- Email: [email protected]
The laser as a tool in environmental problems
Optical Materials 13 (1999) 167±173
The laser as a tool in environmental problems F. Molero, F. Jaque
*
Departamento de Fõsica de Materiales, Universidad Aut onoma de Madrid. 28049 Madrid, Spain
Abstract This work reports on the use of laser as an ecient tool in environmental studies. Application of LIDAR (LIght Detection And Ranging) and LIDAR-DIAL (DIerential Absorption Lidar) technologies in the ultraviolet range related with the tropospheric ozone and sulphur dioxide pro®les in the surroundings of Madrid are given. Sulphur dioxide measurements performed in a coal-burning power plant are also presented. Finally, the LIDAR technique is applied to study the cloud's density and its temporal evolution. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 42.68 Rp; 42.68 Vs Keywords: Laser; LIDAR; Air pollution
1. Introduction Given in densely populated industrial areas air pollution has become a serious problem, the study of pollutants and their reactions with natural components in the atmosphere has become an urgent demand. Historically, air pollution monitoring has been performed using grab sampling techniques for either in-situ or laboratory analysis [1]. More recently, optical instruments, such as DOAS (Dierential Optical Absorption Spectroscopy) [2,3] and COSPEC (COrrelation SPECtrometer) [4], have been incorporated to the task, allowing path-integrated measurement. The grab sampling technique has the disadvantage that it is limited to single point measurements and the delivered data are fully dependent on the site, which must be carefully chosen to be representative. Optical methods used in remote sensing provide three-dimensional pollution measurements but the *
Corresponding author: E-mail: [email protected]
use of searchlights limits the con®guration to direct or retrore¯ected detection, reducing the scanning options to suitable sites. The most powerful of these optical techniques are those that incorporate laser sources. The development of pulsed lasers made possible the generation of very short single pulses of high intensity light, allowing the detection of the light backscattered by the atmospheric constituent and thereby range-resolved measurements. The operating principle is analogous to pulsed microwave RADAR, except that optical rather than microwave components are required to direct, intercept and detect the emitted radiation. The technique has received the acronym LIDAR, standing for LIght Detection And Ranging [5]. Due to the shorter wavelength range kvisible kmicrowave =5, LIDAR provides the opportunity of sensing atmospheric constituents and properties to which RADAR is insensitive. In particular, LIDAR and LIDAR-DIAL techniques oered the capability for sensing scattering by both air molecules and the minute bits of suspended particulate matter, aerosol particles, ever
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 2 6 - 9
168
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
present in the atmosphere. Therefore, these techniques provide a method of mapping the threedimensional distribution of tracer material released into the atmosphere with high sensibility (10 ppb) and high spatial resolution from remote distances [6]. This paper reports some applications of LIDAR and LIDAR-DIAL technologies. Examples of pollution monitoring of Ozone (O3 ) and Sulphur dioxide (SO2 ) in the ultraviolet range in urban as well as in a coal-burning power plant are presented. 2. LIDAR and LIDAR-DIAL theory Two basic con®gurations for laser remote sensors have been considered in LIDAR and LIDAR-DIAL systems. The double-ended (Bistatic) arrangement involves a considerable separation of the transmitter and receiver to achieve spatial resolution in optical probing studies. The de®nite absorption path length can also be performed with a retrore¯ector. This arrangement imitates the lamp-source-based equipment, but today is rarely used, as pulsed lasers are capable of providing enough power to allow single-ended (Monostatic) con®gurations, with the transmitter and receiver at the same location. This arrangement allows device mobility and simplify the scanning procedures [7]. A single-ended LIDAR can either be coaxial or biaxial. In a coaxial system the axis of the laser beam is coincident with the axis of the receiver optics, while in the biaxial arrangement, the laser beam only enters the ®eld of view of the telescope beyond some predetermined range. This avoids the problem of near-®eld backscattered radiation saturating the detector and presents some advantages in stratospheric measurements [8]. The near-®eld intense-signal problem is solved in the coaxial system by gating the detector. This con®guration is used in tropospheric measurement, specially in DIAL measurements. In the case of a pulsed, single-ended coaxial LIDAR, the instantaneous received power at time t 2R/c, assuming single-scattering events, is given by the expression [7]):
