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Structural and light-absorption characteristics of complex water-insoluble organic mixtures in urban submicron aerosols Qingcai Chen, Fumikazu Ikemori, Yuua Nakamura, Petr Vodicka, Kimitaka Kawamura, and Michihiro Mochida Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017
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Structural and light-absorption characteristics of
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complex water-insoluble organic mixtures in urban
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submicron aerosols
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Qingcai Chen1,2, Fumikazu Ikemori1,3, Yuua Nakamura1, Petr Vodicka4, Kimitaka Kawamura4,
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Michihiro Mochida*1
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1 Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
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2 School of Environmental Science and Engineering, Shaanxi University of Science and
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Technology, Xi’an 710021, China
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3 Nagoya City Institute for Environmental Sciences, Nagoya 457-0841, Japan
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4 Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japan
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*Corresponding author e-mail:
[email protected] 12
Phone/fax: 052-788-6157
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Address: Department of Earth and Environmental Sciences, Graduate School of Environmental
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Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
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ABSTRACT: Submicrometer aerosols in the urban atmosphere of Nagoya, Japan, were
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collected in late winter and early spring, and the water-insoluble organic matter (WISOM) in the
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samples were fractionated into six subfractions based on their polarities by using solvent and
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normal-phase solid-phase extractions: non-polar (F1), low-polar (F2 and F3), and medium-polar
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(F4, F5, and F6) fractions. The overall structural characteristics of these subfractions were then
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analyzed using Fourier transform infrared spectroscopy, nuclear magnetic resonance
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spectroscopy, and high-resolution aerosol mass spectrometry. Quantitative information related to
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the overall chemical characteristics of the WISOM in the different polarity fractions, including
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their elemental compositions, the relative abundances of different functional groups and their
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fragments from electron impact ionization, was obtained. These water-insoluble fractions
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accounted for half of the total light absorption by the extracted aerosol matter at 400 nm. The
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contributions of the medium-polar fractions to both the total organic carbon and light absorption
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by the extracts were dominant among the contributions from the six subfractions. Large
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molecules with aromatic and heteroatomic (O and N) groups, including charge transfer
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complexes, might have greatly contributed to the light absorption by the fraction F4, which is the
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largest fraction of the extracted water-insoluble organic matter.
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KEYWORDS: water-insoluble organic matter, brown carbon, chemical structure, aerosol mass
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spectrometry, Fourier transform infrared spectroscopy
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Environmental Science & Technology
INTRODUCTION
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Submicrometer aerosol particles in the atmosphere contain abundant organic compounds (20–
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90%).1,2 Their presence influences the properties of these aerosols and thereby affects the roles
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of the aerosols in the climate and in human health.3 The chemical characterization of these
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organic compounds is important to understand and predict these roles. However, the organic
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aerosol components remain poorly characterized because of the highly complex nature of their
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composition.4-6 The brown carbon (BrC) in organic aerosols can absorb light at visible and near-
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ultraviolet (UV) wavelengths. Hence, BrC potentially plays important roles in the photoinitiated
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reactions of organic aerosols and thus in the radiative balance of the Earth. The characterization
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of BrC is important to gain insight into their source and formation processes and to evaluate the
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effects of BrC on the climate.7,8
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Over the past decade, much of the work on the characterization of organic aerosol components
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has focused on water-soluble organic matter (WSOM).9-13 However, the overall characteristics of
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the remaining organic matter, i.e., the water-insoluble organic matter (WISOM), have not
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received much attention14 even though the remaining organic matter constitutes a large fraction
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of the organic carbon in urban and marine aerosols (26%–94% in OC; 19%–92% in OM).14-17
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Aerosol WISOM is a complex organic matter that is composed of not only hydrocarbons but also
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N- and O-containing compounds.16,18,19 WISOM also contains high-molecular-weight
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oligomers.20-22 The analysis of the chemical composition of aerosol WISOM has generally been
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performed using gas chromatography (GC) or two-dimensional GC (2D-GC) in combination
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with mass spectrometry.19, 23, 24 However, the composition of a large mass fraction of the organic
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matter, including unresolved complex mixtures (branched and cyclic compounds) and
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nonvolatile or thermally unstable compounds (oxygenated compounds with high molecular
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weights), has not been well characterized.6,
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fractions, and recent studies have shown that WISOM has a significant light absorptivity that is
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stronger than that of WSOM.9,25 The light absorption by the methanol-extracted matter from the
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ambient aerosols in Los Angeles was ~3 (365 nm) and 21 (532 nm) times higher than that
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extracted by water.9 Among the organic compounds in WISOM, unsaturated compounds, such as
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polycyclic aromatic hydrocarbons (PAHs) and oxygenated and nitrated PAHs, have electron-
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conjugated systems and absorb light at near-ultraviolet and -visible wavelengths.26-28 It is
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important to clarify which classes or structures of the organic compounds contribute largely to
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the light absorption of WISOM.
BrC is in both the WSOM and WISOM
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In this study, we employed Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic
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resonance (NMR) spectroscopy and high-resolution aerosol mass spectrometry (HR-AMS) to
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characterize the structures of a WISOM comprised of a wide variety of organics. These
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analytical techniques are suitable to obtain the overall structural information of complex organic
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matter.6 A normal-phase solid-phase extraction (SPE) method was employed to separate the
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WISOM into six subfractions, i.e., non-polar (F1), low-polar (F2 and F3), and medium-polar (F4,
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F5 and F6) fractions. The fractions were quantified, and the characteristic compound classes
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were identified using the above analytical techniques. Furthermore, the light-absorption
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characteristics of the WISOM and its subfractions were investigated using the UV-visible spectra.
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The major organic compound groups contributing to light absorption in the WISOM were
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identified, and their relationship to the chemical structures is discussed.
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EXPERIMENTAL SECTION
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Collection, extraction and separation of aerosol components
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Submicrometer urban atmospheric particles (aerodynamic diameter: