multiwavelength lidar for measurements of atmospheric aerosol

Optics and Lasers in Engineering 37 (2002) 91–99

Multiwavelength lidar for measurements ofatmospheric aerosol

S. Chudzy !nski, A. Czy’zewski, K. Ernst, G. Karasi !nski,K. Kolacz, A. Pietruczuk, W. Skubiszak, T. Stacewicz*,

K. Stelmaszczyk, A. Szyma !nski

Institute of Experimental Physics, Warsaw University, ul. Ho ’za 69, 00-681 Warsaw, Poland

Received 13 April 2001; accepted 12 July 2001

Abstract

We represent the results of our investigations on size distribution of atmospheric aerosol

particles by means of a multiwavelength lidar based on Nd :YAG and Ti : Sa lasers equippedwith frequency multipliers. The measurements were performed in mountainous areas.r 2002Elsevier Science Ltd. All rights reserved.

Keywords: Lidar; Aerosol particles size distribution; Atmosphere pollution

1. Introduction

Remote investigation of atmospheric aerosol by means of lidar technique is one ofthe most important optical methods in environmental studies. It can provide a widespectrum of information, which contribute significantly towards a better under-standing of various phenomena occurring in the atmosphere, like pollution emission,its transformation and migration, inversion layer formation, cloud physics andchemistry, etc. Such information are useful for many branches of science, fromphysics of the atmosphere and the Earth sciences to the interaction of aerosolparticles on living organisms. Recently, particular attention has been given to theinfluence of aerosol particles on human health and its dependence on their sizes andchemical composition [1,2].

*Corresponding author. Tel.:+48-22-621-0985; fax: +48-22-625-6406.

E-mail address: [email protected] (T. Stacewicz).

0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 9 1 - 4

2. Simple aerosol lidar

A simplified scheme of a single wavelength aerosol lidar system is shown in Fig. 1.The main part of the lidar sender is a source of the light pulses (laser). The pulses arethen scattered in the atmosphere. Among various processes of the lightFmatterinteraction, the Rayleigh and the Mie scattering [3] on aerosol particles play the mostimportant role. The receiver which registers the backscattered pulse consists of atelescope, which collects the light and a photomultiplier. The filter matches thereceiver spectral range to the laser frequencies. When the laser pulse penetrates aregion of higher aerosol concentration, the optical echo rises. The distance from theregion of interest can be easily found by multiplication of the light speed by half of

Fig. 1. Principle of operation of the aerosol lidar.

S. Chudzy !nski et al. / Optics and Lasers in Engineering 37 (2002) 91–9992

the time interval between the moments of the light pulse sending and the echoregistering.The lidar signal, i.e. the mean number of photons registered by the photodetector

as a function of distance ðNeðRÞÞ from the investigated object can be described by thefollowing formula:

NeðRÞ ¼ NLA0R2

bðRÞdx exp �2Z R

0

aðzÞ dz� �

; ð1Þ

where NL denotes the number of photons emitted in the laser pulse, A0 the telescopesurface area, b the differential cross section for backward scattering in Mie andRayleigh processes, d ¼ cDt=2 half of the optical pulse spatial length, and x thegeometrical and spectral efficiency of the optical receiver. The exponential factordescribes diminution of the light intensity, i.e. the extinction, according to theLambert–Beer law. The extinction coefficient a refers here to the light scatteringonly. For the remote investigation of aerosol, the wavelength is chosen in such a waythat the light absorption is negligible.Inversion of the lidar data provides an opportunity to find the extinction

coefficient (a) for the sounding light. This coefficient can be related to the aerosolnumber density. An example of results of the atmospheric aerosol investigation by asingle wavelength lidar is presented in Fig. 2. However, in order to solve the lidarequation, one has to assume a relation between the total backscattering coefficient band the total extinction coefficient a [5,6]. For the white light, the relation is of theform

b ¼ constak; ð2Þ

where k is a number (1pkp0.5).1

3. Light scattering by aerosols

Radii (r) of particles suspended in the atmosphere span over several ordersof magnitude, from single nanometers up to a few millimeters [8]. These particlesinclude dust, fog, haze, clouds, ice crystals as well as a variety of aerosolswith contribution of special particles produced by local sources like industry,volcanoes, etc. The distribution of sizes of the aerosol particles (f(r))varies depending on many local conditions. In general, three peaks (the so-calledmodes), which can be classified according to the mean radii, characterize the sizedistribution of the continental aerosols. They are called nuclei, accumulation, andcoarse modes and their mean particle radii are about 0.015, 0.15 and 3 mm,respectively. In volume distribution (4=3 pr3f ðrÞ), the last two modes arepredominant in the planetary boundary layer (PBL) [9,10]. There is only a smallvariability in the distribution of these modes for various continental aerosol types.

