Speciation and Sources of Brown Carbon in Precipitation at Seoul

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Speciation and Sources of Brown Carbon in Precipitation at Seoul, Korea: Insights from Excitation-Emission Matrix Spectroscopy and Carbon Isotopic Analysis Ge Yan, and Guebuem Kim Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02892 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Environmental Science & Technology

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Speciation and Sources of Brown Carbon in Precipitation at Seoul, Korea: Insights

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from Excitation-Emission Matrix Spectroscopy and Carbon Isotopic Analysis

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Ge Yan*,† and Guebuem Kim*

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School of Earth & Environmental Sciences/RIO, Seoul National University, Seoul

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151-747, South Korea

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† Present Address: Department of Marine Sciences, Texas A&M University Galveston

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Campus, Galveston, Texas 77553, United States

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Corresponding Authors

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*G.Y. e-mail: [email protected].

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*G.K. e-mail: [email protected]. Phone: +82 2 880 7508.

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ABSTRACT: Brown carbon (BrC) plays a significant role in the Earth’s radiative bal-

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ance, yet its sources and chemical composition remain poorly understood. In this work,

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we investigated BrC in the atmospheric environment of Seoul by characterizing dissolved

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organic matter in precipitation using excitation-emission matrix (EEM) fluorescence

30

spectroscopy coupled with parallel factor analysis (PARAFAC). The two independent

31

fluorescent components identified by PARAFAC were attributed to humic-like substance

32

(HULIS) and biologically-derived material based on their significant correlations with

33

measured HULIS isolated using solid-phase extraction and total hydrolyzable tyrosine.

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The year-long observation shows that HULIS contributes to 66 ± 13% of total fluores-

35

cence intensity of our samples on average. By using dual carbon (13C and

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analysis conducted on isolated HULIS, the HULIS fraction of BrC was found to be pri-

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marily derived from biomass burning and emission of terrestrial biogenic gases and parti-

38

cles (>70%), with minor contributions from fossil-fuel combustion. The knowledge de-

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rived from this study could contribute to the establishment of a characterizing system of

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BrC components identified by EEM spectroscopy. Our work demonstrates that, EEM

41

fluorescence spectroscopy is a powerful tool in BrC study, on the basis of its chromo-

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phore resolving power, allowing investigation into individual components of BrC by oth-

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er organic matter characterization techniques.

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C) isotopic

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TOC ART

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48

INTRODUCTION

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While atmospheric organic aerosol mostly exerts a cooling effect on the climatic system

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by scattering solar radiation, a portion is capable of absorbing light in the ultraviolet and

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lower visible spectral regions.1-3 The compounds in this fraction exhibit strong absorption

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wavelength dependence and appear brownish or yellowish, collectively referred to as

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“brown carbon” (BrC).4 There is a growing weight of evidence suggesting that BrC is

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ubiquitously present in the atmosphere and contributes substantially to aerosol radiative

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forcing,5-7 which is thought to be one of the largest uncertainties in current climate mod-

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els.8 In addition to direct absorption of radiation, BrC could indirectly perturb the Earth’s

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radiative balance by influencing cloud formation, albedo, and lifetime.9 It has been found

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that BrC can be formed either as primary aerosol through burning of biomass, combus-

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tion of fossil-/bio-fuels, and emission of biological particles, or as secondary aerosol from

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anthropogenic and biogenic volatile organic compounds via gas-to-particle conver-

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sions.4,10-15

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Owing to the inherent complexity of compounds that make up organic aerosols, the

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chemical constituents responsible for BrC absorption remain largely unknown.9 Though

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not characterized at the molecular level, a significant fraction of BrC has been attributed

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to humic-like substances (HULIS),9,16,17 which is named for its similar properties to those

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of humic substances found in aquatic and terrestrial environments.18 HULIS is an opera-

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tionally defined complex mixture of heterogeneous compounds extracted from wa-

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ter-soluble organic matter present in atmospheric aerosols, fog, rain, and clouds. The ma-

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jor formation pathways for HULIS include direct emission from biomass burning, sea

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spray, and soil resuspension, as well as secondary oxidation of gaseous biogenic and an-

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thropogenic precursors followed by oligomerization or polymerization and atmospheric

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aging16,18,19 (and references therein). In addition to HULIS, another important type of

