Impacts of the Solar Eclipse of 29 March 2006 on the Surface Ozone Concentration, the Solar Ultraviolet Radiation and the Meteorological Parameters at Athens, Greece

In this study the variations in the surface ozone concentration, the solar ultraviolet radiation and the meteorological parameters at the ground before, during and after the total solar eclipse of 29 March 2006 have been examined. This analysis is based on the measurements performed at four stations located in the greater Athens basin in Greece. The experimental data demonstrated that the solar eclipse phenomenon affects the surface ozone concentration as well as the temperature, the relative humidity and the wind speed near the ground. The decrease in the surface ozone concentration that observed after the beginning of the eclipse event lasted almost two hours, probably due to the decreased efficiency of the photochemical ozone formation. The reduction of the solar ultraviolet radiation at 312 and 365 nm reached 97% and 93% respectively, while the air temperature dropped, the relative humidity increased and the wind speed decreased.


Introduction
The solar eclipse being a rare natural phenomenon gives an opportunity to investigate how the photochemical processes react to the comparatively fast solar radiation changes.
However, the number of studies exploring the effects of solar eclipse on surface temperature and surface winds as well as on surface ozone (SOZ) and its precursors is relatively small (Srivastava et al., 1982;Fernadez et al., 1993;Zerefos et al., 2001;Kolev et al., 2005;Tzanis, 2005, Gerasopoulos et al., 2007).
The experimental data obtained from different observational sites in Bulgaria during the solar eclipse of 11 August 1999, demonstrated that the influence of the phenomenon was manifested with a certain delay and the duration of the impact of the eclipse on the ground ozone concentration was almost two hours (Kolev et al., 2005).
A decrease of around 10-15 ppbv in SOZ concentration has been observed at Thessaloniki, Greece, during the solar eclipse of 11 August 1999 (Zerefos et al., 2001), while the percentage change of SOZ concentration at Athens, Greece was maximized one hour after the solar eclipse maximum and the greater values of SOZ percentage change were observed at the Patision station, an urban station located in the central part of the Athens basin (Tzanis, 2005).The ozone profile measurements over Thessaloniki during the solar eclipse of 11 August 1999 indicated also an ozone decrease up to 2 km with a lag-time between the maximum of the eclipse and the maximum of the induced ozone decrease (Zerefos et al., 2001).
The effect of various meteorological parameters on the variability of the surface ozone and its precursors in the Greater Athens area (a site in the Mediterranean region, where very frequent photochemical pollution episodes occur), have been discussed in a number of recent publications (Kondratyev and Varotsos, 1995;Varotsos and Kondratyev, 1995;Ziomas et al., 1998;Varotsos et al., 2001;Varotsos et al., 2003Varotsos et al., , 2005;;Varotsos and Kirk-Davidoff, 2006).
respectively, during the solar eclipse maximum while the change in SUVR at 365 nm was about 93%.This is in close agreement with the observed reduction in the incoming solar radiation as derived from measurements conducted at Thission station (center of Athens) by Founda et al. (2007).Although the most important chemical mechanisms involved in photochemical pollution have been already identified and studied, further investigation is necessary because this atmospheric phenomenon is a very complex process involving meteorological, topographic, emission and chemical parameters (Varotsos, 1989(Varotsos, , 1994(Varotsos, , 2005b;;Varotsos et al., 2004).
In this work we examine the behavior of the surface ozone concentration as well as the variations in various meteorological parameters (temperature, relative humidity and wind speed and direction) during the solar eclipse that took place on 29 March 2006 over Athens, Greece (38 • N, 23 • E).The results obtained are compared with those that revealed from the observational analysis of the solar eclipse that took place on 11 August, 1999 at Athens.

