On the attenuation of direct solar irradiance in Athens due to aerosols

Summary In this study the trend of the time sequence of the integral aerosol optical depth (k _a), as proposed by Unsworth and Monteith, was determined for clear days in summer for the period 1962–1988 in Athens. The trend was found by fitting a third degree polynomial curve and it was concluded that (k _a) showed a considerable increase (i.e. from the value of 0.18 to 0.31) in the period 1962–1976 and remained approximately constant until 1979, after which it started decreasing again slowly until 1988. The increase of (k _a) in the period 1962–1976 is likely attributable to the rapid development of the city in this period, while the decrease of (k _a) after 1979 probably reflects the efficiency of some restrictions which were imposed on the pollutant emissions during this period. In addition, an analysis of the percentage frequency distribution found that while 95% of the values of (k _a) ranged from 0.100 to 0.400 in the beginning of the period (1964–1967), in recent years (1984–1987) the same percentage of the values of (k _a) ranged from 0.100 to 0.500.With 3 Figures