Preprints
https://doi.org/10.5194/acp-2021-463
https://doi.org/10.5194/acp-2021-463

  22 Jun 2021

22 Jun 2021

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

Bimodal distribution of size-resolved particle effective density in a rural environment in the North China Plain

Yaqing Zhou1,2, Nan Ma1,2, Qiaoqiao Wang1,2, Zhibin Wang3, Chunrong Chen4, Jiangchuan Tao1,2, Juan Hong1,2, Long Peng1,2, Yao He1,2, Linhong Xie1,2, Shaowen Zhu1,2, Yuxuan Zhang5,6, Guo Li5, Wanyun Xu7, Peng Cheng8, Uwe Kuhn5, Guangsheng Zhou7, Pingqing Fu9, Qiang Zhang4, Hang Su5, and Yafang Cheng5 Yaqing Zhou et al.
  • 1Institute for Environmental and Climate Research, Jinan University, Guangzhou, 511443, China
  • 2Guangdong-Hongkong-Macau Joint Laboratory of Collaborative Innovation for Environmental Quality, Guangzhou, 511443, China
  • 3Research Center for Air Pollution and Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, 310058, China
  • 4Department of Earth System Science, Tsinghua University, Beijing, 100084, China
  • 5Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz, 55128, Germany
  • 6School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China
  • 7State Key Laboratory of Severe Weather, Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition and Environmental Meteorology, Chinese Academy of Meteorological Sciences, Beijing,100081, China
  • 8Institute of Mass Spectrometry and Atmospheric Environment, Jinan University, Guangzhou, 510632, China
  • 9Institute of Surface-Earth System Science, Tianjin University, Tianjin, 300072, China

Abstract. Effective density is one of the most important physical properties of atmospheric particles. It is closely linked to particle chemical composition and morphology, and could provide special information on particle emissions and aging processes. In this study, size-resolved particle effective density was measured with a combined DMA-CPMA-CPC system in Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN) in autumn 2019. With a new developed flexible Gaussian fit algorithm, frequent (77–87 %) bimodal distribution of particle effective density is identified, with a low-density mode (named sub-density mode) accounting for 22–27 % of total observed particles. The prevalence of the sub-density mode is closely related to fresh black carbon (BC) emissions. The geometric mean for the main-density mode (eff,main) increases from 1.18 g cm−3 (50 nm) to 1.37 g cm−3 (300 nm) due to larger fraction of high-density components and more significant restructuring effect at large particle sizes, but decreases from 0.89 g cm−3 (50 nm) to 0.62 g cm−3 (300 nm) for the sub-density mode (eff,sub) ascribed to the agglomerate effect. eff,main and eff,sub show similar diurnal cycles with peaks in the early afternoon, mainly attributed to the increasing mass fraction of high material density components associated with secondary aerosol production, especially of secondary inorganic aerosol (SIA). To investigate the impact of chemical composition, bulk particle effective density was calculated based on measured chemical composition (ρeff,ACSM) and compared with the average effective density at 300 nm (eff,tot,300 nm). The best agreement between the two densities is achieved when assuming a BC effective density of 0.60 g cm−3. The particle effective density is highly dependent on SIA and BC mass fractions. The influence of BC on the effective density is even stronger than SIA, implying the importance and necessity of including BC in the estimate of effective density for ambient particles.

Yaqing Zhou et al.

Status: open (until 03 Aug 2021)

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  • RC1: 'Comment on acp-2021-463', Anonymous Referee #1, 13 Jul 2021 reply

Yaqing Zhou et al.

Yaqing Zhou et al.

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Short summary
This study characterizes size-resolved particle effective density and its evolution associated with sources and aging processes in a rural area of the North China Plain. Particle effective density exhibits high frequency of bimodal distribution, and two density modes exhibit opposite trend with increasing particle size. SIA and BC mass fractions are key factors dominating particle effective density, and a value of 0.6 g cm−3 is appropriate to represent BC effective density in bulk particles.
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