Estimates of effective aerosol optical depths from spectral solar radiation measurements

This study reports a 37-year long record of direct beam spectral irradiance measurements made in Athens, Greece. An analysis of aerosol effects on the spectral distribution of solar radiation through effective optical depths, are presented. Thus, spectrally resolved aerosol optical depths were calculated and analyzed for the period 1954-1990. Summertime aerosol optical depths were found to be larger than winter values, while their seasonal variations were related to varying weather conditions throughout the year. The interrelationships between effective optical depths were found to be linear and were related strongly to microphysics of aerosol loading in the atmosphere. For the period 1962-1983 as wavelength exponent alpha(o) values ranged between 0.76-1.14 the spectrally resolved optical depths were found to increase markedly with respect to remaining periods 1954-1961 and 1984-1990 in which alpha(o) values ranged between 1.16-1.39. A minimum in aerosol optical depths, believed to be near background levels, was reached during period 1954-1957, while there was some indication that both optical depths continued to decrease reaching background levels at the end of the study period. From the long-term variation of aerosol effective optical depths some interesting information on the time evolution of air quality in Athens was gained. In addition, their frequency distribution, temporal daily variations and some remarks on photosynthetically active radiation for plant development, are presented and discussed.