Abundance and Light Absorption Properties of Brown Carbon Emitted

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Characterization of Natural and Affected Environments

Abundances and light absorption properties of brown carbon emitted from residential coal combustion in China Meiju Li, Xingjun Fan, Mengbo Zhu, Chunlin Zou, Jianzhong Song, Siye Wei, Wanglu Jia, and Ping'an Peng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05630 • Publication Date (Web): 25 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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Environmental Science & Technology

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Abundances and light absorption properties of brown carbon emitted from

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residential coal combustion in China

3 4

Meiju Li1,3, Xingjun Fan2,4, Mengbo Zhu1,3, Chunlin Zou1,3, Jianzhong Song1*, Siye Wei1,5,

5

Wanglu Jia1, Ping’an Peng1,3

6

1State

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of Environmental Protection and Resources Utilization, Guangzhou Institute of

8

Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

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2College

Key Laboratory of Organic Geochemistry and Guangdong Provincial Key Laboratory

of Resource and Environment, Anhui Science and Technology University, Anhui

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233100, China

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3University

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4Anhui

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233400, China

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5South

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Guangzhou 510655, China

of Chinese Academy of Sciences, Beijing 100049, China

Province Key Laboratory of Biochar and Cropland Pollution Prevention, Bengbu

China Institute of Environmental Sciences, Ministry of Environmental Protection,

16 17 18

* Correspondence to: Jianzhong Song ([email protected])

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1

ACS Paragon Plus Environment


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TOC Art

23 24 25

UV-Vis absorption

26

28

Multi-solvent Extraction

2 Specific absorbance (m /gC)

27

YL (chunk)

0.12

WSOC HULISw ASOC MSOC

0.10 0.08 0.06 0.04 0.02

29

0.00 250

300

350

400

Wavelength (nm)

30 31 32 33 34 35

2


450

500

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ABSTRACT

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Brown carbon (BrC) fractions, including water soluble organic carbon (WSOC), water

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soluble humic-like substances (HULISw), alkaline soluble organic carbon (ASOC) and

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methanol soluble organic carbon (MSOC) were extracted from particles emitted from the

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residential combustion of coal with different geological maturities. The abundances and light

41

absorption properties of these BrC fractions were comprehensively studied. The results

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showed that the abundances of the different constituents of the BrC fraction varied greatly

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with the extraction solvent, accounting for 4.3%–46%, 2.3%–23%, 3.2%–14%, and 76%–

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98% of the total carbon content in particles. The specific UV-Vis absorbance (SUVA254) of

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BrC fractions followed the order of MSOC > ASOC > HULISw > WSOC. The WSOC and

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MSOC fractions from the combustion of low maturity coal had relatively low SUVA254 and

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high SR values. The mass absorption efficiencies (MAE365) for ASOC and MSOC were

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higher than for WSOC, and WSOC and MSOC from low maturity coal combustion had

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relatively low levels of light absorption. These findings indicated that coal combustion is a

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potential source of atmospheric BrC and the abundance and light absorption of the coal

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combustion-derived BrC fractions were strongly dependent on the extraction methods used

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and the coal maturity rather than the coal shapes.

53 54

INTRODUCTION

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In recent decades, a type of light absorbing organic carbon (OC) has received much

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global attention because of its significant effects on atmospheric chemistry, air quality, and

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climate change.1-3 This organic fraction is commonly referred as brown carbon (BrC), and is 3



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strongly wavelength dependent, mainly absorbing light in the low wavelength visible and

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near ultraviolet regions.1, 4-9 The chemical composition of BrC is very complex, the physical,

60

chemical, and optical properties of BrC vary greatly depending on the sources.2, 4, 9, 10 Many

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studies have demonstrated that the sources of atmospheric BrC are mainly primary emissions

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from the incomplete combustion of biomass and fossil fuels

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formation processes.

64

considered to be the most significant source of atmospheric BrC.

65

coal) are consumed in substantial amounts around the world, and are also believed to

66

contribute significantly to atmospheric BrC.

67

comprehensively investigate BrC materials emitted from the combustion of fossil fuels to

68

better understand the impact of these anthropogenic emissions on the atmospheric

69

environment and climate change.

2, 12, 13

1, 4, 7, 10, 11

and secondary

Among the primary sources, biomass burning (BB) is generally

10,

15

10, 11, 14

Fossil fuels (e.g.,

Therefore, it is necessary to

70

In some countries, including China, coal is the dominant fuel used for energy production.

