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Abstract: In-situ aircraft measurements of aerosols and clouds of 7 flights during the period from July to September of 2008 are analyzed in Beijing, China. The measured aerosol concentrations indicated that the Beijing region was highly polluted by aerosol particles. The impact of heavy aerosol particle loadings on cloud formation is studied. The microphysical characters of clouds (including number concentrations of cloud droplets (Nc), cloud droplet radius (Rc), liquid water content (LWC)), and number concentrations of aerosol particles (Na) were measured during the experiments. The aircraft measurements indicated that under high aerosol particle conditions, large number of cloud droplets was formed with small size of droplets. By contrast, under low aerosol particle conditions, the formation of cloud droplets was limited by the number of Na. In this case, small number of cloud droplets was formed with large size of droplets. The number concentrations of cloud droplets were sensitivity to LWC under high aerosol particle conditions. With low LWC value (<0.1gm−3), the Nc was slowly increased with the Na. For example, the cloud droplets were increased from 200 to 300cm−3, while aerosol particles were increased from 1000 to 6000cm−3, indicating that a large amount of aerosol particles were unchanged under low LWC condition. By contrast, with high LWC value (>0.5gm−3), the activation of aerosol particles to become cloud droplets was significantly enhanced. For example, the cloud droplets were increased from 700 to 1900cm−3, while aerosol particles were increased from 1000 to 6000cm−3, suggesting that a large amount of aerosol particles were converted to cloud droplets when there were enough LWC for supporting such conversions. This study also suggests that the Rc was also very sensitivity to the LWC values. The increase in the LWC values led to significant increase in the size of cloud droplets. [Copyright &y& Elsevier]

Organic aerosol constituents can influence the surface tension of nucleating cloud droplets and thereby modify the critical supersaturation necessary to activate aerosol particles. Model calculations, based on experiments carried out with different surrogates, led to contradictory conclusions on the effect of organic components on activation, indicating that the results depend very much on the surrogate selected. In order to reduce this uncertainty surface tension measurements were performed on real atmospheric aerosol components. Humic-like substances (HULIS) that accounted for 60% of the water-soluble organic carbon present in rural aerosol were isolated from 32 samples covering different seasons. The isolated organic matter present in a concentration of about 1 g L-1, decreased the surface tension of the aqueous solutions by 25-42% as compared to pure water. This effect was further enhanced when humic-like substances were mixed with ammonium sulphate. In order to support model calculations Szyskowski functions were fitted to the data to formulate the surface tension effect as a function of concentration.Finally, natural humic substances (fulvic and humic acids) were investigated under the same conditions. The experiments revealed that the surface tension decreasing effect of atmospheric humic-like substances differed from that of the studied terrestrial and aquatic humic substances: the latter substances decreased the surface tension of the aqueous solution to a lesser extent (7-23%) than atmospheric HULIS. This deviation can be explained with the different composition (e.g. the ratio of aromatic to aliphatic moieties) of the substances investigated. [ABSTRACT FROM AUTHOR]