahmed, Musahid; Leone, Stephen R.; Worsnop, Douglas R.; Wilson, Kevin R.

doi:10.5194/acp-9-3209-2009

Cited by 221 publications

(263 citation statements)

References 40 publications

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“…Since the concept of potential aerosol mass (PAM) was proposed, the PAM reactor, operated as a flow tube reactor, has been widely used in laboratory and field studies of SOA formation (Chen et al, 2013;Kang et al, 2007Kang et al, , 2011Keller and Burtscher, 2012;Kroll et al, 2009;Lambe et al, 2011aLambe et al, , 2012Lambe et al, , 2015Ortega et al, 2016Ortega et al, , 2013Palm et al, 2016;Slowik et al, 2012;Smith et al, 2009). A powerful attribute of the PAM and subsequent flow reactors is the capability to generate hydroxyl radical (OH) levels that lead to integrated OH exposure ranging as high as ∼ 10 12 molecules cm −3 s, at which it is possible to simulate atmospheric oxidation conditions comparable to those occurring over ∼ 1 week.…”

Section: Y Huang Et Al: the Caltech Photooxidation Flow Tube Reactormentioning

confidence: 99%

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

et al. 2017

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Abstract. Flow tube reactors are widely employed to study gas-phase atmospheric chemistry and secondary organic aerosol (SOA) formation. The development of a new laminar-flow tube reactor, the Caltech Photooxidation Flow Tube (CPOT), intended for the study of gas-phase atmospheric chemistry and SOA formation, is reported here. The present work addresses the reactor design based on fluid dynamical characterization and the fundamental behavior of vapor molecules and particles in the reactor. The design of the inlet to the reactor, based on computational fluid dynamics (CFD) simulations, comprises a static mixer and a conical diffuser to facilitate development of a characteristic laminar flow profile. To assess the extent to which the actual performance adheres to the theoretical CFD model, residence time distribution (RTD) experiments are reported with vapor molecules (O 3 ) and submicrometer ammonium sulfate particles. As confirmed by the CFD prediction, the presence of a slight deviation from strictly isothermal conditions leads to secondary flows in the reactor that produce deviations from the ideal parabolic laminar flow. The characterization experiments, in conjunction with theory, provide a basis for interpretation of atmospheric chemistry and SOA studies to follow. A 1-D photochemical model within an axially dispersed plug flow reactor (AD-PFR) framework is formulated to evaluate the oxidation level in the reactor. The simulation indicates that the OH concentration is uniform along the reactor, and an OH exposure (OH exp ) ranging from ∼ 10 9 to ∼ 10 12 molecules cm −3 s can be achieved from photolysis of H 2 O 2 . A method to calculate OH exp with a consideration for the axial dispersion in the present photochemical system is developed.

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Section: Y Huang Et Al: the Caltech Photooxidation Flow Tube Reactormentioning

confidence: 99%

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

et al. 2017

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“…For example, the heterogeneous OH oxidation of insoluble or sparingly soluble organic compounds (e.g., squalane and bis(2-ethylhexyl)sebacate) produces water-soluble reaction products that enhance the CCN activity of corresponding aerosols (George et al, 2009;Harmon et al, 2013). A recent study has suggested that low-temperature OH oxidation may increase CCN activity of amorphous solid organic particles (Slade et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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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 particlephase 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 (C 5 H 8 O 4 ) 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 highresolution mass spectrometer reveal two major products: a five carbon atom (C 5 ) hydroxyl functionalization product (C 5 H 8 O 5 ) and a C 4 fragmentation product (C 4 H 6 O 3 ). 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 methylsubstituted 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 highoxidation stages.

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“…While flow tube reactors have been used to study the chemistry of inorganic particles since the 1950s (Robbins and Cadle, 1958;Hanson and Lovejoy, 1995), their application to the study of organic surfaces (Cooper and Abbatt, 1996;de Gouw and Lovejoy, 1998;Bertram et al, 2001) and organic particles (Morris et al, 2002;Katrib et al, 2005;Hearn and Smith, 2006; Published by Copernicus Publications on behalf of the European Geosciences Union. McNeill et al, 2008;Smith et al, 2009) is relatively recent.…”

Section: Introductionmentioning

confidence: 99%

Characterization of aerosol photooxidation flow reactors: heterogeneous oxidation, secondary organic aerosol formation and cloud condensation nuclei activity measurements

et al. 2011

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Abstract. Motivated by the need to develop instrumental techniques for characterizing organic aerosol aging, we report on the performance of the Toronto Photo-Oxidation Tube (TPOT) and Potential Aerosol Mass (PAM) flow tube reactors under a variety of experimental conditions. The PAM system was designed with lower surface-area-tovolume (SA/V) ratio to minimize wall effects; the TPOT reactor was designed to study heterogeneous aerosol chemistry where wall loss can be independently measured. The following studies were performed: (1) transmission efficiency measurements for CO 2 , SO 2 , and bis(2-ethylhexyl) sebacate (BES) particles, (2) H 2 SO 4 yield measurements from the oxidation of SO 2 , (3) residence time distribution (RTD) measurements for CO 2 , SO 2 , and BES particles, (4) aerosol mass spectra, O/C and H/C ratios, and cloud condensation nuclei (CCN) activity measurements of BES particles exposed to OH radicals, and (5) aerosol mass spectra, O/C and H/C ratios, CCN activity, and yield measurements of secondary organic aerosol (SOA) generated from gas-phase OH oxidation of m-xylene and α-pinene. OH exposures ranged from (2.0 ± 1.0) × 10 10 to (1.8 ± 0.3) × 10 12 molec cm −3 s. Where applicable, data from the flow tube reactors are compared with published results from the Caltech smog chamber. The TPOT yielded narrower RTDs. However, its transmission efficiency for SO 2 was lower than that for the PAM. Transmission efficiency for BES and H 2 SO 4 particles was size-dependent and was similar for the two flowCorrespondence to: T. B. Onasch (onasch@aerodyne.com) tube designs. Oxidized BES particles had similar O/C and H/C ratios and CCN activity at OH exposures greater than 10 11 molec cm −3 s, but different CCN activity at lower OH exposures. The O/C ratio, H/C ratio, and yield of m-xylene and α-pinene SOA was strongly affected by reactor design and operating conditions, with wall interactions seemingly having the strongest influence on SOA yield. At comparable OH exposures, flow tube SOA was more oxidized than smog chamber SOA, possibly because of faster gas-phase oxidation relative to particle nucleation. SOA yields were lower in the TPOT than in the PAM, but CCN activity of flow-tubegenerated SOA particles was similar. For comparable OH exposures, α-pinene SOA yields were similar in the PAM and Caltech chambers, but m-xylene SOA yields were much lower in the PAM compared to the Caltech chamber.

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The heterogeneous reaction of hydroxyl radicals with sub-micron squalane particles: a model system for understanding the oxidative aging of ambient aerosols

Cited by 221 publications

References 40 publications

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

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

Characterization of aerosol photooxidation flow reactors: heterogeneous oxidation, secondary organic aerosol formation and cloud condensation nuclei activity measurements

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