STUDY OF THE AEROZOL OPTICAL CHARACTERISTICS AND OZONE CONCENTRATIONS USING LIDAR, RADIOMETER AND OZONOMETER (ECOLOGICAL ASPECTS)

I. Iliev (1) , N. Kolev (2) , I. Kolev (2) , V. Grigorieva (2) , Ts. Evgenieva (2) , B. Kaprielov (2)

(1) Solar–Terrestrial Influences Central Laboratory , Bulgarian Academy of Sciences, Acad.G.Bonchev Str., bl.3, Sofia 1359, Bulgaria

e-mail: ilko@stil.acad.bg (2) Institute of Electronics, Bulgarian Academy of Sciences72 Tsarigradsko shosse Blvd., Sofia 1784, Bulgaria; e-mail:

blteam@ie.bas.bg

Introduction A three-month (from April 1 till June 30, 2004) obser-vation of the atmosphere over the urban area of the Sofia city (located in a mountain valley) was carried out. An EARLINET scanning aerosol lidar, an ozone analy-ser, a spectrophotometer, and a ground meteorological sta-tion were used during the observation. The location of the devices is shown on the Fig.1. Multiple aerosol layers of variable thickness (200-600)m were systematically observed in the planetary boundary la-yer. The aerosol extinction coefficient, the aerosol optical depth and the ozone concentration are determined and their variations are followed during the convective boundary la-yer formation.

This study aimed at determining of:

• the ABL height over an urban area using the aerosol asa tracer;

• the ground level ozone concentration and estimating itsrelation with the ABL development in different meteorolo-gical situations;

• the variation of the aerosol optical characteristics over the area during the ABL formation.

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Fig.1. Map of the observational site

Methods and apparatus Three devices were used to carry out the current study, namely, an aerosol lidar, a multiwavelength optical radio-meter and an ozonemeter. Specifications of the lidar (developed in the Institute ofElectronics): transmitter a standard Nd-YAG laser (ope-rational wavelength 532 nm, pulse duration and energy 1520 ns and 10–15 mJ, repetition rate 12.5 Hz; receivingantenna a Cassegrainian telescope (main mirror diameter 150 mm, equivalent focal length 2250 mm); photodetectora PMT with an interference filter (1nm FWHM); data ac-quisition and processing set 10 bit ; 20 MHz ADC and a PC [2]. Specifications of the radiometer (developed in the Cen-tral Laboratory of SolarTerrestrial Influences): spectral range 6001100 nm; spectral resolution 0.245.4 nm; photographic objective; prism monochromator; photodio-de array detector (512 pixels); 10 bit ADC, 8 bit speciali-sed computer (44 KB RAM); data accumulation time 50 ms10 s [6]. Specifications of the ozonemeter: Surface ozone concen-tration was measured with an ozone analyser model 3-02P(Russia), employing the chemi-luminescent method of O3

detection, sensitivity 2 g/m3, response time < 1 sec [7].

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Fig.2. Height-range images obtained on 11.06.2004(a clear sunny day with certain air pollution existed over the

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Fig.3. Heighttime images obtained on 25.06.2004 (a clear sunny day without aerosol pollution within the nocturnal stable boundary layer SBL over the area)

Fig.4. Sun spectra obtained at the earth surface on 11.06.2004.

Fig.5. Variation in the aerosol optical depth obtained on 11.06.2004.

Fig.6. Variation of the height of the residual layer and mixing layer and in the aerosol optical depth obtained

by lidar on 11.06.2004.

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Fig.7. Variation in the ground level ozone concentration on 17.06.2004.

Fig.8. Variation in the ground level ozone concentration on 25.06.2004.

Fig.9. Variation of the ML and RL heights and of the ozone concentration on 23.06.2004.

Conclusions Lidarradiometer observations and ozone concentration measurements in the atmosphere over an urban area are carried out. The planetary boundary layer dynamics is followed using an aerosol lidar from a stable nocturnal stratification to a well developed mixing layer during two days under different meteorological conditions. The AOD is obtained from lidar and radiometric data. The comparison between the data from the two instruments shows that the AOD obtained by radio-meter is higher than that obtained by lidar because of the different range of work. The vertical mixing, representing the matter of the CBL development, affects the ground level ozone concentration; the latter increases during/after the RL (where the ozone content is higher) destruction. That increase is slight in the case of an advection process and is most powerful in the case of cloudiness. Juxtaposing and analysing the obtained results the possibility of quantitatively estimating the air quality using the combined lidarradiometric apparatus is determined.

