A new mobile and portable scanning lidar for profiling the lower troposphere

Abstract

An in-house developed mobile and portable three-dimensional scanning lidar system is discussed in this work. The system uses a stimulated Raman-scattering technique for the continuous observation of atmospheric aerosols, clouds and trace gases. This system has a fast scanning technique with a high-speed data acquisition, and permits the real-time measurement of atmospheric pollutants with the temporal resolution of 1 min. This scanning lidar system provides typical horizontal coverage of about 8-10 km while scanning; however, in zenith mode, good quality backscattered signals can be from 20 km, depending upon the laser power and sky conditions. This versatile lidar system has also overcome the drawbacks which are popular in the traditional scanning lidar systems such as complicated operation, overlap height between laser beam and telescope field of view In this system, the optical damage is reduced by using an integral coaxial transmitter and receiver. Some of the initial results obtained from the scanning lidar system are also presented. This study shows that boundary-layer structure and land-sea breeze circulation can be resolved from the developed scanning lidar system. The application of this lidar system to measure the pollutants over an industrial area is also discussed.

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A new mobile and portable scanning lidar for proling the lower troposphere

1 Introduction

Aerosols, clouds, and trace gases are among the principal atmospheric variables which alter the radiative balance of the earthatmosphere system. Therefore, over the last several decades, scientists who study the atmosphere have shown great interest in characterizing aerosol, clouds and trace gases to understand their impact on climate change and weather systems (Bach, 1976; Twomey, 1991; Kaufman et al., 2002; Mattis et al., 2004). Various remote sensing techniques are used to have prole measurements of atmospheric variables such as aerosol, clouds and trace gases. The optical remote sensing instruments like light detection and ranging (lidar), which is based on the principle of laser spectroscopy, has been proven to have a great potential to probe earths atmosphere due to their sensitivity, selectivity and range-resolved data of meteorological variables and atmospheric constituents (Fredriksson et al., 1981; Muruyama et al., 2001;Ansmann et al., 2003). The lidar system provides real-time monitoring of various atmospheric variables (such as aerosol, cloud, temperature, water vapour, optical depth of particulate matter, etc.) and meteorological processes (boundary-layer growth, aerosol and cloud layering, etc.).

In the last 15 years, various in-house developed lidar systems were set up at the National Central University (NCU), Chung-Li (25 N, 121 E), Taiwan, to understand the vertical distribution of aerosol (Chiang et al., 2004, 2007, 2008a, b;

2012), cloud (Nee et al., 1998; Das et al., 2009) and other me-

Published by Copernicus Publications on behalf of the European Geosciences Union.

C.-W. Chiang1,3, S. K. Das2,3, H.-W. Chiang1, J.-B. Nee3, S.-H. Sun1, S.-W. Chen4, P.-H. Lin4, J.-C. Chu4, C.-S. Su4, and L.-S. Su5

1Department of Mechanical Engineering, Kun Shan University, Tainan, Taiwan

2Indian Institute of Tropical Meteorology, Pune, India

3Department of Physics, National Central University, Chungli, Taiwan

4Taoyuan County Government Environment Protection Bureau, Taoyuan, Taiwan

5EI-LIDAR Corporation Limited, Taichung, Taiwan Correspondence to: C.-W. Chiang ([email protected])

Received: 20 February 2014 Published in Geosci. Instrum. Method. Data Syst. Discuss.: 9 April 2014 Revised: 22 September 2014 Accepted: 23 January 2015 Published: 17 February 2015

Abstract. An in-house developed mobile and portable three-dimensional scanning lidar system is discussed in this work.The system uses a stimulated Raman-scattering technique for the continuous observation of atmospheric aerosols, clouds and trace gases. This system has a fast scanning technique with a high-speed data acquisition, and permits the real-time measurement of atmospheric pollutants with the temporal resolution of 1 min. This scanning lidar system provides typical horizontal coverage of about 810 km while scanning; however, in zenith mode, good quality backscattered signals can be from 20 km, depending upon the laser power and sky conditions. This versatile lidar system has also overcome the drawbacks which are popular in the traditional scanning lidar systems such as complicated operation, overlap height between laser beam and telescope eld of view In this system, the optical damage is reduced by using an integral coaxial transmitter and receiver. Some of the initial results obtained from the scanning lidar system are also presented. This study shows that boundary-layer structure and landsea breeze circulation can be resolved from the developed scanning lidar system. The application of this lidar system to measure the pollutants over an industrial area is also discussed.

