Trends of visibility on sunny days in China in the recent 50 years

The visibility on sunny days in 543 stations of China were analyzed, the results shows: The annual mean visibility on sunny days is higher in Northwest China and lower in Southeast China, which is similar to the distribution of aerosol optical depth. Visibility in summer is lower than in winter in southeast China, which relates to the stronger solar radiation in summer benefits to aerosol formation; Visibility in large cities is lower than small cities, and visibility of large and medium cities in summer is lower than winter but for small cities is opposite: The visibility decrease trend is revealed for most stations in recent 50 years, and the maximum trend coefficient -0.8 is found in delta regions of Pearl River and Yangtze River with climatic trend of 10 km/10a. The obvious visibility decrease for large and medium cities was from 1960s' to 1990s' accompanied by a slow increase later, and it was after the middle of 1990s' visibility began to decrease for small cities: The diffuse radiation on sunny days show increase trend in recent 50 years, and it significantly correlates to visibility on sunny days with a correlation coefficient of -0.59 passing t-test at 0.001 significance level: Visibility on sunny days also correlates to total fuel consumption data and number of vehicles in China. Aerosol pollution should account for the obvious decrease of sunny visibility. (C) 2012 Elsevier Ltd. All rights reserved.