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According to the National Bureau of Statistics of China (2016), coal consumption (4000 Tg)

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accounts for 68% of China’s total primary energy, ~ 93 Tg of which is used as fuel for

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households throughout the year.16 Because of the poor combustion conditions and lack of

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emission control facilities, the widespread use of coal for residential purposes can lead to

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huge emissions of carbonaceous particles, including OC, black carbon (BC), and polycyclic

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aromatic hydrocarbons (PAHs).

77

BrC released from coal combustion.

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abundance and optical properties in ambient aerosols and BB aerosols.

79

gaps in our knowledge regarding the abundance, chemistry, and optical properties of primary

14, 15, 17, 18

However, there have been few studies of primary

10, 15

Most current studies of BrC have focused on its

4


19-22

There are still

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BrC from coal combustion, especially in the residential sector.

81

BrC contains a complex mixture of light absorbing organic compounds, which can be

82

extracted and separated from the insoluble residues. The solvent-extractable BrC fraction can

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then be measured with UV–Vis spectrometry and high-resolution mass spectrometry, and so

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on. 4, 23, 24 This approach has the advantage of excluding interference from BC and providing

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the complete absorption spectra of BrC, but also has some limitations, such as the inability of

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water and even organic solvents (e.g., methanol and acetone) to extract all of the organic

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species.4,

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studies. 4, 23-26 For example, pure water is commonly used as an extraction solvent to separate

89

water soluble OC (WSOC) and water soluble humic like substance (HULISw) from the other

90

fractions in atmospheric aerosols. For samples of atmospheric particles, alkaline solutions

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and organic solvents have been found to extract more light absorbing organic substances than

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pure water.

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solvents are generally different. It has been reported that methanol soluble OC (MSOC) has a

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higher light absorbing ability than WSOC in atmospheric aerosols and smoke particles. 4, 23-25

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Therefore, to fully investigate BrC in smoke particles, an intensive extraction with water,

96

alkaline solution, and organic solvent is necessary.

25

Despite this, the approach has been frequently used to separate BrC in many

4, 23, 27

The chemical and optical properties of BrC extracted using different

97

The aim of this study was to further understand the abundance and optical properties of

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primary BrC emitted from household coal combustion in China. For this purpose, smoke

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particles emitted from the combustion of household coal, with low to high geological

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maturity, were collected in a laboratory sampling system. Two shapes of each type of coal

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were tested: raw-coal chunks and honeycomb briquettes. The extractable BrC fractions in 5



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particles were extracted and isolated with water, alkaline solution, and organic solvent. Then,

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their abundance, and optical properties were determined using a total organic carbon (TOC)

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analyzer and UV-Vis spectroscopy. As a laboratory controlled study, the work was easier to

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control and repeat than field tests, which allowed us to test the effects of coal maturity and

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briquettes by fixing other conditions. The information obtained will enable a better

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understanding of primary BrC emitted from household coal combustion, and its role in the

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importance to the atmospheric environment and climate change.

109 110

MATERIALS AND METHODS

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Coal samples. Six coal samples were prepared for the study (Table S1). They consisted

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of five bituminous coals (Yulin [YL], Shuangyashan [SYS], Yongcheng-1 [YC-1],

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Yongcheng-2 [YC-2], Hebi [HB]), and one anthracitic coal (Jiaozuo [JZ]). These six coals

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covered a wide range of geological maturities (vitrinite reflectance (Ro), 0.55%–2.0%) (Table

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S1) and were selected to be representative of residential coals in China.

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Each type of coal was prepared in samples of two shapes: a raw-coal chunk and a

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honeycomb briquette. They were combusted in traditional chunk and briquette stoves. The

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details of the two household stoves are presented in the SI (Figure S1).

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Collection of particle samples from coal combustion. The particle samples emitted

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from residential coal combustion were collected in a laboratory sampling system, the details

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of which are given in the SI (see Section S2, Figures S1 and S2).

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Extraction and fractionation of BrC. In this study, the BrC fractions (WSOC,

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HULISw, alkaline soluble OC (ASOC), and MSOC) were obtained using solvent extraction 6


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and solid-phase extraction isolation methods. Although these four BrC fractions (WSOC,

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HULISw, ASOC, and MSOC) contained not only the light absorbing BrC fractions but also a

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certain quantity of non-light-absorbing OC, they have been widely accepted and used as

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surrogates for BrC.

