aerosol optical depth, water vapour content and total ...

Доклади на Българската академия на науките Comptes rendus de l’Académie bulgare des Sciences Tome 66, No 11, 2013 GEOPHYSIQUE Physique de l’atmosphère

AEROSOL OPTICAL DEPTH, WATER VAPOUR CONTENT AND TOTAL OZONE MEASUREMENTS OVER SOFIA (BULGARIA) FROM THREE CAMPAIGNS 2010–2012 Nikolay Kolev, Tsvetina Evgenieva, Nikolay Miloshev∗ , Plamen Muhtarov∗ , Doino Petkov∗∗ , Evgeni Donev∗∗∗ , Danko Ivanov∗∗∗ , Ventsislav Danchovski∗∗∗ , Ivan Kolev (Submitted by Corresponding Member D. Yordanov on July 5, 2013)

Abstract This contribution presents the results from three experimental campaigns carried out in June 2010, June 2011 and June 2012 at three sites in the city of Sofia (Institute of Electronics, Astronomical Observatory in the Borisova Gradina Park and National Institute of Geophysics, Geodesy and Geography (NIGGG)). A ceilometer CHM15k, two sun photometers Microtops II and an automatic meteorological station were used during the experiments. Ceilometer backscatter analyses reveal that the height of the mixing layer varied from 1500 m to 2500(3000) m above ground level. The height of the residual layer ranged from 800 m to 2000 m. The stable boundary layer extended to 200–400 m over the campaigns. The aerosol optical depth (AOD) at wavelength λ = 500 nm ranged from 0.38 to 0.66 in the first campaign and from 0.24 to 0.55 in the second one and from 0.11 to 0.23 in the third one. Corresponding ranges for the water vapour content (WVC) were from 1.26 cm to 2.6 cm. Different types of aerosol optical depth and water vapour content behaviour were observed. The total ozone content (TOC) varied from 240 DU to 370 DU during the campaigns. The groundbased observation from ozonemeter Microtops II with satellite observation of Ozone Monitor Instruments (OMI) over Sofia (Bulgaria) are compared. Our results have implications for further studies of regional climate variability. Key words: ceilometer, sun photometer, aerosol optical depth, water vapour content, total ozone content This work was carried out within the project “City versus Mountain Tropospheric Ozone: The Sofia–Plana region in an air quality and ecological sustainability perspective” (Contract DO 02-127/08).

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1. Introduction. Observations of the total atmospheric column aerosol optical depth (AOD) and precipitable water vapour content (WVC) are fundamental in determining aerosol optical properties and their influence on the global as well as regional-to-local radiation budgets and thus climate [1 ]. The planetary boundary layer (PBL) climate may be considered as a component of the local climate. The height, structure and processes taking place in the PBL depend directly on the solar radiation that reaches the Earth’s atmosphere and surface [2 ]. The radiation-scattering and absorption characteristics of the atmospheric aerosol, water vapour and the total ozone content act as principal agents on the solarenergy and heat to and from the ground surface [3 ]. Most of the aerosol and the water vapour normally reside within the first 3 to 5 km of the atmosphere over urban areas. In that context, this paper presents some results from studies of the PBL, AOD, WVC and TOC over the large city of Sofia. The results emanate from three campaigns, the first one between 10.06.2010 and 30.06.2010, the second one between 07.06.2011 and 18.06.2011 and the last between 31.05.2012 and 09.07.2012. 2. Instruments. Specifications of the ozonemeter Microtops II: optical channels: λ = 305.5 nm, λ = 312.5 nm, λ = 320 nm, λ = 936 nm and λ = 1020 nm, viewing angle – 2.5◦ , dynamic range > 3 × 105 , data storage – 800 records, power source – four AA alkaline batteries [4 ]. Specifications of the sun photometers Microtops II: optical channels: λ = 380 nm, λ = 500 nm, λ = 675 nm, λ = 936 nm and λ = 1020 nm, viewing angle – 2.5◦ , dynamic range > 3 × 105 , data storage – 800 records, power source – four AA alkaline batteries [5 ]. Specifications of the ceilometer-lidar: light source – a microchip Nd: YAG laser with a central wavelength of 1064 nm; measuring range 30–15 000 m; resolution 15 m; measuring time 60 s; pulse duration about 1 ns; pulse repetition rate 5–7 kHz; energy per pulse 8 µJ [7 ]. 3. Experimental results and discussion. Our results show that in clear sunny days the aerosol optical depth values differed at the two sites in the morning hours but came close when a mixing layer was developed. In the region of the Astronomical Observatory (AO) the aerosol optical depth values were approximately two times higher then these obtained in the region of the Institute of Electronics (IE). In case of haze there was not such a big difference in the morning hours. The AOD values were similar at the two sites after 12:00 LST. The differences observed in AOD behaviour at the two regions are related to the particular terrain and respective meteorological parameters and to the appearance of low haze or high clouds in the morning hours, and are also likely to reflect the sites’ differing distance to local pollution sources [5 ]. 3.1. Ceilometer data. The ceilometer is based at the Astronomical Observatory at the Borisova gradina Park, Sofia, Bulgaria. 1604

