ÓPTICA PURA Y APLICADA Vol. 39, núm. 006 3rd-Workshop LIDAR Measurements in Latin América Monitoring of air pollution and atmospheric parameters using a mobile backscatter lidar system G. Georgoussis (), G. Chourdakis (), E. Landulfo (), K. Hondidiadis (), A. Ikonomou (). Raymetrics S.A., Lidar Systems, info@raymetrics.gr, Kanari 5, 53 54 Glyka Nera, Athens, Greece.. Instituto de Pesquisas Energéticas e Nucleares, Centro de Lasers e Aplicações, Avenida Lineu Prestes, 4, Cidade Universitária, CEP 05508-900, São Paulo, SP, Brazil. ABSTRACT: In this paper data obtained using a Raymetrics backscatter lidar were used in order to retrieve information related to the structure of the lower atmosphere and the evolution of air pollution in the area of Athens, Greece. This system is the result of an effort to develop a portable lidar instrument that is affordable, compact, accurate and naturally easy to use. A typical value of the measurement range is 5 km with a spatial and temporal resolution of 3.75 m and 30 sec, respectively. This work demonstrates that a Raymetrics LB-Series lidar system is a powerful tool for long-term monitoring of the air pollution and the structure of the troposphere, which can be used to extend the capabilities of many monitoring sites worldwide. Key words: lidar, laser, remote sensing, aerosols, air pollution, meteorology, backscatter, Raman. REFERENCES AND LINKS. [] D. Aglio, M. Kholodnykh, A. Lassandro, R. D. Pascale, Development of a Ti:Sapphire DIAL system for pollutant monitoring and meteorological applications, Optics and Lasers in Engineering 37, 33 44 (00). [] D. Whiteman, Examination of the traditional Raman lidar technique I. Evaluating the temperature-dependent lidar equations, Applied Optics 4, 57-59 (003). [3] G. Vaughan, Observations for chemistry (remote sensing): lidar, Encyclopaedia of Atmospheric Sciences, Academic press, 509-56 (00). [4] D. Althausen, D. Müller, A. Ansmann, U. Wandinger, H. Hube, E. Clauder and S. Zörner, Scanning 6-Wavelength -Channel Aerosol Lidar, Journal of Atmospheric Oceanic Technology, 7, 469-48 (000). - 35 - Recibido: 6 october 005
[5] G. Chourdakis, A. Papayannis, J. Porteneuve, Analysis of the receiver response for a non-coaxial lidar system with fiber-optic output, Applied Optics, 4. 75-73 (00). [6] M. Vrekoussis, N. Kalivitis, N. Mihalopoulos, M. Kanakidou, T. Kluepfel, J. Lelieveld and G. Chourdakis, Factors controlling the diurnal variation of ozone in the marine boundary layer of the Eastern Mediterranean during summertime, Proceedings Quadrennial Ozone Symposium, 53-55 (004). [7] J.D. Klett, Lidar inversion with variable backscatter/extinction ratios, Applied Optics 4, 638 643 (985). [8] J. H. Seinfeld and S. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, J. Wiley and Sons Inc., NewYork, USA (998). [9] Y. J. Kaufman, D. Tanre, O. Dubovik, A. Karnieli, L.A. Remer, Absorption of sunlight by dust as inferred from satellite and ground-based remote sensing, Geophysical Research Letters 8, 479 48 (00). [0] J. Hansen, M. Sato, A. Lacis, R. Ruedy, The missing climate forcing, Philosophical Transactions of the Royal Society of London, Series B, 3 40 (997). [] A.L Quijano, I.N Sokolik, O.B Toon, Radiative heating rates and direct radiative forcing by mineral dust in cloudy atmospheric conditions, Journal of Geophysical Research 05, 07 9 (000). [] G.P Gobbi,, Polarization lidar returns from aerosols and thin clouds: a framework for the analysis, Applied Optics 37, 5505 5508 (998). [3] G.P Gobbi, F. Barnaba, R. Giorgi, A. Santacasa, Altitude-resolved properties of a Saharan dust event over the Mediterranean, Atmospheric Environment 34, 59 57 (000). [4] S. Rodriquez, X. Querol, A. Alastey, A. Kallos, O. Kakaliagou, Saharan dust contributions to PM 0 and TSP levels in southeastern and eastern Spain, Atmospheric Environment, 35, 433-447 (00). [5] L. Menut, Urban boundary-layer height determination from lidar measurements over the Paris area, Applied Optics 38, 945-953 (999).. INTRODUCTION Laser remote-sensing techniques (lidar systems), have gained high acceptance as long-range non-invasive probes of the chemical composition and physical properties of the atmosphere. The detection and analysis of the received lidar signals permits the retrieval of the relative concentration of the suspended aerosol particles, and of the absolute concentration of several air pollutants (i.e. O 3, NOx, SO etc.), with high temporal and spatial resolution ( min and 3,75 m respectively) along the propagating laser beam []. For the measurements of gases and suspended aerosols, laser light extinction, as well as elastic and inelastic (Raman) scattering are used []. Therefore, the lidar technique has become a powerful tool to visualize, in real time: a) the atmospheric dynamical transport processes, using the aerosol particles as tracers, b) the vertical distribution of aerosol particles, either produced locally over the measuring site (car traffic, domestic heating, industrial activities etc.), or transported by the - 36 -
