Seasonal variation in oxygenated organic molecules in urban Beijing and their contribution to secondary organic aerosol
<p>Oxygenated organic molecules (OOMs) are crucial for atmospheric new particle formation and secondary organic aerosol (SOA) growth. Therefore, understanding their chemical composition, temporal behavior, and sources is of great importance. Previous studies on OOMs mainly focus on environment...
| Main Authors: | , , , , , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/10077/2022/acp-22-10077-2022.pdf |
| Summary: | <p>Oxygenated organic molecules (OOMs) are crucial for atmospheric new particle formation and secondary organic aerosol (SOA) growth. Therefore, understanding their chemical composition, temporal behavior, and sources is of great importance. Previous studies on OOMs mainly focus on environments where biogenic sources are predominant, yet studies on sites with dominant anthropogenic emissions, such as megacities, have been lacking. Here, we conducted long-term measurements of OOMs, covering four seasons of the year 2019, in urban Beijing. The OOM concentration was found to be the highest in summer (<span class="inline-formula">1.6×10<sup>8</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>), followed by autumn (<span class="inline-formula">7.9×10<sup>7</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>), spring (<span class="inline-formula">5.7×10<sup>7</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>) and winter (<span class="inline-formula">2.3×10<sup>7</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>), suggesting that enhanced photo-oxidation together with the rise in temperature promote the formation of OOMs. Most OOMs contained 5 to 10 carbon atoms and 3 to 7 effective oxygen atoms (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="normal">nO</mi><mi mathvariant="normal">eff</mi></msub><mo>=</mo><mi mathvariant="normal">nO</mi><mo>-</mo><mn mathvariant="normal">2</mn><mo>×</mo><mi mathvariant="normal">nN</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="96pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="e448e69a29e58bb1b5c0b93368142ea4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-10077-2022-ie00001.svg" width="96pt" height="13pt" src="acp-22-10077-2022-ie00001.png"/></svg:svg></span></span>). The average nO<span class="inline-formula"><sub>eff</sub></span> increased with increasing atmospheric
photo-oxidation capacity, which was the highest in summer and the lowest in
winter and autumn. By performing a newly developed workflow, OOMs were
classified into the following four types: aromatic OOMs, aliphatic OOMs, isoprene OOMs, and monoterpene OOMs. Among them, aromatic OOMs (29 %–41 %) and aliphatic OOMs (26 %–41 %) were the main contributors in all seasons, indicating that OOMs in Beijing were dominated by anthropogenic sources. The contribution of isoprene OOMs increased significantly in summer (33 %), which is much higher than those in the other three seasons (8 %–10 %). Concentrations of isoprene (0.2–<span class="inline-formula">5.3×10<sup>7</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>) and monoterpene (1.1–<span class="inline-formula">8.4×10<sup>6</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>) OOMs in Beijing were lower than those
reported at other sites, and they possessed lower oxygen and higher nitrogen contents due to high <span class="inline-formula">NO<sub><i>x</i></sub></span> levels (9.5–38.3 ppbv – parts per billion by volume) in Beijing. With regard to the nitrogen content of the two anthropogenic OOMs, aromatic OOMs were mainly composed of CHO and CHON species, while aliphatic OOMs were dominated by CHON and <span class="inline-formula">CHON<sub>2</sub></span> ones. Such prominent differences suggest varying formation pathways between these two OOMs. By combining the measurements and an aerosol dynamic model, we estimated that the SOA growth rate through OOM condensation could reach 0.64, 0.61, 0.41, and 0.30 <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span> h<span class="inline-formula"><sup>−1</sup></span> in autumn, summer, spring, and
winter, respectively. Despite the similar concentrations of aromatic and
aliphatic OOMs, the former had lower volatilities and, therefore, showed
higher contributions (46 %–62 %) to SOA than the latter (14 %–32 %). By contrast, monoterpene OOMs and isoprene OOMs, limited by low abundances or high volatilities, had low contributions of 8 %–12 % and 3 %–5 %, respectively. Overall, our results improve the understanding of the concentration, chemical composition, seasonal variation, and potential atmospheric impacts of OOMs, which can help formulate refined restriction policy specific to SOA control in urban areas.</p> |
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| ISSN: | 1680-7316 1680-7324 |