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https://doi.org/10.5194/acp-2020-177
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-177
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Apr 2020

Submitted as: research article | 06 Apr 2020

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This preprint is currently under review for the journal ACP.

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

Wei Tao1,2, Hang Su1, Guangjie Zheng2, Jiandong Wang2, Chao Wei1, Lixia Liu2, Nan Ma3, Meng Li1, Qiang Zhang4, Ulrich Pöschl1, and Yafang Cheng2,1 Wei Tao et al.
  • 1Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz 55128, Germany
  • 2Minerva Research Group, Max Planck Institute for Chemistry, Mainz 55128, Germany
  • 3Institute for Environmental and Climate Research, Jinan University, Guangzhou 511443, China
  • 4Department of Earth System Science, Tsinghua University, Beijing 100084, China

Abstract. 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 spatio-temporal variability of aerosol pH and sulfate formation during winter in the North China Plain (NCP), we have developed a new aerosol water chemistry module (AWAC) 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 spatio-temporal variation in the relative abundance of acidic and alkaline fine particle components and their gaseous counterparts. Over Beijing, the average aerosol pH at the surface layer was ~ 5.4 and remained nearly constant around ~ 5 up to ~ 2 km above ground level; further aloft, the acidity rapidly increased to pH ~ 0 at ~ 3 km. The pattern of aerosol acidity increase with altitude persisted over the NCP, while the specific levels and gradients of pH varied between different regions. In the region north of ~ 41° N, the mean pH values at surface level were typically > 6 and the main pathway of sulfate formation in aerosol water was S(IV) oxidation by ozone. South of ~ 41° N, the mean pH values at 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 NO2 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 H2O2 pathway dominated in the region extending further south (Shandong and Henan Provinces). In all of these regions, the O3 and TMI pathways in aerosol water as well as the gas-particle partitioning of H2SO4 vapor became more important with increasing altitude. Although pH is sensitive to the abundance of NH3 and crustal particles, we show that the rapid production of sulfate in the NCP can be maintained over a wide range of aerosol acidity (e.g., pH = 4.2–5.7) with transitions from TMI pathway dominated to NO2/O3 pathway dominated regimes.

Wei Tao et al.

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Latest update: 27 May 2020
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Short summary
We simulated the thermodynamics and multiphase reactions in aerosol water during a wintertime haze event over the North China Plain. It was found that aerosol pH exhibited a strong spatio-temporal variability, and multiple oxidation pathways were predominant for particulate sulfate formation in different locations. Sensitivity tests further showed that ammonia, crustal particles and dissolved transition metal ions were important factors for multiphase chemistry during haze episodes.
We simulated the thermodynamics and multiphase reactions in aerosol water during a wintertime...
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