Analysis of the formation of fog and haze in North China Plain (NCP)

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<sup>−1</sup>, 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<sup>−1</sup>), 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<sup>−3</sup>. 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<sup>−3</sup>) with small size (5–6 μm) were observed during the fog period, resulting in extremely low visibility (less than 100 m)