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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.

13 14 15 16 17 18 19 20 21 22 23 24 25 1 ACS Paragon Plus Environment


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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

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

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

32

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

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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-

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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-

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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

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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-

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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

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

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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

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

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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,

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

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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

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which the chromophore corresponding to C1 was predominantly associated with biomass

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

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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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(1) Kirchstetter, T. W.; Novakov, T.; Hobbs, P. V. Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon. J. Geophys. Res. D Atmos. 2004, 109, (D21), D21208. (2) Moosmüller, H.; Chakrabarty, R. K.; Arnott, W. P. Aerosol light absorption and its measurement: A review. J. Quant. Spectrosc. Radiat. Transfer 2009, 110, (11), 844–878. (3) Chung, C. E.; Ramanathan, V.; Decremer, D. Observationally constrained estimates of carbonaceous aerosol radiative forcing. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (29), 11624−11629. (4) Andreae, M. O.; Gelencsér, A. Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols. Atmos. Chem. Phys. 2006, 6, 3131−3148. (5) Bahadur, R.; Praveen, P. S.; Xu, Y. Y.; Ramanathan, V. Solar absorption by elemental 17 ACS Paragon Plus Environment


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and brown carbon determined from spectral observations. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (43), 17366−17371. (6) Feng, Y.; Ramanathan, V.; Kotamarthi, V. R. Brown carbon: a significant atmospheric absorber of solar radiation? Atmos. Chem. Phys. 2013, 13, 8607−8621. (7) Lin, G.; Penner, J. E.; Flanner, M. G.; Sillman, S.; Xu, L.; Zhou, C. Radiative forcing of organic aerosol in the atmosphere and on snow: Effects of SOA and brown carbon. J. Geophys. Res. Atmos. 2014, 119, 7453−7476. (8) Stocker, T. F., Qin, D., Plattner, G. K., Tignor, M., Allen, S. K., Boschung, J., Nauels, A., Xia, Y., Bex, V.; Midgley, P. M. Climate Change 2013: The Physical Science Basis. Intergovernmental Panel on Climate Change, Working Group I Contribution to the IPCC Fifth Assessment Report (AR5); Cambridge Univ. Press: New York, 2013. (9) Laskin, A.; Laskin, J.; Nizkorodov, S. A. Chemistry of atmospheric brown carbon. Chem. Rev. 2015, 115 (10), 4335−4382. (10) Chen, Y.; Bond, T. C. Light absorption by organic carbon from wood combustion. Atmos. Chem. Phys. 2010, 10 (4), 1773−1787. (11) Lack, D. A.; Bahreni, R.; Langridge, J. M.; Gilman, J. B.; Middlebrook, A. M. Brown carbon absorption linked to organic mass tracers in biomass burning particles. Atmos. Chem. Phys. 2013, 13, 2415−2422. (12) Lin, P.; Liu, J.; Shilling, J. E.; Kathmann, S. M.; Laskin, J.; Laskin, A. Molecular characterization of brown carbon (BrC) chromophores in secondary organic aerosol generated from photo-oxidation of toluene. Phys. Chem. Chem. Phys. 2015, 17 (36), 23312−23325. (13) Bond, T. C.; Bussemer, M.; Wehner, B.; Keller, S.; Charlson, R. J.; Heintzenberg, J. Light absorption by primaryparticle emissions from a lignite burning plant. Environ. Sci. Technol. 1999, 33(21), 3887–3891. (14) Updyke, K. M.; Nguyen, T. B.; Nizkorodov, S. A. Formation of brown carbon via reactions of ammonia with secondary organic aerosols from biogenic and anthropogenic precursors. Atmos. Environ. 2012, 63, 22−31. (15) Andreae, M. O.; Crutzen, P. J. Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry. Science 1997, 276 (5315), 1052–1058. (16) Hoffer, A.; Gelencsér, A.; Guyon, P.; Kiss, G.; Schmid, O.; Frank, G. P.; Artaxo, P.; Andreae, M. O. Optical properties of humic-like substances (HULIS) in biomass-burning aerosols. Atmos. Chem. Phys. 2006, 6 (11), 3563−3570. (17) Lukacs, H.; Gelencsér, A.; Hammer, S.; Puxbaum, H.; Pio, C.; Legrand, M.; Kasper-Giebl, A.; Handler, M.; Limbeck, A.; Simpson, D.; Preunkert, S. Seasonal trends and possible sources of brown carbon based on 2-year aerosol measurements at six sites in Europe. J. Geophys. Res. Atmos. 2007, 112, D23S18. (18) Graber, E. R.; Rudich, Y. Atmospheric HULIS: How humic-like are they? A comprehensive and critical review. Atmos. Chem. Phys. 2006, 6 (3), 729−753. 18 ACS Paragon Plus Environment

