1. Introduction
Ž Solar irradiance is attenuated during its passage through the atmosphere Iqbal, 1983;
. Goody and Yung, 1989 . The major extinction processes are the molecular absorption
Ž .
and scattering Rayleigh , absorption by ozone, nitrogen oxides, water vapor and sulfur Ž
. dioxide, as well as absorption and scattering by aerosols Mie scattering . In addition to
these, a variety of chemical species are also responsible for absorbing solar irradiance in Ž
. the UV and VIS spectral regions Jacobson, 1999 .
Ž .
The radiative-transfer codes, e.g. MODTRAN Berk et al., 1989 and SMARTS2 Ž
. Gueymard, 1995 , can model atmospheric properties of the various atmospheric con-
stituents based on standard atmospheric conditions. However, the air composition exhibits large temporal and spatial variations in urban areas like Athens. This may cause
substantial differences between modeled and real data. Therefore, knowledge of the real Ž .
atmospheric optical properties is important for i estimating the impact of a polluted Ž .
atmosphere on weather and climatic change, ii studying air pollution and energy Ž .
exchange, iii correcting satellite images for the atmospheric effect in the visible Ž .
spectral range especially over polluted regions, and iv
simulating spectral solar irradiance. The latter is, in turn, helpful in the design of photovoltaics, window glazing
for energy-efficient buildings, calculation of heatingrcooling loads in architecture and Ž
. the design of flat-plate collectors. Moreover, the selected spectral range UVrVIS is
Ž . Ž
suitable for i estimating atmospheric constituents like O and NO , Kambezidis et al.,
3 2
. Ž . 1996, 1997a, 1998a , ii studying part of the solar erythemal UV band and its effect on
Ž .
biological tissues Lorente et al., 1994; Repapis et al., 1998; Papayannis et al., 1998 and Ž .
Ž .
iii assessing the reduction of part of the photosynthetically active radiation PAR , that Ž
is the radiation required by plants for the photosynthesis process Blackburn and Proctor, .
1983 .
2. Topography and local climatology
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