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chromophores comprising BrC is associated with biological aerosols, which refer to or-

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ganisms and their fragments or excretions (e.g., viruses, bacteria, algae, fungal and fern

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spores, pollen, plant debris, and animal dander) emitted from the biosphere to the atmos-

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phere through various mechanisms.20,21 These ubiquitous biological aerosols have been

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shown to contain several optically active compound classes, including amino acids, co-

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enzymes and vitamins, structural polymers, pigments, and secondary metabolites.20


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Measurement of BrC is generally conducted by using analytical techniques developed

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based on its light-absorption properties. It is expected that the compounds accounting for

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BrC absorption have the same molecular characteristics as those of fluorescent

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(i.e., a high degree of conjugation across the molecular skeleton and large-absorption

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sections).9 Therefore, BrC chromophores could potentially act as efficient fluorophores,

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which makes fluorescence spectroscopy a promising indirect tool for BrC measurement.

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Although not yet widely employed, the existing literature suggests that fluorescence is

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not only a sensitive probe of BrC but also is sensitive to the molecular (or supramolecular)

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identity of BrC compounds9 (and references therein). Among the fluorescence-based

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techniques,

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three-dimensional excitation-emission matrix (EEM) spectroscopy, in which fluorescence

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intensities are given as a function of both excitation and emission wavelength. In combi-

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nation with the multivariate data analysis technique, parallel factor analysis (PARAFAC),

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EEM spectral signature can be decomposed into independent underlying components –

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groups of similar fluorophores (or chromophores) of BrC,22,23 which are generally not

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distinguished in absorbance-based measurements. Nevertheless, applications of EEM

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fluorescence spectroscopy in BrC studies are rather limited.9 In addition, the unambigu-

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ous identification of fluorophores in previous studies was often hindered by the lack of a

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classification system for fluorescent components in the atmospheric environment re-

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solved by EEM.

liquid-phase

measurements

were

commonly

conducted

using



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Although being investigated extensively through field observations and laboratory

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measurements over the past decade, origins, chemical identity, atmospheric evolution,

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formation mechanisms, and optical properties of BrC remain largely unknown, hindering

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our understanding of its potentially significant impact on the Earth’s climate.9 In this

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work, we investigated the speciation of BrC in precipitation samples collected at Seoul by

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using EEM fluorescence spectroscopy coupled with PARAFAC analysis. Study on pre-

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cipitation could provide a comprehensive view of BrC chromophores within the atmos-

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pheric boundary layer, since precipitation incorporates BrC suspending in the atmosphere

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(i.e., associated with aerosols) as well as residing in clouds through below-cloud and

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in-cloud scavenging. Based on the relevance of isolated HULIS with fluorescing compo-

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nents identified by EEM spectroscopy, the potential sources for HULIS fraction of BrC

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were also explored using stable (13C) and radioactive carbon (14C) isotopic analyses on

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HULIS.

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EXPERIMENTAL SECTION

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A one-year sampling campaign (from March 2012 to February 2013) was conducted at

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Seoul (37.5°N, 127°E), which is a typical Asian metropolis located in the midwestern

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part of the Korean peninsula. The prevailing wind over Seoul blows from land (especially

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mainland Asia) for most of the year except summer, during which air masses originate 6 ACS Paragon Plus Environment

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dominantly from the Pacific Ocean.24 Precipitation samples were collected on an event

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basis on a four-storey building rooftop at the Gwanak campus of Seoul National Univer-

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sity using a home-made sampler, which consists of a polypropylene funnel (dia. 250 mm)

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connected to a high-density polyethylene (HDPE) bottle placed in a covered bucket via

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Tygon FEP-lined tubing. The apparatus was acid-cleaned and manually deployed prior to

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onset of precipitation events and retrieved after cessation. The contribution by dry depo-

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sition was minimized by restricting exposure to dry conditions (i.e., < 1h for daytime

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events or ≤ 5h for events end at night). The collected samples were transferred to a lami-

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nar flow clean room and filtered through pre-combusted (5h at 500 °C) Whatman 0.7µm

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GF/F glass fiber filters.