References[1] Appl.Optics, vol. 33, 1994, 7132-7140; [2] Atmosph.Environment, Vol. 34, 2000, 3223–3235; [3] Atmosph.Environment, 21/12, 1987, 2681-2693; [4] Boundary-Layer Meteorology, 1996, 81, 1-9; [5] J. aerosol Sci. vol. 27, No 4, 1996, 621-632; [6] Compt. Rend. Acad. Bul. Sci, T 45, No 12, 1992, 41-44; [7] International Journal of remote Sensing, vol. 26, N 16, 20 Aug. 2005, 3567-3584; [8] Appl.Optics. 36, 1999, 357-375; [9] Phys. of the atmoshere and ocean, 3, No 2, 1987, 121-129; [10] Proc. of SPIE, vol. 4397, 2000, 471-475; [11] Appl. Optics, 38/6, 1999, 945-949.

Experimental results and data processing

The conception of the experiment is the air quality over the urban area to be estimated on the basis of a combined analysis of the lidar, ozonemeter, radiometric and meteorological data. To determine the mixing layer (ML) height the lidar data are processed following the standard procedures. The aerosol ex-tinction coefficient and its variation during the convective boundary layer (CBL) formation are also determined. The ae-rosol optical depth obtained from different solar zenith angles during the different stages of the CBL formation on the aerosol and humidity is estimate on the basis of the radiometric data.

Lidar Data 6000 profiles were recorded at a period of about 9-10 min. Each 150 profiles were averaged. The 40 profiles thus obtai-ned were transformed into S-functions, for which the standard deviation and the first and second derivatives were calculated and were used to determine the mixing layer height. Several aerosol layers could be distinguished, namely at heights of about H1 = 100200 m, H2 = 300400 m and H3 = 600800 m at 07:00 LST (Fig. 2a) and 07:30 LST (Fig. 2b) as well as the beginning of their destruction at 08:00 LST (Fig.2c) (about two hours after sunrise). In Fig.3, similarly to the situation in Fig.2 a SBL is observed up to a height of about H=300 m followed by a residual layer (RL) up to about H=800900 m. The ML development starts around 09:30 within the layer up to height of about H=300 m; at 13:10 it raised up to 14001500 m. The process of the RL destruction finishes at about 11:00 when ML reaches its top boundary.

Ozone measurement data Figs. 7 and 8 present data of ground level ozone concentrati-on measurements obtained in different meteorological situati-ons, characteristic of the summer period in the region, at a height of about 12 m on 17.06.2004 and 25.06.2004 from 08:00 LST till 13:00 LST. The data could be divided into two cases (groups). When an advection presents on 17.06.2004, the ozone concentration was C=38 g/m3 at 08:00 LST reaching values of C=60 g/m3 at 13:00; in clear sunny days, at 08:00 LST, the ozone concen-tration was C=23 g/m3 gradually reaching value of C=90 g/m3 at 14:00 LST. It should be noted here that in both cases a peak (of various intensities) is being observed in the ground level ozone concentration at 10:3011:30 LST. In Fig. 9 the variation in the heights of the ML and RL on 23.06.2004 determined from the lidar data employing the gra-dient method [8] are shown. Such type of graphs gives possibi-lity the time of the RL destruction and the height reached by the ML to be determined. During the summer period of the campaign the RL was being destroyed between 10:30 and 11:30 LST. During the same time, the ML height was within the 500m and 800m interval.

Radiometer Data The spectroradiometer, located at about 4 km apart from the lidar was directly pointed to the Sun. The main goal of the spectroradiometer measurements was the total AOD to be determined and compared with the lidar data and the AOD in the ABL. For the purpose the Langley method [11] was used. The sun spectra, obtai-ned on 11.06.2004 is shown on Fig.4. The behaviour of the AOD at λ=550 nm, during the day of 11.06.2004 is shown on Fig.5. The high values of the AOD could be due to higher humidity and polluti-on in summer period. The increase in the AOD from τar=0.48 at 12:03 LST to τar= 0.66 at 12:58 LST could be due to advection because of the mountain valley circulation. Fig. 6 shows the time variations in the RL, the development of the new ML after 09:00 and the AOD obtained by lidar on 11.06.2004. It could well be seen from the vertical scanning data that in the case of stable nocturnal stratifi-cation of the atmosphere over the region the RL reach heights of about H=1000 m gradually being destroyed from above after 08:00. After 09:00 LST the develop-ment/formation of the new mixing layer, starts due to the solar radiation, slowly at the beginning and rapid after 11:00 LST. The AOD varies from τal=0.15 at 7:00 LST to τal=0.18 at 12:00 LST.