cs
b R; kn kf RAr Rÿ2 Z R exp ÿ 2 a r; kdr
P R; k P0
2
0
1
where P0 is the transmitted power at time t0 0, c the velocity of light, s the pulse duration and R ct/2 the range of measure; n(k) represents the receiver's spectral transmission factor, f(R) the geometrical factor taken as the probability that radiation scattered at range R reaches the detector, b(R, k) the volume backscattering coecient of the atmosphere and a(r, k) the volume extinction coecient of the atmosphere, that can be expressed as: a r; k rm knm r ra kna r
2
where rm and ra are the wavelength-dependent extinction cross section of molecules and aerosols, respectively, including contributions from both scattering and absorption, and nm and na the range-dependent concentration of molecules and aerosols, respectively. In Eq. (1), Ar /R2 is the acceptance solid angle of the receiver optics, with Ar the eective receiver's area. This term shows explicitly the inverse square range dependence of the return signal, that produces its wide dynamic range and eventually, the saturation of the detector at the near-®eld. The time between the transmission of the laser pulse and the arrival of the scattered return signal can be directly related, through the velocity of light, to the range at which the scattering occurs. The crucial point in Eq. (1) is that the return signal is proportional to the term: exp [ÿ2òrm (k) ´ nm (r) dr], which depends on the concentration of the molecule of interest in the atmosphere. In general, interpretation of the LIDAR signal presented by Eq. (1) is complicated because of the simultaneous dependence of a(r, k) and b(r, k). A more readily interpretation can be reached by the combination of dierential absorption and scattering (formally named DAS but the acronym DIAL has gained considerable popularity recently) [9]. In a dierential approach, a comparison is made between the atmospheric backscattered laser radiation monitored when the wavelength of the
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
laser is tuned to closely match that of an absorption line, denoted as kon , and when it is detuned to lie in the wing of the absorption line, denoted as koff . The comparison between these two wavelengths is necessary to separate the absorption by the molecules of interest from other causes of attenuation. Considering Eq. (1), the pollutant concentration averaged in DR-long cells N~ DR can be calculated from the ratio of the received power at these two wavelengths: 1 Pr kon ; R ln N DR 2DrDR Pr kon ; R DR Pr koff ; R BT 3 ÿ ln Pr koff ; R DR where P(ki , Rj ) is the received power at wavelength ki (index i can be either the maximum absorption wavelength (on) or the minimum (o)), DR the length of the atmospheric cell over which the concentration is integrated, and Dr the dierence in absorption cross-section between the on and o wavelengths, r(kon ) ± r(koff ). In Eq. (3), B and T are represented by the expressions: b kon ; R DR b koff ; R DR ÿ ln ; B ln b kon ; R b koff ; R T ÿ2aA kon ; R ÿ aA koff ; RDR: Since the Rayleigh and Mie scattering cross-sections do not dier appreciably in a frequency interval corresponding to the linewidth of a molecular absorption line, the terms B and T can normally be neglected. In conclusion, using two close wavelengths, the concentration of a kind of pollutant molecules could be calculated knowing the ratio of the backscattered received power at these two close wavelengths. 3. LIDAR-DIAL diagram Fig. 1 shows the LIDAR-DIAL system layout developed at Autonoma University of Madrid (UAM), that works in the ultraviolet and visible ranges. The light source, located at the right side of
169
Fig. 1. LIDAR-DIAL diagram developed at the Autonoma University of Madrid.
