Light-Absorbing Brown Carbon Aerosol Constituents from Combustion

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Light-absorbing brown carbon aerosol constituents from combustion of Indonesian peat and biomass Sri Hapsari Budisulistiorini, Matthieu Riva, Michael Williams, Jing Chen, Masayuki Itoh, Jason Douglas Surratt, and Mikinori Kuwata Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00397 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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Environmental Science & Technology

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Light-absorbing Brown Carbon Aerosol Constituents from

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Combustion of Indonesian Peat and Biomass

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Sri Hapsari Budisulistiorini†,*, Matthieu Riva‡,#, Michael Williams‡, Jing Chen†,

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Masayuki Itoh¶, Jason D. Surratt‡, Mikinori Kuwata†,¶,*

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†

9 10 11

‡

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¶

Center for Southeast Asian studies, Kyoto University, Kyoto 6068501, Japan

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*

To whom correspondence should be addressed

14

#

Now at the Department of Physics, University of Helsinki, 00014 Helsinki, Finland

Earth Observatory of Singapore, Nanyang Technological University, Singapore 639798, Singapore Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, North Carolina 27599, United States

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Corresponding Author

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* Email: [email protected]

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Phone: +65 6592 3177

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20

Phone: +65 6592 3606

21 22

For submission to Environmental Science and Technology

1 ACS Paragon Plus Environment


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ABSTRACT

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Light-absorbing brown carbon (BrC) constituents of organic aerosol (OA) have been

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shown to significantly absorb ultra-violet (UV) and visible light, and thus, impact

26

radiative forcing. However, molecular identification of the BrC constituents is still

27

limited. In this study, we characterize BrC constituents at the molecular level in: i)

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aerosols emitted by combustion of peat, fern/leaf, and charcoal from Indonesia and ii)

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ambient aerosols collected in Singapore during the 2015 haze episode. Aerosols were

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analyzed using ultra-performance liquid chromatography interfaced to a diode array

31

detector and electrospray ionization high-resolution quadrupole time-of-flight mass

32

spectrometer (UPLC/DAD-(-)ESI-HR-QTOFMS) operated in the negative ion mode. In

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the laboratory-generated aerosols, we identified 41 compounds that can potentially

34

absorb near-UV and visible wavelengths, such as oxygenated-conjugated compounds,

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nitroaromatics, and S-containing compounds. The sum of BrC constituents in peat,

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fern/leaf, and charcoal burning aerosols are 16%, 35%, and 28% of the OA mass,

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respectively, giving an average contribution of 24%. On average, the BrC constituents

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account for 0.4% of the ambient OA mass; however, large uncertainties in mass closure

39

remain due to the lack of authentic standards. This study highlights the potential of light-

40

absorbing BrC OA constituents from peat, fern/leaf, and charcoal burning, and their

41

importance in the atmosphere.


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INTRODUCTION

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The presence of light-absorbing carbonaceous species in organic aerosol (OA), referred

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to as brown carbon (BrC), have been highlighted in past decades. 1-4 The BrC constituents

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of OA have strong absorption in near ultraviolet (UV) and visible regions of the

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electromagnetic spectrum, causing OA to appear brownish or yellowish. 1 A recent study

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associated BrC with aqueous secondary organic aerosol (SOA) formation from residential

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biomass burning (BB) in Europe, which contributed up to 20% of total OA mass.

49

Moreover, studies have shown a significant contribution of BrC to radiative forcing,

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leading to a net warming effect. 6-8 Lin et al. 9 estimated that BrC exerts a direct forcing,

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ranging from + 0.22 to + 0.57 W m-2, that accounts for 27–70% of black carbon (BC)

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forcing.

5

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Molecular characterization of OA constituents from BB is necessary to evaluate the

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impact of light-absorbing BrC species on air quality and climate, as the presence of BrC

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chromophores, such as imidazoles, can significantly impact the chemical composition

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and mass of OA through multiphase chemistry. 10 Unlike BC, BrC is less characterized

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due to its complex composition and properties. 1,11 Light-absorbing BrC constituents can

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be produced by primary sources, including biomass and fossil fuel burning. 4,12 Secondary

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sources of light-absorbing BrC are multiphase chemistry, including reactions of

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ammonium ions and amino acids with carbonyls and dicarbonyls,

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aldol condensation, 14,17 and nitration of aromatic compounds. 18,19

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13-16

acid-catalyzed

Fuel type (e.g. wood, grass, and peat) influences the physical and chemical properties 4,20,21

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of light-absorbing BrC constituents of OA from BB.

For example, at visible

64

wavelengths of 405–532 nm, carbonaceous particles generated from boreal peat burning



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have a higher absorption Ångstrom exponent (AAE = 9) 4 compared with the estimated

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absorption for tree duffs (AAE = 4–6). 21 Previous studies have also shown that aerosol

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nitroaromatics from combustion of biomass22-25 and fossil fuel26 could be associated with

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light-absorbing BrC. An enhanced contribution of nitrated aromatic compounds to the

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light-absorbing particulate BrC has been shown in areas influenced by BB aerosol.

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Additionally, light-absorbing constituents from solid fuel pyrolysis show some polar

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characteristics; however, many of them are insoluble in water. 28

27

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Human-induced fires in Indonesia date back to the 1960s29 and are primarily caused by

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clearing of forests and degraded lands (including peatlands) for agriculture or plantations.

