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
Ž . Ž
. The total spectral atmospheric transmittance t l, z
in the sun-sensor direction , considering single scattering can be derived from:
t l,u s H l,u rH l D. 1
Ž .
Ž .
Ž . Ž .
z z
o
Ž .
y2
Ž where H l,u
stands for the measured calibrated irradiance in W m function of
z
. Ž .
wavelength, l, and zenith angle, u , H l is the extraterrestrial radiation taken from
z o
Table 1 Selected dates and flow characteristics
Ž .
Date Times of measurements LST
Prevailing flow 20th May 1997
0800–1800 hours except Pure sea breeze with speeds
y1
at 1030 and 1100 less than 2.8 m s
5th September 1997 0730–1800
Pure Etesians with speeds
y1
less than 6.8 m s 1st July 1997
0730–1800 Competition between Etesians and
y1
Ž .
sea breeze speeds less than 5.1 m s
y1
11th July 1997 0730–1800
Southerly flow with speeds less than 5.5 m s
Ž .
Ž Gueymard 1995 , and D is the correction factor for the sun–earth distance Kambezidis
. et al., 1997a .
X
Ž . Ž .
The total spectral optical thickness, d l , towards zenith is obtained from:
X
d l s 1rmln 1rt l
2
Ž . Ž .
Ž .
where m is the pressure-corrected atmospheric optical mass calculated via the formula Ž
. given by Gueymard
1995 . The total spectral atmospheric transmittance function Ž
.
X
towards zenith , t , is derived via the following equation:
X X
t l s exp yd
l .
3
Ž . Ž .
Ž .
Thus, the atmospheric transmittance function is corrected for optical mass effects to enable us to compare values obtained under various solar altitudes.
Fig. 1. Total spectral atmospheric transmittance contours in Athens during the 20th of May 1997.
5. Results and discussion
Based on the method presented above, spectral total atmospheric transmittance curves Ž
. were derived during some distinct days case studies under different wind conditions.
Ž Wind data were taken from the inventory of the National Observatory of Athens NOA,
. 1997 . This is located at a distance of 2 km from the measuring place with no barriers in
between and same climatic features. Table 1 gives a list of the clear-sky days selected and the prevailing flow.
Fig. 1 presents the total spectral atmospheric transmittance during the 20th of May 1997. On that day, the synoptic wind flow was very weak and favored the development
of a sea-breeze circulation in the Athens basin. Under such circumstances a brownish
ˇ
Ž photochemical cloud is very likely to occur Cvitas et al., 1985; Lalas et al., 1983, 1987;
. Gusten et al., 1988; Katsoulis, 1988; Suppan et al., 1998 . The reduction in visibility
¨
Ž .
was severe and the air quality was degraded. The contour lines Fig. 1 show steep gradients indicating rapid changes in the atmosphere. An intense maximum of air
pollution appears around 0900 hours, while a second one shows up at 1300. The former is associated with traffic circulation since it is only observed in the VIS. The spectral
range 366–406 is an atmospheric window for O , while NO
that exhibits great
3 2
absorption in this spectral range, is released by the exhaust pipes of vehicles at a percentage around 70. In addition, the morning peak is also associated with the
appearance of temperature inversions; the mixing-layer height reaches approximately the
Fig. 2. As in Fig. 1 but for the 5th of September 1997.
altitude of the summits of the highest mountains during the mornings of the warm period Ž
. of the year when the synoptic flow is absent or weak Kambezidis et al., 1995 . Other
Ž works confirm this morning peak in air pollution as well Kambezidis et al., 1995;
. EARTH, 1998 . The midday peak coincides with intensification of the local sea breeze.
Secondary photochemical pollutants are then produced. Unlike the morning peak, the midday peak is observed throughout the considered spectral range. From an analysis of
air-pollution measurements performed by a network of ground-based stations belonging to the Ministry of Environment, it turns out that secondary pollutant, such as ozone,
Ž .
peak at 1300 EARTH, 1998 . Afterwards, air pollution turns out to drop because of the air-pollutant transport to the northern suburbs of Athens via the sea-breeze mechanism.