the ®gure, consists of a Nd:YAG laser that pumps, either with the second or third harmonic, a dye laser, the tunable element that permits the emission of radiation at the wavelength that is absorbed by the chosen pollutant. After it, a doubling crystals set extent the wavelength range further in the ultraviolet (UV). The beam is expanded ®ve times in order to decrease its divergence, providing longer useful measurement range. The light probe can be directed to any direction within the top half-sphere with 0.2 mrad resolution, by means of a motorised periscope. Usual energies vary between 10 and 40 mJ/pulse of tunable radiation for tropospheric DIAL measurements, and can be as high as 150 mJ for stratospheric applications. On the left hand side of Fig. 1, the detection line is shown. The light backscattered by the atmosphere is collected by a 1 m focal length Newtonian telescope and sent to a photomultiplier tube (PMT) through an iris. This iris reduces the background light by decreasing the telescope ®eld of view and also geometrically compresses the signal dynamic range [10]. Despite this eect, further compression is necessary in order to avoid the saturation of the PMT by the intense near-®eld signal, retaining enough resolution to accurately process the far-®eld signal. That was done by electronically gating the PMT dynode chain using two transistors in switching mode. It uses a standard 5 V pulse generator, triggered by the laser, to
170
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
control the gate. This system allows selection of the delay between the laser pulse and the start of the electronic detection window. A second high voltage supply is used to feed the switched dynodes. The signal is digitised with 10 ns resolution (equivalent to 1.5 m) and recorded in a computer for later processing. As an example, Fig. 2 shows the eect of the electronic gate commented before. The pulse generator, triggered by the Q-switch signal of the Nd:YAG laser, provides a 0±5 V square pulse to the switching electronics. This pulse switches a couple of transistors, allowing that the second high voltage supply feeds the blocked dynodes. This opens the circuit, starting the detection window. Therefore, the delay between the laser pulse and the start of the detection window, and also the duration of this, is controlled by a 5 V standard pulse generator. This delay protects the PMT from the intense signal produced by scattering on the driving mirrors and the near®eld signal, allowing a clear detection of the far®eld light. The 9 ls lag between the end of the square pulse and the detection window is due to the accumulation of spatial charge in the base of the transistors. The return signal of dierential absorption LIDAR systems is usually so weak that the concentration pro®le calculated from a single pulse pair is very noisy, and temporal averaging is necessary to improve the signal-to-noise ratio (SNR).
Fig. 2. Illustration of the electronic gating eect coupled to the photomultiplier.
4. LIDAR and LIDAR-DIAL examples In this section some experimental results obtained with the LIDAR-DIAL technique are brie¯y exposed. Fig. 3 shows the vertical pro®le of the ozone concentration measured in the Campus of the Autonoma University (20 Km from the city centre of Madrid).The inset in Fig. 3 shows the Hartley ozone absorption bands which are centred at ~250 nm. For tropospheric DIAL applications the wavelengths must be selected on the low energy side of the Hartley ozone bands, being kon 294 and koff 300 nm the chosen wavelengths on this work. Both wavelengths are still close enough to consider negligible the scattering terms and at the same time show a value of Dr 45.32 ´ 10ÿ20 cm2 [11], which is adequate in order to obtain an accurate value of the ozone concentration. As can be seen in Fig. 3, the pro®le of the ozone concentration is very constant with a value near to 50 ppb from 200 to 700 m of altitude, dropping then to a few ppb. The altitude value for observed ozone concentration in¯exion depends on the temperature inversion and therefore on the atmospheric conditions. The errors for this ozone measurements were around 10 ppb, that is sucient for most of urban applications.
Fig. 3. Vertical pro®le of the ozone concentration measured in the Campus of the Autonoma University (Madrid). The inset shows the Hartley ozone absorption band and the two selected wavelengths for the ozone measurements.
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
Fig. 4. Vertical pro®le of sulphur dioxide concentration measured in the Campus of the Autonoma University of Madrid. The inset shows the SO2 absorption spectrum. The two chosen wavelengths selected are denoted in the ®gure.
In the measurements of the atmospheric SO2 content, the couple of wavelengths chosen were kon 300.03 and koff 299.3 nm (Dr 101 ´ 10ÿ20 cm2 ) [12]. Inset in Fig. 4 shows the SO2 absorption spectrum in the range 296±305 nm, obtained using a optical cell with SO2 gas at 1% in synthetic air. The solid line corresponds to the spectrum recorded using a standard spectrophotometer, while the points were obtained by coupling the optical
171
cell to the DIAL set up. This procedure is necessary when two very close wavelengths are used, in order to obtain good laser wavelength calibration. In these conditions the error associated with the absorption cross-section is minimised. The SO2 measurements in the University area show similar pro®les to that presented in Fig. 3 for the variation of the ozone concentration. The SO2 content was found to be in the range of few ppb in agreement with values obtained at ground level by standard methods. The study of the SO2 content in the atmosphere and its spatial and temporal evolution appears very critical in areas close to intense sources of SO2 . As was commented in the introduction, the LIDAR-DIAL technique provides a very powerful tool to study the diusion of a speci®c pollutant over large areas. Sulphur dioxide measurements have been performed in the coal-burning power plant of Compostilla II, Ponferrada (Spain), where coke with a high content of sulphur is used. For this purpose the LIDAR-DIAL equipment was located at approximately 300 m from the chimneys of the power plant. Fig. 5 shows a transverse
Fig. 5. Sulphur Dioxide concentration map corresponding to the two chimneys of the coal-burning power plant of Compostilla II (Spain).
