Abstract. 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 3 Exp) campaign conducted at Xianghe, China in summer 2013. A gradual increase in size-resolved activation ratio (AR) with particle diameter (D p ) suggests that aerosol particles have different hygroscopicities. During pollution events, the activation diameter (D a ) 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 (N CN ) 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 N CN during pollution events, but was closely related to the fraction of the total organic mass signal at m/z 44 (f 44 ), 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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