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Characterization of chromophoric water-soluble organic matter in urban, forest and marine aerosols by HR-ToFAMS analysis and excitation–emission matrix spectroscopy Qingcai Chen, Yuzo Miyazaki, Kimitaka Kawamura, Kiyoshi Matsumoto, Sean C. Coburn, Rainer Volkamer, Yoko Iwamoto, Sara Kagami, Yange Deng, Shuhei Ogawa, Sathiyamurthi Ramasamy, Shungo Kato, Akira Ida, Yoshizumi Kajii, and Michihiro Mochida Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01643 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 12, 2016

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Environmental Science & Technology

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Characterization of chromophoric water-soluble organic

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matter in urban, forest and marine aerosols by HR-ToF-

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AMS analysis and excitation–emission matrix

4

spectroscopy

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Qingcai Chen1, Yuzo Miyazaki 2, Kimitaka Kawamura2,3, Kiyoshi Matsumoto4, Sean Coburn5,6,7,

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Rainer Volkamer5,6, Yoko Iwamoto8,9, Sara Kagami1, Yange Deng1, Shuhei Ogawa1,

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Sathiyamurthi Ramasamy10, Shungo Kato11, Akira Ida10,12, Yoshizumi Kajii10,13, Michihiro

8

Mochida*1

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

1

Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan 3 Now at Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japan 4 Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Kofu 400-8510, Japan 5 Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA 6 Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309-0215, USA 7 Now at Department of Mechanical Engineering, University of Colorado, Boulder, CO 803090215, USA 8 Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan 9 Now at Faculty of Science Division I, Tokyo University of Science, Tokyo 162-8601, Japan 10 Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan 11 Department of Applied Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, Japan 12 Now at Seraku Co. Ltd., Tokyo 160-0023, Japan 13 Center for Regional Environmental Research, National Institute for Environmental Studies, Tsukuba 305-8506, Japan 2

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*Corresponding author e-mail: [email protected]

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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

1 Environment ACS Paragon Plus


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ABSTRACT: Chromophoric water-soluble organic matter in atmospheric aerosols potentially

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plays an important role in aqueous reactions and light absorption by organics. The fluorescence

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and chemical-structural characteristics of the chromophoric water-soluble organic matter in

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submicron aerosols collected in urban, forest and marine environments (Nagoya, Kii Peninsula,

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and the tropical Eastern Pacific) were investigated using excitation-emission matrices (EEMs)

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and a high-resolution aerosol mass spectrometer. Three types of water-soluble chromophores,

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two with fluorescence characteristics similar to those of humic-like substances (HULIS-1 and

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HULIS-2) and one with fluorescence characteristics similar to those of protein compounds

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(PLOM), were identified in atmospheric aerosols by parallel factor analysis (PARAFAC) for

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EEMs. We found that the chromophore components of HULIS-1 and -2 were associated with

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highly and less-oxygenated structures, respectively, which may provide a clue to understanding

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the chemical formation or loss of organic chromophores in atmospheric aerosols. Whereas

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HULIS-1 was ubiquitous in water-soluble chromophores over different environments, HULIS-2

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was abundant only in terrestrial aerosols and PLOM was abundant in marine aerosols. These

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findings are useful for further studies regarding the classification and source identification of

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chromophores in atmospheric aerosols.

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KEYWORDS: chromophores, organic aerosol, chemical characteristics, aerosol mass

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spectrometry, excitation-emission matrices

49 50 51 52


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INTRODUCTION

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Water-soluble organic matter (WSOM) constitutes a large fraction of the organics in

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submicron aerosols in the atmosphere and can affect the role of aerosols in terms of climate

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processes and human health.1-6 Chromophoric water-soluble organic matter is a component of

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WSOM that has the ability to absorb sunlight at visible and near-ultraviolet (UV) wavelengths

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and takes part in photoinitiated reactions. The chromophoric water-soluble organic matter

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therefore potentially plays important roles in some aqueous reactions of organics in aerosols and

