Aerosol pH and chemical regimes of sulfate formation in aerosol water during winter haze in the North China Plain

• Main Authors: W. Tao, H. Su, G. Zheng, J. Wang, C. Wei, L. Liu, N. Ma, M. Li, Q. Zhang, U. Pöschl, Y. Cheng
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/20/11729/2020/acp-20-11729-2020.pdf
• ISSN: 1680-7316; 1680-7324

<p>Understanding the mechanism of haze formation is crucial for the development of deliberate pollution control strategies. Multiphase chemical reactions in aerosol water have been suggested as an important source of particulate sulfate during severe haze (Cheng et al., 2016; Wang et al., 2016). While the key role of aerosol water has been commonly accepted, the relative importance of different oxidation pathways in the aqueous phase is still under debate mainly due to questions about aerosol pH. To investigate the spatiotemporal variability of aerosol pH and sulfate formation during winter in the North China Plain (NCP), we have developed a new aerosol water chemistry (AWAC) module for the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry). Using the WRF-Chem-AWAC model, we performed a comprehensive survey of the atmospheric conditions characteristic for wintertime in the NCP focusing on January 2013. We find that aerosol pH exhibited a strong vertical gradient and distinct diurnal cycle which was closely associated with the spatiotemporal variation in the abundance of acidic and alkaline fine particle components and their gaseous counterparts. Over Beijing, the average aerosol pH at the surface layer was <span class="inline-formula">∼5.4</span> and remained nearly constant around <span class="inline-formula">∼5</span> up to <span class="inline-formula">∼2</span>&thinsp;km above ground level; further aloft, the acidity rapidly increased to pH <span class="inline-formula">∼0</span> at <span class="inline-formula">∼3</span>&thinsp;km. The pattern of aerosol acidity increasing with altitude persisted over the NCP, while the specific levels and gradients of pH varied between different regions. In the region north of <span class="inline-formula">∼41</span><span class="inline-formula">^∘</span>&thinsp;N, the mean pH values at the surface level were typically greater than 6, and the main pathway of sulfate formation in aerosol water was S(IV) oxidation by ozone. South of <span class="inline-formula">∼41</span><span class="inline-formula">^∘</span>&thinsp;N, the mean pH values at the surface level were typically in the range of 4.4 to 5.7, and different chemical regimes and reaction pathways of sulfate formation prevailed in four different regions depending on reactant concentrations and atmospheric conditions. The <span class="inline-formula">NO₂</span> reaction pathway prevailed in the megacity region of Beijing and the large area of Hebei Province to the south and west of Beijing, as well as part of Shandong Province. The transition metal ion (TMI) pathway dominated in the inland region to the west and the coastal regions to the east of Beijing, and the <span class="inline-formula">H₂O₂</span> pathway dominated in the region extending further south (Shandong and Henan provinces). In all of these regions, the <span class="inline-formula">O₃</span> and TMI pathways in aerosol water, as well as the gas-particle partitioning of <span class="inline-formula">H₂SO₄</span> vapor, became more important with increasing altitude. Sensitivity tests show that the rapid production of sulfate in the NCP can be maintained over a wide range of aerosol acidity (e.g., pH <span class="inline-formula">=4.2</span>–5.7) with transitions from dominant TMI pathway regimes to dominant <span class="inline-formula">NO₂∕O₃</span> pathway regimes.</p>