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30,31

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Southern Oscillation (ENSO) episodes in Indonesia. 29,32,33 For instance, during the 2015

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ENSO event, Indonesian fires released an estimated 380 Tg C, which is equivalent to 1.5

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billion metric tons of CO2. 34 Indonesian fires have also been shown to generate light-

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absorbing aerosols that warm the upper troposphere up to 20 W m-2. 34,35 These studies

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reveal the urgent need for characterization of light-absorbing BrC constituents in aerosols

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emitted by combustion of biomass and peat from Indonesia.

Severe fires have been associated with prolonged droughts caused by El Niño

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In this study, we focus on chemical characterization of aerosols generated from

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combustion of Indonesian peat. Peatland fires emit large amounts of carbonaceous

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particles36,37 and can last longer than surface fires, as burning occurs below surface

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deposits.

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chemical and optical characteristics in both laboratory20,39,40 and ambient environments,

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41-43

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Therefore, we focus on molecular characterization of BrC OA constituents from

38

Aerosols from Indonesian peat combustion have been characterized for

but the light-absorbing BrC constituents have not been thoroughly investigated.


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Indonesian peat and biomass combustion of the surface of peatland (i.e., fern, leaf, and

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charcoal). Thirteen types of biomass described in Table S1 were burned, and aerosols

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were collected and chemically characterized by ultra performance liquid chromatography

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interfaced to diode array detector and high-resolution quadrupole time-of-flight mass

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spectrometer equipped with electrospray ionization (UPLC/DAD-(-)ESI-HR-QTOFMS),

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operated in the negative ion mode. In order to evaluate the atmospheric relevance of the

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BrC OA constituents characterized from the laboratory-generated aerosols, we

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chemically characterized ambient fine aerosol samples collected in Singapore between

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October 10 to 31, 2015 for these potential marker (tracer) compounds. During that period,

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smoke from peatland fires in Indonesia was estimated to influence air quality in

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Singapore, as shown in Figure S1.

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EXPERIMENTAL SECTION

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Burning Experiments. We conducted burning experiments on thirteen types of

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biomass, including peats, ferns, leaf, and charcoal, from burned peatland in Indonesia.

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Biomass descriptions and sampling locations are provided in Table S1 and Figure S2,

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respectively. Burned and unburned peat samples were collected from ground surfaces (0

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to 10 cm), except one sample (Riau 1.2), which was collected 30 to 40 cm below the

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surface. The latter is referred to as Riau 1.2-deep. Biomass was combusted in a 100 L

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stainless steel container at room temperature and atmospheric pressure, as illustrated in

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Figure S3. 1 g of raw fuel (without pre-drying treatment) was placed on ceramic crucible

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wrapped in a heating coil, and then put inside the burning container. The heating coil was

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set at 350 °C to reproduce the actual conditions of Indonesian peatland fires, 38 using a

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thermocouple and PID controller for 50–60 min. Our preliminary tests of peat



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combustion in open air showed that this method mainly resulted in smoldering

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combustion with only a short period (1–2 min) of glowing at the beginning. Smoke was

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well mixed inside the combustion chamber by addition of particle-free mixing air, such

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that smoke emitted during glowing and smoldering phases would have mixed before the

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start of sampling. Particle-laden air samples were then collected approximately 1–2 min

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after fuel combustion. The particle-free air was continuously supplied to maintain

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atmospheric pressure within the combustion chamber during particle sampling. We

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unfortunately did not have a scrubber to remove gaseous components in the particle-free

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air. Also the gases concentrations in the laboratory and burning chamber were not

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monitored due to the unavailability of instruments. According to the National

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Environment Agency (NEA), ambient concentrations of ozone (O3), NO2, and SO2 were

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on average 30, 20, and 10 ppb respectively, from September to October 2015,

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(http://www.nea.gov.sg/). Average ambient relative humidity (RH) was 82% during this

124

period. Inside the air-conditioned laboratory, these ambient gases and RH levels are

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lower, and hence, do not significantly affect burning experiments.

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The particle-laden air samples were collected onto 47 mm diameter, 0.2 µm pore

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Teflon membrane filters (Fluoropore™, Sigma Aldrich) at a flow rate of 0.5 L min-1 for

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30–35 min. The filters were wrapped in pre-baked aluminum foil and stored under dark

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conditions at -20 °C until analysis. At the end of each experiment, the stainless steel

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container was wiped by Milli-Q water and 2-propanol (industrial grade, Kanto Chemical

131

Co., Inc.) at least three times with each solvent in order to remove particles on the inside

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wall of the container.


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Ambient Sampling. During the 2015 haze episode in Singapore, from October 14–30,

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ambient aerosols were drawn through a PM2.5 cyclone to a low-volume particle filter

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sampler. The sampler was operated at 4.2 L min-1 for ~24 h during period of low hourly

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PM2.5 (≤ 50 µg m-3), and for ~12 h (daytime sampling was 08:00–19:45 and nighttime

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was 20:00–07:45) during high loading period (PM2.5 > 50 µg m-3), as reported by the

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NEA website. The sampler used in this study hosted a 47-mm Quartz fiber filter pre-

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heated at 250 °C for 16 hours. The filter samples were then analyzed for light-absorbing

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BrC tracers. Filter blanks were also collected throughout the ambient sampling period.

