[Characteristics of haze and its impact factors in four megacities in China during 2006-2009].

The meteorological and environmental data including visibility, SO2, NO2 and PM10 were collected in four major megacities (Beijing, Shanghai, Guangzhou and Chengdu) in the years 2006-2009. Based on the data, seasonal and annual variations of the haze frequency and the key impact factors were discussed. The results indicated that the highest frequencies of haze occurred in summer, winter, spring and autumn for Beijing, Shanghai, Guangzhou and Chengdu, respectively. The trends of haze frequency decreased in Beijing and Guangzhou, while increased in Shanghai and Chengdu during the studied period. The PM10 concentration and relative humidity were the key factors for visibility degradation in the four megacities. The variation of visibility was sensitive to relative humidity in Beijing, to PM10 in Shanghai and Guangzhou, and to both in Chengdu.