Page 19 of 27

386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424


(19) Lin, P.; Huang, X. F.; He, L. Y.; Yu, J. Z. Abundance and size distribution of HULIS in ambient aerosols at a rural site in South China. J. Aerosol Sci. 2010, 41, 74–87. (20) Pöhlker, C.; Huffman, J. A.; Pöschl, U. Autofluorescence of atmospheric bioaerosols − fluorescent biomolecules and potential interferences. Atmos. Meas. Tech. 2012, 5, 37−71. (21) Després, V. R.; Alex Huffman, J.; Burrows, S. M.; Hoose, C.; Safatov, A. S.; Buryak, G.; Frö hlich-Nowoisky, J.; Elbert, W.; Andreae, M. O.; Pö schl, U.; Jaenicke, R. Primary biological aerosol particles in the atmosphere: a review. Tellus, Ser. B 2012, 64 (15598), 1−58. (22) Stedmon, C. A.; Markager, S.; Bro, R. Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy. Mar. Chem. 2003, 82 (3−4), 239−254. (23) Stedmon, C. A.; Bro, R. Characterizing dissolved organic matter fluorescence with parallel factor analysis: A tutorial. Limnol. Oceanogr.: Methods 2008, 6, 572−579. (24) Yan, G.; Kim, G. Dissolved organic carbon in the precipitation of Seoul, Korea: implications for global wet depositional flux of fossil-fuel derived organic carbon. Atmos. Environ. 2012, 59, 117–124. (25) Yan, G.; Kim, G.; Kim, J.; Jeong, Y.-S.; Kim, Y. I. Dissolved total hydrolyzable enantiomeric amino acids in precipitation: Implications on bacterial contributions to atmospheric organic matter. Geochim. Cosmochim. Acta 2015, 153(0), 1–14. (26) Baduel, C.; Voisin, D.; Jaffrezo, J. Seasonal variations of concentrations and optical properties of water soluble HULIS collected in urban environments. Atmos. Chem. Phys. 2010, 10, 4085–4095. (27) St-Jean, G. Automated quantitative and isotopic (13C) analysis of dissolved inorganic carbon and dissolved organic carbon in continuous-flow using a total organic carbon analyser. Rapid Commun. Mass. Spectrom. 2003, 17, 419–428. (28) Stuiver, M.; Polach, H. A. Discussion: Reporting of 14C data. Radiocarbon 1977, 19 (3), 355−363. (29) Zhao, R.; Lee, A. K. Y.; Huang, L.; Li, X.; Yang, F.; Abbatt, J. P. D. Photochemical processing of aqueous atmospheric brown carbon. Atmos. Chem. Phys. 2015, 15, 6087−6100. (30) Kieber, R. J.; Willey, J. D.; Whitehead, R. F.; Reid, S. N. Photobleaching of chromophoric dissolved organic matter (CDOM) in rainwater. J. Atmos. Chem. 2007, 58 (3), 219−235. (31) Coble, P. G. Marine optical biogeochemistry: The chemistry of ocean color. Chem. Rev. 2007, 107 (2), 402−418. (32) Stubbins, A.; Lapierre, J.-F.; Berggren, M.; Prairie, Y. T.; Dittmar, T.; del Giorgio, P. A. What’s in an EEM? Molecular signatures associated with dissolved organic fluorescence in boreal Canada. Environ. Sci. Technol. 2014, 48, 10598−10606. 19 ACS Paragon Plus Environment