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Immediately after collection, EEM spectra were obtained using a Scinco (FS-2) spec-

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trofluorometer for excitation wavelength from 210 to 500 nm at 5-nm increments and

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emission wavelength from 220 to 550 nm at 2-nm increments (excitation and emission

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slit widths at 10 nm and integration time of 50 ms). The EEM results were not corrected

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for inner filter effects, which were found to be negligible (Table S1). Raman scattering

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peaks in EEM spectra were eliminated by subtracting the Milli-Q water blank. The fluo-

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rescence intensities were calibrated against the signal of quinine sulfate dihydrate stand-

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ard at Ex/Em = 350/450 nm and reported in quinine sulfate units (QSU). The EEM data

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were modeled with PARAFAC analysis using the DOMFluor toolbox in MATLAB with

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non-negativity constraints for two to eight components.23 The two component model was 7 ACS Paragon Plus Environment


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found to best fit the data set, through verifying these models using split-half and residual

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analyses, random initialization, and visual examination of the component spectra to en-

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sure they are representative of plausible fluorophores.23 Total dissolved hydrolyzable

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amino acids in our samples were analyzed by using reverse-phase high performance liq-

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uid chromatography (HPLC) on a Waters 2695 HPLC system equipped with an Alltech

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Alltima HP C18 column and a Waters 2475 fluorescence detector. The detailed analytical

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protocols for amino acids were described in our previous study.25

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HULIS in precipitation samples was isolated by solid-phase extraction using DEAE

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resin (a weak anion exchanger) following the method described in Baduel et al.26 Briefly,

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the sample was loaded onto a DEAE column (GE Healthcare®, HiTrapTM DEAE FF, 0.7

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cm ID × 2.5 cm length) without any pre-treatment. After an initial elution with 12 mL

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0.04 M NaOH to remove neutral compounds, hydrophobic bases, inorganic anions, and

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mono-/di-carboxylic acids, HULIS (polyacidic compounds) retained on the column was

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eluted using 4 mL high ionic strength 1 M NaCl solution. The organic carbon content of

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extracted HULIS was then quantified by high-temperature catalytic oxidation (HTCO)

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using a Shimadzu TOC analyzer (Model TOC-VCPH/CPN). Stable carbon isotopic compo-

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sitions (δ13C) of isolated HULIS were determined by using an isotope ratio mass spec-

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trometer (Isoprime) coupled with a Vario TOC Cube analyzer.27 The values were report-

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ed in standard delta notation (δ) as per mil (‰) deviation from the Pee Dee Belemnite

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(PDB) standard. The quality of δ13C data was assured by measurements of the reference 8 ACS Paragon Plus Environment

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standards sucrose (-10.5‰) and Suwannee River Fulvic Acid (-27.6‰) in the sample run.

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For radiocarbon (14C) analysis of HULIS, individual samples from each season were

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combined into one composite before extraction (to obtain enough amount of organic car-

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bon) in a manner that the proportion of individual sample reflects total deposited dis-

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solved organic carbon amount during that precipitation event. After being lyophilized us-

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ing a freeze-dryer, the dehydrated products containing HULIS extracted from four com-

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posite samples were shipped to Beta Analytic Radiocarbon Dating Laboratory (Miami,

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Florida), where they were combusted and CO2 generated was converted to graphite for

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analysis using accelerator mass spectrometry. The 14C results are expressed as fraction of

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modern carbon (fM), which is defined as

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the reference year 1950.28 All of the reported fM values were corrected for δ13C fractiona-

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tion as well as for 14C decay during the period between 1950 and the year of measure-

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ment.

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RESULTS AND DISCUSSION

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Two independently varying fluorescent components in DOM of collected rainwater sam-

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ples were resolved using PARAFAC modeling (Table 1). Component 1 (C1) has an exci-

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tation maximum around 300 nm and an emission peak around 408 nm. The maximum

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fluorescence intensity of Component 2 (C2) occurs at 275 nm/296 nm (excita-

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tion/emission). The intensities ranged from 0.6 to 22.2 QSU for C1 and from 0.5 to 9.9

14

C/12C ratio of a sample normalized to that of



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QSU for C2, respectively (Table S2). Rather similar seasonal variations were shown for

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these two chromophores, with relatively higher intensities observed in spring (Mar to

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May), fall (Sep to Nov), and winter (Dec to Feb) and lower intensities in summer (Jun to

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Aug) (Figure 1). This distribution pattern can be partially attributed to the precipitation

