Abstract: In this study, nearly 50 years of diurnal surface solar radiation data and nearly 60 years of diurnal meteorological data, including total cloud amount, visibility, water vapor pressure and humidity, and 10 years of MODIS aerosol products, have been analyzed to examine the regional surface solar radiation change and its impact factors over Xi’an City,Shaanxi Province,China. We find that there is a significant decreasing of surface solar radiation from 1970s to 1980s. From 1974 to 1984, surface solar radiation over Xi’an decreased by approximately 18.2 %·(10 a)^-1, and increased by 14.9 %·(10 a)^-1 from 1985 to 1995, form 1996 to 2005 there also a decrease by 15.2 %·(10 a)^-1. The average decreasing trend of surface solar radiation over Xi[JP8]’a[JP]n was larger than that over the mainland in China and the time of “brightening” period was a little earlier over Xi’an. The seasonal variation of surface solar radiation shows that although there was some difference in variation scope of total solar radiation, there was a basically downtrend in four seasons. The variation in summer and autumn was most similar with annual change. While in spring there was an obviously decreasing from 1963 to the end of 1980s, then it began to increase slowly. In winter there was a steady decline of surface solar radiation since 1963. In order to find out the main course of decrease in surface solar radiation, we analyzed the principal meteorological factors contributed to surface solar radiation. First we studied the variation of total cloud amount, which may be one of the dominant affecting factors. Generally the increasing of total cloud amount can reduce the amount of solar radiation reaching the surface of the earth. The average decreasing trend of annual total cloud amount suggested that there may be another important factor for the decrease of annual solar radiation. Further more, after analyzed water vapor pressure, relative humidity, visibility and aerosol optical depth. We concluded that there also a decrease of water vapor pressure and relative humidity, so these two factors have little effect on decrease of total solar radiation over Xi’an. While there were an obviously decrease of visibility and an increase of AOD over Xi’an. This means that from 1980s there was an increase of anthropogenic aerosol due to the development of cities. Moreover with the effect of special terrain and prevailing winds, anthropogenic aerosols accumulated at east part of Guanzhong in Shaanxi province, thus there is a high aerosol concentration above the surface layer at Xi’an and its east region. It implies that AOD plays a major role in reducing the solar radiation reaching the surface. In generally, we notice that impact factors of solar radiation were various in different seasons. In spring and summer, the total cloud amount and direct radiation force of aerosol play the same role for the variation of solar radiation, total cloud amount limited the variation range while the direct radioactive stress resulting from the increase of anthropogenic aerosols determined the downward trend. And in autumn and winter the direct radioactive stress of anthropogenic aerosols may be more important.

Key words: surface solar radiation, aerosols, total cloud amount, direct radiative stress