425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463

Page 20 of 27

(33) Kowalczuk, P.; Durako, M. J.; Young, H.; Kahn, A. E.; Cooper, W. J.; Gonsior, M. Characterization of dissolved organic matter fluorescence in the South Atlantic Bight with use of PARAFAC model: Interannual variability. Mar. Chem. 2009, 113 (3−4), 182−196. (34) Chen, Q.; Miyazaki, Y.; Kawamura, K.; Matsumoto, K.; Coburn, S.; Volkamer, R.; Iwamoto, Y.; Kagami, S.; Deng, Y.; Ogawa, S.; Ramasamy, S.; Kato, S.; Ida, A.; Kajii, Y.; Mochida, M. Characterization of chromophoric water-soluble organic matter in urban, forest, and marine aerosols by HR-ToF-AMS analysis and excitation-emission matrix spectroscopy. Environ. Sci. Technol. 2016, 50, 10351−10360. (35) Duarte, R. M. B. O.; Santos, E. B. H.; Pio, C. A.; Duarte, A. C. Comparison of structural features of water-soluble organic matter from atmospheric aerosols with those of aquatic humic substances. Atmos. Environ. 2007, 41, 8100−8113. (36) Birdwell, J. E.; Valsaraj, K. T. Characterization of dissolved organic matter in fogwater by excitation-emission matrix fluorescence spectroscopy. Atmos. Environ. 2010, 44, 3246. (37) Salve, P. R.; Lohkare, H.; Gobre, T.; Bodhe, G.; Krupadam, R. J.; Ramteke, D. S.; Wate, S. R. Characterization of chromophoric dissolved organic matter (cdom) in rainwater using fluorescence spectrophotometry. Bull. Environ. Contam. Toxicol. 2012, 88, 215−218. (38) Mladenov, N.; Alados-Arboledas, L.; Olmo, F. J.; Lyamani, H.; Delgado, A.; Molina, A.; Reche, I. Applications of optical spectroscopy and stable isotope analyses to organic aerosol source discrimination in an urban area. Atmos. Environ. 2011, 45 (11), 1960−1969. (39) Zhang, Y.; Gao, G.; Shi, K.; Niu, C.; Zhou, Y.; Qin, B.; Liu, X. Absorption and fluorescence characteristics of rainwater CDOM and contribution to Lake Taihu, China. Atmos. Environ. 2014, 98, 483−491. (40) Muller, C. L.; Baker, A.; Hutchinson, R.; Fairchild, I. J.; Kidd, C. Analysis of rainwater dissolved organic carbon compounds using fluorescence spectrophotometry. Atmos. Environ. 2008, 42, 8036−8045. (41) Yue, S.; Ren, H.; Fan, S.; Sun, Y.; Wang, Z.; Fu, P. Springtime precipitation effects on the abundance of fluorescent biological aerosol particles and HULIS in Beijing. Sci. Rep. 2016, 6, 29618. (42) Fu, P.; Kawamura, K.; Chen, J.; Qin, M.; Ren, L.; Sun, Y.; Wang, Z.; Barrie, L. A.; Tachibana, E.; Ding, A.; Yamashita, Y. Fluorescent water-soluble organic aerosols in the High Arctic atmosphere. Sci. Rep. 2015, 5, 9845. (43) Pan, Y.-L. Detection and characterization of biological and other organic-carbon aerosol particles in atmosphere using fluorescence. J. Quant. Spectros. Radiat. Transfer 2015, 150, 12–35. (44) Rincon, A. G.; Guzman, M. I.; Hoffmann, M. R.; Colussi, A. J. Thermochromism of 20 ACS Paragon Plus Environment