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All filters were wrapped in pre-baked aluminum foil and stored under dark conditions at -

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20 °C until analysis.

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Filter Extraction and Chemical Analysis. Due to high mass concentrations (5–90 mg

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m-3; Table S2) of OA collected from the laboratory experiments, filters were cut into

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quarter fractions prior extraction. One quarter was extracted by 45 min of sonication in

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22 mL of methanol (LC-MS CHROMASOLV-grade, Sigma Aldrich ≥ 99.9%). The

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methanol extracts were blown dry under gentle N2 (g) stream at room temperature. The

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dried extracts were reconstituted in 150 µL of 50:50 (v/v) solvent mixture of methanol

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and high-purity water (Milli-Q, 18.2 MΩ). The samples were then analyzed by

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UPLC/DAD-ESI-HR-QTOFMS (6520 Series, Agilent) equipped with a Waters Acquity

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UPLC HSS T3 column (2.1 × 100 mm, 1.8 µm particle size) operated in the negative ion

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mode. Operating conditions are described elsewhere.

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standards used in quantification of OA tracers identified by this method. Surrogate

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chemical standards were selected based on structural similarities and elution times of the

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identified constituents. Solvent and filter blanks were included in the analysis and

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Table S3 lists the chemical



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subtracted from the actual samples used for the chemical characterization and

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quantification of BrC OA constituents. Ketopinic acid and camphor-10-sulfonic acid

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were injected onto filters as internal standards to estimate the extraction recovery. We

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obtained 90±12% recovery for both internal standards, suggesting that losses due to the

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extraction procedure and/or constituents degradation during the analytical process are

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minimal. We applied the extraction recovery value to correct mass quantification of each

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compound.

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Another quarter of filter was extracted and dried in a similar manner. The dried extracts

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were

trimethylsilylated

by

addition

of

100

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bis(trimethylsilyl)trifluoroacetamide (BSTFA) + trimethylchlorosilane (TMCS) (99 : 1,

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v/v , Supelco) and 50 µL pyridine (Sigma-Aldrich, 98%, anhydrous), and heated at 70 °C

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for 1 h. Within 24 h following trimethylsylilation, samples were analyzed by GC/EI-MS

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at 70 eV (Hewlett 5890 Packard Series II Gas Chromatograph interfaced to a HP 5971A

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Series Mass Selective Detector, Econo-CapTM-ECTM-5 column, 30 m × 0.25 mm ×

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0.25 µm). The operating conditions and temperature program are described elsewhere. 45

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Levoglucosan was quantified from this sample by an authentic standard listed in Table

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S3.

µL

of

N,O-

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Real-time Chemical Characterization of Aerosol. The time-of-flight aerosol

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chemical speciation monitor (ToF-ACSM) measured compositions of non-refractory PM1

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(NR-PM1) during each burning experiments.

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sampling flow rate of 0.1 L min-1 and obtained data every 200 sec. Chemical composition

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data was acquired using IgorDAQ v2.0.20 and analyzed by Tofware v2.5.6 written in

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Igor Pro (WaveMetrics). In addition to laboratory sampling, the ToF-ACSM also

46

The ToF-ACSM was operated at a


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measured the chemical compositions of ambient NR-PM1 between October 10–31, 2015,

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which overlapped with the ambient PM2.5 filter sampling described above. Calibrations

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conducted after ambient and laboratory experiments showed consistent values of

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sampling flow rate and ionization efficiency of nitrate and sulfate. Average composition-

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dependent collection efficiencies were 0.7 for combustion experiments and 0.5 for

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ambient samplings.

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from combustion experiments and ambient samplings are presented in the following

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discussion.

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RESULTS AND DISCUSSION

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Compositions of organic and inorganic species in fine aerosols

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Mass concentrations of NR-PM1 emitted from burning experiments vary with fuel type.

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The variation could be influenced by mineral compositions as well as water content of the

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fuel. Carbon content in peat samples from Riau and Central Kalimantan Provinces used

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in this study are around 50 g C g-1 of dry peat. The fuel was not pre-dried prior to the

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experiment. Hence, it is likely that the moisture content influences the amount of

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particles emitted during each experiment. However, variability in mass loading would not

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significantly affect chemical characterization of the light-absorbing BrC constituents. In

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this study, we aim to characterize the light-absorbing BrC constituents at the molecular

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level and do not estimate the emission factor for different fuel types. Thus, the average

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particle mass reported here is mainly used to estimate the relative contribution of light-

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absorbing BrC constituents to the total OA mass emitted by each fuel type.

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On average, combustion of peat, fern/leaf, and charcoal generate approximately 38, 18,

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and 25 mg m-3 of NR-PM1 mass, respectively (Table S2). The drained and burned peat

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originated from Riau Province, Sumatra Island and Central Kalimantan Province, Borneo



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Island generates ~55 mg m-3 (Experiments 1-5) and ~15 mg m-3 of NR-PM1 mass

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(Experiment 9), respectively. The unburned peat from Riau generates ~29 mg m-3

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(Experiments 11–12), and that from Central Kalimantan generates ~9 mg m-3

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(Experiment 10). Fern/leaf and charcoal (Experiments 6–8 and 13) generate 5–30 mg m-3

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of NR-PM1 suggesting that vegetation and charcoal could contribute substantially to total

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aerosol loadings emitted during peatland fires.

