Determination of Ångström and Schüepp's Parameters from Ground-based Spectral Measurements of Beam Irradiance in the Ultraviolet and Visible Spectrum in Athens, Greece

Abstract

Ð This work estimates the A Ê ngstroÈ m coecients, assuming a linear behavior of the natural logarithm of aerosol optical thickness, ln d a (k), versus the natural logarithm of wavelength, ln k, in the urban environment of Athens for a period of one year. The calculation of the coecients a, b, was carried out using records of spectral beam irradiance in the 320±575 nm spectral range. The seasonal and diurnal variation of the coecients shows dierent features with generally higher values of b and lower values of a in summertime. The A Ê ngstroÈ m coecients are investigated with respect to various wind regimes and the results are discussed with a view to air pollution sources, climatic and topographic characteristics of the Athens basin. A similar study is also carried out for the SchuÈ epp's turbidity parameter.

Total Atmospheric Transmittance in the UV and VIS Spectra in Athens, Greece

“…During the afternoon hours the temperature inversion disappears and the sea breeze sweeps air pollutants to the north, consequently the city center becomes less polluted (LALAS et al, 1983;GÜ STEN et al, 1988). This diurnal variation observed is in agreement with the diurnal variation of turbidity parameters calculated for Athens (KAMBEZIDIS et al, 2001b(KAMBEZIDIS et al, , 1998c. and Madrid (CASTRO, 1982).…”

“…Afterwards some short variations occur until 14:30 h LST and then the atmosphere becomes cleaner. The above results are also confirmed by atmospheric turbidity estimations (KAMBEZIDIS et al, 2001b).…”

“…This is supported by the fact that the peak is observed above 400 nm where NO x gases play a stronger role in depleting solar radiation if one takes into account that vehicles are responsible for 67% of the total NO x emission in Athens (SCHEFF and VALIOZIS, 1990). Another work (KAMBEZIDIS et al, 2001b), which is based on the same data set and estimates the atmospheric turbidity factors of Å ngstro¨m and Schu¨epp in the Athens area, also confirms the higher air-pollution levels around midday. These results are in accordance with those of another work (CASTRO, 1982) that found smaller turbidity values in the morning and greater at midday in Madrid.…”

“…15, as expected (KAMBEZIDIS et al, 1998c(KAMBEZIDIS et al, , 2001bGAGAOUDAKI, 1979;LALAS et al, 1983;GÜ STEN et al, 1988;CLAPPIER et al, 2000;GROSSI et al, 2000;SUPPAN et al, 1998). This phenomenon is not observed before 9:00 h LST due to a delay in the dispersion of air pollutants by the Etesians.…”

“…Transmittance values corresponding to other wind directions or speeds have not been taken into account. KAMBEZIDIS et al (1998cKAMBEZIDIS et al ( , 2001b) also used these selection criteria. The wind data were taken from the inventory of the National Observatory of Athens.…”