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thus in the radiative balance of Earth.7-10 Although the origin, composition and chemistry of

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WSOM in aerosols have attracted great attention over the past decade, the characteristics of the

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chromophoric fraction of water-soluble organic matter are poorly understood. 1-6

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Brown carbon in atmospheric aerosols, which contain chromophoric water-soluble organic

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matter, is primarily emitted from biomass burning and is formed secondarily.11-13 Chromophoric

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organic matter in aerosol can be formed via a variety of reaction pathways. Nitrophenols are

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major chromophores in chamber-generated secondary organic aerosols from toluene

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photooxidation in the presence of NOx.14 The reactions of carbonyl compounds with amines and

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ammonia can form nitrogen-containing chromophoric organics.15-20 In view of its role in the

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atmosphere, an important characteristic of chromophoric organic matter is its sensitivity to light.

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Photoinitiated reactions with chromophoric organic matter include both degradation and

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polymerization.21-23 Substantial photodegradation of chromophoric organic matter in rainwater

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and laboratory-generated solutions containing brown carbon occurs when the matter is exposed

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to simulated sunlight.21,

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excited states through the capture of the excitation energy from photons and thus plays a

24, 25

Chromophoric organic matter has the potential to be in triplet



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significant role in aqueous-phase oxidation of organics.8, 10 The triplet state of 1-nitronaphthalene

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reacts with phenol to form humic-like substances (HULIS).26

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Excitation-emission matrix (EEM) fluorescence spectroscopy is a commonly used technique

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to investigate the optical and structural characteristics of chromophores that are responsible for

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the light absorption and fluorescence by complex organic matter. Although the method has been

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widely applied to the characterization of chromophoric water-soluble organic matter in terrestrial

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and oceanic systems,27,

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Chromophore components associated with HULIS and protein-like organic matter (PLOM) have

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been statistically determined for complex organic matter in terrestrial and oceanic systems as

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well as atmospheric aerosols by parallel factor analysis (PARAFAC) for EEMs. Although

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chromophoric water-soluble organic matter in terrestrial and oceanic systems are relatively well

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characterized by EEMs, the results of the origins and chemical structures of PARAFAC

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components may not apply to organic matter in the atmosphere because the sources and

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underlying physicochemical processes are different.30, 32 The organic chromophores in different

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types of aerosols (e.g., urban, forest and ocean aerosols) may also be different, which must result

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in differences in the optical properties and the photochemical reactivity of the organic

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components. Investigating the relationships between the optical properties and structural

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characteristics of WSOM in different types of aerosols is also important to better understand the

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structures, sources and chemistry of chromophoric water-soluble organic matter in atmospheric

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aerosols.

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it has not been extensively used in atmospheric aerosol research.29-31

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Chromophoric water-soluble organic matter in atmospheric aerosols may contain an array of

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chemical species, as in the case of WSOM.33 For the chemical characterization of such complex

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matter, high-resolution aerosol mass spectrometers (HR-AMSs) can be used in combination with


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the nebulization technique,34,

35

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complex organic matter, e.g., elemental composition and ion groups of the mass spectra. In this

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study, EEMs and HR-AMS analyses were performed for WSOM in urban, forest and marine

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aerosols to characterize the chromophoric water-soluble organic matter therein. Statistical

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analyses of PARAFAC and non-negative matrix factorization (NMF) were applied to EEMs and

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HR-AMS spectra to resolve the different components of WSOM that exhibit different chemical-

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structural and fluorescence characteristics. The chemical structures and possible sources of

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chromophoric water-soluble organic matter in different types of submicron aerosols are analyzed

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and discussed based on hierarchical cluster analysis for the WSOM samples, backward air mass

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trajectory analysis, and correlation analysis using the relative intensity of ion groups from the

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HR-AMS spectra. Application of the results to future studies is suggested.

