Nocturnal Pbl Lidar

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Aerosol observations by lidar in the nocturnal

boundary layer

Paolo Di Girolamo, Paolo Francesco Ambrico, Aldo Amodeo, Antonella Boselli,

Gelsomina Pappalardo, and Nicola Spinelli

Aerosol observations by lidar in the nocturnal boundary layer NBL were performed in Potenza, South-ern Italy, from 20 January to 20 February 1997. Measurements during nine winter nights wereconsidered, covering a variety of boundary-layer conditions. The vertical profiles of the aerosol back-scattering coefficient at 355 and 723.37 nm were determined through a Klett-modified iterative proce-dure, assuming the extinction-to-backscattering ratio within the NBL has a constant value. Aerosolaverage size characteristics were retrieved from almost simultaneous profiles of the aerosol backscat-tering coefficient at 355 and 723.37 nm, the measurements being consistent with an accumulation mode

radius not exceeding 0.4 m. Similar results in terms of aerosol sizes were obtained from measurementsof the extinction-to-backscattering ratio profile at 355 nm performed on six nights during the measure-ment campaign. Backscattering profiles at 723.37 nm were also converted into profiles of aerosol liquidwater content. 1999 Optical Society of America

OCIS codes: 280.3640, 010.1110, 280.1100, 290.1310, 290.1350, 290.4020.

1. Introduction

The planetary boundary layer PBL, i.e., the lowerlayer of the atmosphere that is sensitive to the effectof the Earths surface, controls the flow of heat andmomentum between the surface and the free atmo-sphere, thus playing a key role in atmospheric circu-lation. Aerosol and moisture have a crucial role inthese mechanisms; they tend to be trapped within thePBL and can be used as tracers for the study of theboundary-layer vertical structure and time variabil-ity.

Diurnal aerosol measurements by lidar in the PBLhave been reported by several authors, covering a

variety of sites over land,13 sea,46 and urbanareas.79 In contrast, a few lidar studies reportingon nighttime measurements have appeared in theliterature.1013

The dimensional characteristics of aerosol particlescan be studied by monostatic lidars using severaltransmitting wavelengths. The use of multiple-wavelength lidar measurements to retrieve aerosolsizes has been investigated by several authors, bothin the troposphere14,15 and in the stratosphere,1618

with limited attention to the PBL.In this paper the vertical profiles of the aerosol

backscattering coefficient at 723.37 and 355 nmhereafter A,723z and A,355z are determinedthrough an iterative procedure based on the assump-tion of a constant value for the extinction-to-backscattering ratio within the nocturnal boundary

layer NBL. Vertical profiles of A,723z are re-ported and discussed for nine nights of measure-ments from 20 January to 20 February 1997.Profiles of A,723z are also expressed in terms ofliquid water content, assuming there is a linear re-lationship between the two quantities.

Aerosol average size characteristics are deter-mined by two different methods: The first is basedon simultaneous measurements of the backscatteringratio A,723zA,355z. The accumulation moderadius of the aerosol particles is retrieved by compar-ing measured and theoretical values of A,723z

All the authors are with the Istituto Nazionale perla FisicadellaMateria, Unita di Napoli, Pad. 20, Mostra dOltremare, I-80125

Napoli, Italy. P. Di Girolamo is also with the Dipartimento diIngegneria e Fisica dellAmbiente, Universita della Basilicata,I-85100 Potenza, Italy. His e-mail address is [email protected]. P. F. Ambrico, A. Amodeo, A. Boselli, and G. Pappa-lardo are also with the Istituto di Metodologie Avanzate di AnalisiAmbientale, Consiglio Nazionale delle Ricerche, Area della Ricercadi Potenza, I-85050 Tito Scalo Pz, Italy. N. Spinelli is also withthe Dipartimento di Scienze Fisiche, Universita di Napoli, I-80100Napoli, Italy.

Received 23 October 1998; revised manuscript received 5 April1999.

0003-693599214585-11$15.000 1999 Optical Society of America

20 July 1999 Vol. 38, No. 21 APPLIED OPTICS 4585


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A,355z with theoretical values obtained throughapplication of the Mie-scattering theory.

Measurements of the extinction-to-backscatteringratio at 355 nm, k355z, are obtained from simulta-neous measurements of A,355z and of the aerosolextinction coefficient profile A,355z, this latter pa-rameter being retrieved from the N2 Raman back-scatter signals. Measurements of k355z arereported for six nights during the JanuaryFebruary1997 measurement campaign. Information on aero-

sol sizes is also obtained from comparisons of exper-imental and theoretical values of k355z. Resultsfrom the two techniques are discussed and compared.

