Preprints
https://doi.org/10.5194/acp-2020-1146
https://doi.org/10.5194/acp-2020-1146

  27 Nov 2020

27 Nov 2020

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

Inter-annual variations of wet deposition in Beijing during 2014–2017: implications of below-cloud scavenging of inorganic aerosols

Baozhu Ge1,5, Danhui Xu1, Oliver Wild2, Xuefeng Yao3, Junhua Wang1,4, Xuechun Chen1, Qixin Tan1,4, Xiaole Pan1, and Zifa Wang1,4,5 Baozhu Ge et al.
  • 1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), Beijing 100029, China
  • 2Lancaster Environment Centre, Lancaster University, LA1 4YQ, UK
  • 3PLA 96941 Army, Beijing 102206, China
  • 4University of Chinese Academy of Sciences, Beijing, 100049, China
  • 5Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

Abstract. Wet scavenging is an efficient pathway for the removal of particulate matter (PM) from the atmosphere. High levels of PM have been a major cause of air pollution in Beijing but have decreased sharply under the Air Pollution Prevention and Control Action Plan launched in 2013. In this study, four years of observations of wet deposition have been conducted using a sequential sampling technique to investigate the detailed variation in chemical components through each rainfall event. We find that the major ions, SO42−, Ca2+, NO3 and NH4+, show significant decreases over the 2013–2017 period (decreasing by 39 %, 35 %, 12 % and 25 %, respectively), revealing the impacts of the Action Plan. An improved sequential sampling method is developed and implemented to estimate the contribution of below-cloud and in-cloud wet deposition over the four-year period. Overall, below-cloud scavenging accounts for between half and two thirds of wet deposition of the four major ions, with the highest contribution for NH4+ at 65 % and lowest for SO42− at 50 %. The contribution of below-cloud scavenging for Ca2+, SO42− and NH4+ decreases from above 50 % in 2014 to below 40 % in 2017. This suggests that the Action Plan has mitigated PM pollution in the surface layer and hence decreased scavenging due to the washout process. In contrast, we find little change in the annual volume weighted average concentration for NO3 where the contribution from below-cloud scavenging remains at ~44 % over the period 2015–2017. While highlighting the importance of different wet scavenging processes, this paper presents a unique new perspective on the effects of the Action Plan and clearly identifies oxidized nitrogen species as a major target for future air pollution controls.

Baozhu Ge et al.

 
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Baozhu Ge et al.

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Short summary
In this study, an improved sequential sampling method is developed and implemented to estimate the contribution of below-cloud and in-cloud wet deposition over the four-year measurements in Beijing. We find that the contribution of below-cloud scavenging for Ca2+, SO42− and NH4+ decreases from above 50 % in 2014 to below 40 % in 2017. This suggests that the Action Plan has mitigated PM pollution in the surface layer and hence decreased scavenging due to the washout process.
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