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Discussion papers
https://doi.org/10.5194/acp-2019-292
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-292
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 Apr 2019

Submitted as: research article | 29 Apr 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Enhanced sulfate formation through SO2+NO2 heterogeneous reactions during heavy winter haze in the Yangtze River Delta region, China

Ling Huang1,*, Jingyu An2,*, Bonyoung Koo3, Greg Yarwood4, Rusha Yan2, Yangjun Wang1, Cheng Huang2, and Li Li1 Ling Huang et al.
  • 1School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China
  • 2State Environmental Protection Key Laboratory of the Cause and Prevention of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai, 200233, China
  • 3Bay Area Air Quality Management District, San Francisco,94105, USA
  • 4Ramboll, Novato, California, 95995, USA
  • *These two authors contributed equally to this work.

Abstract. Rapid sulfate formation is recognized as key characteristics of severe winter haze in China. However, air quality models tend to underestimate sulfate formation during heavy haze periods and heterogeneous formation pathways have been proposed as promising mechanisms to reduce gaps between observation and model simulation. In this study, we implemented a reactive SO2 uptake mechanism through the SO2+NO2 heterogeneous reactions in the Comprehensive Air Quality Model with extensions (CAMx) to improve simulation of sulfate formation in the Yangtze River Delta (YRD) region for the first time. Parameterization of the SO2+NO2 heterogeneous reactions is based on observations in Beijing and considered both impact of relative humidity and aerosol pH on sulfate formation. Ammonia is reported to be critical for the formation of secondary inorganic aerosols and estimation of ammonia emissions is usually associated with large uncertainties. Sensitivity tests were conducted to evaluate the importance of the SO2+NO2 heterogeneous reactions as well as ammonia emissions on modelled sulfate concentrations during a period with several heavy haze episodes in the YRD region. Base case model results show large underestimation of sulfate concentrations by 36 % under polluted conditions in the YRD region. Adding the SO2+NO2 heterogeneous reactions or doubling ammonia emissions alone leads to slight model improvement (~ 6 %) on simulated sulfate concentrations in the YRD region. However, model performance significantly improved when both the SO2+NO2 heterogeneous reactions and doubled ammonia emissions were included in the simulation: predicted sulfate concentrations during polluted periods increased from 23.1 µg m−3 in the base scenario to 29.1 µg m−3 (representing an increase of 26 %). Aerosol pH is crucial for the SO2+NO2 heterogeneous reactions and our calculated aerosol pH is always acidic and increased by 0.7 with doubled ammonia emissions. Modelling results also show that this reactive SO2 uptake mechanism enhanced sulfate simulations by 1 to 5 µg m−3 for the majority of eastern and central part of China, with more than 20 µg m−3 increase of sulfate concentrations over the north-eastern plateau. These findings suggest that the SO2+NO2 heterogeneous reactions could be important for sulfate formation in the YRD region as well as other parts of China. In addition, ammonia emissions need to be carefully estimated. More studies are needed to improve the parameterization of the SO2+NO2 heterogeneous reactions based on local data further evaluate this mechanism in other regions. Substantial efforts are needed to improve the accuracy of ammonia emissions inventory.

Ling Huang et al.
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Short summary
Severe haze events characterized by extremely high concentrations of particulate matter occurred frequently in the Yangtze River Delta (YRD) region, China. Rapid sulfate production during these severe haze episodes was observed via atmospheric measurements but air quality models tend to underestimated sulfate. Our study suggests that the SO2+NO2 heterogeneous reactions could be potentially important for sulfate formation in the YRD region and ammonia emissions need to be carefully estimated.
Severe haze events characterized by extremely high concentrations of particulate matter occurred...
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