Aerosol and moisture are strictly related compo-nents in the PBL since water vapor affects the di-mension and composition of PBL aerosols. Twohumidity-related meteorological parameters are cor-related with the aerosol loading in the PBL: relativehumidity and potential temperature. While relativehumidity RH gives a direct indication of the amountof water uptake on aerosols, potential temperature,undergoing limited changes in the case of dry, andsaturated, adiabatic processes, is expected to be al-

most constant within the PBL aerosol layer and to gothrough a rapid change at the PBL top. Compari-sons between aerosol measurements by lidar and ra-diosonde data of potential temperature andor RHhave been the objectives of a few papers,4,10,19,20 allreporting daytime measurements characteristic ofconvective boundary layers. Few lidar studies cov-ering nighttime measurements have been reported inthe literature.1013 In this paper we focus on theNBL and report the comparison between lidar mea-surements of the aerosol backscattering coefficient at723.37 nm and simultaneous radiosonde data of po-tential temperature and RH for a specific case study11 February 1997 to show the typical behavior of

these parameters and their correlations in the NBL.

2. Lidar Data Analysis Procedure and Tests

Data presented in this paper are expressed in termsof the aerosol backscattering coefficient at 723.37 and355 nm. The former wavelength is located within aspectral region of major absorption for water vapor.However, this wavelength was carefully selected tofall into the gap between two adjacent absorptionlines, so that water-vapor absorption at 723.37 can beneglected.

The aerosol backscattering coefficient at wave-length is given by

A,zRA,znzRay,, (1)

where RA,z is the aerosol scattering ratio at wave-length and nz and Ray, are the molecular num-ber density profile and the Rayleigh-backscatteringcross section at , respectively.

In the hypothesis of single scattering the lidarequation can be expressed in simplified form:

Pz CAzz

z2exp2

0

z

zdz , (2)

where z represents the total backscattering coef-ficient z Ray,z A,z, z is the totalextinction coefficient

z Ray,z A,z,

Az is the effective receiver area, and C is a constantterm inclusive of the laser pulse energy and durationand of the transmission and the detection efficiencies.Ray,z and Ray,z are the molecular contribu-tions to the backscattering and the extinction coeffi-cients, respectively, and can be evaluated byRayleigh-scattering theory through Ray,z

nzRay, and Ray,z 83nzRay,. The ef-fective receiver areaAz is related to the geometry ofthe lidar system and has been calculated by means ofray-tracing-based software,21 taking into account thelaser spatial profile, the exit pupil, and the laserbeam dimension. Because of the geometry of thelidar system, no lidar return is detected at less than120 m Az 0.

The stable analytical inversion procedure proposedby Klett,22 which is particularly effective in the caseof optically thick conditions, does not present anyparticular advantage in the case of small opticaldepths, as in the present case, and is sensitive to the

selection of the extinction-to-backscattering ratio k.This procedure was therefore not applied to thepresent data for the retrieval of the aerosol-backscattering coefficient. In this paper an iterativeapproach for the retrieval ofA,z is considered. Insuch a procedure RA,z is obtained from the elasticbackscattered signal P

z through

RA,zPz

Pmz 1. (3)

Pmz represents the molecular contribution to Pzand takes the form

PmzKnzz2

exp2Ray,z A,z, (4)where Ray,z and A,z are the molecular and theaerosol contributions to the optical thickness, respec-tively, and Kis a calibration factor. K can be deter-mined by normalizing the lidar signal to Pmz in anaerosol-free region above the PBL aerosol layer.Ray,z and nz can be obtained from either theradiosonde data or an atmospheric model, the formerestimate being characterized by a lower uncertainty.In the present analysis Ray,z and nz were deter-mined from radiosonde pressure and temperatureprofiles.

The iterative procedure is based on the assumption

within the PBL of a constant value of the extinction-to-backscattering ratio k

defined as

kz A,zA,z, (5)

where A,z is the aerosol extinction coefficient.Values ofk

are dependent mainly on aerosol micro-

physical properties and to a lesser extent on the lasersounding wavelength. The microphysical charac-teristics of aerosol particles depend on the location ofthe lidar station an urban or rural site, a marine orcontinental site and on local meteorological condi-

4586 APPLIED OPTICS Vol. 38, No. 21 20 July 1999


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tions. Literature values ofk

range from 20 to 100sr.2326 Sasano and Browell15 proposed a value ofkof 36 sr at 300 nm, representative of rural continentalaerosols in the PBL. This value is considered belowin the application of the data-retrieval method forA,z. Lidar measurements of k at 355 nm havebeen performed on six nights during the present mea-surement campaign on the basis of simultaneous andindependent measurements of A,355z and of theaerosol extinction coefficient A,355z. Measure-

ments ofk355z, temporally averaged over six nightsand vertically averaged over the NBL, lead to a mean

value of 40 sr, in good agreement with the valueproposed by Sasano and Browell.15

