monitoring o3 with solar-blind raman lidars

Monitoring O3 with solar-blind Raman lidars

Ferdinando de Tomasi, Maria R. Perrone, and Maria L. Protopapa

The benefits of retrieving ozone concentration profiles by a use of a single Raman signal rather than theRaman differential absorption lidar ~DIAL! technique are investigated by numerical simulations appliedeither to KrF- ~248 nm! or to quadrupled Nd:YAG- ~266 nm! based Raman lidars, which are used for bothdaytime and nighttime monitoring of the tropospheric water-vapor mixing ratio. It is demonstrated thatozone concentration profiles of adequate accuracy and spatial and temporal resolution can be retrievedunder low aerosol loading by a single Raman lidar because of the large value of the ozone absorption crosssection both at 248 nm and at 266 nm. Then experimental measurements of Raman signals provided bythe KrF-based lidar operating at the University of Lecce ~40° 209N, 18°69E! are used to retrieve ozoneconcentration profiles by use of the Raman DIAL technique and the nitrogen Raman signal. © 2001Optical Society of America

OCIS codes: 010.3640, 010.4950.

6,7

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1. Introduction

Ozone is an atmospheric component that deservesspecial attention.1,2 Whereas stratospheric ozone,which constitutes ;90% of the total atmosphericozone content, resides in a layer of the stratospherebetween approximately 10 and 50 km above theEarth’s surface and protects life on Earth from UVsolar radiation,3 tropospheric ozone ~;10% of the to-tal ozone content! contributes to greenhouse warm-ing, initiates the formation of photochemical smog,and in high concentrations can damage vegetationand other life on Earth.4 Therefore it is desirable tomake frequent measurements of changes in the ozonelayer and to quantify the amount of change that canbe attributed to human activity. Ozonesonde mea-surements have been made for decades, but it hasbeen shown that ozonesondes are impractical for fre-quent repetitive measurements.4 The differentialabsorption lidar ~DIAL! technique, however, is wellsuited for repetitive measurements of gaseous com-pounds such as ozone in the atmosphere,5 andground-based or aircraft-based DIAL systems thatuse UV wavelengths to monitor tropospheric andstratospheric ozone have been developed by several

The authors are with the Dipartimento di Fisica, Istituto Nazio-nale di Fisica della Materia, Universita di Lecce, 73100 Lecce,Italy. The email address for F. de Tomasi is [email protected].

Received 22 February 2000; revised manuscript received 15 Au-gust 2000.

0003-6935y01y091314-07$15.00y0© 2001 Optical Society of America

1314 APPLIED OPTICS y Vol. 40, No. 9 y 20 March 2001

groups of scientists. The main characteristics ofan UV DIAL system for ground-based and airborneozone measurements in the troposphere and thelower stratosphere for both daytime and nighttimeoperation were defined in Ref. 7. Indeed, Ref. 7 re-ports an assessment of both statistical and system-atic errors associated with the DIAL technique toidentify the appropriate wavelength with which tomeasure both the ozone concentration in the entiretroposphere and various atmospheric pollution con-ditions in the planetary boundary layer ~PBL!.

Raman-DIAL systems have also proved their capa-bilities for tropospheric ozone sounding.8,9 Raman-DIAL measurements8,10 are taken mainly to estimatethe ozone content and eliminate the effect of ozoneabsorption on water-vapor profiles retrieved by solar-blind Raman lidars. A method for calculating thewater vapor mixing ratio by the O2, N2, and H2ORaman lidar signals that takes the ozone UV atten-uation into account was presented in Ref. 10. UVlasers for which the Raman-scattered light remainsin the solar-blind region of the spectrum ~220–290

m! are generally required for daylight operation of aaman lidar.11,12 Lidar systems based both on KrF

~248-nm! excimer13,14 and on quadrupled Nd:YAG~266-nm! lasers15 have been developed for daytimeand nighttime measurements of water vapor. KrFand Nd:YAG lasers are the most reliable high-powerrepetition radiation sources available for implemen-tation of solar-blind Raman lidars. The utilizationof KrF lasers in solar-blind lidars is the less preferredof the two as a consequence of the rather poor per-formance of the Raman DIAL technique O3 sounding,

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owing to the large absorption cross section ~10.7 310218 cm2! of ozone at 248 nm.

