Abstract

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 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 ∼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 increasing 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 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 ∼41∘ 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 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. 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 =4.2–5.7) with transitions from dominant TMI pathway regimes to dominant NO2∕O3 pathway regimes.

Highlights

  • Persistent haze shrouding Beijing and its surrounding areas in the North China Plain during cold winters is one of the most urgent and challenging environmental problems in China

  • Depending on the aerosol pH and pollutant compositions, the major multiphase oxidation pathways may change from reactions of NO2 and O3 at pH greater than 4.5 to O2 and H2O2 at pH less than 4.5 (Cheng et al, 2016)

  • We have developed a new aerosol water chemistry (AWAC) module and implemented an improved version of ISORROPIA II into the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry) to better account for the different sulfate formation pathways

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Summary

Introduction

Cheng et al (2016) suggested and quantified that during severe haze, multiphase reactions in aerosol water can produce a remarkable amount of sulfate over a wide range of aerosol pH values which complements or even exceeds the contribution from gas-phase and cloud chemistry during the haze events. Laboratory studies of Wang et al (2016) provide an experimental proof of the importance of the NO2 oxidation pathway in sulfate formation in aerosol water. Depending on the aerosol pH and pollutant compositions, the major multiphase oxidation pathways may change from reactions of NO2 and O3 at pH greater than 4.5 to O2 (catalyzed by transition metal ion, TMI) and H2O2 at pH less than 4.5 (Cheng et al, 2016). J. Zheng et al, 2020; Su et al, 2020; Cheng et al, 2016)

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