1Stelmaszczyk et al. [7] found that for typical aerosols with broad distribution of sizes of the particles,

relation (2) can also be used for a narrowband laser light.

S. Chudzy !nski et al. / Optics and Lasers in Engineering 37 (2002) 91–99 93

The size distribution of the aerosol particles for a single mode can be represented bya log-normal function [11]:

dNðlnðrÞÞ ¼N0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2plnðsÞ

p exp �½lnðrÞ � lnðrmedÞ�2

2ln2ðsÞ

� �dlnðrÞ; ð3Þ

where N0 denotes the total number concentration of the aerosol particles, rthe particle radius, s the geometric standard deviation and rmed the medianradius.The Rayleigh theory is valid only if the so-called size parameter x ¼ 2pr=l (l

denotes the light wavelength) is much smaller than unity. According tothis approach, the cross section for the light scattering is proportional to 1/l4.The Mie theory that should be applied for larger particles is mathematicallymuch more advanced. It takes into account interference of the light scatteredon different parts of the particle. Then, the scattering properties of aerosolparticles depend on their shapes, chemical constitutions as well as on thelight wavelength, absorption and refraction coefficients, conductivities, etc.Analytic formulaes exist only for high symmetry particles (spherical orellipsoidal). For other shapes, time-consuming modeling and calculations arenecessary.Due to the Rayleigh and Mie approach, the extinction Qext and the backward

scattering Qp efficiencies can be found. Using both the above functions, thebackscattering coefficient b and the total extinction coefficient a as a function of the

Fig. 2. Cloud of aerosol formed below the inversion layer located at an altitude of 1600m (Karkonosze

Mountains, Poland, June 6, 1997, 23 h 53). The extinction is presented in gray scale. Measurements were

carried out with the lidar system described in [4].


light wavelength l can be found as follows:

bðlÞ ¼ pZ

N

0

r2Qpðr; lÞf ðrÞ dr; ð4Þ

aðlÞ ¼ pZ

N

0

r2Qextðr; lÞf ðrÞ dr: ð5Þ

The cross section distribution is the product of number size distribution and thegeometric cross section of the particles. Since in the surface distribution ðpr2f ðrÞÞ ofthe aerosol particles the accumulating mode is dominating in the PBL, the light ismainly scattered by particles from this mode. For the nuclei mode, the surface of theparticles is small and in spite of their large number the light scattering is weak. Forthe coarse mode, the scattering coefficient of a single particle can be very large butthe number of particles is relatively very small [9]. Such a situation is characteristiconly for a typical continental aerosol. For local cases of the size distribution, thelight scattering properties can be different.When the multiwavelength lidar is used, both the extinction a and the

backscattering b coefficient can be determined for each wavelength li: By solvingthe inverse problem, the aerosol size distribution can be found. The description ofthese procedures can be found in [12–15]. One of the possible methods can beperformed by the conversion of integral equations (4) or (5) into the system of linearequations

aðz; l1Þ ¼Xnj¼1

Dr f ðrj ; zÞpr2j Qðrj ; l1Þ

aðz; l2Þ ¼Xnj¼1

Dr f ðrj ; zÞpr2j Qðrj ; l2Þ ð6Þ

aðz; lnÞ ¼Xnj¼1

Dr f ðrj ; zÞpr2j Qðrj ; lnÞ;

where n is the wavelength number. In this way, the stepwise distribution of theaerosol size distribution can be found. Finally one of the commonly used predefinedfunction (for example the log-normal) can be fitted to the experimental data. It isclear, that in order to determine the real size distribution with a good precision asmany wavelengths as possible should be applied.

