LIDAR AND GROUND OZONE MEASUREMENTS IN THE PBL DURING THE AUGUST 11, 1999, SOLAR ECLIPSE

Nikolay Kolev(1), Vera Grigorieva(1), Vasil Umlensky(2), Boyan Tatarov(3), Boiko Kaprielov(1), Ivan Kolev(1)

(1)Institute of Electronics, Bulgarian Academy of Sciences 72, Tsarigradsko shosse Blvd., 1784, Phone: +359 (2) 7144-514; FAX: +359 (2) 9753201; e-mail: [email protected] (2)Institute of Astronomy, Bulgarian Academy of Sciences (3)National Institute for Environmental Studies, Tsukuba, Japan

ABSTRACT shows some isolines: the isochrones, which connect the points where the eclipse begins and ends at the same The experiment was carried out on August 11, 1999, moment, and the isophases (the points where the during the 94 % solar eclipse, using a lidar, an ozone maximal eclipse phase is the same). By interpolation, meter, and a ground meteorological station. The lidar one can determine approximately the characteristics was used to measure the height of the mixing layer after moment and the maximal phase at each point. sunrise and, in particular, before, during and after the solar eclipse [1]. The ozone meter measured the ground On the territory of Bulgaria, the eclipse began at 12:35 ozone concentration during the phenomenon observed. hours local standard time (LST) to the west of the town The ground meteorological station took the of Kula (point with coordinates 22°21′ eastern longitude meteorological parameters of the atmospheric ground and 43°51′ northern latitude) and ended at 15:34 hours layer. The weather conditions in the region of Sofia, LST on the shore to the east of the town of Bulgaria were favorable for observation. The data of the Shabla (point with coordinates 28°36′ eastern longitude three types of measurements demonstrate with certainty and 43°32′ northern latitude). The central line of the that the solar eclipse affects the meteorological eclipse path started on the bank of River near parameters of the atmosphere near the ground, the the town of Popina and ended on the Black Sea shore to ozone concentration near the ground, and the height of the east of the town of Shabla. The Lunar shadow the mixing layer. It was found out that a certain time “stepped” on Bulgarian soil on 14:07 hours LST at a delay exists of the solar eclipse impact on the point with coordinates 26°57′ e.l. and 44°08′ n.l. and meteorological parameters, the ozone concentration and left at 14:14 hours at 28°36′ e.l. and 43°32′ n.l. the mixing layer height, which delay was different for the different parameters. Fig. 2 shows a schematic diagram of a total solar eclipse. The Moon enters the solar disk from the right- 1. EXPERIMENTAL SETUP, EQUIPMENT AND hand side and gradually blocks an ever-increasing TECHNIQUES portion of it. Fig. 1 presents the path of the solar eclipse totality band across Bulgaria’s territory. The figure also Sun Moon Earth

Shabla

Sofia Fig. 2. Schematic diagram of a total solar eclipse; 1 – cone of the lunar umbra; 2 – cone of the lunar penumbra; 3 – zone where partial solar eclipse is observed; 4 – zone of total solar eclipse.

The illuminance decreases as the portion of the Sun Fig. 1. Totality band (the shaded area), isochrones and screened by the Moon increases. The temperature drops isophases of the eclipse over Bulgaria’s territory on August by several degrees. The data for Sofia are as follows: 11, 1999 [2]. partial eclipse beginning 12:36 hours, maximal phase Solar eclipse Solar eclipse Solar eclipse moment 14:03; maximal surface phase 0.944, partial beginning maximum end 2500 eclipse end 15:27. 2400 2300 2200 In the experiments reported, we used an aerosol lidar 2100 2000 with the following main parameters: tarnsmitter – a Q- 1900 switched frequency-doubled Nd-YAG laser, wavelength 1800 1700 532 nm, pulse energy 10 - 15 mJ, pulse duration 1600 15 - 20 ns, pulse repetition rate 12.5 Hz, laser beam 1500 1400 Mixing layer height (m) Mixing layer divergence 3 mrad; receiving antenna – Cassegrain-type 1300 1200 Second derivative Standard deviation telescope with main mirror diameter 150 mm and 1100 equivalent focal length 2250 mm; photodetector – 1000 12: 00 12: 30 13: 00 13: 30 14: 00 14: 30 15: 00 15: 30 16: 00 photoelectron multiplier type FEU 84 with interference Local time (hh:mm) filter with FWHM 1 nm; data acquisition and processing system – 10-bit 20 MHz analog-to-digital converter Fig. 3. Atmospheric mixing layer height during the solar (ADC) and a PC. eclipse on August 11, 1999 from 11:50 to 16:00 hours LST, as determined by the second derivative minimum and the standard deviation maximum. During the lidar experiment, we recorded 6000 profiles in periods of about 9 minutes. Each 150 profiles were Fig. 3 illustrates the variation of the mixing layer height averaged in view of increasing the signal-to-noise ratio. during the experiment, calculated by using the The 40 profiles thus obtained were transformed into maximum of the root-mean-square deviation and the S-functions, for which the standard deviation and the second derivative of the lidar signal. The mixing layer is second derivative were calculated; these were later used being developed after sunrise due to the heating of the to determine the mixing layer height [3, 4]. earth surface. At the start of the lidar experiment, both techniques yield a mixing layer height 1300 m; it then The meteorological data used were those from the grows steadily up to about 1600 m (at 12:49), remains standard ground measurements carried out by the close to this height until 13:42, and then resumes National Institute of Hydrology and Meteorology, growing to 1900 m (at 14:06), calculated from the located in a close vicinity to the Institute of Electronics, standard deviation. This is followed by a period of a fall and in the Shabla region. in the mixing layer height, until it reaches 1350 m (at 14:55). In the next 10 minutes, one can see a climb to a To measure the ground ozone concentration, a height of 1500 m, followed by another fall during the chemiluminescence ozone analyzer type 3-02P1 next 21 minutes, when the mixing layer height reaches (Russia) was used. The detection method is based on its minimum of 1200 m. The subsequent 33 minutes are measuring the chemiluminescence arising due to the a period of rising up to 1450 m. ozone reacting with a sensitive luminophor adsorbed on a solid substrate. The detector’s main parameters are: 3 The above results can be summarized as follows: sensitivity 2 µkg/m , response time <1 sec, relative error starting from the moment of solar eclipse beginning at 7 %. It has been shown that the chemiluminescence 12:36 till it reached the maximum at 14:03, the mixing technique of O3 concentration measurement is layer height increases from 1600 m to 1900 m; from substantially free from the interference of other 14:03 to 15:27, when the eclipse ends, the mixing layer atmospheric components, which largely determines its height decreases down to 1200 m, as a result of the advantage with respect to the chemical techniques and diminished irradiation of the earth surface. This is UV optical absorption technique [5, 6, 7]. followed by another increase of the mixing layer height.