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Organic species are predominant (> 98%) in the NR-PM1 mass generated during

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laboratory biomass burning experiments. On the other hand, inorganic species only

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contribute up to 1% of the total NR-PM1 mass. Amongst the inorganic aerosol

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constituents, nitrate is predominant, followed by chloride, and ammonium, respectively.

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The contribution of nitrate generated from fern/leaf burning (~57%; Experiments 6–8) to

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the total inorganic species mass is slightly higher, when compared with drained and

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burned peat (~53%; Experiments 1–5, 9) and drained and unburned peat (56%;

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Experiment 10); however it is lower than the un-drained and unburned peat (61%;

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Experiment 11–12), and charcoal (~58%; Experiment 13)). Concentrations of chloride

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and ammonium in all experiments are 18–43% and 1–20% of the total inorganic mass,

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respectively. A significant sulfate concentration was detected from burning of leaves but

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not from the other biomass types. Fragments from thermal decomposition of

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organonitrates and organosulfates could contribute to nitrate and sulfate ions measured by

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the ToF-ACSM, 48 and thus, these ions may not be entirely inorganic. Inorganic aerosol

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constituents could be associated with the formation of light-absorbing BrC constituents as

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discussed in the subsequent sections.


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Chemical Characterization of BrC Species. UPLC-DAD absorption chromatograms

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at 300–700 nm were extracted for each burning sample and are plotted in Figures 1 and

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S4. Absorption intensities on these figures were normalized by filter sampling volumes to

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account for varying aerosol mass loadings collected onto filters. UPLC-DAD absorption

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chromatograms of aerosols generated from combustion of peat and fern in Figures 1b and

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1d, respectively, exhibit intense absorptions at near-UV wavelengths (300–400 nm).

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Similarly, absorption patterns in the near-UV spectral region are observed in the other BB

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aerosol types with various intensities (Figure S4). Absorption of visible light (400–700

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nm) is also observed in all experiments, but at lower intensities than near-UV light. Based

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on the intensities observed in Figures 1b and 1d, we extracted absorption chromatograms

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for peat (Riau 1.2) and fern (Pteridium) aerosols at selected wavelengths (i.e., 365, 400,

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500, and 580 nm) and plotted them in Figures 1a and 1c, respectively. The extracted

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chromatograms reveal a broad peak associated with absorption at 365 nm and retention

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times (RTs) of 6–13 min. A previous study found a broad peak from the combustion of

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peat and other biomass types at an earlier retention time (4.5–6.5 min), 20 which is likely

239

due to differences in analytical techniques. The broad peak suggests the presence of a

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chemically complex mixture containing isomeric chromophores. 49 As a result, we focus

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our chemical characterization efforts on constituents that have chromatographically

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resolved peaks. For absorption at 365 nm, resolved chromatographic peaks with intensity

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≥ 5000 mAU are observed at ~9 and 12 min RTs for BrC constituents in peat burning OA

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and at ~9 min for fern burning. For absorptions at 400, 500, and 580 nm, the resolved

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chromatographic peaks are observed at RTs of ~6.5, 10, and 11 min, respectively. The

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resolved chromatographic peaks are also observed at RTs ~10 and 11 min for absorption



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at 365 nm from the fern burning OA (Figure 1c). However, their abundances are quite

248

similar or slightly smaller than absorption at 500 nm for similar RT. Hence, the BrC

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constituents characterized at these regions are assigned to absorption at 500 nm rather

250

than at 365 nm. Similar peaks and RTs are observed in approximately similar abundances

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in our other experiments with different absorption intensities (Figure S4).

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Using ESI-HR-QTOFMS analysis, we characterized 41 BrC constituents, which are

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tentatively identified as oxygenated and nitrogen (N)- and sulfur (S)-containing

254

compounds in Tables S4 and S5. The compound assignments are tentative due to the lack

255

of authentic standards. Most light-absorbing species identified in the different

256

experiments exhibit a high degree of unsaturation (double bond equivalent (DBE) > 4),

257

which is indicative of conjugated compounds. These compounds likely contribute to

258

absorption at near-UV and visible wavelengths. Typical distributions of N-, O-, and S-

259

atoms observed in the aerosols from combustion of peat and fern are presented in Figure

260

S5. This figure shows that absorption at near-UV and visible wavelengths is primarily

261

associated with highly oxygenated (O atom = 3–7) compounds, as well as S-containing

262

(S atom = 1) and N-containing compounds (N atom = 1–3). N-containing compounds

263

found in peat burning OA are mostly nitroaromatic compounds that have been previously

264

observed in biomass burning experiments and field measurements.

265

containing compounds might be detected as organosulfur fragments by ToF-ACSM,

266

and thus, they did not contribute to the insignificant inorganic sulfate fraction in peat and

267

fern burning OA.

