Solar dimming and brightening over Thessaloniki, Greece, and Beijing, China

This work presents evidence that ultraviolet (UV)-A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV-A irradiances over Thessaloniki show a downward trend of -0.5% yr(-1), which reverses sign and becomes positive in the last decade (+0.8% yr(-1)). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV-A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short-wave radiation over Beijing showed negative changes of -0.4% (1984-1991) and -0.1% yr(-1) during 1994-2006. The negative trend in solar radiation continued even during 2000-2006. Satellite-derived aerosol optical depth (AOD) increased by +1.0% yr(-1) during 2000-2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future.