36 C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere

Figure 2. The mobile lidar system and vehicle interior.

and Spuler, 2004; Radlach et al., 2008; Behrendt et al., 2009, 2011). In this context, a mobile and portable three-dimensional scanning lidar system has been developed to investigate the properties of aerosol and trace gases in the lower troposphere, which is capable of daytime and nighttime operation. The scanning measurement technique will help in understanding the chemical and physical processes of the atmospheric pollutants caused by the planetary boundary layer (PBL) dynamics/evolution, where the conventional point-sampling instruments are insufcient. This three-dimensional scanning lidar is mobile and portable and have many added advantages over the conventional lidar system.

The purpose of this paper is to report the design of a newly developed scanning lidar with operating features and data analysis techniques. Some of the preliminary results and the potential applications of this scanning lidar are also presented. The paper is presented as follows: the system description and capabilities are discussed in Sect. 2. The methodology and analysis technique are described in Sect. 3. Some of the initial results of the measurements are presented in Sect. 4. Finally, Sect. 5 summarizes the paper.

2 System description and capabilities

2.1 Lidar and auxiliary equipment

An image and a prototype model of the scanning lidar system are shown in Fig. 1a and b, respectively. The system includes the laser as a transmitter, SchmidtCassegrain telescope as a receiver, photomultiplier tube as a detector and real-time data acquisition and signal processing unit. The laser, telescope and scanner are mounted on a vibration-isolated platform (to protect the laser and optical instrument against bumps in mobile application) in an aluminum framework to have a good structural stability. The power supply required for the operation of lidar is met either from commercial mains or a Honda diesel power generator (rating 220 VAC [notdef] 10 %,

50 Hz [notdef] 10 %). The lidar system can also be housed in a ve

hicle, shown in Fig. 2. Technical specications of the lidar system are given in Table 1.

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Figure 1. (a) Image and (b) prototype model of the 3-D scanning lidar system. In (b), the arrows (i) and (ii) show the zenith and azimuth scanning direction. Red line shows the path of the transmitting laser beam. The numbers 1 6 show the position of the reection mirror. The receiver section is shown in the enlarged box (green).

teorological variables such as temperature (Nee et al., 1995; Chen et al., 2004) and water vapour (Chiang et al., 2009). The NCU lidar system is xed at the ground and can operate in the nighttime only, which limits the investigation of spatial distribution and the diurnal cycle (associated with orography and atmospheric conditions) of the atmospheric variables.

To understand the variability of aerosol, cloud and trace gases on different scales, both spatially and temporally, lidar techniques are now advancing with the development of powerful computer-controlled instruments. In view of the importance of aerosol and cloud measurements, the three-dimensional scanning lidar is the most appropriate and foremost tool to do routine observations automatically (Mayor

C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere 37

The different Nd:YAG (neodymium : yttrium aluminum garnet)-pulsed solid state lasers are used for different purposes. The commercial Nd:YAG laser (New Wave Polaris) is used as a transmitter for the RayleighMie lidar system. The laser pulse width is 10 ns and repetition rate is adjustable

from 10 to 20 Hz. The laser operation is based on the second, third and fourth harmonic frequency generation at 532, 355 and 266 nm, respectively. The in-house developed laser has a pulse energy of about 500, 387, 160 and 95 mJ at 1064, 532, 355 and 266 nm, respectively, and is used as a pumping laser for a Raman lidar (shown in Fig. 3a). The laser beam divergence is about 0.5 mrad. The laser is mounted on an optical bench having the facility of height adjustment. Seven reection mirrors (17, shown in Fig. 1b), which are of the thick, hard-coated and at type, are mounted at 45 with respect to the lidar system surface to direct the laser beam into the atmosphere. The mirrors have the provision of precise azimuth adjustment and tilting of the laser beam. In case of a breakdown of the laser transmitter, a safety interlock is provided to shut down the laser unit.