20,22,23

The S48

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Formation of the light-absorbing BrC OA constituents could be explained through

269

various reactions that take place during combustion. Biomass combustion releases large


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numbers and concentrations of oxygenated volatile organic compounds (OVOCs) and

271

trace gases. For example, aldehydes (e.g., formaldehyde, glycoaldehyde, and

272

acetaldehyde), ketones (e.g., methyl vinyl ketone, acetone, and hydroxy acetone),

273

carboxylic acids (e.g., formic acid and acetic acid), aromatics (e.g., benzene and toluene),

274

furan (e.g., methylfuran and methyl furfural), biogenic VOCs (e.g., isoprene, α- and β-

275

pinene), and trace inorganic gases (e.g., nitric oxide (NO), nitrogen dioxide (NO2),

276

nitrous acid (HONO), and ammonia (NH3)) have been previously measured from

277

combustion of peat and biomasses in laboratory and ambient environments.

278

Some of these compound classes yield chromophores under atmospherically relevant

279

conditions. 16,17,54 Furthermore, formation of hydroxyl radicals during combustion55 might

280

intensify chromophore formation. In the following, we discuss possible mechanisms to

281

form chromophores during biomass combustion based on prior atmospheric studies.

20,40,50-53

282

Carbonyls13,17,56,57 and dicarbonyls16,54 could react with ammonium ions, ammonia,

283

and/or amino acids in the aqueous phase of aerosols. The resulting oxygenated and N-

284

containing organic compounds typically exhibit conjugated systems, such as imidazole,

285

N-heterocycle, imine, or aldol condensate, which absorb at wavelengths within the range

286

of 300–500 nm. In this study, 10–90 µg m-3 of NH4+ (Table S2) was measured during the

287

experiments; however, NH3 concentrations were not measured. The presence of

288

ammonium in BB aerosols suggests that reactions with aldehydes and/or ketones could

289

take place during combustion experiments. S-containing organic compounds could be

290

attributed to acid-catalyzed heterogeneous reactions under dry conditions during

291

combustion. This reaction has been shown to form aldol condensates and unsaturated

292

oligomers, which absorb at wavelengths of 300–500 nm. 49,58



293

Heterogeneous reactions of aromatics with NO2 and nitrate (NO3)

Page 14 of 36

18,19

and/or

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oligomerization by dicarbonyls reacting with ammonium ions16 could also take place

295

during combustion and produce nitrated-aromatics and/or PAHs that absorb at 300–500

296

nm. As illustrated in Figure 2, most of the identified compounds with m/z values > 120

297

have DBE ≥ 4, indicating the likely presence of aromatic compounds. Additionally,

298

reactions of aromatics and/or aromatic hydroxyacids with hydroxyl radicals (OH) could

299

form functionalized (carboxylic, phenolic) aromatic compounds and high molecular

300

weight (MW) species/humic-like substances (HULIS) that absorb from the UV to the

301

lower range of the visible spectrum.

302

resolved. This may be a result of low sensitivity of the negative-ion ESI-HR-QTOFMS to

303

polar compounds. 49 OH radicals could be produced through multiple gas-phase reactions

304

during combustion, 55 causing oxidation during combustion experiments. A small amount

305

of OH radicals is sufficient to initiate autoxidation, through intra- or inter-molecular

306

hydrogen abstraction of VOCs and/or OVOCs by peroxy radicals, leading to formation of

307

light-absorbing highly oxidized multifunctional compounds (HOMs). 11,61-63

23,59,60

The HULIS were not chromatographically

308

Contribution of Light-absorbing BrC Constituents to OA. Figure S6 shows that

309

most of the light-absorbing BrC constituents identified here absorb in the near-UV region

310

(365 nm). No significant differences in composition of BrC constituents are observed

311

from the different peat burning aerosols (Experiments 1–5, 9–12) and the fern/leaf

312

burning aerosols (Experiments 6–8). Our results suggest that the BrC constituents

313

characterized in this study absorb mostly at near-UV and short visible wavelengths,

314

which is consistent with prior work. 3 It should be noted that although some constituents


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absorb at both 365 and 400 nm (Table S4), the absorption at 400 nm is much lower.

316

Hence, the BrC constituents in Table S4 are all attributed to absorption at 365 nm.

317

Figures 3 and S7–S9 show average mass concentrations of the organic compounds that

318

absorb at 365, 400, 500, and 580 nm in the aerosols generated from thirteen burning

319

experiments. The BrC constituents in Tables S4 and S5 are quantified using surrogate

320

standards exhibiting somewhat similar RTs and chemical structures; therefore, large

321

uncertainties remain in the OA mass closure. The three most abundant BrC constituents

322

absorbing at 365 nm are tentatively assigned as C8H7O3− (vanillin, m/z 151), C10H9O4−

323

(ferulic acid, m/z 193) and C9H9O4− (homovanillic acid, m/z 181). These constituents are

324

known tracers for biomass and peat burning aerosols. 5,20,64 For absorption in the visible

325

light spectrum, the tentative assignment of the most abundant constituents are C9H11O2−

326

(ethyl guaiacol, m/z 151), C7H7O3− (m/z 139), C10H9O3− (coniferyl aldehyde, m/z 177),

327

C10H11O3− (coniferyl alcohol, m/z 179), and C10H13O2− (m/z 165). Coniferyl aldehyde and

328

coniferyl alcohol are products of lignin pyrolysis characterized from biomass burning

329

experiments, 20,65 and ethyl guaiacol is a derivative of guaiacol previously observed from

330

field observations.