Data
We have used the measurements of SOZ concentration along with meteorological measurements, taken at four monitoring stations (Patision, Smyrni, Geoponiki and Zografou).The four sites are located into the greater area of the Athens basin as follows: Patission site in a street of heavy traffic in the centre of the city, Smyrni site at about 5 km SE of the centre of the city in an area of low traffic, Geoponiki site at about 2 km SW from the centre of the city in an area of low traffic and light industrial activities and Zografou site at about 3 km NE of the centre of the city in an 20 area of low traffic inside the campus of the Athens University.SOZ measurements were made with conventional analyzers (APOA,   HORIBA Inc., UV absorption) with time resolution 30 s and accuracy ±2µg/m 3 ppb.
At the same locations similar analyzers were also used to obtain observations of the ozone precursors.The selection of the above mentioned monitoring stations was made on the basis that their respective locations give a representative picture of the urban area since they cover four opposite sections.It should be clarified that our data were obtained by the National Service for Air Pollution Monitoring, which performs 120 measurements per hour reporting hourly means.Therefore, our measurements are the arithmetic mean value of 120 measurements obtained in an hour.The above-mentioned time resolution of our data is adequate because we are going to search for a signal with duration of almost two hours (see figure at http://www.cc.uoa.gr/∼ nsarlis/gndlayerleveloz/ Figure1.doc in Tzanis et al., 2007).
In addition, the MICROTOPS II sun-photometer (Solar Light Co., Inc.) was used for measurements of SUVR-B at 312 nm.This filter instrument is a continuation of a series of hand-held ozonometers with TOPS at the beginning.The new generation of those instruments is MICROTOPS II, that is a 5-channel sun-photometer with centre wavelengths of 300, 305, 312, 940, and 1020 nm for measurements of TOZ, total water vapour and aerosol optical thickness measurements (Kondratyev and Varotsos, 2000).With this instrument TOZ is derived from measurements for three wavelengths in the UV region, given the site's latitude and longitude, universal time, altitude and pressure.
As in Dobson instrument the measurement at an additional third wavelength enables a correction for particulate scattering and stray light (Varotsos et al., 1995).The total water vapour is determined through the measurements at 940 nm and 1020 nm.The angle of view of each of optical channels is 2.5 • and the resolution is 0.001µW cm −2 .The typical agreement between various MICROTOPS II instruments (accuracy) is within 1-2%.The repeatability of consecutive ozone measurements is better than 0.5%.A 21-months intercomparison of the MICROTOPS II filter ozonometer with the Dobson and Brewer spectrometers resulted that Mtops can measure TOZ with an accuracy comparable to the conventional spectrometers (agreement is better than ±1%), over a reasonable range of µ.Adverse conditions (clouds, haze, and low sun) result in deviations of more than ±2% or even ±3% (Kondratyev et al., 1995;Kondratyev and Varotsos, 2000;Varotsos et al., 2000).

Discussion and results
The solar eclipse of 29 March, 2006 at Athens, Greece (38 • N, 23 • E) started at 12:31 LT, reached the maximum solar coverage (84%) at 13:48 and ended at 15:04 LT. Figure 1 presents the SOZ measurements and the expected SOZ values as derived from the fitted curve of the measurements, before, during and after the solar eclipse of 29 March 2006, at four stations (Patision, Smyrni, Geoponiki and Zografou) in the Athens basin.The expected values were calculated by applying a 6th degree polynomial fit (best fit) to the observed SOZ concentrations when the eclipse event was absent.All percents in Table 1 were calculated by subtracting the observed SOZ concentrations just before, during and just after the eclipse event from the expected ones.According to Fig. 1  almost two hours and is maximized one hour after the solar eclipse maximum at each of the stations.The greater values of SOZ percentage change are observed at the Patision station, an urban station located in the central part of the Athens basin.This is in agreement with SOZ measurements before, during and after the solar eclipse of 11 August 1999 at Patision, Geoponiki and Smyrni stations as it shown in Fig. 2 and Table 2 (Tzanis, 2005).The above mentioned behavior of SOZ during the solar eclipse may be related to photochemical processes due to the fact that the gradual decrease in the solar radiation affects the photochemical reactions within the planetary boundary layer.The decrease in SOZ concentration started after the beginning of the eclipse and maximized after the solar eclipse maximum as a consequence of the further fall in sunlight intensity that decreased the efficiency of the photochemical ozone formation.The solar radiation started to increase after the eclipse totality while the SOZ concentration started to increase about one hour later and returned to its ordinary behavior several minutes after the end of the solar eclipse.The SOZ variations detected at Athens, before, during and after the solar eclipse of 29 March, 2006 demonstrated that the influence of the phenomenon was manifested with a certain delay and lasted almost two hours.The observed SUVR measurements at 312 and 365 nm as derived from the MICROTOPS II sun-photometer and the VLX-3W radiometer, before, during and after the solar eclipse of 29 March, 2006 at Athens are shown in Figs. 3  and 4. As can be seen, the percentage decrease of SUVR-B (312 nm) which was measured by the MICROTOPS II and VLX-3W instruments reached the value of 97% and 93%  respectively, during the solar eclipse maximum while the change in SUVR at 365 nm was about 93%.This is in close agreement with the observed reduction in the incoming solar radiation as derived from measurements conducted at Thission station (center of Athens) by Founda et al. (2007).
Figure 5 shows the temporal variation of the relative humidity, temperature and wind velocity during the eclipse at Patision station.Air temperature near the ground decreased from 20.1 • C to 19.4 • C during the eclipse event and after that it began to increase abruptly.Relative humidity started to increase at 13:00 (55%) as a result of the temperature decrease reaching a maximum (58%) at the end of the solar eclipse.It is notable that the observed anti-correlation of the temperature and relative humidity curves in Fig. 5 would indicate that the absolute humidity (important parameter to the photochemistry did not change throughout the day. The fact that the wind speed started to decrease after the solar eclipse maximum, may be attributed to the cooling and the stabilization of the atmospheric boundary layer.The wind speed continued to decrease for two hours after the eclipse event without any significant change in direction.Kolev et al. (2005) observed similar changes in the wind speed in Bulgaria during the solar eclipse of 11 August 1999.The temporal variation of the above mentioned meteorological parameters during the solar eclipse of 29 March, 2006 is in close agreement with observations at three other stations (Thission, Penteli, Markopoulo) in the Athens basin reported by Founda et al. (2007).