It is shown in this paper that under clear-sky con-ditions ~low aerosol loading in absence of clouds!, aingle Raman signal may permit retrieval of ozoneoncentration profiles of better accuracy than thosebtained by the Raman-DIAL technique, if KrF oruadrupled Nd:YAG-based lidar systems are used.ndeed, the proposed technique represents a simpleool with which to use a two-channel Raman lidar toroduce ozone profiles of relatively good accuracy inddition to water-vapor profiles. Eliminating the ef-ect of ozone absorption on water-vapor profiles re-rieved by Raman-DIAL measurements will require ahree-channel Raman lidar.10

The statistical and systematic uncertainties of O3profiles retrieved under various pollution conditionsin the PBL by the single Raman signal ~extinction!method and by the Raman DIAL method are dis-cussed and compared here. To this end, numericalsimulations applied either to KrF- or to quadrupledNd:YAG-based Raman lidars are presented. Fi-nally, to further support the numerical results, O3concentration profiles retrieved from experimentalmeasurements of Raman signals either by theRaman-DIAL technique or by the extinction methodare given. Numerical studies are presented in Sec-tion 2; experimental results are given in Section 3.

2. Numerical Investigations

Let the Raman lidar equation be written as9

PR~z! 5 ho~lR!B@O~z!yz2#NR~z!

3 expH2*0

z

@aL~j! 1 aR~j!#djJ , (1)

where PR~z! is the return signal from distance z atRaman wavelength lR, ho~lR! represents the overallefficiency of the receiving optical system at lR, Bcontains all the other depth-independent parame-ters, O~z! is the overlap function between the laserbeam and the receiver field of view, NR~z! is the mol-ecule number density of the Raman-active gas, andaL and aR describe the extinction of light by atmo-spheric gas molecules and aerosol particles at thelaser and the Raman wavelengths, respectively. Itfollows from Eq. ~1! that

aL~z! 1 aR~z! 5 dydz ln$O~ z! NR~ z!y@ z2PR~ z!#%.(2)

ach extinction coefficient in Eq. ~2! can be written as

a~z! ; sR~z! 1 sA~z! 1 sO3~ z! 1 sO2

~ z!, (3)

here sR~z! and sA~z! represent the Rayleigh andaerosol extinction coefficients, respectively, sO3

~z!and sO2

~z! represent the extinction contributions ofO3 and O2, respectively. It is believed that in thesolar-blind spectral region the absorption that is dueto other contaminants such as SO2 can be considered

negligible in nonpolluted ~SO2 loading of fewer than100 parts in 109! air.6

It follows from Eq. ~1! that the O3 concentration,NO3

~z!, can be determined from the N2 Raman signal,PN2

, by the relation ~extinction method!

@NO3~ z!#S 5 1y@sO3~lL! 1 sO3~lN2

!#

3 (dydz ln$O~ z!N2~ z!y@ z2PN2~ z!#%

2 sR,L~ z! 2 sR,N2~ z! 2 sA,L~ z! 2 sA,N2

~ z!

2 sO2,L~ z!), (4)

here sO3~l! is the O3 absorption cross section atwavelength l.

As is well known, O3 concentration profiles can alsobe retrieved from N2 and O2 Raman signals ~the

aman-DIAL technique! by use of the expression8

@NO3~ z!#D 5 1yDsN2,O2

$dydz ln@PN2~ z!yPO2

~ z!#%,(5)

here DsN2,O2! is the O3 differential absorption cross

section for the Raman wavelengths lN2and lO2

.One retrieves Eq. ~5! by taking the ratio of the Ramansignals that are due to N2 and those to O2 and as-suming that O3 is the only factor of wavelength-dependent attenuation. The small separationbetween the N2 and the O2 Raman wavelength andthe large absorption cross section of O3 at these wave-lengths can cause these measurements to be ratherdifficult to make, even if it diminishes the influence ofthe aerosol differential extinction. This problem be-comes important in KrF-based Raman lidars becausethe Raman wavelengths for N2 and O2 are 263.7 and258.4 nm, respectively. The Raman wavelengths forN2 and O2 are at 277.5 and 283.6 nm, respectively, inquadrupled Nd:YAG-based lidars.

