ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-13423-2014 Aerosol hygroscopicity and cloud condensation nuclei activity during the AC<sup>3</sup>Exp campaign: implications for cloud condensation nuclei parameterization Zhang F. 1 Li Y. https://orcid.org/0000-0001-6737-382X 1 Li Z. 1 2 Sun L. 3 Li R. 1 Zhao C. 1 Wang P. 3 Sun Y. 4 Liu X. 5 Li J. 6 7 Li P. 6 Ren G. 6 Fan T. 1 College of Global Change and Earth System Science, Beijing Normal University, 100875 Beijing, China Earth System Science Interdisciplinary Center and Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029 Beijing, China State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029 Beijing, China State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, 100875 Beijing, China Weather Modification Office of Shanxi Province, 030032 Taiyuan, China Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, 210044 Nanjing, China 16 12 2014 14 24 13423 13437 Copyright: © 2014 F. Zhang et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/13423/2014/acp-14-13423-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/13423/2014/acp-14-13423-2014.pdf

Aerosol hygroscopicity and cloud condensation nuclei (CCN) activity under background conditions and during pollution events are investigated during the Aerosol-CCN-Cloud Closure Experiment (AC<sup>3</sup>Exp) campaign conducted at Xianghe, China in summer 2013. A gradual increase in size-resolved activation ratio (AR) with particle diameter (<i>D</i><sub>p</sub>) suggests that aerosol particles have different hygroscopicities. During pollution events, the activation diameter (<i>D</i><sub>a</sub>) measured at low supersaturation (SS) was significantly increased compared to background conditions. An increase was not observed when SS was > 0.4%. The hygroscopicity parameter (κ) was ~ 0.31–0.38 for particles in accumulation mode under background conditions. This range in magnitude of κ was ~ 20%, higher than κ derived under polluted conditions. For particles in nucleation or Aitken mode, κ ranged from 0.20–0.34 for background and polluted cases. Larger particles were on average more hygroscopic than smaller particles. The situation was more complex for heavy pollution particles because of the diversity in particle composition and mixing state. A non-parallel observation CCN closure test showed that uncertainties in CCN number concentration estimates ranged from 30–40%, which are associated with changes in particle composition as well as measurement uncertainties associated with bulk and size-resolved CCN methods. A case study showed that bulk CCN activation ratios increased as total condensation nuclei (CN) number concentrations (<i>N</i><sub>CN</sub>) increased on background days. The background case also showed that bulk AR correlated well with the hygroscopicity parameter calculated from chemical volume fractions. On the contrary, bulk AR decreased with increasing total <i>N</i><sub>CN</sub> during pollution events, but was closely related to the fraction of the total organic mass signal at <i>m/z</i> 44 (<i>f</i><sub>44</sub>), which is usually associated with the particle's organic oxidation level. Our study highlights the importance of chemical composition in determining particle activation properties and underlines the significance of long-term observations of CCN under different atmospheric environments, especially regions with heavy pollution.

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