12 Nov 2021

12 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

High atmospheric oxidation capacity drives wintertime nitrate pollution in the eastern Yangtze River Delta of China

Han Zang1, Yue Zhao1, Juntao Huo2, Qianbiao Zhao2, Qingyan Fu2, Yusen Duan2, Jingyuan Shao3, Cheng Huang4, Jingyu An4, Likun Xue5, Ziyue Li1, Chenxi Li1, and Huayun Xiao1 Han Zang et al.
  • 1School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
  • 2Shanghai Environmental Monitoring Center, Shanghai 200235, China
  • 3College of Flight Technology, Civil Aviation University of China, Tianjin 300300, China
  • 4Shanghai Academy of Environmental Sciences, Shanghai 200233, China
  • 5Environment Research Institute, Shandong University, Qingdao, Shandong, 266237, China

Abstract. Nitrate aerosol plays an increasingly important role in wintertime haze pollution in China. Despite intensive research on the wintertime nitrate chemistry in recent years, quantitative constraints on the formation mechanisms of nitrate aerosol in the Yangtze River Delta (YRD), one of the most developed and densely populated regions in eastern China, remain inadequate. In this study, we identify the major nitrate formation pathways and their key controlling factors during the winter haze pollution period in the eastern YRD using two-year (2018–2019) field observations and detailed observation-constrained model simulations. We find that the high atmospheric oxidation capacity, coupled with high aerosol liquid water content (ALWC), made both the heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) and the gas-phase OH oxidation of nitrogen dioxide (NO2) important pathways for wintertime nitrate formation in this region, with contribution percentages of 69 % and 29 % in urban areas and 63 % and 35 % in suburban areas, respectively. We further find that the gas-to-particle partitioning of nitric acid (HNO3) was very efficient so that the rate-determining step in the overall formation process of nitrate aerosol was the oxidation of NOx to HNO3 through both heterogeneous and gas-phase processes. The atmospheric oxidation capacity (i.e., the availability of O3 and OH radicals) was the key factor controlling the production rate of HNO3 from both processes. During the COVID-19 lockdown (January–February 2020), the enhanced atmospheric oxidation capacity greatly promoted the oxidation of NOx to nitrate and hence weakened the response of nitrate aerosol to the emission reductions in urban areas. Our study sheds light on the detailed formation mechanisms of wintertime nitrate aerosol in the eastern YRD and highlights the demand for the synergetic regulation of atmospheric oxidation capacity and NOx emissions to mitigate wintertime nitrate and haze pollution in eastern China.

Han Zang et al.

Status: open (until 24 Dec 2021)

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Han Zang et al.


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Short summary
Particulate nitrate plays an increasingly important role in wintertime haze pollution in eastern China, yet quantitative constraints on detailed nitrate formation mechanisms remain limited. Here we quantified the contributions of the heterogeneous N2O5 hydrolysis (66 %) and gas-phase OH + NO2 reaction (32 %) to nitrate formation in this region and identified the atmospheric oxidation capacity (i.e., availability of O3 and OH radicals) as the driving factor of nitrate formation from both processes.