172
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
section of the two plumes corresponding to the two operatives chimneys of this power plant. The section was obtained by ®ring the laser in nine dierent direction, starting from the vertical and fanning in both direction in 5 degrees spacing. The concentrations obtained in the dierent directions were interpolated using the Kriging method. As can be observed, the maximum SO2 content found in the centre of the plume reached a value around 8 ppm. Both plumes split in different lobes due to atmospheric turbulence. The temporal evolution of the two plumes was also studied showing the high capacity of the DIAL technique in the study of environmental pollution.
The system was also utilised for LIDAR measurements. A fraction of the third harmonic radiation from the Nd:YAG laser (k 355 nm) was split before pumping the dye laser, expanded and sent to the atmosphere. The intensity of the return signal, after range-correction, was used to study the cloud density evolution. In Fig. 6, 45 vertical measurements, one taken each minute and averaged 256 laser pulses, were interpolate using the Kriging method, and used to estimate the density of the clouds that over¯y the LIDAR system. Wind speed measurement at that altitude will be required in order to validate the data. In conclusion, it is necessary to mention that both methods LIDAR and LIDAR-DIAL show a
Fig. 6. Example of the cloud density temporal and spatial evolution. The data map was obtained using third harmonic radiation from the Nd:YAG laser.
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
high capacity in monitoring environmental problems, specially if spatial and temporal distribution are required. This technique permits studies of the diusion of pollutant plumes in the atmosphere, providing direct measurements of its impact on the ground as it evolves. References [1] R. Perry, R.J. Young, (Eds.), Handbook of Air Pollution Analysis, Chapman & Hall, London, 1977. [2] H.K. Roscoe, K.C. Clemitshaw, Science 276 (1997) 1065. [3] U. Platt, in: Air Monitoring by Spectroscopic Techniques, Chapter 2, Wiley, New York, 1994.
173
[4] H.G. Reichle, H.A. Wallio, J. Geophys. Res. 91 (1986) 9841. [5] R.M. Measures, Laser Remote Sensing, Wiley, New York, 1983. [6] E.R. Murray, Opt. Eng. 16 (1977) 284. [7] E.D. Hinkley, Laser Monitoring of the Atmosphere, Springer, Berlin, 1976. [8] Northam et al., Appl. Optics 13 (1974) 2416. [9] R.M. Schotland, Proceedings of the Fourth Symposium on Remote Sensing of the Environment, 12±14 April 1966, University of Michigan, Ann Arbor, p. 273. [10] Harms, J. Appl. Optics 18 (1979) 1559. [11] L.T. Molina, M.J. Molina, J. Geophys. Res. 91 (1986) 14501. [12] D.J. Brassington, Appl. Optics 20 (1981) 3774.
The laser as a tool in environmental problems F. Molero, F. Jaque
*
Departamento de Fõsica de Materiales, Universidad Aut onoma de Madrid. 28049 Madrid, Spain
Abstract This work reports on the use of laser as an ecient tool in environmental studies. Application of LIDAR (LIght Detection And Ranging) and LIDAR-DIAL (DIerential Absorption Lidar) technologies in the ultraviolet range related with the tropospheric ozone and sulphur dioxide pro®les in the surroundings of Madrid are given. Sulphur dioxide measurements performed in a coal-burning power plant are also presented. Finally, the LIDAR technique is applied to study the cloud's density and its temporal evolution. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 42.68 Rp; 42.68 Vs Keywords: Laser; LIDAR; Air pollution
1. Introduction Given in densely populated industrial areas air pollution has become a serious problem, the study of pollutants and their reactions with natural components in the atmosphere has become an urgent demand. Historically, air pollution monitoring has been performed using grab sampling techniques for either in-situ or laboratory analysis [1]. More recently, optical instruments, such as DOAS (Dierential Optical Absorption Spectroscopy) [2,3] and COSPEC (COrrelation SPECtrometer) [4], have been incorporated to the task, allowing path-integrated measurement. The grab sampling technique has the disadvantage that it is limited to single point measurements and the delivered data are fully dependent on the site, which must be carefully chosen to be representative. Optical methods used in remote sensing provide three-dimensional pollution measurements but the *
Corresponding author: E-mail: [email protected]