The atmospheric species are sensitive to different wavelengths. Therefore, a multi-wavelength laser must be in practice for the measurement of different atmospheric species. To achieve multi-wavelength, the Nd:YAG laser is allowed to pass through various Raman active gases enclosed in a cell.The cell is known as a Raman cell (shown as Fig. 3b) and the method is known as the Raman cell technique. We have locally designed and developed a Raman cell (single pass) of1.5 m long and 2 cm diameter. Two lenses (L1 and L2) are used in the Raman cell. L1 is used at the head of the Raman cell to control the confocal parameter of the pump laser beam. L2 is congured at the end of the cell to collimate the output beam from the Raman cell. The focal length of the input and output cell lenses is about 75 cm. Two similar congurations of the Raman cells are used. The cell is lled with H2 at 5 atm and pumped with the fourth harmonic (266 nm)

of a Nd:YAG laser generating a wavelength of about 298 nm.

The partial pressure of gas in the Raman cell is variable and must be chosen for their better conversion efciency. Then, another cell is lled with CH4 at 20 atm and pumped with fourth and third harmonics (266/355 nm) of a Nd:YAG laser generating a wavelength of about 289 and 395 nm, respectively. The transmitted Raman wavelength energy is about 1530 mJ at 298, 289 and 395 nm. Fast switching of the Raman cells are with the piezoelectric drivers. This allows for the availability of multi-wavelength lidar for simultaneous measurements of several spectral, overlapping atmospheric species. Therefore, the developed lidar system can be also used as a differential absorption lidar (DIAL).

A SchmidtCassegrain telescope (Celestron-G8) is used as an optical receiver with a focal length of f/10. The receiver telescope is capable to scan in azimuth (from 0 to 360 ) and zenith (from 0 to 180 ) directions through servo motors i and ii, respectively (Fig. 1b), with a minimum precision of 0.04 . At the back of the telescope there is a colli-

(a)

(b)

Figure 3. (a) The in-house developed laser (500 mJ at 532 nm).(b) Lidar system with the Raman cell (Raman lidar).

mating lens (Fig. 1b), which focuses the entire eld of view (FOV) of the telescope onto the photomultiplier tube (PMT, which is used as a detector) head. The background noise level is suppressed by using the narrowband interference lter. Behind the collimating lens, a rotating interference lter wheel (Fig. 1b) is used. The rotating wheel has six lters, which enables the different wavelength selection. The signals from the PMT are fed directly on to a multi-channel transient recorder (Licel TR20-160). The Licel recorder combines an A/D converter (12 bit at 20 MHz) with 250 MHz fast photon counting system in the acquisition, which allows for a high, dynamic range, louder signal. The spatial and temporal resolution of the data acquisition in the Licel recorder is 7.5 m and 1 min, respectively.

In the scanning lidar system, a major concern is to maintain the parallel optical axes between telescope and laser

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38 C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere

Table 1. Technical specications of the scanning lidar.

Transmitter

Nd:YAG Laser New wave Polaris laser In-house developed laser Wavelength (nm) 266/355/532 nmPulse energy (mJ) 10/11/50 mJ 95/160/387 mJ Repetition rate (Hz) 1020 Hz 10 HzBeam divergence 0.5 mradPulse duration 10 nsRaman cell length 1.5 m

Raman output wavelength

H2 (5 atm) at 266 nm 298 nm CH4 (20 atm) at 266/355 nm 289/395 nm

Receiver

Telescope type SchmidtCassegrain (Diameter 20 cm; FOV of 0.5 mrad)

Detector Head-on PMT, Hamamatsu R7400Filters 0.23 nm FWHM

Scanner

Scan type Azimuth-elevation Scan rate 6000.25 min1

Signal processor

Type Digital processing Sample rate 250 MHz

beam and make the alignment easier in the process of scanning. The coaxial mode lidar is used to maintain the parallel optical axes between telescope FOV and laser beam, which makes the alignment easier in the process of scanning. However, in most of the scanning lidar where it has a coaxial mode, the backscattered light will be transmitted along the light path. This approach will generate strong background noise in the detector (i.e. PMT) and will limit the detection of backscattered signals. During continuous, long-run lidar operation, the coaxial-mode scanning can damage the detector.Moreover, the complex procedure to guide the emitted laser beam is also inconvenient (Eichinger et al., 1999; McGill et al., 2002).