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regime of Korea, of which the extraordinarily high precipitation amount in summer di-

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lutes the fluorescence intensity. Source regions could also contribute significantly to the

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much lower intensities observed during summer, since the prevailing air masses originat-

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ing from ocean in summer are relatively more pristine than those derived from continent

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for the rest of the year.24 In addition, removal of these chromophores through photo-

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chemical processing (e.g., direct photolysis and OH radical oxidation)29 might be en-

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hanced by intensive solar radiation in summer, as being demonstrated by Kieber et al.30

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using laboratory experiments with simulated mid-summer sunlight. Nevertheless, the un-

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derlying mechanism for this pattern is still ambiguous and further work on this regard is

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thus warranted. It is worth noting that the extent of C1 intensity variation is about 2-fold

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greater than that of C2, suggesting that the strength of input source and/or removal pro-

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cess for C1 is relatively more variable during the sampling period. Of the two chromo-

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phores identified, C1 was found to be the dominant species in most of the samples (Fig-

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ure 1), representing on average 66 ± 13% of total fluorescence intensity. The small varia-

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tion in the relative contributions of these two components suggests minor compositional

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changes of fluorescent DOM in our samples. 10 ACS Paragon Plus Environment

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The EEM spectral location of C1 is similar to that of “peak M” identified in aquatic

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and terrestrial system, which is characterized as microbially processed HULIS; whereas

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C2 matches closely with the classically defined “peak B”, resembling spectral features of

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the amino acid tyrosine.31-33 However, these classifications on EEM fluorescence spectra

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employed by previous studies might not be applicable to organic matter in the atmosphere,

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since their sources, chemical composition, and transformation pathways are different

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from those in terrestrial and aquatic environments.25,34,35 Table 1 summarizes the atmos-

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pherically-relevant fluorophores identified in the literature which are associated with

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EEM spectral characteristics similar to those of C1 and C2. Although the chromophore

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corresponding to C1 is commonly linked to atmospheric HULIS, certain coenzyme (i.e.,

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pyridoxine) and polycyclic aromatic hydrocarbons (i.e., phenanthrene) could exhibit sim-

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ilar fluorescence features. C2 was consistently assigned to organic matter containing flu-

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orescing aromatic amino acid tyrosine in the atmosphere. Nevertheless, the spectral char-

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acteristics used for qualitative investigation of fluorescent components are subject to in-

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fluence of the local environment (e.g., pH value and interactions with other molecules),43

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atmospheric conditions (e.g., humidity and temperature)20,44,45 and physicochemical aging

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processes,43,46,47 which result in the variation of excitation/emission spectra observed in

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previous studies. As such, the potential uncertainty of fluorophore spectral profile and the

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presence of interfering species could influence the identification of PARAFAC compo-

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We compared excitation and emission loadings between C1 and HULIS extracted

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from our samples, and between C2 and dissolved pure free tyrosine (Figure 2A,B). The

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spectral characteristics of C1 matched closely with those of HULIS, whereas discrepan-

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cies in peak locations were observed between C2 and tyrosine, with a shift of over 20 nm

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in emission maximum. The blue shift in emission of C2 with respect to that of free tyro-

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sine solution might be mainly due to the fact that amino acids in the atmosphere are

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mostly bounded in larger organic molecules (e.g., protein) rather than residing in free

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form,48,49 as the fluorescence characteristics of amino acids are known to be influenced

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by the proximity of other residues.50 Nevertheless, other changes in local environment

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such as solvent polarity and pH may also contribute to the spectral shift observed here.51

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We further compared fluorescence intensities of C1 and C2 with concentrations of HU-

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LIS and total hydrolysable tyrosine in collected rainwater samples (Figure 2C,D). The

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strong positive correlations obtained confirmed our speculations on identify of C1 and C2

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that fluorescence of C1 is derived from HULIS and C2 is associated with tyro-

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sine-containing biological aerosols. The significant positive intercept on X-axis repre-

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senting HULIS concentrations in Figure 2C suggests that a portion of HULIS is

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non-fluorescent. It is concluded that organic matter in the atmosphere of Seoul mainly

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contains two types of fluorophores – HULIS and biologically-derived material, with HU-

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LIS as the dominant species. This pattern was consistently observed in the atmospheric


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environments at other locations in previous studies, and the chromophore corresponding