Ž .
Klemm et al., 1998; Suppan et al., 1998 . The diurnal changes are largely attributed to NO mainly over 350 nm and particulate
2
matter throughout the considered spectral range. Especially over 530 nm, the higher values of transmittance observed after 1000 hours indicate that there is a substantial load
of marine aerosols present in the atmosphere as a result of sea-breeze circulation. This conclusion is supported by the fact that marine aerosols possess fair attenuation
Ž .
coefficients in the range 500–575 nm Vaxelaire et al., 1991 . The role of marine Ž
aerosols in atmospheric chemistry was found to be significant for Athens Eleftheriadis .
et al., 1998 . Much higher values of atmospheric transmittance are obtained during the 5th of
Ž .
September 1997, a pure Etesian day, Fig. 2 compared to those calculated during the
Fig. 3. As in Fig. 1 but for the 1st of July 1997.
pure sea-breeze day. The day was characterized by the dominance of relatively strong northerly Etesian winds with speed varying in the range 4.5–6.8 m s
y1
. The higher values of total atmospheric transmittance observed are consistent with the results of
Ž other studies Kambezidis et al., 1998c; Ziomas, 1998; Suppan et al., 1998; Svensson
. and Klemm, 1998 . The apparent effect of the Etesians on air pollution is also in line
with simultaneous visibility observations. A morning maximum of air pollution is observed in this case as well. After this morning peak, the Etesians sweep the air
pollutants seawards thus cleansing the atmosphere over the basin. This is clear from Fig. 2 showing continually improving transmittances in the rest of the day. After 1700, they
reach the value of 0.8 above 500 nm. The results are in accordance with the high visibility observed and the low levels of air-pollutant concentrations recorded by
Ž .
EARTH 1998 on that day. During the 1st of July 1997, a high-pressure system was centered over Italy and the
day was characterized by the competition between Etesian and sea-breeze flows. As a Ž
. result, the transmittance contours Fig. 3 have characteristics of both wind regimes
Ž .
Figs. 1 and 2 . The values obtained lie in between those observed during pure-sea-breeze Ž
. and pure-Etesian day as expected Suppan et al., 1998 . Concerning the diurnal variation
of the total transmittance, the morning maximum in air pollution mainly due to traffic circulation is still observed but it is of longer duration in this case. The midday peak is
not well pronounced, although it seems to exist. After 1500, the atmosphere becomes
Ž .
cleaner higher transmittances as also observed in both former cases.
Fig. 4. As in Fig. 1 but for the 11th of July 1997.
During the 11th of July 1997, the predominant wind direction was from the southern sector. However, this had nothing to do with sea breeze. This wind flow was generated
by the influence of a low-pressure system located north of Greece. The calculated total Ž
. Ž
transmittances Fig. 4 are much greater than those under the sea-breeze circulation Fig. .
Ž 1 in the whole spectral range, although both winds are southerly in origin Katsoulis,
. Ž
. 1988 . In addition, the total transmittance values Fig. 4 are comparable to those
Ž .
obtained during the pure-Etesian day Fig. 2 especially on the top of the spectral range. The morning peak in air pollution appears at 0830 and it is in line with the rush hour of
the city. The 1300 peak is still observed but it is less intense than that during the sea-breeze day. Another rigorous maximum in air pollution shows up at 1600 in the VIS
range. This might be due to chemical processes and transport phenomena. Kambezidis et
Ž .
al. 1998d have shown the formation of an O reservoir at an altitude of 1200 m a.g.l.
3
over the basin at the end of the day. Nevertheless, the absence of pollutant-concentra- tion-profile measurements does not allow any approach. However, this may be due to a
delayed appearance of secondary photochemical pollutants, together with dust and marine-particle accumulation.
6. Concluding remarks
