Chemical composition, optical properties, and oxidative potential of water-and methanol-soluble organic compounds emitted from the combustion of biomass materials and coal

Abstract. Biomass burning (BB) and coal combustion (CC) are important sources of brown
carbon (BrC) in ambient aerosols. In this study, six biomass materials and
five types of coal were combusted to generate fine smoke particles. The BrC
fractions, including water-soluble organic carbon (WSOC), humic-like
substance carbon (HULIS-C), and methanol-soluble organic carbon (MSOC), were
subsequently fractionated, and their optical properties and chemical
structures were then comprehensively investigated using UV–visible
spectroscopy, proton nuclear magnetic resonance spectroscopy (1H NMR),
and fluorescence excitation–emission matrix (EEM) spectroscopy combined with
parallel factor (PARAFAC) analysis. In addition, the oxidative potential
(OP) of BB and CC BrC was measured with the dithiothreitol (DTT) method. The
results showed that WSOC, HULIS-C, and MSOC accounted for 2.3 %–22 %,
0.5 %–10 %, and 6.4 %–73 % of the total mass of combustion-derived
smoke PM2.5, respectively, with MSOC extracting the highest
concentrations of organic compounds. The MSOC fractions had the highest
light absorption capacity (mass absorption efficiency at 365 nm
(MAE365): 1.0–2.7 m2/gC) for both BB and CC smoke, indicating
that MSOC contained more of the strong light-absorbing components.
Therefore, MSOC may represent the total BrC better than the water-soluble
fractions. Some significant differences were observed between the BrC
fractions emitted from BB and CC with more water-soluble BrC fractions with
higher MAE365 and lower absorption Ångström exponent values
detected in smoke emitted from BB than from CC. EEM-PARAFAC identified four
fluorophores: two protein-like, one humic-like, and one polyphenol-like fluorophores. The
protein-like substances were the dominant components of WSOC
(47 %–80 %), HULIS-C (44 %–87 %), and MSOC (42 %–70 %). The
1H-NMR results suggested that BB BrC contained more oxygenated
aliphatic functional groups (H-C-O), whereas CC BrC contained more
unsaturated fractions (H-C-C= and Ar−H). The DTT assays indicated that BB
BrC generally had a stronger oxidative potential (DTTm, 2.6–85 pmol/min/µg) than CC BrC (DTTm, 0.4–11 pmol/min/µg), with
MSOC having a stronger OP than WSOC and HULIS-C. In addition, HULIS-C
contributed more than half of the DTT activity of WSOC (63.1 % ± 15.5 %), highlighting that HULIS was a major contributor of reactive oxygen species (ROS) production
in WSOC. Furthermore, the principal component analysis and Pearson
correlation coefficients indicated that highly oxygenated humic-like
fluorophore C4 may be the important DTT active substances in BrC.



Biomass and coal samples
19%, respectively) and one anthracitic coal (with a volatile fraction of 3.3%). These five 66 coals represented the major types of coal used for residential CC in China. After collection, 67 the coals were washed with water three times to remove dust and then air-dried. Then raw 68 coal was crushed, fully mixed, and made into coal briquettes. Co., Ltd, Qindao, China), and exhaust port. The smoke samples emitted from BB and CC 77 were then collected as follows:

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(1) Biomass burning smoke samples. The biomass fuels were first prepared as small 79 pieces (length ~10 cm) and then placed on a combustion stove. After dropping 1 mL of 80 alcohol on the biomass fuels they were ignited with an electronic gas lighter. The smoke 81 particles were diluted and transported into the mixing chamber. Finally, smoke particles were 82 collected on quartz fiber filters (Ø 90 mm: Whatman, Maidstone, UK) in a PM 2.5 sampler at a 83 flow rate of 80 L/min. Five complete experiments were conducted for each biomass fuel and 84 five smoke PM 2.5 filter samples were obtained.

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(2) Coal combustion smoke samples. The smoke particles emitted from the CC samples 86 were also collected in the same combustion and sampling system. Sample collection was S6 ultrasonically extracted three times with 20 mL ultrapure water for 30 min.    The OC and EC in smoke filter samples were measured using an OC/EC analyzer (TOT, 145 The UV-visible absorption spectra of the BrC fractions (i.e., WSOC, HULIS, and MSOC) 146 were recorded between the wavelengths of 200 to 700 nm using a UV-2600 UV-vis 147 spectrophotometer (Shimadzu, Japan). The sample solution was placed in a 1-cm quartz 148 cuvette and analyzed at 1 nm intervals. Ultrapure water was used as a blank reference for the 149 WSOC and HULIS solutions, while pure methanol was used for the MSOC fraction. The 150 field blank sample solution was also used as the blank sample, and the interference from the 151 instrument and operating blank was determined.

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The absorption Ångström exponent (AAE) is a measure of the spectral dependence of S8 the light absorption of BrC solutions (Cheng et al., 2016), which was calculated by the 154 following equation: where A λ is the absorbance derived from the spectrophotometer at a given wavelength λ (330-157 400 nm) and K is a constant.

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The mass absorption efficiency at 365 nm (MAE 365 ) is an important parameter used to 159 characterize the light absorbing ability of BrC. It was obtained using the following equation: where A λ is the absorbance at λ nm, c is the carbon concentration of BrC in solution (gC 162 mL −1 ), and L is the absorbing path length. respectively. The repeatability of analysis procedure was obtained based on one blank filter 181 sample, which were 3.1%, 2.2% and 4.5% for WSOC, HULIS and MSOC, respectively.

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The field blank filter (ambient and ignition coal (for coal combustion only)) were μgC/cm 2 , respectively. They were much less than the values of that in smoke particle.