Preprints
https://doi.org/10.5194/gmd-2022-62
https://doi.org/10.5194/gmd-2022-62
Submitted as: model evaluation paper
21 Mar 2022
Submitted as: model evaluation paper | 21 Mar 2022
Status: this preprint is currently under review for the journal GMD.

On the simulations of aerosol pH in China using WRF-Chem (v4.0): sensitivities of aerosol pH and its temporal variations in haze episodes

Xueyin Ruan1, Chun Zhao1,2,3, Rahul A. Zaveri4, Pengzhen He5, Xinming Wang6, Jingyuan Shao7, and Lei Geng1,2,3 Xueyin Ruan et al.
  • 1School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, Anhui, China
  • 2CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei 230026, Anhui, China
  • 3Frontiers Science Center for Planetary Exploration and Emerging Technologies, University of Science and Technology of China, Hefei, China
  • 4Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  • 5School of Environment and Tourism, West Anhui University, Lu'an 237012, Anhui, China
  • 6Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China
  • 7Flight branch, Civil Aviation University of China, Tianjin 300300, China

Abstract. Precise estimation of aerosol pH in chemical transport models (CTMs) is critical to aerosol modeling and thus influencing policy development. We reported WRF-Chem simulated PM2.5 pH over China during a period with heavy haze episodes in Beijing, and explored the sensitivity of the modeled aerosol pH to factors including emissions of nonvolatile cations (NVCs) and NH3, aerosol phase state assumption and heterogeneous production of sulfate. We found default WRF-Chem could predict spatial patterns of PM2.5 pH over China similar to other CTMs, but with generally lower pH values largely due to the underestimates of alkaline species (NVCs and NH3) and the difference in thermodynamic treatments between different models. Increasing NH3 emissions in the model would improve the modeled pH in comparison with offline thermodynamic model calculations of pH constrained by observations. In addition, we found that aerosol phase state assumption and heterogeneous sulfate production are important in aerosol pH predictions for regions with low relative humidity (RH) and high anthropogenic SO2 emissions, respectively. These factors should be better constrained in model simulations of aerosol pH in the future. Analysis of the modeled temporal trend of PM2.5 pH in Beijing over a haze episode revealed a clear decrease in pH from 5.21 ± 0.88 in clean period to 3.56 ± 0.49 in heavily polluted period. The increased acidity in more polluted conditions is largely due to the formation and accumulation of secondary species including sulfuric acid and nitric acid, even though being modified by alkaline species (NVCs, NH3). Our result suggests that NO2 oxidation is unlikely to be important for heterogeneous sulfate production in Beijing haze as the effective pH for NO2 oxidation of S(IV) is at higher pH of ~6.

Xueyin Ruan et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2022-62', Shaojie Song, 25 Apr 2022
  • RC2: 'Comment on gmd-2022-62', Anonymous Referee #2, 27 Apr 2022

Xueyin Ruan et al.

Model code and software

xyruan/wrfchem_ustc4.0: wrfchem_ustc 4.0 including heterogeneous sulfate production in aerosol water and modified cation speciation profiles Xueyin Ruan, Chun Zhao, Rahul A. Zaveri, Pengzhen He, Xinming Wang, Jingyuan Shao, and Lei Geng https://doi.org/10.5281/zenodo.6359417

WRF Source Codes and Graphics Software Downloads WRF Users Page http://www2.mmm.ucar.edu/wrf/users/download/get_source.html

Xueyin Ruan et al.

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Short summary
Accurate prediction of aerosol pH in chemical transport models is essential to aerosol modeling. This study examines the performance of Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) on aerosol pH predictions and the sensitivities to emissions of nonvolatile cations and NH3, aerosol phase state assumption and heterogeneous sulfate production. Temporal evolution of aerosol pH during haze cycles in Beijing and the driving factors are also presented and discussed.