Page 21 of 27

464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502


model organic aerosol matter. J. Phys. Chem. Lett. 2010, 1 (1), 368−373. (45) Lee, A. K. Y.; Zhao, R.; Li, R.; Liggio, J.; Li, S. M.; Abbatt, J. P. D. Formation of light absorbing organo-nitrogen species from evaporation of droplets containing glyoxal and ammonium sulfate. Environ. Sci. Technol. 2013, 47 (22), 12819−12826. (46) Sareen, N.; Moussa, S. G.; McNeill, V. F. Photochemical aging of light-absorbing secondary organic aerosol material. J. Phys. Chem. A 2013, 117 (14), 2987−2996. (47) Lee, H. J.; Aiona, P. K.; Laskin, A.; Laskin, J.; Nizkorodov, S. A. Effect of solar radiation on the optical properties and molecular composition of laboratory proxies of atmospheric brown carbon. Environ. Sci. Technol. 2014, 48 (17), 10217−10226. (48) Samy, S.; Robinson, J.; Rumsey, I. C.; Walker, J. T.; Hays, M. D. Speciation and trends of organic nitrogen in southeastern US fine particulate matter (PM2.5). J. Geophys. Res. Atmos. 2013, 118, 1996–2006. (49) Zhang, Q.; Anastasio, C. Free and combined amino compounds in atmospheric fine particles (PM2.5) and fog waters from Northern California. Atmos. Environ. 2003, 37, 2247–2258. (50) Lakowicz, J. R. Plasmonics in biology and plasmon-controlled fluorescence. Plasmonics 2006, 1, 5. (51) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum publishers: New York, 1999. (52) Matos, J. T. V.; Freire, S. M. S. C.; Duarte, R. M. B. O.; Duarte, A. C. Natural organic matter in urban aerosols: Comparison between water and alkaline soluble components using excitation-emission matrix fluorescence spectroscopy and multiway data analysis. Atmos. Environ. 2015, 102, 1−10. (53) Mladenov, N.; Williams, M. W.; Schmidt, S. K.; Cawley, K. Atompsoheric deposition as a source of carbon and nutrients to an alpine catchment of the Colorado Rocky Mountains. Biogeosciences 2012, 9, 3337–3355. (54) Song, J.; He, L.; Peng, P.; Zhao, J.; Ma, S. Chemical and isotopic composition of humic-like substances (HULIS) in ambient aerosols in Guangzhou, South China. Aerosol Sci. Technol. 2012, 46, 533−546. (55) Fry, B.; Sherr, E. B. δ13C measurements of carbon flow in marine and fresh-water ecosystems. Contrib. Mar. Sci. 1984, 27, 13–47. (56) Miyazaki, Y.; Kawamura, K.; Jung, J.; Furutani, H.; Uematsu, M. Latitudinal distributions of organic nitrogen and organic carbon in marine aerosols over the western North Pacific. Atmos. Chem. Phys. 2011, 11(7), 3037–3049. (57) Smith, B. N.; Epstein, S. Two categories of 13C/12C ratios for higher plants. Plant Physiol. 1971, 47(3), 380–384. (58) Kohn, M. J. Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate. Proc. Natl Acad. Sci. USA 2010, 107, 19691–19695. (59) Turekian, V. C.; Macko, S.; Ballentine, D.; Swap, R. J.; Garstang, M. Causes of bulk 21 ACS Paragon Plus Environment


503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541

Page 22 of 27

carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies. Chem. Geol. 1998, 152, 181–192. (60) Das, O.; Wang, Y.; Hsieh, Y.-P. Chemical and carbon isotopic characteristics of ash and smoke derived from burning of C-3 and C-4 grasses. Org. Geochem. 2010, 41, 263–269. (61) Kawashima, H.; Haneishi, Y. Effects of combustion emissions from the Eurasian continent in winter on seasonal δ13C of elemental carbon in aerosols in Japan. Atmos. Environ. 2012, 46, 568–579. (62) Agnihotri, R.; Mandal, T.; Karapurkar, S.; Naja, M.; Gadi, R.; Ahammmed, Y. N.; Kumar, A.; Saud, T.; Saxena, M. Stable carbon and nitrogen isotopic composition of bulk aerosols over India and northern Indian Ocean. Atmos. Environ. 2011, 45(17), 2828–2835. (63) Widory, D. Combustibles, fuels and their combustion products: A view through carbon isotopes. Combust. Theor. Model. 2006, 10, 831–841. (64) López-Veneroni, D. The stable carbon isotope composition of PM2.5 and PM10 in Mexico city metropolitan area air. Atmos. Environ. 2009, 43, 4491–4502. (65) Anderson, R. S.; Iannone, R.; Thompson, A. E.; Rudolph, J.; Huang, L. Carbon kinetic isotope effects in the gas-phase reactions of aromatic hydrocarbons with the OH radical at 296 ± 4 K. Geophys. Res. Lett. 2004, 31, L15108, (66) Aggarwal, S. G.; Kawamura, K. Molecular distributions and stable carbon isotopic compositions of dicarboxylic acids and related compounds in aerosols from Sapporo, Japan: Implications for photochemical aging during long-range atmospheric transport. J. Geophys. Res. 2008, 113, D14301. (67) Bosch, C.; Andersson, A.; Kirillova, E. N.; Budhavant, K.; Tiwari, S.; Praveen, P.; Russell, L. M.; Beres, N. D.; Ramanathan, V.; Gustafsson, Ö. Source-diagnostic dual-isotope composition and optical properties of water-soluble organic carbon and elemental carbon in the South Asian outflow intercepted over the Indian Ocean. J. Geophys. Res. Atmos. 2014, 119, 11,743–11,759. (68) Kirillova, E. N.; Andersson, A.; Tiwari, S.; Srivastava, A. K.; Bisht, D. S.; Gustafsson, Ö. Water-soluble organic carbon aerosols during a full New Delhi winter: Isotope-based source apportionment and optical properties. J. Geophys. Res. Atmos. 2014, 119, 3476–3485. (69) Wozniak, A. S.; Bauer, J. E.; Dickhut, R. M.; Xu, L.; McNichol, A. P. Isotopic characterization of aerosol organic carbon components over the eastern United States. J. Geophys. Res. 2012, 117, D13303. (70) Szidat, S.; Jenk, T. M.; Synal, H. A.; Kalberer, M.; Wacker, L.; Hajdas, I.; Kasper-Giebl, A.; Baltensperger, U. Contributions of fossil fuel, biomass-burning, and biogenic emissions to carbonaceous aerosols in Zurich as traced by 14C. J. Geophys. Res. 2006, 111 (7), 206. 22 ACS Paragon Plus Environment