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EXPERIMENTAL SECTION

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Collection of aerosol samples and WSOM extraction

which provides the overall structural characteristics of the

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This study used sixty-three PM0.95 samples (50% cutoff: 0.95 µm) collected on 8 × 10 inch

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quartz fiber filters using high-volume samplers in three different environments. Twenty-seven

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samples were collected at an urban site in Nagoya, Japan (Nagoya University, 35.15°N,

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136.97°E) from 11 August to 14 September 2013 (sample ID: 1–17), and from 4 to 24 March

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2014 (sample ID: 18–27). Sixteen samples were collected at a forest site on the Kii Peninsula,

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Japan (Wakayama Forest Research Station (WFRS), Kyoto University, 34.06°N, 135.52°E), five

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from 20 to 30 August 2010 (sample ID: 28–32), and eleven from 28 July to 8 August 2014

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(sample ID: 33–43). Twenty samples were collected aboard the NOAA RV Ka’imimoana as part

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of the Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOCs

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(TORERO) field experiment over the eastern equatorial Pacific ocean during the TORERO/KA-



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12-01 cruise in February 2012 (sample ID: 44–63). Blank filters were also collected at different

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sites and periods. More information on the samples and the environment of the sampling sites are

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given in Table S1 and section S2 of the Supporting Information (SI), respectively.

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WSOM for the EEM analysis and HR-AMS analysis was extracted via the ultrasonication of

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filter punches (diameter: 34 mm) with Milli-Q water for 1 h, followed by filtration through 0.2

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µm PTFE filters (Millex). Different extraction conditions, i.e., the number of filter punches

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(diameter: 34 mm) and the mass of Mill-Q water, were applied to different samples, as

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summarized in Table S1. Quantification of WSOC was performed using a TOC analyzer (Model

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TOC-VCHS, Shimadzu, Japan), as detailed in the SI (section S3). Analyses of amino acids and

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biogenic secondary organic aerosol (BSOA) tracers were also performed for urban and forest

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samples, respectively (see section S3, SI).

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HR-AMS measurements

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Each extract was atomized with argon (purity: 99.9999%). The generated aerosol was passed

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through two diffusion scrubbers to remove water vapors. Then, the aerosol was introduced to the

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HR-AMS (HR-ToF-AMS, Aerodyne Research Inc., Billerica, MA, USA). The HR-AMS spectra

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were acquired in V-mode, and the data were analyzed using Squirrel v.1.56A and Pika v1.15A

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Software (http://cires.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware/). In the

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analysis using Pika, all of the CO+ ion signals were considered to have originated from organics

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because pure argon was used as the carrier gas (Figure S1a). All of the organic ions were

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classified into CH, CHN, CHO1, CHOgt1, CHO1N, CHOgt1N, CS, Cx, HO, CO, and CO2 groups.

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Elemental analyses were performed to determine the atomic ratios (O/C, H/C, and N/C), and the

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mass ratio of organic matter (OM) to organic carbon (OC). N/C was derived according to Aiken

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et al,36 and O/C, H/C, and OM/OC were derived according to Canagaratna et al.37


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UV-visible absorption spectra and EEM fluorescence spectra The extracts were placed in a 1-cm path-length quartz cell and subjected to analysis using a

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UV-visible

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spectrophotometer (Model FP-6600, JASCO, Japan). A UV-visible absorption spectrum was

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recorded in the range of 200 to 800 nm. Before the scans for a series of extracts, Milli-Q water

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was also analyzed for a baseline correction. The mass absorption efficiency (MAE, m2 g–1 OC)

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of the organics in the extracts at λ nm (MAEλ) was calculated using the following equation: 38

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spectrophotometer

(Model

V-570,

JASCO,

Japan)

and

MAEλ = Absλ ln(10)/CWSOC,

a

fluorescence

(1)

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where Absλ is the light absorption coefficients (m–1) of the extract solutions at λ, and CWSOC is the

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concentration of water-soluble organic carbon (WSOC, g m-3) in the extracts. Because the MAE

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of the extracts from the forest and marine samples were low in the visible range (Figure S2), the

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MAE at 266 nm (MAE266) was used as a measure of absorptivity of the extracts.