Our scanning lidar is designed in the coaxial mode to reduce the overlap height ( 260 m) between transmitter and

receiver. However, a separate path has been used for the laser transmission. The laser source is kept xed, but the beam of the laser light (path is indicated by the red line) is made to scan by using seven reecting mirrors as shown in Fig. 1. All the reecting mirrors used for the beam steering are numbered 17. The laser beam can be steered in both azimuth and zenith directions by rotating the suitable reecting mirror. This technique helps to protect the laser transmitter from being getting damaged while scanning. In traditional lidar systems, the laser transmitter and telescope rotated together while scanning (Sasano, 1985). This approach can easily

damage the laser transmitter, or require frequent calibration for the long-term operation.

All the hardware sections of the lidar system are controlled automatically via a computer with the Microsoft Windows platform. The laser is in auto-control with the computer via an RS232 serial port. The Licel recorder is connected to a computer via a network with TCP/IP (transmission control protocol/Internet protocol) protocol. The servo motors are connected to a computer via GPIB card. The auto-controlled software of lidar has a user-friendly graphical interface that makes the system operation easier (shown in Fig. 4).

2.2 Experimental set-up and procedure

The Raman cell technique is used for generating multi-wavelength signals. The automatic switching of the Raman cell is through piezoelectric drivers. The rotating lters placed in front of the detector (i.e. PMT) and are automatically controlled via a computer, which allows the lidar system to function as a multi-wavelength lidar.

For pollution measurements, the lidar system can be automatically switched ON depending upon the air quality. This is done by following certain steps: (a) set scan strategy of the lidar system (e.g. scan angle, scan speed, laser power, and temporal and spatial resolution; as shown in Fig. 4), and(b) the rotating zoom camera image recognition system as an early warning (whether to switch ON or not) for the lidar

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C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere 39

3.2 DIAL technique

The differential absorption lidar (DIAL) technique is based on the signal at two different wavelengths, viz. ON and OFF, corresponding to large and small absorption by the target species, respectively. The DIAL equation for the species concentration estimation is expressed as

N(z) =

1

2[Delta1]

d dz

lnP ( ON,z)

P ( OFF,z)

[bracketrightbigg] + EP

Figure 4. The interface control program of the lidar system.

system. The camera takes a photo and identies the smoke area. If the intensity of smoke is over the set threshold value, then the lidar transmitter can be triggered and the lidar system will switch ON. The lidar will continue to operate until the air quality improves (i.e. the intensity comes below the set threshold value). All the operations are auto-controlled by a computer.

3 Methodology and data analysis

3.1 Lidar equation

The optical power measured with lidar is proportional to the signal backscattered by the atmospheric particles and molecules. The detected lidar signal can be expressed as

PM(z) = PL [notdef]

, (4)

where P ( ON, z) and P ( OFF, z) are the return signals from range z at ON and OFF, respectively.

[Delta1] = ( ON) ( OFF) is the differential absorption cross

section of the measured target species at temperature T ; and EP is the correction term for the contribution of differential backscatter and extinction by the aerosols and molecules.Detailed methodology of the DIAL technique for the atmospheric trace gas measurement can be found in Gimmestad (2005, and references therein). We have used ON and OFF as 289 and 298 nm, respectively for the sulfur dioxide (SO2) measurement.

3.3 Errors and uncertainties

The sources of errors and uncertainties associated with the measurements of the aerosol backscattering coefcient ( a(z)) are the noise from the signal, uncertainty in the molecular backscattering coefcient ( r(z)), consideration of initial value for reference distance and lidar ratio, etc. The signal-to-noise ratio of our lidar system can lead to an uncertainty in a(z) not exceeding 0.5 %. The r(z), which is calculated using the temperature and density of air derived from the radiosonde data have an uncertainty of within 0.5 % based on Pratt (1985). An uncertainty in the calculation of the lidar ratio is within 30 % and the extinction coefcient is estimated with an error of less than 30 % (Chiang et al., 2008a and references therein).