use of searchlights limits the con®guration to direct or retrore¯ected detection, reducing the scanning options to suitable sites. The most powerful of these optical techniques are those that incorporate laser sources. The development of pulsed lasers made possible the generation of very short single pulses of high intensity light, allowing the detection of the light backscattered by the atmospheric constituent and thereby range-resolved measurements. The operating principle is analogous to pulsed microwave RADAR, except that optical rather than microwave components are required to direct, intercept and detect the emitted radiation. The technique has received the acronym LIDAR, standing for LIght Detection And Ranging [5]. Due to the shorter wavelength range kvisible kmicrowave =5, LIDAR provides the opportunity of sensing atmospheric constituents and properties to which RADAR is insensitive. In particular, LIDAR and LIDAR-DIAL techniques oered the capability for sensing scattering by both air molecules and the minute bits of suspended particulate matter, aerosol particles, ever
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 2 6 - 9
168
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
present in the atmosphere. Therefore, these techniques provide a method of mapping the threedimensional distribution of tracer material released into the atmosphere with high sensibility (10 ppb) and high spatial resolution from remote distances [6]. This paper reports some applications of LIDAR and LIDAR-DIAL technologies. Examples of pollution monitoring of Ozone (O3 ) and Sulphur dioxide (SO2 ) in the ultraviolet range in urban as well as in a coal-burning power plant are presented. 2. LIDAR and LIDAR-DIAL theory Two basic con®gurations for laser remote sensors have been considered in LIDAR and LIDAR-DIAL systems. The double-ended (Bistatic) arrangement involves a considerable separation of the transmitter and receiver to achieve spatial resolution in optical probing studies. The de®nite absorption path length can also be performed with a retrore¯ector. This arrangement imitates the lamp-source-based equipment, but today is rarely used, as pulsed lasers are capable of providing enough power to allow single-ended (Monostatic) con®gurations, with the transmitter and receiver at the same location. This arrangement allows device mobility and simplify the scanning procedures [7]. A single-ended LIDAR can either be coaxial or biaxial. In a coaxial system the axis of the laser beam is coincident with the axis of the receiver optics, while in the biaxial arrangement, the laser beam only enters the ®eld of view of the telescope beyond some predetermined range. This avoids the problem of near-®eld backscattered radiation saturating the detector and presents some advantages in stratospheric measurements [8]. The near-®eld intense-signal problem is solved in the coaxial system by gating the detector. This con®guration is used in tropospheric measurement, specially in DIAL measurements. In the case of a pulsed, single-ended coaxial LIDAR, the instantaneous received power at time t 2R/c, assuming single-scattering events, is given by the expression [7]):
cs
b R; kn kf RAr Rÿ2 Z R exp ÿ 2 a r; kdr
P R; k P0
2
0
1
where P0 is the transmitted power at time t0 0, c the velocity of light, s the pulse duration and R ct/2 the range of measure; n(k) represents the receiver's spectral transmission factor, f(R) the geometrical factor taken as the probability that radiation scattered at range R reaches the detector, b(R, k) the volume backscattering coecient of the atmosphere and a(r, k) the volume extinction coecient of the atmosphere, that can be expressed as: a r; k rm knm r ra kna r
2
where rm and ra are the wavelength-dependent extinction cross section of molecules and aerosols, respectively, including contributions from both scattering and absorption, and nm and na the range-dependent concentration of molecules and aerosols, respectively. In Eq. (1), Ar /R2 is the acceptance solid angle of the receiver optics, with Ar the eective receiver's area. This term shows explicitly the inverse square range dependence of the return signal, that produces its wide dynamic range and eventually, the saturation of the detector at the near-®eld. The time between the transmission of the laser pulse and the arrival of the scattered return signal can be directly related, through the velocity of light, to the range at which the scattering occurs. The crucial point in Eq. (1) is that the return signal is proportional to the term: exp [ÿ2òrm (k) ´ nm (r) dr], which depends on the concentration of the molecule of interest in the atmosphere. In general, interpretation of the LIDAR signal presented by Eq. (1) is complicated because of the simultaneous dependence of a(r, k) and b(r, k). A more readily interpretation can be reached by the combination of dierential absorption and scattering (formally named DAS but the acronym DIAL has gained considerable popularity recently) [9]. In a dierential approach, a comparison is made between the atmospheric backscattered laser radiation monitored when the wavelength of the