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The EEMs were measured in the range of 200 to 400 nm for excitation and in the range of 250

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to 650 nm for emission at a scan speed of 1000 nm min–1. The band-passes for excitation and

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emission were both set as 10 nm.22 The EEMs were calibrated according to the procedures

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described by Lawaetz and Stedmon, and Murphy. 39, 40 The EEMs were normalized according to

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the Raman peak of water, so the data are reported in Raman units (RU).39 The fluorescence

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index, biological index, and humification index, which are the ratios of the fluorescence intensity

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at excitation/emission wavelengths (Ex/Em) of 370/470 nm to that at 370/520 nm, the intensity

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at 310/380 nm to that at 310/430 nm, and the intensity at 255/434–480 nm (average of the values

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on the line between 255/434 and 255/480 nm) to that at 255/300–344 nm (average of the values

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on the line between 255/300 and 255/344 nm), respectively, were obtained from the EEMs.41, 42

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To compare the fluorescence efficiency of different water-extract samples, the fluorescence



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intensity over excitation wavelengths of 200–400 nm and emission wavelengths of 250–650 nm

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were normalized by the OC concentration of the extracts (abbreviated to NFV, RU-nm2 [mg L−1

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OC]–1).

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Factor analysis for EEMs and HR-AMS spectra

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PARAFAC analysis with non-negativity constraints of EEMs was performed with the drEEM

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toolbox version 0.2.0 for MATLAB (http://www.models.life.ku.dk/dreem).27 For the analysis,

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the EEM data at the excitation wavelengths longer than or equal to those on the line between

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Ex/Em of 200/570 nm and that of 235/650 nm were omitted. The three-component PARAFAC

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model was selected from two- to ten-component PARAFAC models because the residual errors

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decreased noticeably when the number of components increased from two to three (Figure S3).

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In addition, the three-component model has reasonable spectra of fluorescent components, and it

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passes the assessment of a split-half analysis with the split style of ‘S4C6T3’ based on the

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PARAFAC tutorial (Figure S4).27 The seven-component PARAFAC solution, which also

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exhibits a noticeable decrease in residual errors with the increase in the number of components

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among four- to ten-component PARAFAC solutions (Figures S3), was used to obtain more

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detailed information on the chromophores. The seven-component PARAFAC solution passes the

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split analysis with the split style of ‘S3C3T3’ (Figure S5), indicating that the solution is stable.

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Non-negative matrix factorization (NMF) analysis was performed for the HR-AMS dataset

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using the NMF toolbox version 1.4 in MATLAB (https://sites.google.com/site/nmftool/).43 First,

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a gradient-descent-based multiplicative algorithm was used to find solutions from multiple

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random starting values, and then a least-squares active-set algorithm was used to find final

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solutions from the first solutions. To find a final solution, the model was run 100 times with a


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different starting point each time. The four-factor NMF model was validated by comparisons of

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residual errors, split analysis and visualization of spectral loadings (Figures S6, S7 and S8).

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Hierarchical cluster analysis

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A hierarchical cluster method was performed to classify aerosol samples based on the relative

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contributions of PARAFAC components and NMF factors to the respective samples. The

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Euclidean distance was used to evaluate the distances between samples, and the centroid method

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was chosen for the hierarchical cluster analysis by comparing the multiple correlation

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coefficients for nearest neighbor, farthest neighbor, median neighbor and Ward’s method. Six

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clusters were determined for EEMs and HR-AMS data and were named clusters a-f and clusters

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A-F, respectively (Figures S9 and S10).

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RESULTS AND DISCUSSION

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EEMs and PARAFAC components

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To identify and quantify the underlying chromophore components in aerosols, the PARAFAC

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model was applied to separate independently varying fluorescent components. Figure 1 presents

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the EEMs of the three- (3CM-C1~3) and seven- (7CM-C1~7) component PARAFAC solutions.

