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Residential Coal Combustion as a Source of Levoglucosan in China Caiqing Yan, Mei Zheng, Amy P. Sullivan, Guofeng Shen, Yingjun Chen, Shuxiao Wang, Bin Zhao, Siyi Cai, Yury Desyaterik, Xiaoying Li, Tian Zhou, Örjan Gustafsson, and Jeffrey L Collett Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05858 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017
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Environmental Science & Technology
Residential Coal Combustion as a Source of Levoglucosan in China
1 2 3
Caiqing Yan†, Mei Zheng†*, Amy P. Sullivan‡, Guofeng Shen∥,a, Yingjun Chen⊥,
4
Shuxiao Wang#, Bin Zhao#, Siyi Cai#, Yury Desyaterik‡, Xiaoying Li†, Tian Zhou†,
5
Örjan Gustafsson§, Jeffrey L. Collett Jr.‡
6 7 8 9 10 11 12
†
SKL-ESPC and BIC-ESAT, College of Environmental Sciences and Engineering,
Peking University, Beijing 100871, China ‡
Department of Atmospheric Science, Colorado State University, Fort Collins,
Colorado 80523, USA ∥
College of Urban and Environmental Sciences, Peking University, Beijing 100871,
China ⊥
Key Laboratory of Cities’ Mitigation and Adaption to Climate Change in Shanghai
13
(China Meteorological Administration), College of Environmental Science and
14
Engineering, Tongji University, Shanghai 200092, China.
15
#
16
School of Environment, Tsinghua University, Beijing 100084, China
17
State Key Joint Laboratory of Environment Simulation and Pollution Control,
§
Department of Environmental Science and Analytical Chemistry (ACES) and the
18
Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden
19
a
20
Laboratory (NRMRL), U.S. EPA, RTP, 200709, U.S.
21
*
presently ORISE postdoctoral fellow at National Risk Management Research
Corresponding author:
[email protected] 1
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TOC
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ABSTRACT
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Levoglucosan (LG) has been widely identified as a specific marker for biomass
26
burning (BB) sources, and frequently utilized in estimating the BB contribution to
27
atmospheric fine particles all over the world. However, this study provides direct
28
evidence to show that coal combustion (CC) is also a source of LG, especially in the
29
wintertime in Northern China based on both source testing and ambient measurement.
30
Our results show that low temperature residential CC could emit LG with emission
31
factors (EF) ranging from 0.3 to 15.9 mg kg-1. Ratios of LG to its isomers, mannosan
32
and galactosan, differ between CC and BB emissions, and the wintertime ratios in
33
Beijing ambient PM2.5 and source specific tracers including carbon isotopic signatures
34
all indicated a significant contribution from CC to ambient levoglucosan in winter in
35
Beijing. The results suggest that LG cannot be used as a distinct source marker for
36
biomass burning in special cases such as some cities in the northern China, where coal
37
is still widely used in the residential and industrial sectors. Biomass burning source
38
could be overestimated although such overestimation could vary spatially and
39
temporally.
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INTRODUCTION
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Levoglucosan (LG, 1, 6-anhydro-β-D-glucopyranose, C6H10O5), a major pyrolysis
42
product of cellulose, has been proposed as a specific molecular marker of biomass
43
burning aerosols.
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contribution of biomass burning sources, and assess the impacts of wood burning,
45
agricultural waste burning and wildfire emissions on ambient air quality in urban,
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rural background and remote areas. 5-8
1-4
It has been widely utilized in numerous studies to identify the
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Previous studies have reported high-levels of atmospheric LG in China, with
48
maxima in cold seasons (fall and winter) and a minimum in the warm season
49
(summer), at urban, suburban and background.
50
studies
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usually occurred in fall followed by winter, and were about 2-5 times higher than in
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summer. The highest LG concentrations in fall in China could be closely associated
53
with the burning of agricultural crop residues in farmland during the autumn
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post-harvest period. 17,18 In winter high ambient LG levels were primarily attributed to
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residential biomass/biofuel (e.g., straws and wood branches) burning for household
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cooking and heating, especially in rural and suburban regions, coupled with more
57
stagnant air and other unfavorable meteorological conditions. 16,19
9,10,15
9-16
For example, some year-long
at urban sites in Beijing reported that the highest LG concentrations
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A recent study based on positive matrix factorization modeling implied that the
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biomass burning source factor only explained 84% of the total variation of LG in
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Barcelona, Spain, 20 suggesting that there might be other potential sources of ambient
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LG. It is known that LG could be produced from combustion and pyrolysis of
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cellulose,
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burning, as well as charcoal broiling.