To evaluate the SO2 measurement errors, the systematic errors from aerosols and other gaseous constituents of the atmosphere, beam misalignment, and statistical errors, etc., must be considered. The error analysis can be referred to in the literature (Schotland, 1974; Vandaele et al., 1994;Fukuchi et al., 1999). In this work, an inaccuracy of less than 30 % was found by comparing the concentration of SO2 recorded between the continuous emission monitoring systems and DIAL technique.

4 Initial results and discussion

4.1 Determination of overlap function with a scanning lidar

The overlap function of lidar denes the efciency with which the laser beam is coupled with the receiver FOV as a function of height (Povey et al., 2012, and references

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O(z) [notdef] AT

z2 [notdef] atm(z) [notdef] e2[integraltext]

z0 (z[prime])dz[prime], (1)

where PM and PL are the power received from distance z and laser output energy, respectively; O(z) is the overlap function; AT is a constant which accounts for system optical efciency, the telescope receiver area, and the PMT spectral efciency; and atm(z) and (z) are the backscatter and extinction coefcients of the atmosphere, respectively. The integral is taken in between heights 0 and z to derive the atmospheric transmission. The backscattering and extinction coefcients, atm(z) and (z) are contributed both from aerosols and air molecules as shown below:

atm(z) = a(z) + r(z), (2) (z) = a(z) + r(z), (3) where the subscripts r and a represent the air molecules and aerosols, respectively.

40 C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere

Figure 5. Overlap function determination by keeping the scanning lidar xed in a horizontal direction. (a) shows a linear t (y = 2.43 [notdef] 10

4 [notdef] x + 1.03, shown as a solid red line) applied to the horizontal data ln(P m [notdef] z2) to determine the expected signal re

sponse for the range below overlap height ( 260 m). (b) shows the resultant overlap function obtained from horizontal data ln(P m [notdef] z2)

divided by expected signal.

therein). Accurate estimation of the overlap function describes the accuracy with which the lidar can be used to study the PBL, where the aerosol distribution is inhomogeneous. In a coaxial lidar system, the backscattered signal at short distances is partly blocked by the secondary mirror of the SchmidtCassegrain telescope; thus, indicating that the overlap will depend on the spatial intensity distribution of the beam. Therefore, the horizontal operation of lidar can be used to derive the overlap function. The correction is done as atm and (see Eq. 1) are assumed to be constant for the horizontal atmospheric path. When there is a complete overlap between transmitter and receiver FOV, the overlap function can be cancelled and the correction function O(z) becomes 1 (Sasano et al., 1979; Campbell et al., 2002). By taking the natural logarithm on the both sides of Eq. (1),

ln

PM(z) [notdef] z2[parenrightBig] =ln(PL [notdef] AT [notdef] atm) 2 [notdef] z. (5)

Because the horizontal atmosphere is assumed to be homogeneous, the term (PL [notdef] AT [notdef] atm) becomes constant. Equa

tion (5) describes the expected linear dependence form of the BeerLambert relationship, where 2 is the slope

and ln(PL [notdef] AT [notdef] atm) the intercepted coefcients. These lin

ear coefcients are determined by tting a straight line to ln(PM(z) [notdef] z2) over some interval beyond z, as shown in

Fig. 5. From a linear t coefcient, the expected signal ln(PM(z) [notdef] z2) for a homogeneous path z < z0 ( 260 m) can

be calculated. O(z) is then determined by normalizing the measured horizontal signal Sh(z) by the expected signal

Se(z),

O(z) = Sh(z)/Se(z). (6)

Figure 5b shows the resultant overlap function obtained from the linear t as shown in Fig. 5a. This result can be used to correct the overlap functions for scanning lidar observation at short distances. The above technique can be applied to re-

duce the systematic error in the DIAL system where the overlap height with telescope FOV is different for two different wavelengths.