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to C1 was often found to be the most abundant species.38,39,52,53

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While it is explicit that tyrosine-like C2 is of biological origin, the source of C1 or

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HULIS is rather ambiguous. We employed stable carbon (13C) isotope analysis to explore

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the source of HULIS extracted from our samples. The δ13C values varied from -28.1 to

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-21.2‰ with an average of -25.4 ± 1.6‰ (Figure 3, Table S2), which is in good agree-

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ment with that reported for HULIS (-25.7 ± 0.3‰) isolated from aerosols in Guangzhou,

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China.54 The exceptionally high δ13C signature observed on July 4 is likely due to the in-

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clusion of significant amount of

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-18‰)55,56 in that precipitation sample, which often occurs in summer at our study site.24

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C4 plants are also characterized by relatively heavier δ13C (-19 to -9‰).57 However, its

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contribution to HULIS in our samples is expected to be insignificant, since C4 type plants

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(e.g., maize and sugar cane) are rare in Korea. The δ13C values for HULIS obtained in

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this study generally fall within the range of isotopic compositions for C3 type vegetation

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(-37 to -20‰),57,58 burning of biomass (e.g., fuelwood and crop-residue) derived from C3

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plants (-35 to -24‰),59-62 and combustion of gaseous (e.g., natural gas), liquid (e.g., gaso-

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line and diesel), and solid (e.g., coal) fossil-fuels (-32 to -21‰),61-64 indicative of the ter-

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restrial biomass and fossil origins for HULIS. Nevertheless, it is rather difficult to distin-

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guish the contributions between these sources based solely on stable carbon isotopic data,

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due to their overlapping and poorly-constrained δ13C signatures. In addition, the δ13C 13

13

C enriched marine-derived organic carbon (-22 to

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isotopic ratio has been found to be subject to significant influences of atmospheric pro-

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cessing (e.g., photochemical aging and secondary formation),65-68 resulting in large un-

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certainties in δ13C-based source attribution.

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Radiocarbon (14C) analysis has been shown to be a powerful technique for quantita-

260

tively differentiating contemporary (14C abundance comparable to that of atmospheric

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CO2) versus fossil (devoid of

14

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phere.68-70 Due to the excess

C produced by the nuclear bomb tests in the 1950s and

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1960s,70,71 the fraction of modern carbon (fM) for contemporary (non-fossil) samples with

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reference to the year 1950 can be overestimated and greater than 1 (e.g., 1.16 for 30－

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50-year-old trees and 1.05 for biogenic emission and recent-grown biomass).72 Assuming

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non-fossil carbon in isolated HULIS was mostly (70%) derived from recently produced

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biomass, a normalization factor of 1.08 was employed to convert the reported fM values to

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fraction of contemporary carbon (fC) for HULIS.73 The calculated fC values are therefore

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subjected to uncertainties introduced by the variabilities in fM of the two end members of

270

contemporary organic carbon and their relative contributions. By applying extreme values

271

of normalization factor used in the literature as well as two end member fM values (corre-

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sponding to scenario that non-fossil carbon entirely stems from old trees or from fresh

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biomass), the uncertainty of fC calculated is estimated to be within 6% (Table S3). The fC

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values were 0.72 for spring, 0.70 for summer, 0.92 for fall, and 0.70 for winter (Figure 3),

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respectively, indicating that HULIS in our samples was mostly derived from non-fossil 14

14

C) contributions to carbonaceous matter in the atmos-


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sources (i.e., emission of biogenic gases and particles and biomass burning). The fC val-

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ues for HULIS derived from this study are higher than those found for HULIS extracted

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from aerosols in an urban area of Southern China (i.e., Guangzhou), which ranged from

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0.51 to 0.65 with an average of 0.58.54 These radiocarbon based source apportionment

280

results indicate the dominant contributions from contemporary carbon sources to HULIS

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even in regions significantly impacted by fossil-fuel combustion, such as Seoul and

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Guangzhou. The remarkably high fC of HULIS in fall probably reflects the enhanced con-

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tribution from contemporary carbon sources rather than reduced emissions from fos-

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sil-fuel combustion, since it is unlikely that fossil-fuel consumption in fall is significantly

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(~70%) lower than in rest of the year. Further, the high contemporary fraction of HULIS

