Topography and local climatology Data collection

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

Athens is a city of about 4 million people located in a basin of 450-km 2 area. It is surrounded by high mountains on three sides and the sea on the south. Its topographic characteristics together with Mediterranean climatological conditions and intense pollu- tant emissions often make air quality as bad as that of Los Angeles, and other cities ˇ Ž . Cvitas et al., 1985; Mantis et al., 1992 . Thus, under favorable meteorological condi- Ž . tions Lalas et al., 1982; Katsoulis, 1988; Adamopoulos et al., 1996a,b,c , the concentra- tion of air pollutants may exceed the air quality standards of the European Union and the Ž . World Health Organization EARTH, 1998 . Sea breeze and calms are weather condi- tions that cause the so-called ‘‘nephos’’, a brownish photochemical cloud that is visible by the residents. Athens basin has two openings at the SW and NE edges, so that the predominant winds and consequently the aeration of the basin are accomplished in the NE–SW direction. Ž . During the warm period of the year, there are two contradicting wind regimes: a the sea-breeze circulation that appears when the synoptic flow is weak and is associated Ž . with high levels of air pollution Lalas et al., 1983; Gusten et al., 1988; Ziomas, 1998 ; ¨ Ž . and b the northerly ‘‘Etesian’’ winds generated under the influence of the anticyclone of Azores and the low-pressure system of India often extended to Cyprus. These winds of high speed and persistence are characteristic summer winds in the eastern part of the Ž . Mediterranean having a cleansing effect on the atmosphere Gagaoudaki, 1979 .

3. Data collection

In the warm period of 1997 the beam component of spectral solar irradiance was recorded during some cloudless days with a sampling rate of 30 min. The measuring site was on the roof of the Department of Electrical Engineering, National Technical Ž . University in the city center of Athens w s 37.978N, l s 23.728E . The measurements Ž . were performed using a passive pyrheliometric scanner PPS . In general, a pyrheliome- ter is a solar radiation instrument that records the direct component of the solar radiation. The term ‘‘passive’’ indicates that no energy from PPS is required for recording solar radiation. The term ‘‘scanner’’ implies that PPS performs spectral measurements. A detailed description of this system is given in Kambezidis et al. Ž . 1996 . In brief, solar radiation enters through two concentric lenses and is guided by Ž two optical fibers to two different diffraction gratings one in the UV and VIS spectral regions and the other in the VIS–NIR regions; the latter was not in use during this . study . A CCD sensor is then used to imprint the two spectral regions one at a time; the Ž . individual spectral values in mV are then converted from analog to digital form by an Ž y2 . ArD converter and further into solar radiation units W m . To achieve accuracy in the measurements, PPS was calibrated against a halogen spectral prototype lamp. In addition, another very important point was to keep the diffraction-grating unit in a Ž . constant-temperature environment Kambezidis et al., 1996 . A number of applications Ž . using PPS are involved with the estimation of: i the spectral total atmospheric Ž . transmittance and total optical thickness; ii the individual spectral atmospheric trans- Ž . mittances due to different atmospheric constituents; iii the total O and NO columns; 3 2 Ž . and v the aerosol influences on climatic changes and other properties of the atmo- Ž . Ž sphere e.g. albedo, effect on visibility etc , Adamopoulos et al., 1998a,b,c,d; Kam- . bezidis et al., 1996, 1997a,b, 1998a,b,1999 .

4. Total spectral transmittance retrieval