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of other materials (e.g., fossil fuels), biodegradation or hydrolysis of cellulose do not
4
and unquestionable that it could be emitted from crop straw and wood 21
Previous studies have stated that combustion
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produce any LG.
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emissions, as coal is formed through geologic processes from dead plant matter and
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other organic matter living millions of years ago. A few studies have mentioned that
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combustion of some low quality coals, such as peat, lignite and brown coal could also
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be an additional source of LG in regions where these materials are utilized as major
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household fuels.
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and residential coal combustions in China, but the measured LG was attributed to
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emissions from combustion of the ignition materials, such as woods or paper, whose
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influence was not anticipated and removed in those earlier studies. 8,26
[24,25]
However, in theory, LG might exist in coal combustion
LG was previously reported in the emissions from industrial
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Coal and biomass are two major sources of energy in China. An overwhelming
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majority of China’s residents (including most rural and suburban areas) still use coal
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and biomass (collectively referred to as solid fuels) for domestic cooking and heating
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purposes, 27 especially during the winter heating season. Particularly residents in rural
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areas in China may be inclined to choose low-ranking coal for residential heating and
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cooking as cost is still a primary concern. Raw coals with low coalification and
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without efficient coal washing are widely used. Based on the statistics of coal usage in
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People’s Republic of China by International Energy Agency, the most important coal
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used in the residential sectors is bituminous coal.
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electricity plants, coke is used besides bituminous coal, and in the industry sector,
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more types of coal, including bituminous coal, coke, and anthracite are used, but still
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with bituminous coal dominated. 28 Wang et al. indicate that coal used by residents in
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Beijing-Tianjin-Hebei area in the northern China are mainly low ranked bituminous
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with a small amount of anthracite.
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sources of LG and the relative contribution of each source type is urgently needed to
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reduce uncertainties in source apportionment studies, to accurately estimate the
29
28
While in heating plants and
Therefore, a better understanding of potential
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impact of biomass burning on air quality, and to formulate effective haze control
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strategies.
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The primary goal of this study is to identify if coal combustion is a significant
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source of LG in Northern China. To achieve this, fine particle (PM2.5, particles with
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aerodynamic diameter less than or equal to 2.5 µm) filter samples from source tests
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(e.g., crop straw, wood and coal burning) conducted in the laboratory using typical
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biomass-fired cooking stoves and coal-fired residential stoves were collected.
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Simultaneously, offline filter-based fine particle samples were also collected during
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winter in Beijing. Saccharides (i.e., anhydrosugars, monosaccharides, disaccharides,
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and alcohol-sugars) were examined in both the ambient and source samples. Source
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specific diagnostic indicators were compared for the different source types and
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ambient samples.
102 103
MATERIALS AND METHODS
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Ambient PM2.5 sampling. Ambient PM2.5 samples (over a 24-hr period on a daily
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basis) were collected at an urban site located on the campus of Peking University in
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Beijing, using a high volume sampler (VFC-PM2.5, Thermo Fisher Scientific Co., U.S.,
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with a flow rate of 1.13 m3 min-1) and a co-located four-channel sampler (TH-16A,
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Tianhong, China, 16.7 L min-1, 47 mm i.d. Teflon and quartz filters) from 11 to 31
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January 2013. Quartz filters (8×10 inch, 2500 QAT-UP, Pall Corp., NY, USA) were
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prebaked at 550℃ for 6 h in a muffle furnace and wrapped in pre-combusted
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aluminum foil. All filter samples were stored in triple plastic bags in a refrigerator at
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-20℃ after sampling until analysis. Field blanks were collected at the beginning and
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end of the sampling period with the same operation procedure but without pump on.
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Source testing and source sample collection. Source testing was performed in the 6
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laboratory of Residential Combustion Simulation at Peking University Shenzhen
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Graduate School. The burning simulation system (see Figure 1) in the laboratory has
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been described in previous studies. 30,31 The difference here is the combustion pan has
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been changed to biomass-fired cooking stoves or coal-stoves respectively.