331

0.02% and 0.5% of the total quantified BrC species respectively, suggesting a dominance

332

of oxygenated-conjugated compounds (~99.4%). The characterization of N-containing

333

species, such as N-heterocyclics, may be more sensitive to positive-ion ESI-MS67 rather

334

than negative-ion applied in this study. Thus, the positive-ion approach might be useful in

335

future studies.

66

On average, the N- and S-containing compounds only make up

336

Figure S10 shows summed concentrations of BrC OA constituents in each biomass

337

burning experiment, in comparison with the total OA mass concentration measured by



Page 16 of 36

338

the ToF-ACSM and levoglucosan measured by GC/EI-MS. On average, total light-

339

absorbing BrC constituents quantified in aerosols from peat, fern/leaf, and charcoal

340

burning account for 16%, 35%, and 28% of their OA mass, respectively. This gives an

341

average BrC constituent contribution of 24% to the total OA mass; however, it should be

342

noted that quantification of BrC constituents is highly uncertain due to a lack of authentic

343

standards. Levoglucosan, a known biomass burning tracer, 64 contributes around 61% of

344

the total OA mass. Adding the average BrC constituent contribution to the average

345

contribution of levoglucosan gives a total contribution of 85% of the total OA mass.

346

The OA mass spectra in this study show that the fraction of the signal at m/z 43 (f43;

347

C3H7+ and C2H3O+) is larger compared with that of m/z 44 (f44; CO2+) (Figure S11). This

348

suggests that OA from peat and biomass burning has low oxidation state. 68 The OA mass

349

spectra of biomass burning aerosols in this study share some similar features with

350

biomass burning OA (BBOA) factor, such as signal fractions at m/z 60 (f60; C2H4O2+) and

351

73 (f73; C3H5O2+). This factor is typically resolved from positive matrix factorization

352

(PMF) of aerosol mass spectrometer (AMS)/ACSM datasets.

353

between f44 and f60 in Figure S12 contrasts with the features shown in the aging BB

354

plume.

355

characteristic. Although OA in this study has a low oxidation state, we observe a

356

significant number of oxygenated-conjugated species. Species with different level of

357

oxidation states could be further resolved using factor analysis, such as PMF or multi-

358

linear engine algorithm. In future work, factor analysis of OA would be useful to

359

deconvolute light-absorbing BrC OA constituents.

360

ATMOSPHERIC SIGNIFICANCE

69

68,69

The positive slope

This discrepancy indicates that OA in this study does not show aged-BBOA


Page 17 of 36


361

This study characterizes light-absorbing BrC constituents within OA formed from

362

combustion experiments under atmospheric-like conditions. Chemical characterization of

363

ambient fine aerosols collected during the 2015 haze episode in Singapore which was

364

originated from peatland fires in Indonesia (Figure S1), suggests an enhancement of

365

aerosol absorption during periods of high OA concentrations (Figure 4a). Although

366

relative absorption of the ambient OA is very low in comparison with OA from peat and

367

biomass burning (Figures 1 and S4), an enhanced absorption at 300–400 nm between RTs

368

of 6 to 13 min is observed. When most of the peatland fires subdued at the end of

369

October 2015, the OA absorption decreased (Figure 4b) along with the OA concentration

370

(Figure 4c). Further investigation resulted in the identification of 10 out of 41 BrC OA

371

constituents listed in Tables S4 and S5, and four nitroaromatic compounds (i.e.,

372

C8H8NO3-, C7H4NO5-, C10H6NO3-, and C7H5N2O5-) that are not observed in laboratory-

373

generated particles. Additional nitroaromatic compounds might come from vegetation

374

burning and fossil fuel; however, these are not the focus of our study. As illustrated in

375

Figure S13, the identified light-absorbing BrC constituents exhibit an enhanced

376

absorption during high-OA loading periods compared with low-OA loading periods.

377

Three compounds, tentatively identified as ferulic acid (C10H9O4−; m/z 193), p-coumaric

378

acid (C9H7O3−; m/z 163) and coniferyl aldehyde (C10H9O3−; m/z 177), were observed

379

during the entire campaign at fairly significant atmospheric concentrations, up to 162,

380

102, and 103 ng m-3, respectively. These compounds have DBE values of 6, which

381

indicates the presence of an aromatic ring likely acting as a chromophore.

382

The sum of identified light-absorbing BrC constituents accounts for 0.3–0.8% of the

383

total ambient OA mass. While the fraction is small, it indicates the presence of light-



Page 18 of 36

384

absorbing BrC constituents during a period of heavy atmosphere pollution from

385

Indonesian peatland fire smoke. These identified BrC constituents can serve as tracers of

386

BrC from peat burning and be used in factor analysis of the ambient OA. As shown in

387

Figure S14, the total mass of the BrC constituents is moderately correlated with OA (r2 =

388

0.5) and ammonium and nitrate (r2 ~0.4). These correlations with OA suggests that the

389

BrC constituents might be attributed to some OA mass fractions and contribute to the

390

reduced visibility during the transboundary smoke pollution from peatland fires in

391

Indonesia. The weaker correlation with sulfate (r2 ~0.3) and the estimated less acidic

392

ambient aerosol (Figure S15) suggest that acidity might not be the limiting factor in BrC

393

formation, as observed for biogenic SOA. 70,71

394

This study highlights the potential for combustion of peat and other biomass types

395

relevant to Indonesia and tropical regions in the formation of chromophores, such as

396

oxygenated-conjugated compounds and nitroaromatics. These compounds could impact

397

OA aging and growth through photochemical oxidation.

