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

• Main Authors: T. Cao, M. Li, C. Zou, X. Fan, J. Song, W. Jia, C. Yu, Z. Yu, P. Peng
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/13187/2021/acp-21-13187-2021.pdf
• ISSN: 1680-7316; 1680-7324

<p>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 (<span class="inline-formula">¹</span>H 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 PM<span class="inline-formula">_2.5</span>, 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 (MAE<span class="inline-formula">₃₆₅</span>): 1.0–2.7 m<span class="inline-formula">²</span>/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 MAE<span class="inline-formula">₃₆₅</span> 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 <span class="inline-formula">¹</span>H-NMR results suggested that BB BrC contained more oxygenated aliphatic functional groups (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">H</mi><mo>-</mo><mi mathvariant="normal">C</mi><mo>-</mo><mi mathvariant="normal">O</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="42pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="ea6d310d597e63d28276ecf8982a35b9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-13187-2021-ie00001.svg" width="42pt" height="11pt" src="acp-21-13187-2021-ie00001.png"/></svg:svg></span></span>), whereas CC BrC contained more unsaturated fractions (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">H</mi><mo>-</mo><mi mathvariant="normal">C</mi><mo>-</mo><mi mathvariant="normal">C</mi><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="7416ee3c5a7b4a25416064a588edbf9b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-13187-2021-ie00002.svg" width="49pt" height="11pt" src="acp-21-13187-2021-ie00002.png"/></svg:svg></span></span> and <span class="inline-formula">Ar−H</span>). The DTT assays indicated that BB BrC generally had a stronger oxidative potential (DTT<span class="inline-formula">_m</span>, 2.6–85 pmol/min/<span class="inline-formula">µ</span>g) than CC BrC (DTT<span class="inline-formula">_m</span>, 0.4–11 pmol/min/<span class="inline-formula">µ</span>g), 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 % <span class="inline-formula">±</span> 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.</p>