Modeling of aerosol property evolution during winter haze episodes over a megacity cluster in northern China: roles of regional transport and heterogeneous reactions of SO₂

• Main Authors: H. Du, J. Li, X. Chen, Z. Wang, Y. Sun, P. Fu, J. Gao, Y. Wei
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/9351/2019/acp-19-9351-2019.pdf

<p>Regional transport and heterogeneous reactions have been shown to play crucial roles in haze formation over a megacity cluster centered on Beijing. In this study, the updated Nested Air Quality Prediction Model System (NAQPMS) and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were employed to investigate the evolution of aerosols – in terms of the number concentration, size distribution, and degree of aging – in Beijing during six haze episodes between 15 November and 15 December 2016, as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) winter campaign of 2016. The model exhibited reasonable performance not only with respect to mass concentrations of PM<span class="inline-formula">_2.5</span> and its components in Beijing but also regarding the number concentration, size distribution, and degree of aging. We revealed that regional transport played a non-negligible role in haze episodes, with contributions of 14&thinsp;%–31&thinsp;% to the surface PM<span class="inline-formula">_2.5</span> mass concentration. The contribution of regional transport to secondary inorganic aerosols was larger than that to primary aerosols (30&thinsp;%–63&thinsp;% and 3&thinsp;%–12&thinsp;%, respectively). The chemical transformation of <span class="inline-formula">SO₂</span> along the transport pathway from source regions to Beijing was the major source of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="28cd4f8c12cf9ef751545712573a522a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-9351-2019-ie00001.svg" width="29pt" height="17pt" src="acp-19-9351-2019-ie00001.png"/></svg:svg></span></span>. We also found that sulfate formed outside Beijing from <span class="inline-formula">SO₂</span> emitted in Beijing; this sulfate was then blown back to Beijing and considerably influenced haze formation. Along the transport pathway, aerosols underwent aging, which altered the mass ratio of the coating of black carbon to black carbon (<span class="inline-formula"><i>R</i>_BC</span>) and the size distribution of number concentrations. During the episodes, the geometric mean diameter (GMD) increased from less than 100&thinsp;nm at the initial site to approximately 120&thinsp;nm at the final site (Beijing), and the <span class="inline-formula"><i>R</i>_BC</span> increased from 2–4 to 4–8. During haze episodes with high humidity, the average contributions of gas and aqueous chemistry, heterogeneous chemistry, and primary emission to sulfate were comparable. However, their relative contributions varied with pollution levels. Primary emissions had the greatest impact under light to moderate pollution levels, whereas heterogeneous chemistry had a stronger effect under high pollution levels.</p>