Numerical simulations applied to either KrF- orquadrupled Nd:YAG-Raman lidars were used for in-vestigating the benefits and the disadvantages of re-trieving O3 concentration profiles from Eqs. ~4! and~5!. To this end, we used the U.S. Standard Atmo-sphere model to calculate N2~z! and O2~z! concentra-tions and Rayleigh extinction coefficients sR,L~z! andsR,N2

~z! at the lidar laser and N2 wavelengths, respec-tively. The aerosol extinction coefficient sA~z! usedin the model was calculated with the U.S. Air ForceBETASPEC program,16 assuming a wavelength de-pendence of the aerosol extinction of l21. Figure 1~solid curve! shows the profile of sA,L~z! at the wave-length of the KrF laser ~optical thickness, tA 5 0.2!.

e included the absorption that is due to O2 ~Ref. 17!at the KrF laser wavelength in the model by settingthe absorption cross section ~sO2!L 5 7.8 3 10225 cm2.We used the concentration profile of Fig. 2 ~solidurve! for O3 to point out more clearly the limits of the

extinction and the DIAL methods for retrieving O3.A hypothetical lidar configuration located at groundand consisting of a 30-cm telescope viewing 0.9 mradin the vertical and a laser delivering laser pulses of50-mJ-energy and 25-ns duration, with a cross sec-tion of 3 cm 3 2 cm and divergences of 0.26 and 0.17

20 March 2001 y Vol. 40, No. 9 y APPLIED OPTICS 1315

ua

c

ct

lSi

1Nt

mf1c

mf

1

mrad along the x and the y directions, respectively, isrepresented in the model. The value ho 5 0.015 was

sed for optical efficiency of the total receiving systemt the Raman wavelengths of N2 and O2. Raman

lidar signals were generated from Eqs. ~1! and byaveraging of 105 laser shots. Fluctuations were gen-erated independently, with a Poissonian noise onphoton counting assumed. Figure 3 shows the rel-ative Poissonian noise εN2

~dotted curve! and εO2~solid

urve! of the N2 and O2 Raman signals, respective-ly generated both with the KrF @Fig. 3~a!# and the@Fig. 3~b!# quadrupled Nd:YAG-based lidar. A com-parison of Figs. 3~a! and 3~b! reveals that a quadru-pled Nd:YAG-based lidar allows Raman signals oflarger intensity and hence of smaller Poissoniannoise to be received if the lidar system parametersare equal to those of a KrF-based lidar. This resultis a consequence of the smaller value of the absorp-tion cross section of O3 at 266 nm ~9.0 3 10218 cm2! aswell as at the N2 and O2 wavelengths.

Fig. 1. Aerosol extinction profile at the KrF laser wavelengthcalculated by the U.S. Air Force. BETASPEC program ~solidcurve!. Dotted curve, a typical extinction profile that is due to thelayering of aerosols in the atmosphere.

Fig. 2. O3 concentration profiles retrieved by ~a! the extinctionethods ~filled circles! and ~b! the DIAL technique ~filled circles!

or a KrF-based lidar and for ho 5 0.015, a 50-mJ laser beam, and05 laser shots. Smoothing over 45 m has been applied. Solidurves, the assumed O3 concentration.


Let us now retrieve the initially assumed distribu-tion of O3 by using the generated Raman lidar sig-nals, a procedure that is fully reversible. Figures2~a! and 2~b! show O3 concentration profiles ~filledircles retrieved by Eqs. ~4! and ~5!, respectively, forhe KrF-based lidar. The O3 profiles retrieved by

Eqs. ~4! and ~5! for the quadrupled Nd:YAG-basedidar are shown in Figs. 4~a! and 4~b!, respectively.moothing over 45 m was applied. The solid curves

n Figs. 2 and 4 represent the assumed O3 concentra-tions. The benefits of getting concentration profilesfrom the extinction profile @Eq. ~4!# are clearly illus-trated Figs. 2 and 4. The O3 profile retrieved by Eq.~4! is quite close to the modeled profile, at least up to300 m, both for the KrF- and for the quadrupledd:YAG-based lidar, whereas the differences be-

ween retrieved O3 concentrations and model valuesare quite large, approaching 200 m, when the DIALtechnique is applied to N2 and O2 Raman signalsgenerated with the KrF- based lidar. This conclu-sion is further supported by Fig. 5 ~solid curves!,which shows the relative statistical errors versus al-

Fig. 3. Relative Poissonian noise εN2and εO2

of the N2 and O2

Raman signals, respectively, generated by consideration of ~a! aKrF-based lidar and ~b! a quadrupled Nd:YAG-based lidar, bothcharacterized by ho 5 0.015, a 50-mJ laser beam, and 105 lasershots.