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The EEMs of 3CM-C1~3 are similar to those identified from water and alkaline soluble matter in

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urban aerosols in Granada, Spain and in Aveiro, Portugal.29, 30 Based on a comparison with the

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EEMs of three fluorescent components defined for terrestrial and oceanic organic matter by Yu

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et al., 28 the chemical components corresponding to 3CM-C1, -C2 and -C3 are named HULIS-1,

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HULIS-2 and PLOM, respectively, in this study. Note that the origins and chemical structures of

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HULIS and PLOM chromophores studied are not necessarily similar to those of the



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chromophores with same names in terrestrial and oceanic organic matter. The origins and

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chemical structures of the chromophores in atmospheric aerosols are discussed below.

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The seven-component PARAFAC solution provides more detailed information on the

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chromophores. By comparing the emission wavelengths with the maximum fluorescence of the

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three-component PARAFAC solution and based on a correlation analysis of the relative

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abundance of the three and seven components (Table S2), chemical components corresponding

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to 7CM-C1 and 7CM-C3 are assigned to HULIS-1, whereas 7CM-C2, 7CM-C4 and 7CM-C5 are

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assigned to HULIS-2, and 7CM-C6 and 7CM-C7 are assigned to PLOM. Among the seven

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components, 7CM-C1, 7CM-C3 and 7CM-C5 have peaks at emission wavelengths of >400 nm

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(Figure 1); these emissions probably originate from conjugated systems, e.g., via quinonoid n-π*

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transitions.44 This claim is supported by the result that the emission wavelength of 7CM-C3

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(~470 nm) is similar to that of the quinone-like component identified in aerosol WSOM in

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Granada, Spain.29 Based on the definition of fluorescent components of terrestrial and oceanic

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organic matter, 7CM-C6 and 7CM-C7 correspond to tryptophan-like and tyrosine-like

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components, respectively.27,

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major portion of the chromophores of 7CM-C6 and 7CM-C7 in atmospheric aerosols, as

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discussed below.

28

However, non-nitrogen-containing species may account for a

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The relative intensities of different fluorescent components in different types of aerosols were

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highly variable depending on the sampling site and period, as shown in Figure 2a. This result

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suggests that the chemical composition of chromophoric water-soluble organic matter differs

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markedly from sample to sample. HULIS-1was the substance with most intense fluorescence in

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all samples, and, on average, accounted for 46% of the total fluorescence intensity of all samples.

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High mass fractions of HULIS were observed in an area similar to that of the marine aerosol


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sampling, supporting the particularly intense fluorescence of HULIS-1 in the marine aerosols

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studied.47 The fluorescence of HULIS-2 was intense in the terrestrial aerosols (urban and forest

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aerosols) (mean: 47% of the total fluorescence intensity of the extracts), particularly in forest

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aerosols (mean: 51%). Conversely, the fluorescence of HULIS-2 in marine aerosols was weak

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(mean: 23%). This result suggests that HULIS-2 primarily has a terrestrial origin. The marine

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aerosols were enriched in the PLOM (mean: 19%), which can be attributed to marine biological

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materials.28, 46 The fluorescent components assigned to PLOM were also present in the urban

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aerosols, among which the fluorescence of tryptophan-like substances (7CM-C6) was the

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strongest. Accordingly, these aerosols differ from the marine aerosols, in which the relative

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strengths of the fluorescences of tryptophan-like and tyrosine-like components (7CM-C7) were

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nearly equal. This result indicates that PLOM in urban and marine aerosols can be attributed to

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different chemical structures and sources.

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HR-AMS spectra and the NMF factors

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The chemical characteristics of WSOM in aerosols were obtained on the basis of the HR-AMS

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spectra of the extracts in various samples. Four factors were determined from the HR-AMS

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spectra of the NMF model. The mass spectra and relative abundances of these factors in different

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types of aerosols are presented in Figures 3 and 2b, respectively. Based on the degree of

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oxidation (O/C ratio) of organics, the four factors were identified as highly oxygenated organic

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aerosols (HOOA-1 and HOOA-2) and low oxygenated organic aerosols (LOOA-1 and LOOA-2).