4.2 Comparison of extinction coefcient derived from a scanning lidar with particulate matter

The total mass of aerosols per unit volume is termed particulate matter (PM). The surface pollution indicated by the PM2.5/PM10 (particulate matter with size smaller than 2/10 m) data are obtained from the Taiwan Environmental Protection Administration (EPA). The EPA monitoring station is about 5 km away from the lidar site. The PM2.5/PM10 data are routinely measured by a tapered element oscillating microbalance PM monitor (TEOM model 1400a, R&P, Albany, NY, USA). Data are collected consecutively on an hourly average for the purpose of legislating, controlling and preventing air pollution.

Figure 6 shows the comparison between the aerosol extinction coefcient derived with lidar at 532 nm and the aerosol mass concentration (in terms of PM2.5 and PM10).

The aerosol extinction coefcient derived from the lidar is for every 1 min, while PM2.5/PM10 is the hourly average data.

It is observed that the aerosol extinction coefcient shows good consistency with the PM value. The correlation coefcients between the aerosol extinction coefcient and PM2.5 concentration, and between aerosol extinction coefcient and PM10 concentration are respectively 0.73 and 0.85. The better correlation between the aerosol extinction coefcient and PM10 concentration may be due to the humidity at our location, which plays a crucial role in the growth of hygroscopic aerosols (Chiang et al., 2008b).

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C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere 41

Figure 6. Comparison of (a) PM2.5 and (b) PM10 concentrations with the aerosol extinction coefcient derived from the scanning lidar at 532 nm. The measurements were taken over the Guanyin industrial area during the period from 29 August to 11 October 2009. The PM data are hourly averages; while the lidar data are shown for every 1 min. Error bars indicate the standard deviation of aerosol extinction coefcient.

Figure 8 shows the time series contour map of the land breeze (top panel) and the sea breeze (bottom panel). The results are averaged for every 30 min. During the night, it is observed that the backscattered signal increases as the night progresses (refer Fig. 8, top panel). This is possibly due to the increase in aerosol concentrations, humidity, descending boundary layer and/or land breeze onset with the passage of time. Increase in the land breeze may sometimes also lead to increase in the turbulence. In addition, more humid air will blow from the sea towards the land and will get mixed with aerosol, causing the aerosol hygroscopic growth. And the amount of growth is dependent on the hygroscopicity factor. The aerosol hygroscopicity factor causes the strong backscattered signal due to their large cross-section area.

In the morning (see Fig. 8, bottom panel), there is a dense layer near the surface, which is formed by a residual layer at night. This layer rises slowly following the sunrise. This phenomenon is referred as convection. Moreover, the sea breeze, which is owing in from the sea underneath the land boundary layer also plays a role in lifting the layer.

4.4 Air pollution measurement (SO2)

Figure 9a shows the photograph of the DIAL measurement site NCU, Chung-Li. The inserted box in the gure shows the horizontal observations as a function of range for two different wavelengths used for SO2 measurement. It can be seen that the differential absorption occurred away from the lidar site, more than 1 km. We infer the different absorption is mainly caused by the human activities since our observing site is located in the outskirts, about 2 km away from the bustling city. The retrieved SO2 concentration (parts per billion) as a function of range is shown in Fig. 9b. Enhanced concentration of SO2 is observed at 1300 and 1800 m. A

comparison of SO2 concentrations simultaneously collected on 29 July 2010 between in situ CEMS (continuous emission monitoring systems) instruments and DIAL measurements is shown in Fig. 10. The lidar is kept xed at 5 elevation and

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Figure 7. Range height indicator (RHI) plot of the lidar backscattered signal at 532 nm of the lower troposphere. The lidar measurements were taken at NCU, Chung-Li, on 26 November 2009.