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provided in Figure S3 and section S3 in the SI.

4, 6, 22-24, 28

The details of the analytical procedures used in the study are

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The TC content of smoke samples and the OC content of HULISw, WSOC, and ASOC

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solutions were measured using a thermal-optical carbon analyzer and a high-temperature

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catalytic oxidation instrument, respectively.

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Measurement of UV-Vis spectra. The UV-Vis absorption spectra of the BrC solutions

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(WSOC, HULISw, ASOC, and MSOC) were measured using a UV-Vis spectrophotometer

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(UV-2600, Shimadzu) between the wavelengths of 200 to 700 nm. To characterize the

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chemical and optical properties of BrC, the absorbance at 250 nm (UV250), specific UV-Vis

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absorbance at 254 nm (SUVA254), slope ratio (SR), absorption Ångström exponent (AAE),

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and the mass absorption efficiencies at 365 nm (MAE365) of BrC fractions were calculated.

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The details of calculation are presented in section S5 in the SI.

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RESULTS AND DISCUSSION

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Abundances of WSOC, HULISw, ASOC, and MSOC in smoke particles. Table 1

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shows the average abundances of WSOC, HULISw, ASOC, and MSOC in coal smoke

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particles. The WSOC was obtained by direct extraction, and the average contribution of

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WSOC to the TC of smoke particles was 4.3%–16% for bituminous coal and 24%–46% for

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anthracite, respectively. This result for bituminous coal was comparable to the WSOC result 7



10

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(11.2%) for household coal reported in our previous study,

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WSOC results (14%–56%) for BB smoke PM2.5 samples.

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WSOC for bituminous coal than anthracite could be ascribed to the fact that the bituminous

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coal contained higher concentrations of volatile and semi-volatile hydrophobic aromatic

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organic components than anthracite.

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emitted from bituminous coal combustion containing a relatively high content of water

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insoluble

153

nitrogen/sulfur-containing heteroatomic PAHs. 29, 30

organic

compounds,

15, 29

but lower than the reported

10, 11

The lower contribution of

These differences might result in the particles

such

as

hydrophobic

parent

PAHs

and

154

The ASOC fraction was subsequently extracted by an alkaline solution extraction of the

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residue particles after the water extraction. As shown in Table 1, the average contributions of

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ASOC to TC in coal smoke particles were 3.3%–14% and 3.2%–6.8% for coal chunks and

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briquettes, respectively. These results suggest that ASOC was an important organic material

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in coal combustion emissions and could potentially contribute to the ASOC fraction in the

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atmospheric aerosol. There was a lesser contribution of ASOC in the smoke from bituminous

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coal chunks than in anthracite chunks, but no significant differences were observed in the

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smoke from bituminous coal briquettes and anthracite briquettes. Moreover, the sum of the

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WSOC and ASOC levels were investigated, and were termed BrC extractable by inorganic

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solvents. The contribution of WSOC + ASOC to TC in smoke particles was 8.3%–59% and

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8.0%–29% for coal chunks and briquettes, respectively (Table 1). The MSOC fraction is often seen as a BrC surrogate in the atmospheric environment.6, 9,

165 166

24

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smoke particles for coal chunks and briquettes, respectively. These large contributions were

As shown in Table 1, the MSOC fraction accounted for 76%–97 and 95%–98% of TC in

8


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comparable with the data (>90%) for particles from BB reported by Chen and Bond (2010).4

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In addition, MSOC has been reported to account for ~90% of the OC in Beijing winter

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aerosols.24 The enrichment of MSOC in particles may be the result of the considerable

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amounts of OC rather than the elemental carbon produced during the combustion of coal in

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household stoves during winter.

