Abstract
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).
Highlights
North China Plain (NCP) is located in northern coast China, and is surrounded by Taihang Mountains, Yanshan Mountains, and Bohai Sea
The NCP region is one of the regions with very heavy aerosol loadings, and haze and fog events are frequently occurred in this region
A long-term trend of fog and haze over the NCP region during the past 56 years is analyzed to study the effect of aerosol loadings on the formations of haze and fog, especially under high aerosol loadings
Summary
North China Plain (NCP) is located in northern coast China, and is surrounded by Taihang Mountains (at the west of NCP), Yanshan Mountains (at the north of NCP), and Bohai Sea (at the east of NCP) (see Fig. 1). In order to better understand the characteristics of haze and fog in the NCP region, meteorological parameters, the size distributions and number concentrations of aerosol particles and fog droplets were simultaneously measured during the experiment. Both the trend data and the experiment result provide valuable information to study several important characteristics of haze and fog, including; (1) the evolution of haze and fog occurrences in the past 56 yr; (2) the relationship between the occurrences of haze and fog; (3) the impact of aerosol particles on the formation of haze and fog; and (4) the impact of haze and fog on visibility.
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