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Light absorption and excitation-emission fluorescence of urban organic aerosol components and their relationship to chemical structure Qingcai Chen, Fumikazu Ikemori, and Michihiro Mochida Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02541 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016
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Environmental Science & Technology
Light absorption and excitation-emission fluorescence of urban organic aerosol components and their relationship to chemical structure
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Qingcai Chen1, Fumikazu Ikemori1,2, Michihiro Mochida*1
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Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan 2
Nagoya City Institute for Environmental Sciences, Nagoya, Japan
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*Corresponding author e-mail:
[email protected] 11
Phone/fax: 052-788-6157
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Address: Department of Earth and Environmental Sciences, Graduate School of Environmental Studies,
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Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
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ABSTRACT: The present study used a combination of solvent and solid-phase extractions to fractionate
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organic compounds with different polarities from total suspended particulates in Nagoya, Japan, and their
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optical characteristics were obtained on the basis of their UV-visible absorption spectra and
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excitation-emission matrices (EEMs). The relationship between their optical characteristics and chemical
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structures was investigated based on high-resolution aerosol mass spectra (HR-AMS spectra), soft ionization
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mass spectra and Fourier transform infrared (FT-IR) spectra. The major light-absorption organics were less
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polar organic fractions, which tended to have higher mass absorption efficiencies (MAEs) and lower
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wavelength dependent Ångström exponents (Å) than the more polar organic fractions. Correlation analyses
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indicate that organic compounds with O and N atoms may contribute largely to the total light absorption and
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fluorescence of the organic aerosol components. The extracts from the aerosol samples were further
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characterized by a classification of the EEM profiles using a PARAFAC model. Different fluorescence
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components in the aerosol organic EEMs were associated with specific AMS ions and with different
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functional groups from the FT-IR analysis. These results may be useful to determine and further classify the
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chromophores in atmospheric organic aerosols using EEM spectroscopy.
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KEYWORDS: Organic aerosol, UV-visible absorption, Excitation-emission matrices, Aerosol mass spectra,
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PARAFAC analysis
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Environmental Science & Technology
1. INTRODUCTION
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Atmospheric aerosols scatter and absorb solar radiation and thereby play an important role in Earth’s
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radiative balance.1 Whereas the light absorption of atmospheric aerosols is dominated by the absorption by
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black carbon (BC),1–4 recent studies have revealed that optical absorption by organics also occurs,
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suggesting that light absorptive organics (i.e., brown carbon, BrC) should be explicitly included in radiative
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forcing models.3, 5 Based on a model simulation, Feng et al. have shown that atmospheric BrC is responsible
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for 7%~19% of the total (global average) aerosol absorption.3 Still, the estimated absorption by BrC has a
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large uncertainty because the amounts and optical properties of BrC, as well as its sources and chemical
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composition, are not well understood.
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Mass absorption efficiency (MAE) as a function of the UV-Vis wavelength and the wavelength
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dependence, as represented by the Ångström exponent (Å) according to a power raw fit, are used to
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characterize aerosol optical properties. The MAEs of the atmospheric aerosols in the visible and UV regions
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increase strongly as the wavelength decreases. Whereas the MAEs of urban and rural aerosols and isolated
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humic-like substances (HULIS) from biomass burning organic aerosol (BBOA) are below 0.1 m2g–1 [OC]
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at >500 nm,6–9 the MAEs at 350–500 nm are as high as 1~3 m2 g–1 [OC].6, 10–13 Some studies have shown
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that the Å values vary in the range of 2–14, depending on the types and origins of the aerosols.6–13 The
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dependence of Å on both the aerosol types and the wavelength suggests that using a single Å value may not
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be sufficient to represent the MAEs of BrC and that an extensive and thorough investigation on the
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variability of Å for BrC is necessary.