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
laser is tuned to closely match that of an absorption line, denoted as kon , and when it is detuned to lie in the wing of the absorption line, denoted as koff . The comparison between these two wavelengths is necessary to separate the absorption by the molecules of interest from other causes of attenuation. Considering Eq. (1), the pollutant concentration averaged in DR-long cells N~ DR can be calculated from the ratio of the received power at these two wavelengths: 1 Pr kon ; R ln N DR 2DrDR Pr kon ; R DR Pr koff ; R BT 3 ÿ ln Pr koff ; R DR where P(ki , Rj ) is the received power at wavelength ki (index i can be either the maximum absorption wavelength (on) or the minimum (o)), DR the length of the atmospheric cell over which the concentration is integrated, and Dr the dierence in absorption cross-section between the on and o wavelengths, r(kon ) ± r(koff ). In Eq. (3), B and T are represented by the expressions: b kon ; R DR b koff ; R DR ÿ ln ; B ln b kon ; R b koff ; R T ÿ2aA kon ; R ÿ aA koff ; RDR: Since the Rayleigh and Mie scattering cross-sections do not dier appreciably in a frequency interval corresponding to the linewidth of a molecular absorption line, the terms B and T can normally be neglected. In conclusion, using two close wavelengths, the concentration of a kind of pollutant molecules could be calculated knowing the ratio of the backscattered received power at these two close wavelengths. 3. LIDAR-DIAL diagram Fig. 1 shows the LIDAR-DIAL system layout developed at Autonoma University of Madrid (UAM), that works in the ultraviolet and visible ranges. The light source, located at the right side of
169
Fig. 1. LIDAR-DIAL diagram developed at the Autonoma University of Madrid.
the ®gure, consists of a Nd:YAG laser that pumps, either with the second or third harmonic, a dye laser, the tunable element that permits the emission of radiation at the wavelength that is absorbed by the chosen pollutant. After it, a doubling crystals set extent the wavelength range further in the ultraviolet (UV). The beam is expanded ®ve times in order to decrease its divergence, providing longer useful measurement range. The light probe can be directed to any direction within the top half-sphere with 0.2 mrad resolution, by means of a motorised periscope. Usual energies vary between 10 and 40 mJ/pulse of tunable radiation for tropospheric DIAL measurements, and can be as high as 150 mJ for stratospheric applications. On the left hand side of Fig. 1, the detection line is shown. The light backscattered by the atmosphere is collected by a 1 m focal length Newtonian telescope and sent to a photomultiplier tube (PMT) through an iris. This iris reduces the background light by decreasing the telescope ®eld of view and also geometrically compresses the signal dynamic range [10]. Despite this eect, further compression is necessary in order to avoid the saturation of the PMT by the intense near-®eld signal, retaining enough resolution to accurately process the far-®eld signal. That was done by electronically gating the PMT dynode chain using two transistors in switching mode. It uses a standard 5 V pulse generator, triggered by the laser, to
170
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
control the gate. This system allows selection of the delay between the laser pulse and the start of the electronic detection window. A second high voltage supply is used to feed the switched dynodes. The signal is digitised with 10 ns resolution (equivalent to 1.5 m) and recorded in a computer for later processing. As an example, Fig. 2 shows the eect of the electronic gate commented before. The pulse generator, triggered by the Q-switch signal of the Nd:YAG laser, provides a 0±5 V square pulse to the switching electronics. This pulse switches a couple of transistors, allowing that the second high voltage supply feeds the blocked dynodes. This opens the circuit, starting the detection window. Therefore, the delay between the laser pulse and the start of the detection window, and also the duration of this, is controlled by a 5 V standard pulse generator. This delay protects the PMT from the intense signal produced by scattering on the driving mirrors and the near®eld signal, allowing a clear detection of the far®eld light. The 9 ls lag between the end of the square pulse and the detection window is due to the accumulation of spatial charge in the base of the transistors. The return signal of dierential absorption LIDAR systems is usually so weak that the concentration pro®le calculated from a single pulse pair is very noisy, and temporal averaging is necessary to improve the signal-to-noise ratio (SNR).
Fig. 2. Illustration of the electronic gating eect coupled to the photomultiplier.