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HULISw are important hydrophobic compounds within the WSOC in ambient aerosols,

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and have received much attention in recent years.20,

26, 31, 32

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HULISw/WSOC (%) values in smoke particles are 30%–63% and 42%–71% based on the

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TOC and UV250 measurements, respectively (Figure S4). This indicates that HULISw was

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the major component of WSOC in smoke particles. These figures were comparable to the

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results (46  2.1% by TOC measurement and 64  3.9% by UV250 measurement) for coal

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smoke particles reported in our previous study.10 In terms of TOC content, they were slightly

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lower compared to the values (57%–66%) reported for smoke particles from the burning of

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rice straw, corn straw, and pine branches10, 11 and the range of 63%–76% reported for ambient

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aerosols heavily impacted by tropical BB.33 In addition, the values of HULISw/WSOC ratios

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determined with UV250 measurements were always higher than those obtained by TOC

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measurements, which could be due to the enrichment of strongly light absorbing compounds

185

in the HULISw fraction as suggested in previous studies. 10, 26, 34, 35

In the present study, the

186

The influence of extraction solvents on the abundances of BrC fractions. As shown

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in Table 1, the average abundances of the coal smoke BrC fractions quantified with water,

188

alkaline solution, and methanol were very different. MSOC accounted for almost all of the

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TC, ranging from 93% to 98% for bituminous coal and 76% to 98% for anthracite. The 9



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abundances of the MSOC fraction were significantly higher than that of the WSOC and

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ASOC fractions and even much higher than the sum of the WSOC and ASOC fractions. This

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indicated that most of the organic compounds in smoke particles were soluble in pure

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methanol, but were insoluble in water and alkaline solutions. These organic fractions

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accounted for 72%–89% of the TC for bituminous coal and for 16%–69% of the TC for

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anthracite, respectively. The results indicated that methanol was the most effective solvent to

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extract OC. However, the chemical and optical properties of BrC extracted with different

197

solvents are generally different and therefore play different roles in the atmospheric

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environment. For example, the water soluble BrC fractions (WSOC and HULIS) can directly

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absorb light and will also have indirect climate effects by enhancing the ability of aerosol

200

particles to act as cloud condensation nuclei.36 Organic solvents, especially methanol, have

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been found to extract much greater amounts of light absorbing organic substances than water.

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Therefore, to better understand BrC fractions in particles, multiple solvent extractions of BrC

203

are necessary.

204

A significant positive correlation was found between coal maturity and the differences in

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BrC fractions (HULISwWSOC, ASOCWSOC, and MSOCWSOC) (r = 0.706, 0.497,

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0.605, p < 0.01; Table S4). In addition, there was a strong negative correlation between the

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maturity and volatile matter content of the coals (r = 0.952, p ASOC >

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HULISw > WSOC. The MSOC fraction contained many more strongly light absorbing 13



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compounds (i.e., aromatic and conjugated systems) than the ASOC, HULISw, and WSOC

279

fractions. As a hydrophobic component of WSOC, HULISw had a relatively higher

280

normalized absorbance than WSOC. Therefore it is obvious that the optical properties of

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extractable primary BrC varied significantly depending on the solvents. This indicates the

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need to use multiple solvents, including water, alkaline solutions, and organic solvents, for

283

the extraction of BrC from smoke particles and for further investigating their chemical and

284

optical properties.

285

In addition to the absorbing intensities, the shapes of the UV-Vis spectra were different,

286

especially within the UV region. As an example, a clear peak in the region of 250 to 300 nm

287

was observed in the UV-Vis spectra of the BrC fractions including WSOC, HULISw, ASOC,

288

and MSOC for coal YL (raw-chunks), although the peak positions were different. This peak

289

in the spectra of the WSOC and HULISw fractions was located at ~275 nm; however, it

290

shifted to a lower wavelength (~255 mm) for the ASOC and MSOC fractions. This difference

291

in the absorbing peak may indicate that some weakly conjugated compounds were contained

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within the MSOC and ASOC fractions.

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SUVA254 and SR. Spectroscopic parameters, such as SUVA254 and SR, obtained from

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UV-Vis spectra, have been found to be correlated with the aromaticity and MW of natural

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humic acids, WSOC, and HULIS in ambient aerosols.26,

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SUVA254 values were 2.6–4.6, 3.1–4.9, 4.3–7.5, and 6.8–13 m2/gC for WSOC, HULISw,

297

ASOC, and MSOC from the combustion of bituminous coal and 3.4–4.6, 4.5–5.6, 4.9–7.8,

298

and 10–15 m2/gC for anthracite coal, respectively. The SUVA254 values of coal smoke

299

HULISw are generally higher than those of atmospheric HULISw10, 26, 35, suggesting that coal

35, 43, 46

14


In the present study, the

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combustion derived HULIS may contain more aromatic structures. For each type of coal, the

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SUVA254 values of the extracted BrC fractions followed the order of WSOC < ASOC

Abundance and Light Absorption Properties of Brown Carbon Emitted

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