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The results obtained on 17.06.2010 are presented in Fig. 1A. The experiment started at 23:58 local standard time (LST) on 16.06.2010 and ended at 23:58 LST on 17.06.2010. The ceilometer data obtained show typical development of a stable boundary layer (SBL) with height 400 m, elevated inversion height 1400 m and a residual layer (RL) with height about 2600 m. The mixing layer (ML) formation began after 08:00 LST and its height reached 1200 m at 12:00 LST. The maximum of ML height 2800 m is reached at 16:30 LST. Figure 1B shows the ceilometer-lidar data obtained on 15.06.2011. The experiment started at 23:59 LST (14.06.2011) and finished at 23:59 LST next day. The elevated inversion height is about 750 m. The height of RL is 1750 m. The ML formation began after 08:00 LST and reached 1300 m at 12:00 LST and 1500 m at 13:30 LST. Afterward formation of clouds over the planetary boundary layer began and clouds at different heights and precipitations were observed about 15:00 LST. In Figure 1C the ceilometer-lidar data from 15.06.2012 are shown. The experiment started at 00:00 LST (15.06.2012) and ended at 23:58 LST. The data obtained show a SBL with height 500 m. The RL height is 2250 m. The ML formation began after 08:30 LST and reached 1000 m at 12:00 LST and 1950 m at 15:00 LST. The maximum of residual layer height 2100 m is reached at 18:00 LST. 3.2. Sun photometer data (AOD and WVC) and ozonometer data (TOC). Sun photometer allows to monitor aerosol optical depth as well as atmospheric precipitable water on real time scale [3 ]. Daily variations in aerosol optical depth, water vapour content and total ozone content obtained during the first campaign (June 2010) are shown from 15.06.2010 (Fig. 2A) and 17.06 2010 (Fig. 2B) in the region of AO. On June 15, measurements commenced at 09:45 LST and ended at 16:00 LST. The AOD at wavelength λ = 500 nm changed from 0.47 to 0.64 over the clear-sky part of the day, and reached 0.75 in cloudy conditions. The water vapour content varied between 2.0 cm and 2.6 cm. Measurements on June 17 were performed between 09:45 LST and 14:45 LST; later cloudy weather thwarted further work. During the measurement period, AOD values at λ = 500 nm declined from around 0.6 to around 0.4, and the WVC values were around 1.8 cm, with an increasing variability beginning around noon. In the next two figures daily variation in aerosol optical depth, water vapour content and total ozone content obtained during the second campaign (June 2011) are presented. The sun photometer data from 14.06.2011 (Fig. 2C) and 15.06.2011 (Fig. 2D) in the region of IE are shown. On June 14, measurements started at 09:15 LST and ended at 14:15 LST. The aerosol optical depth at wavelength λ = 500 nm varied between 0.24 and 0.36. The water vapour content changed from 1.65 to 1.77 cm. The total ozone content values varied between 334 DU Compt. rend. Acad. bulg. Sci., 66, No 11, 2013

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Fig. 1. Height-time indicators constructed from the ceilometer CHM 15K data obtained at the Astronomical Observatory on: A) 17.06.2010, B) 15.06.2011, C) 15.06.2012

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Fig. 2. Daily variations in the aerosol optical depth values at λ = 500 nm, water vapour column values and total ozone obtained at Sofia sites on: A) 15.06.2010, B) 17.06.2010, C) 14.06.2011, D) 15.06.2011

in the beginning of the experiment to 316 DU at the end. A certain relation has been observed between the AOD behaviour obtained at λ = 500 nm and total WVC, especially in the morning hours. Such relation is not observed in the TOC behaviour. This could be explained by the fact that the TOC mainly depends on the ozone content in the atmospheric layers above the troposphere. The significant variations in the AOD and total WVC in the atmospheric column, in case of undisturbed atmosphere, are due to atmospheric processes taking place in the layers up to about 3–4 km. The experiment on June 15 commenced at 09:15 LST and ended at 12:45 LST, cloudy weather then thwarted further work. The aerosol optical depth at wavelength λ = 500 nm changed from 0.35 at 09:15 LST to 0.54 at 12:12 LST. The water vapour content values varied from 1.92 cm in the beginning of the experiment to 1.76 cm at the end. Again, correlation is seen between AOD at λ = 500 nm and WVC, especially after 11:00 LST. Such strong correlation is not observed between these atmospheric characteristics and TOC due to their dependence on the processes taking place at significantly different heights in the atmosphere [9, 11 ]. Compt. rend. Acad. bulg. Sci., 66, No 11, 2013