atmospheric circulation (land-sea breeze circulation, trans-boundary air pollution, desert dust events etc.) and c) the structure of the lower atmosphere (structure of the PBL, identification of several aerosol dust layers), and its correlation with ground air pollution levels [3].. Methodology.a. - The Lidar Technique When a laser beam is sent into the atmosphere, it is widely scattered by the suspended aerosol particles, molecules and atoms present in the air. In a typical lidar arrangement, a telescope in a coaxial or biaxial configuration with respect to the laser emitter, collects the backscattered light. The signal is then focused onto a photodetector through a spectral filter, adapted to the laser wavelength. Since a pulsed laser is used, the intensity of the backscattered light can be recorded as a function of time, and thus provide the required spatial resolution of the measurement [4]. The basic lidar equation is given by: cτ Po z * P () z = β() z AtelΟ() z exp a z dz () z 0 * where, P(z) is the detected backscattered radiation from range z, P o is the laser output power, τ is the laser pulse duration, c is the speed of light, β(z) is the volume backscatter coefficient, a(z) is the total atmospheric extinction coefficient, A tel is the total telescope area and O(z) is the overlap function which takes into account geometrical and optical factors of the receiver arrangement [5]. The extinction term a(z) includes the contribution of the different absorbing atmospheric molecules (O 3, NO x, SO, etc.) and aerosol particles..b. - Lidar System Hardware. The Raymetrics LB lidar system is designed and manufactured by Raymetrics S.A. in order to perform continuous measurements of suspended aerosol particles in the Planetary Boundary Layer (PBL) and the lower free troposphere (Figure ). It is based on the second harmonic frequency of a compact pulsed Nd:YAG laser, which emits pulses of 65 mj output energy at 53 nm with a 0 Hz repetition rate [6]. The optical receiver is a Cassegrainian reflecting telescope with a primary mirror of 00 mm diameter, directly coupled to the lidar signal detection box. Inside the detection box a polarization cube splits the incoming radiation into its two orthogonal polarization components and two photomultipliers detect the incoming radiation. Analog detection of the photomultiplier current and single photon counting is combined in one acquisition system. The combination of a powerful A/D converter ( Bit at 40 MHz) with a 50 MHz fast photon counting system increases substantially the dynamic range of the acquired signal, compared to conventional systems and provides a spatial raw resolution of 3,75 m. The lidar is easily transportable and it is housed in an IP56 environmentally protected enclosure, which includes all optical and electronic components. The enclosure also houses a PC for data acquisition, analysis and onboard data storage, a climate control unit (heating, cooling) and a communication port for telemetry and remote access. The detection unit has an extra window for the optional addition of a Raman channel at 607 nm. Fig. a. - Typical configuration of a Raymetrics LB lidar system; also shown is the portable enclosure with IP56 environmental protection, which houses the complete lidar system. - 37 -
Fig. b.-inside view where the 00 mm telescope of the receiver is shown. Fig. c. - Integrated PC for data acquisition, analysis and on-board data storage..c. - Lidar System Analysis Software. The lidar is equipped with dedicated software for setting-up and operating the system as well as several software modules for automatic data acquisition, analysis and visualization. Raw data are stored in a database providing data mining capabilities and extensive compatibility with other commercial programs. The Klett inversion technique is used to retrieve the vertical profile of the aerosol backscatter coefficient at the respective wavelengths [7]. The final output data include: i) Aerosol optical parameters