Quantification of the effect of urbanization on solar dimming

During the 25-year period (1964-1989), a noticeable decline in surface solar radiation, termed global dimming, over worldwide sites was essentially a local phenomenon associated with human activity as expressed by the sites' population density. Specifically, our findings indicate that solar dimming was observed only over a limited part (similar to 30%) of the total land area, restricted to highly-populated sites with population density higher than 10 person/km(2). Dimming was dominated by anthropogenic aerosol emissions: the decline in surface solar radiation intensified from -0.05 W/m(2)/yr to -0.32 W/m(2)/yr, with population density increasing from 10 to 200 person/km(2). At sites with population density >200 person/km(2), a saturation effect was observed: declining trends were much less pronounced than those over sites with a lower population density. Overall, it is demonstrated that urban areas obtained less solar radiation, compared to rural areas, in the amount of similar to 12 W/m(2) which is equivalent to about 8%.