Mechanistic insights into chloroacetic acid production from atmospheric multiphase volatile organic compound–chlorine chemistry
<p>Chlorine-containing oxygenated volatile organic compounds (Cl-OVOCs) are indicators of atmospheric chlorine chemistry involving volatile organic compounds (VOCs). However, their formation mechanisms are insufficiently understood. Herein, a strong diel pattern of chloroacetic acid (<span...
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| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/25/3753/2025/acp-25-3753-2025.pdf |
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| Summary: | <p>Chlorine-containing oxygenated volatile organic compounds (Cl-OVOCs) are indicators of atmospheric chlorine chemistry involving volatile organic compounds (VOCs). However, their formation mechanisms are insufficiently understood. Herein, a strong diel pattern of chloroacetic acid (<span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span>) was observed with daytime peaks at 19 and 13 ppt (1 h averages) in 2020 and 2021, respectively, at a coastal site in southern China. Ethene was previously proposed as the primary precursor responsible for daytime <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span> levels, but a photochemical box model based on Master Chemical Mechanism (MCM) simulations indicates that ethene accounts for less than 1 %. Quantum chemical calculations suggest that other alkenes also can act as chloroacetic acid precursors. Using an updated gas-phase VOC–Cl chemistry model, we find that isoprene, the most abundant VOC at the sampling site, along with its oxidation products, accounts for 7 % of the observed <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span>. Moreover, the simulation with the updated MCM produces appreciable levels of other Cl-OVOCs, especially chloroacetaldehyde, a precursor of <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span>. We proposed the multiphase reaction of Cl-OVOCs to reconcile the overestimation of Cl-OVOCs and the underestimation of <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span> in our gas-phase model. The estimated reactive uptake coefficients for various Cl-OVOCs range from <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">3.63</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">5</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="cce2e488907f9c5af9bd4078433c43fe"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-3753-2025-ie00001.svg" width="57pt" height="13pt" src="acp-25-3753-2025-ie00001.png"/></svg:svg></span></span> to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">2.34</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4a89d9e21b2603b64d9d7047ed8eacca"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-3753-2025-ie00002.svg" width="57pt" height="14pt" src="acp-25-3753-2025-ie00002.png"/></svg:svg></span></span>, based on quantum chemical calculations and linear relationship modelling. The box model simulation with multiphase chemistry shows that the heterogeneous conversion of chloroacetaldehyde to <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span> can contribute 24 %–48 % of the observed levels. Our study thus proposes a formation mechanism of gaseous <span class="inline-formula">C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>Cl</span> and highlights the potential importance of multiphase processes in atmospheric organic acid formation.</p> |
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| ISSN: | 1680-7316 1680-7324 |