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As in previous campaigns, precipitable WVC obtained during daytime (from 09:15 LST to 16:48 LST) showed consistent and systematic diurnal variation on the majority of days. The behaviour of the water content was qualitatively similar to the behaviour of the aerosol optical depth and connected with the formation of a PBL. The data for the daily variations in aerosol optical depth, water vapour content and total ozone content obtained during the third campaign (June 2012) presented in the following section were taken in the region of the National Institute of Geology and Geophysics, Bulgaria. Figure 3A shows the data obtained on 12.06.2012. The experiment started at 09:36 LST and finished at 17:00 LST. AOD values at λ = 500 nm decreased from 0.26 in the beginning of the experiment to 0.12 at the end. WVC changes between 2.1 cm and 1.24 cm in the same period. TOC values were 310 DU in the beginning and 220 DU at the end of the measurements. The similarity in the behaviour of the three atmospheric characteristics is well seen in the figure.

Fig. 3. Daily variations in the aerosol optical depth values at λ = 500 nm, water vapour column values and total ozone obtained on: A) 12.06.2012 and B) 15.06.2012 and daily variations of the ˚ Angstr¨ om coefficients α and β calculated from the aerosol optical depth data obtained on: C) 12.06.2012 and D) 15.06.2012

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The results obtained by the two Microtops II photometers on 15.06.2012 are shown in Fig. 3B. The measured values of the atmospheric characteristics are the following: AOD at λ = 500 nm changes between 0.22 in the beginning of the experiment and 0.10 at the end. WVC varied between 1.3 cm and 0.75 cm, respectively. TOC decreased from 370 DU in the morning hours to 240 DU in the afternoon. Again, the similar behaviour of the three atmospheric characteristics is well seen. While the correlation between AOD and WVC is more easily to be explained along the whole length of the sounding path as well as in the planetary boundary layer, the TOC depends mainly on the upper atmospheric layers and requires additional information and research to be explained. The behaviour of the water vapour content was qualitatively similar to the behaviour of the aerosol optical depth at the three different sites (AO, IE and NIGGG) and is connected with the formation of a PBL. The WVC values varied from 0.73 cm to 2.6 cm. About 2–3 h after sunrise one can visually see the spreading (horizontally) and lifting of surface haze layers as the mixing layer begins to develop. The nocturnal temperature inversions lift upward and a convective boundary layer starts to form [3 ]. These could be some plausible reasons for the observation of higher values of WVC in the morning and the subsequent decrease in the WVC as the haze starts dissipating (especially in the region of AO). The values of ˚ Angstr¨ om coefficients α and β calculated on the basis of AOD data are also reported [10, 11 ]. In addition, the ˚ Angström coefficients α and β ˚ have also been calculated. The Angström wavelength exponent α is a commonly used parameter to illustrate the wavelength dependence of AOD and to obtain basic information on the aerosol size distribution. It is also an indicator of the average aerosol particle size in the atmosphere. Thus, α ≤ 1 indicates aerosol size distribution mainly dominated by coarse mode aerosols of effective radius usually greater than 0.5 µm. These aerosols mainly originate from dust outflows or seaspray. On the other hand, α > 1 usually indicates a size distribution dominated by fine-mode aerosols of effective radius smaller than 0.5 µm, usually associated with urban pollution and biomass burning. The ˚ Angström turbidity coefficient β is related to the aerosol loading. A value of β > 0.2 usually indicates heavy pollution while β > 0.4 indicates extremely heavy pollution [6, 7 ]. Figures 3C and 3D show the daily variations of the ˚ Angström coefficients α and β obtained on 12.06.2012 and on 15.06.2012, respectively. The ˚ Angstr¨ om exponent α decreased from 1.41 to 1.09 during the measurements on 12.06.1012 (Fig. 3C) and from 1.63 to 1.21 on 15.06.2012 (Fig. 3D). Higher values of ˚ Angstr¨ om exponent α are observed in the hours before noon having maximum 1.52 and 1.80 on 12.06.2012 and 15.06.2012, respectively. The ˚ Angstr¨ om exponent α values show that mostly fine-mode aerosols are presented in the atmosphere over Sofia. The ˚ Angström coefficient β varied between 0.09 in Compt. rend. Acad. bulg. Sci., 66, No 11, 2013