4. Experiment

Our multifrequency lidar was constructed due to the modification of a DIALsystem (ELIGHT 510M) [4]. The simplified scheme of this construction is shown inFig. 3.The lidar uses a pulsed Ti : Sa laser, which alternatively generates pulses at two

wavelengths (lON and lOFF) that are necessary for the DIAL operation. For the


aerosol size distribution monitoring, we have chosen the values of about 750 and860 nm, respectively. The energy of pulses reaches 250mJ. The pulses are frequencydoubled and tripled, then the system as a whole is able to generate six wavelengths.In order to use them for aerosol investigations, some changes in the lidarconstruction were necessary. The laser beams are alternatively sent to theatmosphere after passing through a monochromator made of a Pellin-Broca prismand a slit. The echo signal is registered by means of a 40 cm telescope and aphotomultiplier, then it is digitized by the 9450A LeCroy oscilloscope and stored in acomputer.In order to increase the number of wavelengths used for the determination of the

size distribution of aerosol particles, we used an additional lidar with Nd :YAG laserworking on first- and second-harmonic pulses (1060 and 532 nm, respectively). Thereceiver of this lidar contains a Cassegrain telescope with a mirror of 150mmdiameter and two photomultipliers (with S20 cathode for the visible wavelength andwith S1 photocathode for the fundamental one). The photomultiplier pulses aredigitized with HP 54522 oscilloscope.Our preliminary measurements were carried out with three wavelengths only.

They were performed in Warsaw during wet weather. The results are presented inFig. 4. They describe the aerosol distribution at the altitude of about 300m. In this

Fig. 3. Simplified scheme of the multiwavelength lidar for measurements of the aerosol size distribution.


case, we were only able to approximate the aerosol size distribution by Jungefunction

n½logðrÞ� ¼C

rk: ð7Þ

This function is often used to show the falling slope of the aerosol characteristics[16].The results of measurements carried out by means of 6Fwavelength lidar in July

2000 during the campaign in Karkonosze Mountains are presented in Fig. 5. In thiscase, the aerosol size distributions for different distances from the lidar wereachieved. The number of wavelength (6) is sufficient to approach the aerosol sizedistributions by means of a single mode log-normal function. For both results(Figs. 4 and 5), the model of the aerosol consisting of spherical water droplets wasassumed in our calculations. We believe that this is a proper assumption forconditions accompanying both our observations, i.e. for the aerosol suspendedduring the wet (slightly foggy) weather above the town and above the mountains.Then, the refractive index of the water (1.33) was applied. The lidar equations (1)were solved using assumption (2) with k=1. The software that allows fitting of thetwo mode log-normal distribution is under elaboration.

Fig. 4. Junge distribution fitted to the data obtained by means of three wavelength lidar during a wet

evening in Warsaw 21. 06. 1998. The distance from the lidar was about 0.5 km.


5. Conclusion

Lidar is a unique tool for the remote investigation of atmospheric aerosol. Usingthis instrument, both the aerosol extinction (that could be related to the aerosolconcentration) and the aerosol particle size distribution can be found. For the secondkind of measurements, the multiwavelength operation is required. We believe thatdue to recent progress in lasers, optics and computers, a development of such lidarsystems will be observed. It should allow the simultaneous registration of signals atall wavelengths, and faster (on line) elaboration of experimental data. That shouldalso result in an increase in the lidar ranges and the precision of measurements.By now, reliable results of the aerosol size distribution can be obtained mainly for

the water aerosol. This is connected with poor knowledge of the relation betweenbackscattering and extinction coefficients for the particles that are different fromnonabsorbing spherical droplets. The modeling of these parameters on the basis ofMie theory is complicated and needs high computer capabilities. Good results can beobtained using experimental techniques of modern optics, like Paul traps [17]. Insuch devices, small charged particles (like particles of aerosols) can be imprisonedover long periods of time (even hours). Then the light scattering experiments can beperformed and both the backscattering and the extinction coefficients can bedetermined for particles of different shapes and refraction/extinction index. Weexpect that such a technique will provide information for the successful investigation

Fig. 5. Aerosol size distributions found for different altitudes. The measurements were carried out by

means of 6Fwavelength lidar in July 2000 during the campaign in Karkonosze Mountains.


of continental, urban and industrial aerosol. Moreover, such experiments will allowone to observe the evolution of aerosol particles in various atmospheric conditions.

Acknowledgements

This work was supported by the Polish Committee for Scientific Research project6 P04G 030 17.

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multiwavelength lidar for measurements of atmospheric aerosol

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