286

can be linked to the intensive biomass burning activities, as we didn’t expect particularly

287

higher biogenic contribution in fall than in other seasons. Indeed, the samples collected in

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fall might be subject to significant influences of open burning of agricultural waste,

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which is known to be a common practice (especially in harvest season) in Korea (e.g.,

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rice) as well as in northern and northeastern China (upwind of Korea) (e.g., rice, maize,

291

and millets).74-77 In addition, wildfires may also contribute to the exceptionally high con-

292

temporary fraction of HULIS, since the Korea Peninsula has been shown to be impacted

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by Siberian forest fire via long-range atmospheric transport, which was reported to be the

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most severe in 2012 over the last decade.78,79 Taken together, it is concluded that hu-

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mic-like C1 mainly stems from terrestrial contemporary carbon sources (i.e., biomass 15 ACS Paragon Plus Environment


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burning and biogenic emission), with minor contributions from fossil-fuel combustion.

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Our finding is consistent with that derived from a study on aerosols collected in Japan, in

298

which the chromophore corresponding to C1 was predominantly associated with biomass

299

burning and terrestrial biological origin based on chemical structure analysis.34

300

Due to the high detection sensitivity and chromophore resolving ability, EEM spec-

301

troscopy coupled with PARAFAC analysis plays an increasingly important role in BrC

302

studies. Nevertheless, its application is hampered by the lack of a classification system

303

for BrC components identified by EEM spectroscopy, in which their sources, chemical

304

identity, and optical characteristics should be properly defined. In this work, two com-

305

monly observed fluorescent components in the atmospheric environment with spectral

306

locations at around 300/408 and 275/296 (excitation/emission) were unequivocally at-

307

tributed to HULIS and biologically-derived material based on direct measurements of

308

isolated HULIS and total hydrolyzable tyrosine. The primary origins of the fluorophore

309

corresponding to HULIS were further shown to be biomass burning and terrestrial bio-

310

genic emissions using dual carbon isotopic analysis. The knowledge obtained herein con-

311

tributes to the establishment of a system for classification of EEM resolved fluorophores

312

in the atmospheric environments. The quantitative information on BrC (e.g., composition

313

and source apportionment in this study) derived from EEM spectroscopy coupled with

314

other organic matter analytical techniques could improve our understanding of BrC’s role


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in climate forcing as well as regional air quality, which benefits the development of ef-

316

fective mitigation strategies.

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ASSOCIATED CONTENT

319 320 321

Supporting Information Detailed data shown in Figures 1−3 and estimated range for calculated fraction of contemporary carbon.

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AUTHOR INFORMATION

324 325 326 327 328 329 330

ACKNOWLEDGEMENTS

331 332 333

This work was supported by the National Research Foundation (NRF) of Korea (NRF-2015R1A2A1A10054309) funded by the Korean government. We are grateful to Jeonghyun Kim, Heejun Han, and Shin-Ah Lee for their assistance with sample analysis.

Corresponding Authors *G.Y. e-mail: [email protected]. *G.K. e-mail: [email protected]. Phone: +82 2 880 7508. Notes The authors declare no competing financial interest.

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REFERENCES

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Table 1. Excitation and Emission Maxima for PARAFAC Components Identified and Their Assignments According to Previous Studies PARAFAC component Excitation maxima (nm) Emission maxima (nm) Assignment according to previous studies References C1 300 408 HULIS, pyridoxine, phenanthrene 20, 36−41 C2 275 296 Tyrosine 20, 36, 37, 39, 40, 42

578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595

24


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596 597 598 599 600 601 602 603

Figure 1. Fluorescence intensities (top) and relative abundance (bottom) of each PARAFAC component in precipitation samples collected at Seoul from 2012 to 2013.



604 605 606 607 608 609 610 611 612 613 614 615

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Figure 2. Plots showing comparisons of spectral loadings between C1 and HULIS (A) and between C2 and tyrosine standard solution (B), and correlations of C1 with HULIS (C) and C2 with total hydrolysable tyrosine (D). The empty symbol represents outlier.


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616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642

Figure 3. Temporal variation in δ13C signatures of HULIS extracted from individual precipitation samples and contemporary fraction (fC) of HULIS extracted from composite samples for each season.


Speciation and Sources of Brown Carbon in Precipitation at Seoul

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