Conclusions
During the solar eclipse of 29 March, 2006 measurements of surface ozone concentration, solar ultraviolet radiation and meteorological parameters (relative humidity, temperature and wind velocity) were performed at four sites (Patision, Smyrni, Geoponiki and Zografou) into the greater area of the Athens basin in Greece.As expected, all the parameters mentioned above were affected by the solar eclipse.
The percentage decrease of the SUVR at 312 and 365 nm reached 97% and 93% respectively at the maximum phase of the eclipse.
The SOZ concentration decreased after the eclipse event for almost two hours and the maximum percentage change observed one hour after the maximum of the solar eclipse at all stations.The greater values of SOZ percentage change were observed at Patision station (urban station) located in the center of Athens.The decrease in SOZ concentration was attributed to the dramatic reduction of the solar radiation that affects the photochemical reactions.
The near-ground air temperature dropped about 0.7 • C during the eclipse event at the center of Athens while the relative humidity increased reaching a maximum at the end of the solar eclipse.The wind speed decreased after the solar eclipse maximum without any significant change in direction.
After the end of the solar eclipse of 29 March, 2006, all the aforementioned parameters exhibited a tendency to regain their ordinary behavior.

Figure 1 . 6 Fig. 1 .
Figure 1.Surface ozone measurements and the expected surface ozone values as derived from the fitted curve of the measurements before, during and after the eclipse event of 29 March, 2006 at four stations (Patision, Smyrni, Geoponiki and Zografou) in the Athens basin, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.

Figure 2 .
Figure 2. Surface ozone measurements and the expected surface ozone values as derived from the fitted curve of the measurements before, during and after the solar eclipse of 11 August, 1999 at three stations (Patision, Geoponiki and Smyrni) in the Athens basin, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.

Fig. 2 .
Fig. 2. Surface ozone measurements and the expected surface ozone values as derived from the fitted curve of the measurements before, during and after the solar eclipse of 11 August, 1999 at three stations (Patision, Geoponiki and Smyrni) in the Athens basin, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.

Fig. 3 .
Fig. 3. SUVR-B (312 nm) measurements derived from the MICRO-TOPS II sun-photometer (Solar Light Co., Inc.), before, during and after the solar eclipse of 29 March, 2006, at Athens, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.
speed in Bulgaria during the solar eclipse of 11 August 1999.The temporal variation of the above mentioned meteorological parameters during the solar eclipse of 29 March, 2006 is in close agreement with observations at three other stations(Thission, Penteli,   Markopoulo)  in the Athens basin reported byFounda et al. (2007).

Figure 5 .Fig. 5 .
Figure 5. Meteorological data obtained at Patision station before, during and after the solar eclipse of 29 March, 2006, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.
throughout the eclipse event of 29 March, 2006 at Athens, Greece.

Table 1 .
and Table 1, the percentage change of SOZ lasts for Calculated percentage change of surface ozone (at four stations, separately) throughout the eclipse event of 29 March 2006 at Athens, Greece.

Table 2 .
Calculated percentage change of surface ozone (at three stations, separately) throughout the eclipse event of 11 August 1999 at Athens, Greece.
Figure 3. SUVR-B (312 nm) measurements derived from the MICROTOPS II sunphotometer (Solar Light Co., Inc.), before, during and after the solar eclipse of 29 March, 2006, at Athens, (a) beginning of the solar eclipse, (b) solar eclipse maximum, (c) end of the solar eclipse.