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combustion device was equipped with a vertical chimney. Smoke emissions from the
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chimney were then collected by a fume hood, and went through a flue duct, where
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emission exhaust was then isokinetically withdrawn by a sampling probe and mixed
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with zero air (15 times for crop straws, and 5 times for coals and woods) in a dilution
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system. PM2.5 from the mixing chamber was collected on quartz (pre-baked at 550 ℃
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in muffle furnace) and Teflon filters through four PM2.5 cyclones, each with a flow
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rate of 16.7 L/min. Altogether, 5 types of crop straws, 8 types of wood logs, and 6
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types of domestic coals with different geological maturity (e.g., high-volatile
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bituminous coal (HVB), medium-volatile bituminous coal (MVB), semi-anthracite
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(SA)) were collected from some agricultural provinces and the main coal-mining
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regions in China, such as Shandong, Shanxi, Heilongjiang, and Hunan provinces. The
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main characteristics of these solid fuels are presented in Table S1, and the details of
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the proximate and ultimate analysis results of coals were shown in Table S2. Crop
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straws, woods and coals were burned in commonly used biomass-fired cooking stoves
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(Figure S1a) and residential coal-stoves in China (Figure S1b for bitumenite, and
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Figure S1c for honeycomb briquette), respectively. It is noted that these residential
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stoves were not equipped with any emission control technologies.
32
The
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Approximately 2.35±0.26 kg crop residue, 1.94±0.39 kg wood and 1.62±0.31 kg
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coal were weighed and burned during each testing cycle (each sample) to provide
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enough heat to boil 15 kg water. The pre-weighed crop straws were ignited directly in
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the stove, while woods and coals were ignited outside of the laboratory with 7
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alcohols/biomass and woods/burned coals, respectively. After ignition, the burned
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woods or coals were moved to ignite the pre-weighed woods or coals in the stove,
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respectively, in order to remove the influence of ignition materials. Each fuel was
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tested at least twice to ensure repeatability. Before each combustion test, one 30-min
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system blank operation was conducted, which was similar to the operation of source
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testing but without fuel burning, to reduce the risk of cross contamination between
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samples. A negligible level of particulate matter was found in the blank samples. All
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data presented below were blank corrected.
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Chemical analysis. Organic carbon (OC) and elemental carbon (EC) were
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analyzed by the Desert Research Institute’s (DRI) Thermal/Reflectance Optical
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Carbon Analyzer (Model 2001 A) following the thermal/optical reflectance (TOR)
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IMPROVE A protocol. Total water-soluble organic carbon (WSOC) as well as
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water-soluble saccharides such as anhydrosugars (LG, mannosan and galactosan),
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monosaccharides (arabinose, galactose, glucose, mannose and xylose) and
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alcohols/polyol- sugars (glycerol, inositol, threitol and mannitol) were measured for
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all ambient and source samples. The details of the analytical protocol are available in
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Sullivan et al.
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quartz filter sample was extracted with 10 mL deionized water (DI water, >18 MΩ) by
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sonication for 30 min, then filtered using a 0.2 µm PTFE membrane filter (Whatman
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Inc.). A 1 mL aliquot was used for the WSOC measurement (Sievers Model 800 Turbo
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Total Organic Carbon (TOC) Analyzer). Another 1 mL aliquot was used to analyze for
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saccharides using a Dionex DX-500 series ion chromatograph with an ED-50
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electrochemical detector operating in integrating amperometric mode using waveform
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A and a GP-50 gradient pump. It should be noted that all the carbonaceous component
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analyses for each sample were performed with the same quartz filter, and
33,34
Briefly, one 47-mm punch of each ambient and source testing
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water-soluble species analyses were based on the same water extractions.
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For ambient samples, an aliquot of the water extracts was used for water-soluble
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ion (e.g., K+, NH4+, Mg2+, Ca2+, NO3-, SO42-) analysis by ion chromatography
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(DIONEX, ICS-2500) following the method described by Guo et al. 35 Water extracts
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of eight ambient samples were further treated for
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method described previously. 36,37 Briefly, the water extracts were freeze-dried and
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re-dissolved in 150 µl hydrochloric acid to be de-carbonated, and then transferred into
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pre-combusted silver capsules and evaporated and dried in an oven at 60 ℃. The
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WSOC isolates were then sent to the U.S. National Ocean Sciences Accelerator Mass
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Spectrometry (NOSAMS) facility at Woods Hole Oceanographic Institution (MA,
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USA) for high-precision
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fossil and non-fossil source WSOC (WSOC_F & WSOC_NF) could be found in
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previous studies. 36, 37 Briefly, the fraction modern (Fm) was determined by comparing
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the measured 14C/12C ratios in the blank, ambient samples, and modern standard (i.e.,
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NBS Oxalic Acid I in AD 1950), and then converted into the fraction of the
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contemporary carbon (Fc) by normalization with a conversion factor of 1.06, to
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correct the ambient 14C excess due to the atmospheric nuclear bomb tests in the 1950s.
