Daytime atmospheric oxidation capacity in four Chinese megacities during the photochemically polluted season: a case study based on box model simulation

• Main Authors: Z. Tan, K. Lu, M. Jiang, R. Su, H. Wang, S. Lou, Q. Fu, C. Zhai, Q. Tan, D. Yue, D. Chen, Z. Wang, S. Xie, L. Zeng, Y. Zhang
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/3493/2019/acp-19-3493-2019.pdf

<p>Atmospheric oxidation capacity is the basis for converting freshly emitted substances into secondary products and is dominated by reactions involving hydroxyl radicals (OH) during daytime. In this study, we present in situ measurements of <span class="inline-formula">RO_<i>x</i></span> radical (hydroxy OH, hydroperoxy <span class="inline-formula">HO₂</span>, and organic peroxy <span class="inline-formula">RO₂</span>) precursors and products; the measurements are carried out in four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons. The atmospheric oxidation capacity is evaluated using an observation-based model and radical chemistry precursor measurements as input. The radical budget analysis illustrates the importance of HONO and HCHO photolysis, which account for <span class="inline-formula">∼50</span>&thinsp;% of the total primary radical sources. The radical propagation is efficient due to abundant NO in urban environments. Hence, the production rate of secondary pollutants, that is, ozone (and fine-particle precursors (<span class="inline-formula">H₂SO₄</span>, <span class="inline-formula">HNO₃</span>, and extremely low volatility organic compounds, ELVOCs) is rapid, resulting in secondary air pollution. The ozone budget demonstrates its high production in urban areas; also, its rapid transport to downwind areas results in rapid increase in local ozone concentrations. The <span class="inline-formula">O₃</span>–<span class="inline-formula">NO_<i>x</i></span>–VOC (volatile organic compound) sensitivity tests show that ozone production is VOC-limited and that alkenes and aromatics should be mitigated first for ozone pollution control in the four studied megacities. In contrast, <span class="inline-formula">NO_<i>x</i></span> emission control (that is, a decrease in <span class="inline-formula">NO_<i>x</i></span>) leads to more severe ozone pollution. With respect to fine-particle pollution, the role of the <span class="inline-formula">HNO₃</span>–<span class="inline-formula">NO₃</span> partitioning system is investigated using a thermal dynamic model (ISORROPIA 2). Under high relative humidity (RH) and ammonia-rich conditions, nitric acid converts into nitrates. This study highlights the efficient radical chemistry that maintains the atmospheric oxidation capacity in Chinese<span id="page3494"/> megacities and results in secondary pollution characterized by ozone and fine particles.</p>