2. RESULTS AND DISCUSSION As the lidar data shows, the effect of the eclipse on the mixing layer height (reduced heating of the earth 2.1. Lidar data surface) starts with a delay of about 87 minutes after the first contact, the maximal impact of the phenomenon on The lidar data were taken within the time interval the development of the PBL, i.e.; the minimal mixing 11:57–15:57 hours, bearing in mind the solar eclipse layer height occurs 84 minutes after the solar eclipse duration (the total solar eclipse in Bulgaria was maximum. observed in the region of Shabla from 14:11 to 14:13 hours). The measurements were performed in Sofia, 2.2. Variation of the ground ozone concentration where one could observed a partial solar eclipse, with 94 % of the Sun screened by the Moon; nevertheless, Fig. 4 presents the ozone concentration behavior near the atmospheric processes taking place during the the earth ground during the solar eclipse. The change eclipse exhibited interesting behavior. (decrease) of the O3 concentration starts about 40 minutes following the eclipse first contact. The maximal again up to T = 30 0C (15:20). The relative humidity change in the ozone condition near the ground (minimal near the ground at the eclipse beginning is 67 %. It values of the O3 concentration) is registered about 10 starts growing with the increase of the screening of the minutes after the eclipse maximal phase. The O3 Sun, up to a maximal value of 87 % (14:12), after the variation detected is a manifestation of the fast moment of maximal eclipse. The relative humidity falls photochemical processes taking place in the ground as the screening of the Sun is reduced, down to 69 % at layer under the influence of the changing solar 15:20. The wind speed and direction also change: the irradiation and reveals the efficiency and high rate of speed declines from 3 – 4 m/s to 1 – 2 m/s, while the the anthropogenic ozone photochemical formation in the direction changes from south-east to south to south-west real atmosphere. during the eclipse maximum.

Based on the data in the present section, one can conclude that the solar eclipse affects the meteorological parameters of the atmosphere near the ground, the ground ozone concentration and the height of the atmospheric mixing layer with a certain delay, different for the three types of measurements. The phenomenon observed influences not only the atmospheric layer near the ground, but also the entire planetary boundary layer.

3. CONCLUSIONS

Fig. 4. Dynamics of the ground ozone concentration in the city During the solar eclipse, we performed a complex study of Sofia during the solar eclipse of August 11, 1999. within the planetary boundary layer of the atmosphere by using a lidar, an ozone concentration analyzer, and a 2.3. Meteorological data ground meteorological station. The experimental data of the three sets of measurements demonstrate that the The meteorological situation over the Sofia city during influence of the phenomenon is manifested with a the experiment on August 11, 1999, can be described as certain delay, as follows: for the meteorological undisturbed; the corresponding data are summarized in parameters this delay is about 20 minutes; for the Table 1. ground ozone concentration the delay is about 40 minutes; and for the mixing layer height it is 87 Table 1. Meteorological data for August 11, 1999, ground minutes. The maximal impact of the eclipse on the three weather station, H = 2 m. types of parameters is revealed to be, 7, 10, and 84 Time, Temperature, Relative humidity, minutes after the maximal eclipse, respectively. After hours 0C % the eclipse end, all three sets of parameters show a 12:10 30.4 67 tendency toward restoring their normal values for the 12:30 30.1 69 respective time of the day during the season. 12:40 30.1 70 13:00 30.1 71 The results obtained prove once more the lidar 13:10 29.4 74 equipment potential in the study of various natural 13:20 28.3 75 phenomena and the possibility for using in combination 13:30 27.5 77 with in situ measurements. 14:00 26.5 85 14:10 26.2 86 REFERENCES 14:30 26.5 82 14:40 27.5 78 1. Kolev I., et. al.,. Lidar observation of the nocturnal 15:09 28.8 72 boundary layer formation over Sofia, Bulgaria, 15:10 28.9 69 15:20 29 67 Atmospheric Environment, Vol. 34, 3223–3235, 2000. 15:30 29 67 16:30 29.3 69 2.Mishev D., et al., Total solar ECLIPSE of 11 August 1999. Scientific research programme, Astronomical The near ground temperature at the eclipse beginning is Calendar of 1999, 88–93. T = 30 0C (12:30). As the eclipse progresses, it gradually drops and reaches T = 26 0C (14:12). Past the 3. Menut L., et al., Urban boundary layer height solar eclipse maximum, the temperature starts growing determination from lidar measurements over the Paris area, Appl. Optics, Vol. 36, No. 6, 357–375, 1999.

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