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Finally, the fractional contributions of contemporary biomass/biogenic sources versus
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radiocarbon-extinct fossil fuel sources were determined based on the isotopic mass
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balance.
14
14
C analysis of WSOC with the
C-AMS analysis. The detailed calculation processes for
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In addition, ambient samples were digested by 4.5 mL nitric acid, 1.5 mL
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hydrochloric acid, and 0.2 mL hydrofluoric acid at 190 ℃ for 40 min, and then
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diluted to 10 mL with DI-water after digestion and cooling, and finally 23 trace metals
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(e.g., Al, Fe, Mn, Zn, As, Cu, Co, Se, Mo, Cr) were analyzed by inductively coupled
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plasma mass spectrometry (ICP-MS). Details of this method could be found in 9
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RESULTS AND DISCUSSION
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Direct evidence of LG from residential coal combustion emissions. In this study,
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it was found that LG was detectable in all the tested coal combustion source samples
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and in all crop straw and wood burning source samples. As the ignition procedures
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utilized in the study ensured that the emissions were solely from the combustion of
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the pure tested fuels (e.g., coal), this study provides direct evidence that residential
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coal combustion could also be a contributor of LG.
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In general, LG accounted for the majority of the measured sugar compound mass in
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all kinds of source emissions (Coal: 54.7±7.8%; Crop straw: 74.2±11.1%; Wood:
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77.4±13.3%),
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6-anhydro-β-mannopyranose)
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6-anhydro-β-D-galactopyranose), which are produced from pyrolysis of hemicellulose.
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However, the saccharide profiles observed in the emissions from different solid fuel
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combustions were distinct (see Figure S2). For example, unlike biomass burning
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emissions, arabinose was not detectable in most of the coal combustion emissions.
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Furthermore, the percentages of glycerol and monosaccharides (galactose, glucose,
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mannose and xylose) were much higher in coal combustion samples compared to
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biomass burning samples.
followed
by
its
two and
isomers,
mannosan
galactosan
(MN,
(GA,
1, 1,
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Emission factors of source-specific LG. Emission factor (EF), expressed in
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emitted pollutant mass per kg of fuel consumed, is calculated based on equations (1)
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and (2).
EFf ,i = 213
m f ,i ∆mdry , f
×
Q0 × DR1 × DR2 Qt 10
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(1)
Q0 = v0π ( 215 216
D0 2 ) 2
(2) Where
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EFf ,i
m f ,i
represents EFs of species i from fuel f;
∆mdry , f
is the collected mass of
is the mass of burned fuels f (dry basis); DR1 and
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species i from fuel f;
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DR2
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Qt
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dilution tunnel and determined by equation (2);
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and smoke velocity (m s-1) of the first dilution tunnel, respectively.
represent the dilution ratios in the first and second dilution tunnel, respectively;
is the sampling volume during the sampling time t;
D0
Q0
is the flow rate in the first
v0
represent the diameter (m)
and
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Figure 2 shows the LG emission factors (EFLG, mg/Kg-1) for the different fuels
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(i.e. crop straw, wood, coal). The coefficient of variation (CV) based on the repeated
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tests for each type of fuel was within the range of 0.044-0.49, much lower than that
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for the different fuel types (CV: 0.82-1.61), indicating a larger variation of EFLG due
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to the diversities in the fuels themselves. The EFLG was within the range of 0.3-15.9
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mg Kg-1 for coals, 11.7-429 mg Kg-1 for crop straws, and 5.2-147 mg Kg-1 for woods.
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The averaged EFLG for different solid fuels ranks from high to low as crop straw (Ave.
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± Std.: 133±125 mg Kg-1) > wood (Ave. ± Std.: 46.7±41.6 mg Kg-1) > coal (Ave. ±
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Std.: 3.1±4.9 mg Kg-1) (see Figure 2A). That is, biomass burning sources emit more
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LG per unit fuel weight, about 1-3 orders of magnitude higher than coal combustion
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emissions (see Figure 2B).
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Of different biomass fuels, rice straws collected in north China exhibited the
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highest EFLG with an average of 326±91.6 mg Kg-1, followed by wheat straw
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(173±59.7 mg Kg-1) and rice straw collected in south China (121±37.0 mg Kg-1), 11
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respectively, which were comparable to the reported EFLG of 150±130 mg Kg-1 for the
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simulated open wheat burning conducted in the U.S. EPA burn test facility by
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Dhammapala et al.