398

light-absorbing BrC OA constituents contribute on average 24% and 0.4% to the total OA

399

mass measured from the laboratory biomass burning and to the atmosphere impacted by

400

transboundary peatland fires smoke, respectively. Furthermore, light-absorbing BrC

401

constituents characterized in this study are predominantly oxygenated-conjugated

402

compounds and absorb at near-UV and visible wavelengths. Aging of aqueous-phase

403

SOA has been shown to increase absorption at visible wavelengths due to the presence of

404

chromophores, such as conjugated oligomers and imidazoles. 17,54,58 Photodegradation of

405

SOA has also been shown to impact the OVOCs budget in the atmosphere. 73 Therefore,

72

Additionally, the identified


Page 19 of 36


406

future studies should attempt to investigate the effects of aqueous-phase aging and

407

photodegradation on the light-absorbing BrC OA constituents.

408 409

SUPPORTING INFORMATION

410

Table S1 summarizes biomass fuel types used in the burning experiments. Table S2

411

provides the average mass concentration of organic and inorganic aerosols as well as NR-

412

PM1 from combustion experiments. Table S3 lists the standards used in quantification of

413

light-absorbing BrC constituents by UPLC/DAD-ESI-HR-QTOFMS. Tables S4 and S5

414

list organic compounds that absorb near-UV and visible wavelengths, respectively.

415

Figure S1 depicts a map of fires in Indonesia and back trajectory. Figure S2 shows the

416

biomass sampling locations and Figure S3 shows a schematic of the combustion

417

experimental setup. Figure S4 shows the UPLC-DAD absorption chromatograms at near-

418

UV and visible wavelengths. Figure S5 shows the histograms of atom number in N-, O-,

419

and S-containing compounds identified in the BB aerosols. Figure S6 shows the fraction

420

of constituents absorbing at different wavelengths to total identified compounds. Figures

421

S7–S9 show the concentration of compounds that absorb in the visible region. Figure S10

422

shows the concentration of total light-absorbing BrC constituents and levoglucsaon

423

quantified in BB aerosols and OA mass measured by the ToF-ACSM. Figure S11 shows

424

the typical OA mass spectra from peat and fern combustion. Figure S12 shows the

425

scatterplots of f44 and f43, and f44 and f60 from the experiments. Figure S13 shows the UV

426

spectra of light-absorbing BrC constituents characterized from ambient aerosols. Figure

427

S14 shows the correlation of the sum of BrC constituents against species measured by the



Page 20 of 36

428

ToF-ACSM. Figure S15 shows the estimated acidity of ambient aerosols. This

429

information is available free of charge via the Internet at http://pubs.acs.org.

430 431

AUTHOR INFORMATION

432

Corresponding Author

433


434

Ph./Fax.: +65 6592 3177/+65 6790 1585

435


436

Ph./Fax.: +65 6592 3606/+65 6790 1585

437

Earth Observatory of Singapore

438

Nanyang Technological University

439

50 Nanyang Avenue, Singapore 639798, Singapore

440 441

ACKNOWLEDGEMENTS

442

We acknowledge Dr. H. Gunawan for supporting our research in Indonesia. We thank

443

S. Shiodera for helping biomass collection in Indonesia, G. B. Lebron and W. -C. Lee for

444

assisting in the particles sampling and data collection, and K. Niezgoda and S. R. He for

445

the meteorological data. The research is funded by the National Research Foundation

446

Singapore (NRF) under its Singapore National Research Fellowship scheme (National

447

Research Fellow Award, NRF2012NRF-NRFF001-031), the Earth Observatory of

448

Singapore (EOS), and Nanyang Technological University.

449 450

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S. A. Formation of nitrogen- and sulfur-containing light-absorbing compounds accelerated

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by evaporation of water from secondary organic aerosols. J. Geophys. Res. 2012, 117,

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(59) Gelencśer, A.; Hoffer, A.; Kiss, G.; Tombácz, E.; Kurdi, R.; Bencze, L. In-situ formation of light-absorbing organic matter in cloud water. J. Atmos. Chem. 2003, 45, 25-33.

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(60) Hoffer, A.; Kiss, G.; Blazsó, M.; Gelencsér, A. Chemical characterization of humic-like

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substances (HULIS) formed from a lignin-type precursor in model cloud water. Geophys.

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Res. Lett. 2004, 31, L06115.

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(61) Crounse, J. D.; Nielsen, L. B.; Jørgensen, S.; Kjaergaard, H. G.; Wennberg, P. O.

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Autoxidation of organic compounds in the atmosphere. J. Phys. Chem. Lett. 2013, 4, 3513-

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3520.