4.3 Observation of landsea breeze using a scanning lidar

The scanning lidar data measured on 26 November 2009, at 02:40 LT (LT = GMT + 8 h) through a vertical scan with a 1

angular resolution is shown in Fig. 7. The data are averaged for every 10 min time interval. The gure shows the 2-D lidar backscattered signal intensity representing cross sections of pollutant concentration in the vertical plane. The scanning proles clearly characterize the well-dened boundary layer. The depth of the boundary layer is about 500 m, which is consistent with the mean height of the PBL ( 580 m) observed

over Chung-Li (Chiang et al., 2008b). The aerosol particles were bounded and rather uniformly mixed within the boundary layer.

To investigate the landsea breeze, the scanning lidar is directed towards the sea, which is about 6.8 km from the lidar site. It is seen that the lidar site is affected by the land and sea breezes by observing the movement of aerosol structures.

42 C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere

Figure 8. Range height indicator (RHI) plot of lidar backscattered signal at 532 nm of land (top panels) and sea (bottom panels) breeze cases. The lidar measurements were taken at NCU, Chung-Li, on 21 May 2010.

Figure 9. (a) DIAL measurement of SO2 at NCU, Chung-Li, on 7 June 2010. The range-corrected DIAL signal measured at 289 and 298 nm is shown in the inserted box. (b) The SO2 concentration as a function of range retrieved from the DIAL measurement.

pointed towards the in situ site. Most of the discrepancies in

SO2 concentrations derived from the lidar and CEMS measurements are due to the different properties of the instruments or dynamics, or may be due to ventilation of the atmosphere; however, both measurements show a similar tendency.

4.5 Application of the scanning lidar in monitoring industrial pollutants

Environmental pollution is one of the major concerns in the modern era, especially in the urban and sub-urban areas. Thus, it is of prime importance to monitor the mass concen-

tration of pollutants, their sources and spatial and temporal variations. In this context, the scanning lidar is very useful in order to characterize the plumes or a fugitive emission from the industrial areas and their sources. This could provide information about the areas which can be affected from these particle concentrations. Figure 11 shows a typical example of backscattered signal intensity measured using scanning lidar over the industrial areas at Guanyin in Taoyuan County. The pollutant regions are shown on the topographic map (taken from Google Maps) of the industrial areas. The horizontal scan shows the pollutant covering various emitted sources and concentrations within the targeted scan eld. This type of scanned gure can be useful in tracking the source and

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C.-W. Chiang et al.: A new mobile and portable scanning lidar for proling the lower troposphere 43

Figure 10. Comparison of SO2 concentrations simultaneously measured from the DIAL and in situ instruments (CEMS) on29 July 2010 over the Guanyin industrial area. Error bars indicate the statistical SO2 concentration uncertainty derived from the DIAL measurements.

measuring the diffusion range of pollutants within the specied region. This is a useful technique to help the EPA agency to protect the peoples health from poor air quality.

5 Summary

We have designed and developed a 3-D scanning lidar system for the multi-wavelength measurements of aerosol and trace gases, which will be useful in understanding their temporal and spatial variability. This lidar system is designed with a small size, light weight, and as suitable for installation in various vehicles and therefore will be more handy than the traditional techniques of conducting atmospheric pollutant observations.

The continuous operation of the scanning lidar will gather data, which are useful to examine pollution episodes. Such data are also valuable for understanding the characteristics of pollutant transport, where the conventional point-sampling instruments have limitations. The scanning lidar is a new tool to examine the landsea breeze circulation and variation of the PBL by observing the movement of aerosol structures.

The set-up of the automatic scanning lidar network will be helpful in the real-time observation of air pollution over the urban and industrial zones. Such network data will help the EPA to protect the peoples health and abate the air pollution as quickly as possible. This could also be coupled to future policy directives for air pollution abatement strategies.

Figure 11. Lidar sector scan at 532 nm over the Guanyin industrial area on 6 August 2010. The topographical map (Google Maps) have been overlayed.

Acknowledgements. The authors are grateful to the Taiwan EPA for providing the PM data. One of the authors, S. K. Das, was supported by a Taiwan scholarship during this work and would like to acknowledge the Taiwan Government and National Central University, Chung-Li, Taiwan. Comments by two anonymous reviewers and the editor improved earlier versions of this manuscript, for which we are grateful.

Edited by: A.-M. Harri

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