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The relationships between the optical properties of organic aerosols and their chemical structures have
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been investigated in several recent studies.14 The MAE values of organics can represent the degree of
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conjugation and the amount of electron delocalization in molecules. Whereas the MAE of secondary organic
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aerosol (SOA) from the oxidation of biogenic and anthropogenic volatile organic compounds with OH at 3
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405 nm increases with the O/C ratio,15 the absorptivity of model organics generated via the photolysis of
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pyruvic acid solutions does not increase monotonically with the oxidation level.16 The contributions of some
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individual organic compounds in the atmosphere to light absorption and the dependence of absorption on
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chemical structure have also been investigated. For instance, Desyaterik et al. have reported that
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approximately half of the measured sample absorption in the region of 300–400 nm are attributable to 16
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major light-absorbing compounds, considered to be nitrophenols and aromatic carbonyls, in
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BBOA-dominated fog water.17 Zhang et al. have shown that the ambient concentrations of eight
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nitro-aromatic compounds correlate with the light absorption of water-soluble BrC at 365 nm; yet these
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compounds account for only 4% of the overall water-soluble BrC absorption.13 Recent studies have
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demonstrated that charge transfer complexes are responsible for approximately 50% of the observed
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absorption by the water-soluble fraction of particulate matter collected in Athens, GA.18 However, their
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overall chemical characteristics remain poorly understood.
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Excitation-emission matrices (EEMs) provide some information on chromophores that are responsible for
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the light absorption of organics. They have been widely applied to organics in terrestrial and oceanic
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systems,19, 20 although they have not been widely used by the atmospheric aerosol community.21–24 The
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humic-like and protein-like components have been statistically determined from analysis of EEMs of
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dissolved organic matter in terrestrial and oceanic systems and also from those of atmospheric aerosols.
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Although the EEMs of natural organic matter in terrestrial and oceanic systems are relatively well
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characterized, these results cannot be applied to organics in the atmosphere because the sources and
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undergoing physical-chemical processes are different.22,
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components and chemical structural characteristics of aerosol organics must be studied further to understand
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the chemical structures of chromophores in the atmosphere.24
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Hence, the relationship between the EEM
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In this study, a combination of solvent extraction and solid-phase extraction (SPE) to fractionate the
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organic material was applied to extract the fractions of organic compounds with different polarities from 4
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total suspended particulates (TSP), and their optical characteristics were obtained on the basis of the
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UV-visible absorption spectra and the EEMs. Further, the relationship of the optical characteristics to their
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chemical structures was investigated based on HR-AMS spectra, soft ionization mass spectra and Fourier
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transform infrared (FT-IR) spectra. The extracts from the aerosol samples were further characterized by a
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classification of the EEM profiles using a PARAFAC model. For the first time in the atmospheric aerosol
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science community, we analyzed the relationship of the relative contents of the EEM components and the
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fluorescence intensity of clusters with the intensity of AMS ion groups and the abundance of functional
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groups determined from the FT-IR analysis. This study is based on the analysis of the chemical structural
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characteristics of the TSP samples in Chen et al. 26
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2. EXPERIMENTAL SECTION
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2.1. Sample collection and preparation
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The details of the collection and the extraction and fractionation of the studied aerosols are described
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elsewhere.25 Briefly, TSP samples were collected on a weekly basis in two periods as follows: from 26 July
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to 20 September 2011 (summer and early autumn) and from 20 December 2011 to 13 February 2012
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(winter) on 8 × 10-inch quartz fiber filters at the Nagoya City Institute for Environmental Sciences, Nagoya,
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Japan. For solvent extraction, 9 cm2 TSP filter punches were used. The water-soluble organic matter
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(WSOM) fraction was extracted by ultrasonication with 3 g of Fluka water (Sigma Aldrich) three times.