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D.; Kulmala, M.; Ehn, M.; Herrmann, H.; Berndt, T. Rapid autoxidation forms highly

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oxidized RO2 radicals in the atmosphere. Angew. Chem. Int. Ed. 2014, 53, 14596-14600.

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Spindler, G.; Sipilӓ, M.; Jokinen, T.; Kulmala, M.; Herrmann, H. Highly oxidized

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multifunctional organic compounds observed in tropospheric particles: A field and

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laboratory study. Environ. Sci. Technol. 2015, 49, 7754-7761.

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Rogge, W. F.; Cass, G. R. Levoglucosan, a tracer for cellulose in biomass burning and

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atmospheric particles. Atmos. Environ. 1999, 33, 173-182.



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(65) Oros, D. R.; Simoneit, B. R. T. Identification and emission factors of molecular tracers in

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organic aerosols from biomass burning Part 1. Temperate climate conifers. Appl. Geochem.

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2001, 16, 1513-1544.

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(66) Gaston, C. J.; Lopez-Hilfiker, F. D.; Whybrew, L. E.; Hadley, O.; McNair, F.; Gao, H.; Jaffe,

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D. A.; Thornton, J. A. Online molecular characterization of fine particulate matter in Port

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Angeles, WA: Evidence for a major impact from residential wood smoke. Atmos. Environ.

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2016, 138, 99-107.

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containing aromatics by atmospheric pressure photoionization or electrospray ionization

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fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom.

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2007, 18, 1265-1273.

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(68) Ng, N. L.; Canagaratna, M. R.; Jimenez, J. L.; Chhabra, P. S.; Seinfeld, J. H.; Worsnop, D.

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R. Changes in organic aerosol composition with aging inferred from aerosol mass spectra.

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Atmos. Chem. Phys. 2011, 11, 6465-6474.

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Mikoviny, T.; Riemer, D.; Sachse, G. W.; Sessions, W.; Weber, R. J.; Weinheimer, A. J.;

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Wisthaler, A.; Jimenez, J. L. Effects of aging on organic aerosol from open biomass burning

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smoke in aircraft and laboratory studies. Atmos. Chem. Phys. 2011, 11, 12049-12064.

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photochemical oxidation and direct photolysis of vanillin - a model compound of methoxy

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phenols from biomass burning. Atmos. Chem. Phys. 2014, 14, 2871-2885.

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688

50, 9990-9997.

689 690



691

Page 32 of 36

FIGURES

a

365 nm 400 nm 500 nm 580 nm

λ

b

c

λ

d

0

1000

2000

3000

4000

5000

6000 mAU

692 693

Figure 1. UPLC-DAD absorption chromatograms at near-UV and visible wavelengths

694

for (a,b) peat (Experiment 2) and (c,d) fern (Experiment 6) burning aerosols. b and d

695

show the blank-subtracted absorption chromatograms at 300–700 nm. a and c show the

696

blank-subtracted absorption chromatograms at selected wavelengths (i.e. 365, 400, 500,

697

and 580 nm). Intensity of absorption is expressed in arbitrary units (mAU) and

698

normalized by aerosol sampling volume.

699 700 32 ACS Paragon Plus Environment

Page 33 of 36


a

near-UV Visible

b

701 702

Figure 2. Relationship of the DBE and the m/z values of the identified BrC constituents

703

in OA from combustion of (a) peat (Experiment 2) and (b) fern (Experiment 6). The

704

marker size indicates normalized abundance of the identified BrC constituents.

705 706 707



Page 34 of 36

-

-

C9H7O3

C11H11O5 -

-

C10H9O4

C9H5O3

C8H7O3 C6H5O3

-

C10H7O4 C9H9O4 -

-

C13H15O4

C13H13O6S -

-

C12H13O7S

C18H19O5S

C23H21O7-

C15H7O5 -

C6H11O4S

_ C7H6NO3 _ C7H6NO4

_

C6H4NO3

_

C6H4NO4

708 709

Figure 3. Concentration of BrC OA constituents that absorb light at 365 nm from each

710

biomass burning experiments.

711 712


Page 35 of 36


a

b

mAU 2.0

λ (nm)

1.5 1.0 0.5 0.0

c -

-

C10H9O4

-

C9H7O3 -

-

C10H9O3 -

C10H11O3 -

-

C10H17O4S

C6H4NO3

C7H6NO3

C6H4NO4

C7H6NO4

C7H4NO5

C10H6NO3

C7H5N2O5

-

C14H17O4

-

C8H8NO3

a b

713 714

Figure 4. Top panel shows UPLC-DAD chromatograms at near-UV and visible

715

wavelengths of ambient aerosols collected when OA mass concentrations are (a) high

716

(79 µg m-3) and (b) low (12 µg m-3). Bottom panel (c) shows the BrC constituents and

717

OA mass concentration measured when Singapore was affected by smoke from Indonesia

718

peatland fires in 2015. a and b on the bottom panel refer to the top panel as well as the

719

periods when ambient OA concentrations are high and low, respectively.

720



721

TOC Art Indonesian Peat

500 λ (nm)

722

Page 36 of 36

T=350 °C

450 400 350 300

6

8

10 R.T. (min)

12

723


Light-Absorbing Brown Carbon Aerosol Constituents from Combustion

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