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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 25 Jun 2019

Submitted as: research article | 25 Jun 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).

Contrasting ambient fine particles hygroscopicity derived by HTDMA and HR-AMS measurements between summer and winter in urban Beijing

Xinxin Fan1, Fang Zhang1, Lu Chen1, Don Collins2, Weiqi Xu3,4, Xiaoai Jin1, Jingye Ren1, Yuying Wang1,5, Hao Wu1, Shangze Li1, Yele Sun3,4, and Zhanqing Li1,6 Xinxin Fan et al.
  • 1State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China
  • 2Department of Chemical and Environmental Engineering, University of California Riverside, Riverside, California, USA
  • 3State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 4College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
  • 5School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 6Earth System Science Interdisciplinary Center and Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA

Abstract. The effects of aerosols on visibility through scattering and absorption of light and on climate through altering cloud droplet concentration are closely associated with their hygroscopic properties. Here, based on field campaigns in winter and summer in Beijing, we compare the size-resolved hygroscopic parameter (κgf) of ambient fine particles derived by an HTDMA (Hygroscopic Tandem Differential Mobility Analyzer) to that (denoted as κchem) of calculated by an HR-ToF-AMS (High-resolution Time-of-Flight Aerosol Mass Spectrometer) measurements using a simple rule with a uniform internal mixing hypothesis. We mainly focus on contrasting the disparity of κgf and κchem between summer and winter to reveal the impact of atmospheric processes/sources on aerosols hygroscopicity and to evaluate the uncertainty in estimating particles hygroscopicity with the hypothesis. We show that, in summer, the κchem for 110, 150 and 200 nm particles was averagely ~ 10 %–12 % lower than κgf, with the greatest difference between the values observed around noontime when aerosols experience rapid photochemical aging. In winter, no apparent disparity between κchem and κgf is observed for those > 100 nm particles around noontime, but the κchem is much higher than κgf in the late afternoon when ambient aerosols are greatly influenced by local traffic and cooking sources. By comparing with the observation from other two sites (Xingtai, Hebei and Xinzhou, Shanxi) of north China, we verify that atmospheric photochemical aging of aerosols enhances their hygroscopicity and may induce a coating effect which thereby leads to 10 %–20 % underestimation of the hygroscopic parameter if using the uniform internal mixing assumption. The coating effect is found more significant for these >100 nm particles observed in remote or clean regions. However, local primary sources, which result in an externally mixture of the fine particles with a large number of POA (Primary Organic Aerosol) in urban Beijing, makes the particle much less hygroscopic and cause 20–40 % overestimation of the hygroscopic parameter by the mixing rule assumption. In addition, we also note lower κchem than κgf for 80, 110 and 150 nm particles during the nighttime of winter, particularly in polluted days, probably due to a nighttime coating effect driven by condensation of secondary hygroscopic species on pre-existing aerosols in cold season. Our results suggest that it is critical to parameterize the impacts in model simulations to improve the evaluation of the aerosols indirect effect.

Xinxin Fan et al.
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Xinxin Fan et al.
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Short summary
Aerosols effects on visibility and climate are influenced by their hygroscopicity. By contrasting data from two techniques between summer and winter in Beijing, we identify a coating/aging effect on aerosols hygroscopicity, which is typically presented at noontime in summer. The effect is also verified at other two sites of China. Comprehensive impacts of particle mixing state on estimating its hygroscopic parameter are further revealed. Our results are helpful for parameterization in models.
Aerosols effects on visibility and climate are influenced by their hygroscopicity. By...