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with the highest EFLG of 92.8-147 mg Kg-1 for oak and the lowest EFLG of ~5.2 mg
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Kg-1 for locust; the average EFLG for the other woods was 29.0±5.2 Kg-1. It is noted
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that, the two semi-anthracite
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Shanxi province exhibited the highest EFLG for coals, with averaged EFLG value of
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12.7±4.6 mg Kg-1 and 4.0±0.2 mg Kg-1, respectively, followed by the honeycomb
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briquette made by anthracite from Jaingmen, Guangdong province (JM-SA, 0.63 mg
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Kg-1). They were all higher than those from high-volatile bituminous coals mined in
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Yanzhou, Shandong province (YZ-HVB, 0.43 mg Kg-1), and medium-volatile
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bituminous coals mined in Jixi, Heilongjiang province (JX-MVB, 0.44 mg Kg-1) and
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Baotou, Inner Mongolia (BT-MVB, 0.35±0.06 mg Kg-1). It is reported in previous
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studies that LG was more likely from combustion of low-rank (low geologic maturity
251
coals.
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maturity and LG emission, which could be examined in more details in future study.
24, 25
39
EFLG for different wood fuels had a relatively small variation,
mined in Yangquan (YQ-SA) and Jincheng (JC-SA),
This study suggests that it is not a simple relationship between coal
253
Estimated total emissions of LG from residential solid fuels in northern China.
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Although the EFLG for coal (EFLG_coal) is the lowest among solid fuels in this study,
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the total emission of LG from coal combustion maybe substantial as there is large coal
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combustion in China. China is the leading coal producer in the world. 40 According to
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the China Statistical Yearbook, coals account for 20% of the total residential energy in
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the residential sectors in 2013. 41 Based on the same activity data for biomass and coal
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in domestic sectors described in Ma et al.
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measured in this study, LG emissions from domestic solid fuels were roughly
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estimated (see Text S1). Given that most of the solid fuels tested in this study were
42
and the EFLG_coal, EFLG_crop and EFLG_wood
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mainly collected and used in northern China, especially coals were mined from
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several typical mines in 4 provinces (e.g., Shanxi, Inner Mongolia, Heilongjiang, and
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Shandong) in northern China, estimations here were only performed in northern China
265
(see Figure 3).
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LG emissions from residential coal combustion were lower compared to those from
267
residential biomass burning in all provinces in northern China (see Figure 3A). Total
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LG emissions from domestic coal combustion in northern China estimated here were
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approximately 5.26±4.31 (0.42-14.39) t yr-1 vs. 1.18±0.896 (0.046-2.89) kt yr-1 for
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domestic biomass burning emissions. That is, on the average, coal combustion
271
contributed approximately only ~1 % of the total LG emissions from the domestic
272
sector in northern China in the year of 2013. However, it should be noted that in
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Beijing, the capital of China, the LG emission from residential coal combustion in
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2013 totaled about 3.23 t yr-1, accounting for about 7% of the total LG emissions from
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residential sectors, much higher than the percentages in other provinces (see Figure
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3B and Table S3). This reflects the relatively high usage of coal (432×104 tce yr-1,
277
where tce is ton of standard coal equivalent) compared to biomass (37×104 tce yr-1)
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in residential sectors in Beijing.42 That is, the coal usage in the residential sectors in
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Beijing was about 12 times of the biomass, which was similar to the cases in Neimeng
280
and Shanxi provinces, where coal usage was larger than that of biomass. Besides, it is
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noteworthy that the percentage of coal combustion contribution to total LG emissions
282
in cities in northern China (e.g., Beijing) could be much higher during the wintertime
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due to increased coal consumption for heating and cooking.
284
Diagnostic indicators for solid fuel emissions. Ratios of LG to PM2.5 and other
285
co-emitted carbonaceous components were compared among the different solid fuels
286
and were listed in Table 1. In general, LG constituted 2.0±1.5% and 2.0±1.6% of 13
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emitted PM2.5 mass in crop straws and woods, respectively, consistent with previously
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reported values ranging from 2 to 10%.
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0.3±0.4% of the PM2.5 mass from coal combustion emissions.
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3,39,48,49
By contrast, LG only accounted for
Ratios of LG to carbonaceous components. The LG/OC ratio, a ratio used for 6,50
291
estimating contributions of biomass burning in many previous studies,
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0.018±0.012
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(0.00003-0.025) (carbon/carbon basis, µgC µg-1C) in this study for crop straw, wood
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and coal combustion products, respectively. LG/OC ratios were significantly different
295
between coal and biomass (i.e., crop straw and wood) burning emissions based on a
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t-test (p