Impact of aerosol particles on cloud formation: Aircraft measurements in China

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 (<0.1 g m−3), the Nc was slowly increased with the Na. For example, the cloud droplets were increased from 200 to 300 cm−3, while aerosol particles were increased from 1000 to 6000 cm−3, indicating that a large amount of aerosol particles were unchanged under low LWC condition. By contrast, with high LWC value (>0.5 g m−3), the activation of aerosol particles to become cloud droplets was significantly enhanced. For example, the cloud droplets were increased from 700 to 1900 cm−3, while aerosol particles were increased from 1000 to 6000 cm−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.

Research highlights
► Under high aerosol particle conditions, large number of cloud droplets was formed with small size of droplets, while under low aerosol particle conditions, the formation of cloud droplets was limited by the number of Na.
► The number concentrations of cloud droplets were sensitivity to LWC under high aerosol particle conditions. With low LWC value (<0.1 g m−3), the Nc was slowly increased with the Na. However, with high LWC value (>0.5 g m−3), the activation of aerosol particles to become cloud droplets was significantly enhanced.
► The cloud radius (Rc) is very sensitivity to the LWC values.
► The increase in the LWC values led to significant increase in the size of cloud droplets.