Page 23 of 27

542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571


(71) Levin, I.; Naegler, T.; Kromer, B.; Diehl, M.; Francey, R. J.; Gomez-Pelaez, A. J.; Steele, L. P.; Wagenbach, D.; Weller, R.; Worthy, D. E. Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2 . Tellus, Ser. B 2010, 62 (1), 26−46. (72) Szidat, S.; Ruff, M.; Perron, N.; Wacker, L.; Synal, H.-A.; Hallquist, M.; Shannigrahi, A. S.; Yttri, K.; Dye, C.; Simpson, D. Fossil and non-fossil sources of organic carbon (OC) and elemental carbon (EC) in Göteborg, Sweden. Atmos. Chem. Phys. 2009, 9 (5), 1521−1535. (73) Heal, M. R.; Naysmith, P.; Cook, G. T.; Xu, S.; Duran, T. R.; Harrison, R. M. Application of 14C analyses to source apportionment of carbonaceous PM2.5 in the UK. Atmos. Environ. 2011, 45 (14), 2341−2348. (74) Streets, D. G.; Yarber, K. F.; Woo, J.-H.; Carmichael, G. R. Biomass burning in Asia: annual and seasonal estimates and atmospheric emissions. Global Biogeochem. Cycles 2003, 17 (4), 1099. (75) van der Werf, G. R.; Randerson, J. T.; Giglio, L.; Collatz, G. J.; Mu, M.; Kasibhatla, P. S.; Morton, D. C.; DeFries, R. S.; Jin, Y.; van Leeuwen, T. T. Global fire emissions and the contribution of deforestation, savanna, forest, agricultural, and peat fires (1997–2009). Atmos. Chem. Phys. 2010, 10, 11707–11735. (76) Ryu, S. Y; Kim, J. E.; Zhuanshi, H.; Kim, Y. J.; Kang, G. U. Chemical composition of post-harvest biomass burning aerosols in Gwangju, Korea. J. Air Waste Manage. Assoc. 2004, 54, 1124–1137. (77) Jung, J.; Lee, S.; Kim, H.; Kim, D.; Lee, H. O. S. Quantitative determination of the biomass-burning contribution to atmospheric carbonaceous aerosols in Daejeon, Korea during the rice-harvest period. Atmos. Environ. 2014, 89, 642–650. (78) Jung, J.; Lyu, Y.; Lee, M.; Hwang, T.; Lee, S.; Oh, S. Impact of Siberian forest fires on the atmosphere over the Korean Peninsula during summer 2014. Atmos. Chem. Phys. 2016, 16, 6757–6770. (79) Hao, W. M.; Petkov, A.; Nordgren, B. L.; Silverstein, R. P.; Corley, R. E.; Urbanski, S. P.; Evangeliou, N.; Balkanski, Y.; Kinder, B. Daily black carbon emissions from fires in Northern Eurasia from 2002 to 2013. Geosci. Model Dev. 2016, 9, 4461–4474.

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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.


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