Fig. 4. O3 concentration profiles retrieved by ~a! the extinctionethods ~filled circles! and ~b! the DIAL technique ~filled circles!

or a quadrupled Nd:YAG-based lidar characterized by ho 5 0.015,a 50-mJ laser beam, and 105 laser shots. Smoothing over 45 mhas been applied. Solid curves, the assumed O3 concentration.

tma

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t

ms

taba a

titude εS 5 D@NO3#sy@NO3

#s and εD 5 D@NO3#Dy@NO3

#Dfor ozone retrieved by Eqs. ~4! and ~5!, respectively,and for the KrF-based lidar. D@NO3

#S and D@NO3#D

represent the statistical uncertainties of O3 concen-ration retrieved by the extinction and the DIALethods, respectively. From the law of error prop-

gation, in the absence of background one gets

D@NO3# s 5 ~1y$Dz@sO3~lL! 1 sO3~lN2

!#%!@2yPN2~ z!#1y2,

(6)

D@NO3#D 5 @1y~Dz 3 DsN2,O2

!#@2yPN2~ z!

1 2yPO2~ z!#1y2.

(7)

One can observe from Fig. 5~a! ~solid curve! that thencertainty of the data is less than 10% up to ;1100

when the extinction method is used, whereas thetatistical uncertainties of O3 retrieved by the DIAL

method @Fig. 5~b!, solid curve# are greater than 10%beginning at 200 m. This result is due mainly to thelarge absorption cross section of O3 at the laser andN2 wavelengths as well as to the small value of the O3differential cross section, DsN2,O2

. In fact, for theKrF-based lidar

D@NO3#DyD@NO3

#S < Î2 @sO3~lL! 1 sO3~lN2!#yDsN2,O2

5 23. (8)

It is worth noting that one should increase the ampli-tude of the O3 and N2 Raman signal by ;103 to use theDIAL method to retrieve O3 concentrations with sta-tistical uncertainties of the same order of magnitude ofthose of Fig. 5~a! ~solid curve!. Figures 6~a! and 6~b!show the O3 profile retrieved by Eqs. ~4! and ~5!, re-spectively, for a KrF-based lidar characterized by ho 50.15, a 500-mJ laser beam, and 106 laser shots. Therelative statistical errors versus altitude εS and εD areshown in Figs. 7~a! and 7~b!, respectively ~solid curves!,and one observes that the statistical uncertainties εDare less than a few percent until ;750-m and increaseo 15% at 1000 m, even if the larger values of ho, laser

Fig. 5. Relative statistical errors versus altitude ~a! of O3 re-rieved by Eq. ~4! and ~b! of O3 retrieved by Eq. ~5!, for ~solid curves!KrF-based lidar and for ~dotted curves! a quadrupled Nd:YAG-

ased lidar, both characterized by ho 5 0.015, a 50-mJ laser beam,nd 105 laser shots.

energy, and numbers of laser shots allow N2 and O2Raman signals of 103 larger amplitude to be obtained.It is worth mentioning that it is not so easy to realizesolar-blind lidars having ho $ 0.15. Moreover, themaximum repetition rate of high-power excimer lasersis 100 Hz, even then nearly 3h of continuous monitor-ing would be required for a single O3 profile with sta-tistical errors of the same order of magnitude as thoseof Fig. 7 to be produced, according to numerical simu-lations.

The benefits of obtaining O3 concentration profilesby Eq. ~4! are also revealed by the numerical resultsfor quadrupled Nd:YAG-based lidars ~Fig. 4!, even ifthe difference between the values of εS and εD iseduced ~Figs. 5 and 7!. In fact, the ratio D@N3#Dy@NO3

#s is reduced by a factor of ;3 when quadrupledNd:YAG-based Raman lidars are used, and Fig. 5~dotted curves! reveals a ratio εDyεS ' 10, at least upo 1000 m.