such as the backscatter coefficient, the extinction coefficient and the depolarization ratio of the suspended aerosols, ii) Meteorological parameters such as the optical depth, the cloud height and thickness, the height and the temporal evolution of the PBL and dust layers, iii) Lidar related parameters such as the lidar Range Corrected Signal (RCS), the Logarithm of the RCS (LRCS) and the first and second derivatives of the RCS and the LRCS. 3. Results and discussion 3.a. -Tropospheric Aerosols Monitoring Tropospheric aerosols arise from natural sources, such as wind-borne dust, sea spray and volcanoes, and also from anthropogenic sources, such as combustion of fossil fuels and biomass burning activities. With the increasing urbanization and industrialization the content of aerosols particularly in the lower troposphere increases continuously [8]. Atmospheric particles and particularly mineral dust particles, influence the earth s radiation balance and climate in two ways: (a) by reflecting and absorbing, both incoming and outgoing radiation, depending on their chemical composition, a phenomenon termed as direct aerosol effect, and (b) by acting as cloud condensation nuclei (CCN) and thereby determine the concentration of the initial droplets, albedo, precipitation formation and lifetime of clouds, a phenomenon termed as indirect aerosol effect. These two phenomena are very difficult to quantify and thus large uncertainty exists about the climatic role of aerosols on the Earth s radiative budget and consequently on its climate. These uncertainties are a consequence of our poor knowledge of dust optical properties (as scattering and absorption coefficients) and of its regional and altitude location [9]. In fact, mineral dust can lead to opposite radiative effects, i.e., a warming of the planet, with respect to sulphate aerosols [0]. The magnitude of such a heating has been shown to increase for increasing values of the: aerosol imaginary refractive index, the dust layer altitude, the albedo of the underlying surface and the cloud-free portion of the atmosphere []. A backscatter lidar system is able to provide accurate data on the spatial and temporal evolution of the suspended aerosols in the troposphere. In Figure a the vertical profile of the aerosols backscatter coefficient is presented for one case of high aerosol loading. Figure b presents the temporal evolution of aerosol loading in the troposphere. - 38 -
Altitude (km) ASL 0 9 8 7 6 5 4 3 0,0000 0,0005 0,000 0,005 0,000 0,005 0,0030 0,0035 0,0040 Backscatter coefficient (km*sr) - 53 nm Fig. a. Vertical distribution of the backscatter coefficient for a typical case of high aerosol loading over the area of Athens, Greece on the 0/08/004 (:54 UTC). nonspherical particles introduce some degree of depolarization in the light they backscatter, this technique allows for discrimination of spherical (liquid) vs. nonspherical (solid) particles by evaluating the linear depolarization ratio D = I p I s [3]. A typical result of the vertical profile of the depolarization ratio is presented in Figure 3, for a typical case of high aerosol loading over the area of Athens, Greece on the 0/04/005. 6 5 53 nm Altitude asl (km) 4 3 0 0,00 0,05 0,0 0,5 0,0 0,5 0,30 0,35 0,40 Depolarization ratio (I s /I p ) Fig. b. Temporal evolution of a dust layer at an altitude of.0-.5 km asl. over the area of Athens, Greece on the 8/09/004. The Planetary Boundary Layer (PBL) is also accurately and clearly monitored. 3.b. - Depolarization Polarization lidars represent a unique and affordable tool to retrieve the altituderesolved information on both aerosol nature and scattering properties, and on cloud extent and frequency needed to better evaluate the radiative effects of these particles []. In this paper observations presented are the result of -min (00 laser shots) time averages. At each receiver, both parallel I S and perpendicular I P polarization signals (with respect to the polarized laser emission) are recorded. Since spherical particles do not depolarize while Fig. 3. -Vertical distribution of the depolarization ratio at 53 nm, for a typical case of high aerosol loading over the area of Athens, Greece on the 0/04/005. 3.c. - Sahara Dust Every year large quantities of dust particles are emitted in the atmosphere in desert regions of high convective activity. Most of these particles are coarse (diameter µm) and are thus deposited close to their source, while a large fraction of the smaller particles can be transported over very large distances [4]. Raymetrics has in operation a backscatter lidar station, located at the company s premises in Athens, Greece, which has monitored several extreme aerosol events over the Eastern Mediterranean Sea. The vertical distribution of the backscatter coefficient of the suspended particles in a typical Sahara dust - 39 -
