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Abstract. Size-resolved and bulk activation properties of aerosols were measured at a regional/suburban site in the North China Plain (NCP), which is occasionally heavily polluted by anthropogenic aerosol particles and gases. A Cloud Condensation Nuclei (CCN) closure study is conducted with bulk CCN number concentration (N CCN ) and calculated CCN number concentration based on the aerosol number size distribution and size-resolved activation properties.The observed CCN number concentration (N CCN-obs ) are higher than those observed in other locations than China, with average N CCN-obs of roughly 2000, 3000, 6000, 10 000 and 13 000 cm Size-resolved activation measurements show that most of the 300 nm particles are activated at the investigated supersaturations, while almost no particles of 30 nm are activated even at the highest supersaturation of 0.72%. The activation ratio increases with increasing supersaturation and particleCorrespondence to: C. S. Zhao (zcs@pku.edu.cn) size. The slopes of the activation curves for ambient aerosols are not as steep as those observed in calibrations with ammonium sulfate suggesting that the observed aerosols is an external mixture of more hygroscopic and hydrophobic particles.The calculated CCN number concentrations (N CCN-calc ) based on the size-resolved activation ratio and aerosol number size distribution correlate well with the N CCN-obs , and show an average overestimation of 19%. Sensitivity studies of the CCN closure show that the N CCN at each supersaturation is well predicted with the campaign average of sizeresolved activation curves. These results indicate that the aerosol number size distribution is critical in the prediction of possible CCN. The CCN number concentration can be reliably estimated using time-averaged, size-resolved activation efficiencies without accounting for the temporal variations.

Abstract. North China Plain (NCP) is one of the most populated and polluted regions in China. During the recent years, haze and fog occur frequently and cause severely low visibility in this region. In order to better understand the impact of aerosol particles on the formation of haze and fog, a long-term record of haze and fog occurrences in the past 56 yr (from 1954-2009) over NCP is analyzed. The results show that there are rapid changes in the occurrences of haze and fog over NCP. The occurrences of haze and fog were low during 1970-1980, and reached a maximum during 1981-1998. After 1999, the occurrences of haze and fog slightly decreased. There was a nonlinear relationship between the occurrences of haze and fog. When the occurrence of haze was lower than 40 days yr −1 , the occurrence of fog was strongly proportional to the occurrence of haze. However, when the occurrence of haze was high (larger than 75 days yr −1 ), the occurrence of fog was not sensitive to the occurrence of haze. In order to better understand the relationship between the occurrences of haze and fog as well as the effect of aerosol particles on the formation of haze and fog, an in-situ field experiment was conducted during a period with a mixed occurrence of haze and fog. The analysis of the experiment suggests that there were considerably high aerosol concentrations during the measurement period with an averaged aerosol number concentration of 24 000 cm −3 . The measurement also shows that a large amount of aerosol particles can act as condensation nuclei to enhance the formation of fog droplets. As a result, a large amount of fog droplets (>1000 cm −3 ) with small size (5-6 µm) were observed during the fog period, resulting in extremely low visibility (less than 100 m).

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Severe haze events during which particulate matter (PM) increases quickly from tens to hundreds of microgram per cubic meter in 1–2 days frequently occur in China. Although it has been known that PM is influenced by complex interplays among emissions, meteorology, and physical and chemical processes, specific mechanisms remain elusive. Here, a new positive feedback mechanism between planetary boundary layer (PBL), relative humidity (RH), and secondary PM (SPM) formation is proposed based on a comprehensive field experiment and model simulation. The decreased PBL associated with increased PM increases RH by weakening the vertical transport of water vapor; the increased RH in turn enhances the SPM formation through heterogeneous aqueous reactions, which further enhances PM, weakens solar radiation, and decreases PBL height. This positive feedback, together with the PM-Radiation-PBL feedback, constitutes a key mechanism that links PM, radiation, PBL properties (e.g. PBL height and RH), and SPM formation, This mechanism is self-amplifying, leading to faster PM production, accumulation, and more severe haze pollution.

North China Plain (NCP) is one of the most populated and polluted regions in China. During the recent years, haze and fog occur frequently and cause severely low visibility in this region. In order to better understand the impact of aerosol particles on the formation of haze and fog, a long-term record of haze and fog occurrences in the past 56 years (from 1954–2009) over NCP is analyzed. The results show that there are rapid changes in the occurrences of haze and fog over NCP. The occurrences of haze and fog were low during 1970–1980, and reached a maximum during 1981–1998. After 1999, the occurrences of haze and fog slightly decreased. There was a nonlinear relationship between the occurrences of haze and fog. When the occurrence of haze was lower than 40 days yr⁻¹, the occurrence of fog was strongly proportional to the occurrence of haze. However, when the occurrence of haze was high (larger than 75 days yr⁻¹), the occurrence of fog was not sensitive to the occurrence of haze. In order to better understand the relationship between the occurrences of haze and fog as well as the effect of aerosol particles on the formation of haze and fog, an in-situ field experiment was conducted during a period with a mixed occurrence of haze and fog. The analysis of the experiment suggests that there were considerably high aerosol concentrations during the measurement period, with an averaged aerosol number concentration of 24 000 cm⁻³. The measurement also shows that a large amount of aerosol particles can act as condensation nuclei to enhance the formation of fog droplets. As a result, a large amount of fog droplets (>1000 cm⁻³) with small size in radius (5–6 μm) were observed during the fog period, resulting in extremely low visibility (less than 100 m)