In addition to statistical errors related to the signaleasurements, systematic errors that arise from in-

trumental limitations or from deviations in air den-

Fig. 6. O3 profile retrieved ~a! by the extinction method ~filledcircles! and ~b! by the Raman DIAL technique ~filled circles!, for aKrF-based lidar characterized by ho 5 0.15, 500-mJ laser beam,nd 106 laser shots. The assumed O3 profile is shown by a solid

curve in each figure.

Fig. 7. Relative statistical errors versus altitude of O3 retrievedby ~a! the extinction method and ~b! the Raman DIAL technique fora KrF-based lidar ~solid curves! and a quadrupled Nd:YAG-basedlidar ~dotted curves!, both characterized by ho 5 0.15, a 500-mJlaser beam, and 106 laser shots.


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sity from the standard atmosphere15 also contribute toerrors in O3 measurements. We have assumed in themodel that these last uncertainties are less than sta-tistical errors, so they have been not taken into ac-count. However, it is worth noting that O3 can beretrieved from Eq. ~4! if a standard or a measuredaerosol extinction profile is available, and this mayrepresent the main limit of the proposed technique.The influence of the aerosol distribution on the O3concenetration retrieved by Eq. ~5! can be considered

egligible mainly with KrF-based Raman lidars andecomes less negligible as the difference between lN2

and lO2increases. Let us evaluate the effect of sys-

tematic errors on O3 profiles retrieved by Eq. ~4! as aresult of the use of a modeled aerosol profile instead ofthe real one. To this end, let us assume that the realaerosol extinction profile at the laser wavelength iss*A,L~z!. The total ~statistical plus systematic! errorfor the O3 concentration profile retrieved by Eq. ~4! willbe

D@NO3#T 5 D@NO3

#S 1 $1y@sO3~lL!1sO3~lN2!#

3 @DsA,L~ z! 1 DsA,N2~ z!#%, (9)

here DsA,L~z! 5 us*A,L~z! 2 sA,L~z!u. One gets fromEq. ~9! that the contribution to the error from theaerosol extinction uncertainty amounts to ;14% ifthe aerosol concentration deviates by no more than afactor of 3 from the modeled aerosol profile for an O3concentration of 2 3 1018 molym3. So, under lowaerosol loading, the total ~statistical plus systematic!

ncertainties on O3 profiles retrieved by Eq. ~4! aretill significantly less than the statistical errors thatccur when the Raman-DIAL technique is used toetrieve O3 and are D@NO3

#DyD@NO3#S 5 23 for KrF.

Note that D@NO3#DyD@NO3

#S 5 8 for quadrupled Nd:YAG-based lidars.

Let us now evaluate the effect of the systematicerrors on O3 profiles retrieved by Eq. ~4! that are dueo the layering of aerosols in the atmosphere. To

Fig. 8. O3 concentration profiles retrieved by ~a! the extinctionethods ~filled circles! and ~b! the DIAL technique ~filled circles!,

or a quadrupled Nd:YAG-based lidar characterized by ho 5 0.015,a 50-mJ laser beam, and 105 laser shots. The aerosol profile of

ig. 1 ~dotted curve! was used as the so-called real aerosol profile.moothing over 45 m has been applied. Solid curves representhe assumed O3 concentration.


his end we have assumed that the real aerosol profiles characterized by a gradient at ;900 m, reaching anxtinction value of 1 km21 at the KrF laser wave-

length ~Fig. 1, dotted curve!. Figure 8 shows the O3profiles retrieved by Eqs. ~4! and ~5! for the quadru-

led Nd:YAG-based lidar characterized by ho 50.015, a laser beam energy of 50 mJ, and 105 lasershots. Note that the Raman lidar signals have beengenerated by use of the aerosol extinction profileshown by the dotted curve in Fig. 1 ~real aerosolextinction profile!. In contrast the aerosol extinc-ion profile calculated with the U.S. Air Force BETA-PEC program ~Fig. 1, solid curve! was used in Eq.