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After the extraction, the water-insoluble organic matter (WISOM) was further extracted from the same filter
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punch by ultrasonication with methanol (MeOH) and subsequently with dichloromethane (DCM)/MeOH
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(2/1, v/v). All of the extracts were filtered through 0.2-µm PTFE filters (Millex); the PTFE filter used to
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filtrate the WSM was used again to filtrate insoluble residues in the extracts containing WISOM. Two
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fractions of HULIS showing neutral and acidic natures at the elution steps (HULIS-n and HULIS-a,
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respectively) and high polarity (HP-) WSOM were fractionated from the WSOM fraction using an Oasis 5
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HLB (6cc, 200 mg; Waters) in a manner analogous to that described in Varga et al. and Lin et al.27, 28 The
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recoveries of OC in the WISOM and WSOM were calculated to be 92% ± 5.3% and 102% ± 11% (mean ±
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SD; n = 12) with the application of thermal/optical transmittance (TOT) and reflectance (TOR) methods,
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respectively, for the analysis using the TOC analyzer. The recoveries of the organic mass in the SPE
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procedure were determined to be 106% ± 8% (mean ± SD; n = 12). These results suggest that nearly all of
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the organics were extracted from the TSP samples.26
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2.2. Instrumental analyses
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2.2.1. HR-AMS, FT-IR and ESI-MS analysis
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The details of the chemical analyses and their results are presented elsewhere.26 Briefly, off-line HR-AMS
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measurements were applied to quantify the organic mass and to analyze the ion-group and elemental
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analyses of the extracted matters. The mass concentrations of the organics in the extracts were determined
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by the internal standard method using phthalic acid.29 All of the organic ions were classified into six groups
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as follows: CH, CO (CO + O + HO), CHO1, CHO>1, CHON and CS, and the elemental analyses were
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performed as in Canagaratna et al.30 The concentrations of polycyclic aromatic hydrocarbons (PAHs) in the
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WISOM were also calculated, with minor modifications for the fragment table of organics and PAHs in the
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m/z range of 198 to 479 as in Dzepina et al.31 An FT-IR spectrometer (FT/IR-6100, JASCO) with a diffuse
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reflectance accessory was employed to quantify the chemical functional groups in the extracts. A fully dried
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mixture of the extracts and KBr was ground in an agate mortar and subjected to the FT-IR analysis. Six
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types of functional groups were quantified: aliphatic C–H (saturated and unsaturated), alcoholic C–OH,
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non-acidic carbonyl C=O, carboxylic COOH, amine C–NH2 and organic nitrate C–ONO2. The
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molecular-weight distributions of the fractionated organics were obtained using an electrospray-ionization
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mass spectrometer (ESI-MS). The number-average molecular weight (Mn) estimated from the intensity of
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ions in the negative mode spectra was used in this study. The Mn of the studied organic fractions was also 6
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determined in the positive mode as presented by Chen et al.; 26 the differences of the values (on average
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from −20% to 17% for respective fractions) serve as a guide to estimate the uncertainty.
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2.2.2. UV-visible absorption spectra and EEM fluorescence spectra
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The extract solution in a 1-cm path-length quartz cell was subjected to analysis using a UV-visible
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spectrophotometer (V-570, JASCO) and a fluorescence spectrophotometer (FP-6600, JASCO). The
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UV-visible absorption spectrum in the range from 190 to 800 nm was recorded in triplicate with an interval
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of 0.5 nm. The UV-visible absorption spectra of the solvents were also recorded to subtract their
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contributions from the extract spectra. The EEMs were measured in the range from 235 to 500 nm for
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excitation and from 300 to 600 nm for emission with a scan speed of 600 nm min–1. The wavelength
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increments of the scans for excitation and emission were 5 nm and 2 nm, respectively. The excitation and
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emission slit widths were 5 nm. The EEMs of the solvents for the extracts, water (Fluka, Sigma-Aldrich),
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methanol (MeOH) and dichloromethane (DCM)/MeOH (2/1, v/v) were also recorded and subtracted from
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the extract EEMs. The reproducibility and blank levels of the extraction and analyses are described in the
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Supporting Information (SI). The possible influences of solvents, pH and added ammonia on the optical
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properties of the extracts were assessed, which is explained in the SI.
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2.3. Data analysis
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2.3.1. Analysis of UV-visible absorption spectra
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Based on the light-absorption spectra, the MAEs (m2 g–1) of the organics in the extracts were calculated using MAE(λ ) = ln(10) ⋅ Abs(λ ) / COM ,
(1)
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where Abs(λ) is the light-absorption coefficient (m–1), and COM is the organic mass concentrations in the
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extracts. We assume that the light absorption by extracts were mainly from organic matter. Because the
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cut-off wavelengths of the DCM and MeOH were 230 nm and 210 nm, respectively, the MAEs of the 7
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WISOM, HULIS-n and HULIS-a in Figure 1 were calculated from 700 nm to 230 nm and 210 nm,
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respectively. Because nitrate absorbs light in the wavelength range of