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the beginning and 0.05 at the end of the experiment on 12.06.2012 and between 0.07 and 0.04 on 15.06.2012. These values show low aerosol loading over Sofia. Comparison between Microtops II ozonemeter’s data and NASA’s Earth Observing System’s (EOS) Aura satellite’s (http://ozoneaq.gsfc.nasa.gov/) data has been done. Figure 4 shows the monthly averages of TOC for Bulgaria (42–43◦ N, 23– ◦ 28 E) for the period 2008–2012 according to the OMI instrument. OMI is a nadirviewing near-UV/Visible CCD spectrometer aboard NASA’s Earth Observing System’s (EOS) Aura satellite (http://ozoneaq.gsfc.nasa.gov/). Monthly averages of the total ozone during the campaigns are marked with great circle. Values are: 329 DU in June 2010, 309 DU in June 2011 and 308 DU in June 2012. In 2011 and 2012 there is a decrease in TOC values compared to 2009 and 2010. A good agreement is seen between Microtops II ozonemeter and satellite data [8, 9 ]. The monthly averages of TOC values for Sofia constructed from the ozonometer Microtops II varied as follows: 295 DU in June 2012, 317 DU in June 2011 and 327 DU in 2010. As regards the summer measurements (the June 2010 campaign), the mean AOD values were generally higher than those obtained in the summer of 2011 and 2012 and were in the range from 0.38 to 0.66. WVC values ranged from 1.65 to 2.6 cm. The height of the mixed layer reached 2800 m.

Fig. 4. The monthly averages of the total ozone content for Bulgaria (42–43◦ N, 23–28◦ E) for the period 2008–2012 according to the OMI instrument (NASA’s Earth Observing System’s (EOS) Aura satellite)

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During the second experiment (June 2011), the mean AOD values at wavelength λ = 500 nm varied from 0.24 to 0.55, WVC values changed between 1.65 cm and 1.93 cm and TOC varied from 316 DU to 334 DU. The height of the mixed layer reached 2500 m. During the third campaign (June 2012) the mean AOD values at wavelength 500 nm varied from 0.11 to 0.23, WVC values changed between 0.73 cm to 2.05 cm and TOC values varied from 240 DU to 370 DU. The height of the mixed layer reached 2000 m. 4. Conclusion. The results of previous campaigns show that in clear sunny days the aerosol optical depth (AOD) values (in this study pertaining to the wavelength 500 nm) differed at the two sites in the morning hours and became close when a mixing layer developed. In the region of AO, in a park area in the city centre, the AOD values were approximately two times higher than those obtained in the region of IE. Under hazy conditions, differences during the morning hours were smaller [5 ]. Among plausible explanations of the differences observed in AOD behaviour in the two regions special characteristics of the respective sites, in regard to topography, terrain, local meteorology, haze formation, and local sources of pollution are observed [7, 8 ]. The results of June 2010, June 2011 and June 2012 campaigns show that the height of the mixing layer varied from 1500 to 2500 m (occasionally 3000 m) during the measurements. The height of the residual layer was in the range of 800 to 2000 m. The stable boundary layer did not exceed 200 to 400 m. During the experiments, the mean AOD values varied from 0.38 to 0.66. The water vapour content (WVC) values ranged between 1.26 cm and 2.6 cm, in terms of precipitable water. Qualitatively, the temporal courses of WVC and AOD at the two sites were similar and relatable to the formation of a planetary boundary layer. The ˚ Angstr¨ om exponent α values show the presence of fine-mode aerosol in the atmosphere. Corresponding values of turbidity coefficient β show low aerosol loading over Sofia. In conclusion, our campaigns show the usability of an instrument set-up that combines several types of measurements. In particular, we note that aerosol optical depth, precipitable water content and total ozone content are important parameters in the model of solar-radiation transmission through the atmosphere [12, 13 ]. Regular experimental campaigns investigations of the optical characteristics of atmospheric aerosol in Sofia can be used by, on the one hand, to assess the purity of the air over the area and, on the other hand, to study the impact of climate variability on the aerosol structure of the atmosphere in the valley of Sofia. Compt. rend. Acad. bulg. Sci., 66, No 11, 2013

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Institute of Electronics Bulgarian Academy of Sciences 72, Tsarigradsko shaussée Blvd 1784 Sofia, Bulgaria e-mail: nic [email protected] ∗∗

Space Research and Technology Institute Bulgarian Academy of Sciences Acad. G. Bonchev Str., Bl. 1 1113 Sofia, Bulgaria e-mail: [email protected]

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National Institute of Geophysics, Geodesy and Geography Bulgarian Academy of Sciences Acad. G. Bonchev Str., Bl. 3 1113 Sofia, Bulgaria e-mail: [email protected] ∗∗∗

Faculty of Physics Sofia University “St. Kliment Ohridski” 5, J. Bourchier Blvd 1164 Sofia, Bulgaria e-mail: [email protected] N. Kolev, T. Evgenieva, N. Miloshev et al.