4. LIDAR and LIDAR-DIAL examples In this section some experimental results obtained with the LIDAR-DIAL technique are brie¯y exposed. Fig. 3 shows the vertical pro®le of the ozone concentration measured in the Campus of the Autonoma University (20 Km from the city centre of Madrid).The inset in Fig. 3 shows the Hartley ozone absorption bands which are centred at ~250 nm. For tropospheric DIAL applications the wavelengths must be selected on the low energy side of the Hartley ozone bands, being kon 294 and koff 300 nm the chosen wavelengths on this work. Both wavelengths are still close enough to consider negligible the scattering terms and at the same time show a value of Dr 45.32 ´ 10ÿ20 cm2 [11], which is adequate in order to obtain an accurate value of the ozone concentration. As can be seen in Fig. 3, the pro®le of the ozone concentration is very constant with a value near to 50 ppb from 200 to 700 m of altitude, dropping then to a few ppb. The altitude value for observed ozone concentration in¯exion depends on the temperature inversion and therefore on the atmospheric conditions. The errors for this ozone measurements were around 10 ppb, that is sucient for most of urban applications.
Fig. 3. Vertical pro®le of the ozone concentration measured in the Campus of the Autonoma University (Madrid). The inset shows the Hartley ozone absorption band and the two selected wavelengths for the ozone measurements.
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
Fig. 4. Vertical pro®le of sulphur dioxide concentration measured in the Campus of the Autonoma University of Madrid. The inset shows the SO2 absorption spectrum. The two chosen wavelengths selected are denoted in the ®gure.
In the measurements of the atmospheric SO2 content, the couple of wavelengths chosen were kon 300.03 and koff 299.3 nm (Dr 101 ´ 10ÿ20 cm2 ) [12]. Inset in Fig. 4 shows the SO2 absorption spectrum in the range 296±305 nm, obtained using a optical cell with SO2 gas at 1% in synthetic air. The solid line corresponds to the spectrum recorded using a standard spectrophotometer, while the points were obtained by coupling the optical
171
cell to the DIAL set up. This procedure is necessary when two very close wavelengths are used, in order to obtain good laser wavelength calibration. In these conditions the error associated with the absorption cross-section is minimised. The SO2 measurements in the University area show similar pro®les to that presented in Fig. 3 for the variation of the ozone concentration. The SO2 content was found to be in the range of few ppb in agreement with values obtained at ground level by standard methods. The study of the SO2 content in the atmosphere and its spatial and temporal evolution appears very critical in areas close to intense sources of SO2 . As was commented in the introduction, the LIDAR-DIAL technique provides a very powerful tool to study the diusion of a speci®c pollutant over large areas. Sulphur dioxide measurements have been performed in the coal-burning power plant of Compostilla II, Ponferrada (Spain), where coke with a high content of sulphur is used. For this purpose the LIDAR-DIAL equipment was located at approximately 300 m from the chimneys of the power plant. Fig. 5 shows a transverse
Fig. 5. Sulphur Dioxide concentration map corresponding to the two chimneys of the coal-burning power plant of Compostilla II (Spain).
172
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
section of the two plumes corresponding to the two operatives chimneys of this power plant. The section was obtained by ®ring the laser in nine dierent direction, starting from the vertical and fanning in both direction in 5 degrees spacing. The concentrations obtained in the dierent directions were interpolated using the Kriging method. As can be observed, the maximum SO2 content found in the centre of the plume reached a value around 8 ppm. Both plumes split in different lobes due to atmospheric turbulence. The temporal evolution of the two plumes was also studied showing the high capacity of the DIAL technique in the study of environmental pollution.
The system was also utilised for LIDAR measurements. A fraction of the third harmonic radiation from the Nd:YAG laser (k 355 nm) was split before pumping the dye laser, expanded and sent to the atmosphere. The intensity of the return signal, after range-correction, was used to study the cloud density evolution. In Fig. 6, 45 vertical measurements, one taken each minute and averaged 256 laser pulses, were interpolate using the Kriging method, and used to estimate the density of the clouds that over¯y the LIDAR system. Wind speed measurement at that altitude will be required in order to validate the data. In conclusion, it is necessary to mention that both methods LIDAR and LIDAR-DIAL show a
Fig. 6. Example of the cloud density temporal and spatial evolution. The data map was obtained using third harmonic radiation from the Nd:YAG laser.
F. Molero, F. Jaque / Optical Materials 13 (1999) 167±173
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