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Note that each organic aerosol may represent a group of molecules and/or their moieties.

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The HOOA-1 factor has a high O/C ratio (1.67) and a low H/C ratio (1.1). Approximately half

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of the total HOOA-1 signal was contributed by CO2+ ions, suggesting a large fraction of

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carboxylic acids in HOOA-1.37, 47 The mass fraction of HOOA-1 in the marine aerosols (mean:



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48%) was obviously higher than those in the urban (mean: 20%) and forest aerosols (mean: 9%),

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indicating that WSOM in marine aerosols was dominated by more processed oxygenated

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products.

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The HOOA-2 factor contains abundant CS ions (16%), primarily CHSO+ (m/z 61), CH2SO+

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(m/z 62), CH2SO2+ (m/z 78), CH3SO2+ (m/z 79) and CH4SO3+ (m/z 96) ions (Figures 3 and S1).

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Among these ions, the most abundant CS ion in the mass spectrum of HOOA-2 is CH2SO2+,

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which likely originated from methanesulfonic acid (CH3SO3H, MSA).48, 49 HOOA-2 contributes

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largely to the WSOM of marine aerosols (mean: 30%) and slightly to the WSOM of the urban

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(mean: 2%) and forest samples (mean: 5%). The relative abundance of HOOA-2 is particularly

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high in marine samples collected over the eastern tropical Pacific Ocean in the Southern

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Hemisphere (mean: 46%), which is consistent with the high mass concentration ratios of MSA to

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WSOC.50 The above results suggest that HOOA-2 should be representative of the WSOM of

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marine aerosols. The characteristics of the low abundances of CxHyOz+ (10%) ions and high

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abundances of CxOy+ (31%) and CxHy+ (31%) ions in HOOA-2 differ from those of the PMF-

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derived marine organic aerosols in Paris, France (CxHyOz+, CxOy+ and CxHy+ were 41%, 17% and

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32%,

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difference may be because marine aerosols are enriched in water-insoluble organic matter

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(WISOM), which may consist of hydrophobic sugar-like compounds.52 In fact, the WSOC/TC

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ratio was low (mean: 0.4), thus supporting the high abundance of WISOM in the marine

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aerosols.50

respectively)

(URL:

http://cires.colorado.edu/jimenez-group/HRAMSsd/).51

This

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The LOOA-1 factor was dominated by CxHy+ (38%) and CxHyOz+ (24%) ion groups. In

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contrast, CO2+ was very low in the LOOA-1 spectra, but abundant CO+ (15%) was observed.

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This result suggests that LOOA-1 is associated with alcohols and ketones or aldehydes, but not


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with carboxylic acids and carboxylate esters.37 One of the remarkable characteristics of LOOA-1

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in comparison with the other factors is the higher mass fraction of the CxHyNq+ ion group (6%),

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suggesting that nitrogen-containing compounds were abundant. The relative abundances of

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LOOA-1 in certain urban aerosol samples, particularly in the late winter and early spring

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samples (mean: 47%), were obviously higher than those in the forest and marine aerosol samples

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(Figure 2b). Hence, LOOA-1 is probably anthropogenic.

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The mass spectrum of the LOOA-2 is largely composed of CxHy+ (44%) and CxHyOz+ (31%)

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ion groups and is characterized by the higher signal of C2H3O+ (11%). The profile of the LOOA-

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2 factor was similar to that of the factor of semi-volatile oxygenated organic aerosols,

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determined on the basis of the PMF analysis of the online HR-AMS data at the WFRS forest site,

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which was interpreted as locally formed BSOA.53 The relative abundance of LOOA-2 was

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positively correlated with the ratios of total OC concentrations of BSOA tracers (supporting

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information S3) to WSOC in the forest samples (r = 0.80, p

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