• surface solar radiation;
  • aerosol optical depth;
  • cloud cover


Using surface meteorological data at 20 sites in Beijing area during 1961–2007, the variability and trend of surface solar radiation (SSR) and their potential drivers were investigated. Cloud cover was an important factor determining the interannual variation of SSR reflected in the linear correlation coefficients (R ranged from −0.65 to −0.89) between interannual variations of SSR and cloud cover. Aerosol optical depth (AOD) also contributed to the interannual variability of SSR, as shown by negative correlations between AOD and SSR (R ranged from −0.44 to −0.81). SSR decreased by 2.29 (winter), 3.63 (spring), 7.45 (summer), and 3.76 W m−2 (fall) per decade during 1961–2007. The observed decrease in cloud cover should have resulted in solar brightening instead of dimming, indicating that cloud cover is not the driver of the long-term trend of SSR. AOD was observed to increase by 0.02–0.04 per decade, which was consistent with the SSR trend. Therefore, we argue that AOD, instead of cloud cover, is the major driver of the long-term SSR trend in Beijing metropolitan area. The largest dimming was observed in urban sites as a result of the largest increased AOD trend observed there. SSR trends in rural sites were about 30%–54% of the trends in urban stations, which was mostly due to the smaller decreasing AOD trends that were observed in rural sites.