event is shown in Figure 4a. The temporal evolution of the event is shown in Figure 4b. Altitude (km) ASL 0 9 8 7 6 5 4 3 0,000 0,00 0,00 0,003 0,004 Backscatter coefficient (km*sr) 53 nm Sahara dust Fig. 4a.- Vertical distribution of the aerosol backscatter coefficient for a typical case of Sahara dust transportation over the area of Athens, Greece on the 8/08/003. ecosystem. The concentrations of air pollutants in the PBL are generally orders of magnitudes higher than in the free troposphere. Accurate determination of the PBL height is crucial for photochemical and dispersion models to accurately predict pollutant concentration. This layer is typically moister and has a greater aerosol content than the free troposphere, causing more scattering of laser light. Therefore, lidars can easily detect the boundary between these two layers with high spatial and temporal resolution. In this paper we determine the PBL height using the Inflection Point Method (IPM) proposed by Menut et al., [5]. This technique is applied on a series of lidar data taken over Athens on July, 004. (Figure 5a). The minimum value of the second derivative of the range-corrected signal (RCS) determines the PBL height. In this case the minimum value is at 083,6 m, which corresponds to the actual PBL height over Athens at 06:00 UTC. In Figure 5b the temporal evolution of the RCS is presented over Athens, Greece on the /07/004. The temporal evolution of the PBL height can be accurately determined applying the proposed IPM method on that time plot.,6,4, d (RCS) / dz d(rcs) / dz RCS Fig. 4b. - Temporal evolution of the Sahara dust transportation across the Mediterranean. The parameter shown is the lidar Range Corrected Signal (RCS). The x-axis is UT Time, the y-axis is altitude ASL (m) and the color scale shows the intensity of the lidar RCS. 3.d. -Monitoring of The Planetary Boundary Layer (PBL) Height The Planetary Boundary Layer is the lowest part of the troposphere that is directly influenced by the earth s surface, and responds to surface forcing with a time scale of an hour or less. This layer is of primary importance for our entire Altitude (km),0,8,6,4,,0 h 0,8-0 -8-6 -4-0 4 6 8 0 4 Lidar signal (a.u.) Fig. 5a. - The vertical profile of the lidar RCS (blue-line), the first-derivative (black-line) and the second-derivative (red-line) of the RCS are presented. According to the this calculation the PBL height on the /07/004 at Athens, Greece is 083,6 m. - 40 -
Fig. 5b. - Temporal evolution of the lidar Range Corrected Signal (RCS) over Athens, Greece on the /07/004. The x-axis is UTC time, the y-axis is altitude ASL (m) and the color scale shows the intensity of the RCS. The PBL height can be accurately determined applying the IPM method on these time plot. CONCLUSIONS This study demonstrates that a compact, small-size lidar system can be used to monitor the characteristics of the lower troposphere with great accuracy and reliability. Predicting pollution events in urban areas such as the Athens Area requires a good knowledge of the dynamic processes that take place in the lower troposphere. Laser remote sensing is a useful tool for probing the composition and physical state of the atmosphere and for performing measurements at locations that cannot be reached with conventional detection methods. Lidar offers the only realistic measurement technique that is able to retrieve the basic meteorological and atmospheric parameters, as well as the concentration of the suspended aerosol particles, in real time. In this paper we have demonstrated some of the prospects of the lidar technique. Lidar technology has matured over the last decade and the future of laser remote sensing appears to be very promising. This argument is supported by many factors including: (i) the development of compact, eye-safe lidar systems, (ii) further reduction of lidar system s complexity, cost and size and (iii) the use of lidar systems in air pollution measurements networks. ACKNOWLEDGEMENTS Part of this work was supported by the HIRON Research Project of the Greek General Secretariat for Research and Technology of the Ministry of Development, under contract HR-5. - 4 -