4! to retrieve the O3 concentration. A comparison ofFigs. 4 and 8 reveals that the O3 profile retrieved byEq. ~4! is affected more by the systematic error that isdue to the aerosol layer at 900 m. The total ~statis-tical plus systematic! relative errors

~εS!* 5 D@NO3#Ty@NO3

#S,

and εD of the data of Fig. 8, are shown in Fig. 9, andone can observe that ~εS!* , εD at every altitude,despite the large effect on ~εS!* of the systematic errorthat is due to the aerosol layer. Similar results areobtained for KrF-based lidars, which are affected lessby the aerosol extinction uncertainty.

Let us test the sensitivity of the O3 retrieval by Eq.~4! that is due to the wavelength dependence of theaerosol extinction coefficient. As is well known, it isgenerally assumed that sA,L~z!ysA,R~z! 5 ~lLylR!n,where the n parameter can vary from 21 to 1 for tro-ospheric aerosols.18 Then the total ~statistical plusystematic! error for O3 profiles retrieved by Eq. ~4! is

D@NO3#T 5 D@NO3

#S 1 $1y@sO3~lL! 1 sO3~lN2!#

3 sA,L~lN2ylL 2 lLylN2

!% (10)

if one assumes that n 5 21 for the modeled aerosolprofile and a wavelength dependence of the real aero-sol extinction, such as n 5 1. One gets from Eq. ~10!that the contribution from the wavelength depen-

Fig. 9. Relative statistical errors in the O3 measurements of Fig.8 as a function of the altitude: ~εS!* ~solid curve! and εD ~dottedcurve!.

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Table 1. Main Lidar Characteristics

dence of the aerosol extinction coefficient amounts toless than 1% for an O3 concentration of 2 3 1018

molym3 and is negligible with respect to statisticaluncertainties for both KrF- and Nd:YAG-based li-dars. However, systematic errors for O3 profiles re-trieved by Eq. ~4! owing to the use of a modeledaerosol extinction profile can be significantly reducedif the elastic lidar signal is used to retrieve the aerosolbackscattering coefficient and the aerosol extinctioncoefficient profile, on the assumption of a constantvalue of the extinction-to-backscattering ratio withinthe PBL.19

3. Experimental Results

We present some Raman lidar measurements takenwith the KrF-based lidar operating at the Universityof Lecce ~40° 209N, 18° 69 E!. The relevant parametersf the University of Lecce (UNILE) lidar are listed inable 1. Figure 10 shows the experimental setup.he very low value of the receiving optical system’sfficiency ~ho 5 3 3 1023! is due either to the use of0% beam splitters to divide the collected backscat-ered radiation into the Raman channels or to the usef double grating monochromators ~Jobin-Yvon DH10

UV! characterized by an out-of-band rejection of#1027 to separate the Raman signals spectrally. Inact, each monochromator grating is characterized byrelative efficiency of ;15% in the spectral range of

Fig. 10. Experimental layout of the KrF-based Raman lidar op-erating at the University of Lecce ~40° 209N, 18° 69E!. Threedifferent channels are used to monitor the Raman H2O, N2, and O2

backscattered radiation. L’s, lenses; BS’s, beam splitters; D, dia-phragm; MR’s, monochromator and photosensor system; MCS’s,multichannel scalers; Disc., discriminator; Ampl.’s, amplifiers.

Laser energy 50 mJLaser pulse duration 25 nsRepetition rate 50 HzTelescope collecting area 0.07 m2

Telescope focal length 1.2 mho 3 3 1023

interest, and the quantum efficiency of the photomul-tiplier tube has been estimated to be 0.04.

Figure 11 shows the relative Poissonian noise εN2

~dotted curve! and εO2~solid curve! of the N2 and O2

Raman signals, respectively, monitored on 15 July1999 at 19:00 h local time ~8 3 104 laser shots!. Fig-

re 12~a! shows the O3 concentration profiles retrievedby the DIAL technique ~open triangles! and by theextinction method ~filled circles!. The relative statis-tical errors εS ~dotted curve! and εD ~solid curve! aregiven in Fig. 12~b!. Smoothing over 45 m has beenapplied. We used the modeled aerosol distribution ofFig. 1 ~tA 5 0.2! to retrieve O3 because the lidar islocated in a rural area and measurements were takenin summer on a quite sunny day. We used the U.S.Standard Atmosphere model with measured values ofthe ground temperature and pressure to evaluate N2and O3 concentrations as well as the Rayleigh extinc-tion coefficient. One can observe from Fig. 12~a! thata rather smooth O3 profile that is typical of a nonpopu-lated area14 with statistical errors of less than 20% hasbeen obtained by use of the extinction method,whereas the O3 concentration values provided by Eq.~5! are not of use. The rather short sounding range

Fig. 11. Relative Poissonian noise εN2and εO2

of the N2 and O2

Raman signals, respectively, monitored on 15 July 1999 at 19 hlocal time.

Fig. 12. ~a! O3 concentration profile retrieved by the extinctionethod ~filled circles! and by the Raman DIAL technique ~open

riangles! and ~b! relative statistical errors of O3 retrieved by theextinction method ~dotted curve! and by the Raman DIAL tech-nique ~solid curve!.


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reached with the UNILE Raman lidar is due to thequite low optical efficiency of the receiving system inaccordance with numerical simulations.

4. Conclusions

KrF- and quadrupled Nd:YAG-based Raman lidars aregenerally used for both daytime and nighttime moni-toring of water vapor. But the rather poor perfor-mance of the Raman-DIAL technique in estimatingthe O3 content and eliminating the effect of O3 absorp-tion on water-vapor profiles limits the benefits of thesesolar-blind lidars owing to the high power and shortwavelength of the laser beams delivered by either KrFor quadrupled Nd:YAG lasers. Indeed, the large ab-sorption cross section of O3 at either 248 or 266 nmmakes O3 measurements by the Raman-DIAL methodather difficult, mainly with KrF-based lidars.

We have demonstrated that one can use the N2 Ra-man signal provided by a KrF- or a quadrupled Nd:YAG-based lidar to retrieve O3 concentration profilesof adequate accuracy and spatial and temporal resolu-tion because of the large value of the O3 absorptionross section at the laser wavelength, provided that aodeled or a measured aerosol extinction profile is

vailable. We have discussed and compared statisti-al and systematic uncertainties for O3 profiles re-

trieved under various pollution conditions in the PBLby the single Raman signal ~extinction! and the Ra-man DIAL methods, respectively. In particular, ithas been shown that the systematic uncertainties of O3concentrations that are due to the use of a modeledaerosol profile amount to ;14% if the aerosol concen-ration has a deviation of no more than a factor of 3rom the modeled aerosol profile. It has also beenhown that one can use measurements of the aerosolackscattering coefficient to get the aerosol extinctionoefficient profile on the assumption within the plan-tary boundary layer of a constant value of thextinction-to-backscattering ratio. Then it was dem-nstrated that KrF- and quadrupled Nd:YAG-basedidars can provide water vapor and O3 concentrationrofiles of good accuracy by monitoring N2 and water-apor Raman signals instead of monitoring N2, O3, andater-vapor Raman signals to estimate the O3 content

by the Raman DIAL method, as is generally done whensolar-blind Raman lidars are used to retrieve water-vapor mixing ratio profiles. However, it is worthmentioning that a peculiarity of the Raman DIAL tech-nique is due to its capability of determining O3 profilesalso below the height h* of the complete laser-beamreceiver-field-of-view overlap, whereas one must eval-uate the geometric overlap function to retrieve O3 con-entrations at altitudes of #h* when Eq. ~4! is used.

In conclusion, the proposed method represents asimple tool with which to use KrF- and quadrupledNd:YAG-based lidars for continuous monitoring of O3in the lower troposphere to investigate O3 changesand to quantify the amount of change that can beattributed to human activity, even if Rayleigh DIALsystems that use two different UV laser beams rep-


resent the best-suited systems for both daytime andnighttime measurements of O3 in the troposphere.

We are working to implement the KrF-based Ra-man lidar operating at the University of Lecce toshow experimentally the benefits of the proposedtechnique for measurements of O3 and water-vapormixing ratios.

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3. K. Ya. Kondratyev and C. A. Varotsos, “Total and troposphericozone changes: observations and numerical modelling,”Nuovo Cimento C 22, 219–246 ~1999!.

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monitoring o3 with solar-blind raman lidars

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