In the application of the retrieval technique the kdependency on wavelength is assumed to be negligi-ble25; that is, the same value of k

36 sr was usedin the retrieval of both A,355z and A,723z. Nu-merical computations based on application of theMie-scattering theory have been performed to verifythis hypothesis. Results from these computations,reported in Fig. 8 and discussed in Section 3, showthat k variability with wavelength is limited. In

particular, in the spectral range of 355723 nm,kk does not exceed 15% for aerosol particles withan accumulation mode mean radius in the range of0.11 m. A 15% variability ofk

produces a vari-

ability in the retrieved profiles of A,z not exceed-ing 5% at any height. As a consequence the erroraccomplished in the application of the data-retrievalprocedure in neglecting the wavelength variability ofk

does not exceed 5%.The iterative procedure starts with determining a

first-guess estimate of Pmz obtained by neglectingthe term A,z in Eq. 4. A first-guess estimate ofRA,z and A,z is consequently obtained throughEqs. 3 and 1, respectively. By making use of the

extinction-to-backscattering ratio k, we then obtaina first-guess estimate of A,z through

A,z 0

z

A,zdz. (6)

Such an estimate ofA,z can be introduced in Eq.4, leading to a new profile ofPmz. The procedurecan be iterated and values of A,z are found to bestable after two to three iterations.

Figure 1 shows as an example the vertical profile ofA,723z for 30 January 1997. In Fig. 1a we illus-

trate three successive iterations in the case ofk 36sr with the third iteration appearing to be almostcompletely superimposed on the second one. Simi-lar behavior is also observed for k

equal to 50 and

100 sr. In Fig. 1b we compare the final iterationscorresponding to the values ofk

of 36, 50, and 100 sr.

The three curves almost coincide in the upper regionof the PBL and deviate from one another by less than20% in the lower PBL portion.

The iterative procedure was applied to all the lidarechoes at both 355 and 723.37 nm acquired duringthe nine selected nights of measurements. In all

cases lower values ofk

were found to lead to a fasterconvergence of the iterative procedure, with the sec-ond iteration being a good representation of the finalresult.

The overall error affecting A,z depends on theuncertainty affecting the measurements of nz andRA,z. The use of radiosonde data leads to valuesofnznz within the PBL not exceeding 1%. Theerror affecting RA,z is mainly the result of the sig-

nal statistical error PzPz and the uncertaintyaffecting the estimate ofPmz. For all reported pro-files at 355 and at 723.37 nm, P

zP

z stays

smaller than 5% throughout the PBL; Pmz is itselfdependent on nz, Ray,z, and A,z. The erroraffecting RA,z also depends on the uncertainty as-sociated with the assumption of no aerosol above thePBL aerosol layer: i.e., RA,z zh 0, wherezh isthe PBL height. This latter source is expected todetermine an additional error of5%.27 The overalluncertainty affecting A,z does not exceed 20% atany height. A further error source can be present inthe vertical region where no complete overlap be-

tween laser and telescope field of view is achievedthe lowest 400 m. Accurate knowledge of the laserbeam and the telescope field of views relative posi-tion is important in estimating Az. A deviation of10 rad from the condition of perfect parallelism isexpected to produce an error ofA,z as large as 10%at the lower detectable height28 and not exceeding 5%at 300 m. For the present measurements no at-tempt was carried out to verify experimentally theoccurrence of an alignment deviation of this order ofmagnitude, so the reader must be aware of the pos-sible occurrence of this further error source.

Fig. 1. Vertical profile of the aerosol backscattering coefficient

A,723z for 30 January 1997: a iterative procedure for k 36sr; b final iteration for k

equal to 36, 50, and 100 sr.



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As anticipated above, measurements of theextinction-to-backscattering ratio at 355 nm are ob-tained from simultaneous and independent lidarmeasurements of A,355z and A,355z. Verticalprofiles of A,355z are retrieved from molecular ni-trogen Raman signals at 387 nm,P387z, through thealgorithm proposed by Ansmann et al.29:

A,355z

d

dz lnnz

P387zz2

Ray,355z

Ray,387z

1 355387 ,

(7)

where 1; nz, Ray,355z, and Ray,387z can bedetermined from radiosonde pressure and tempera-ture profiles.

Vertical profiles ofk355z, reported and discussedin detail in Section 3, were measured on six nightsduring the measurement campaign. Vertically av-eraged values of k355z are found to range between

21 and 57 sr with a temporal mean value of 40 sr, ingood agreement with the value of k at 300 nm pro-posed by Sasano and Browell15 36 sr. The extreme

values of A,z obtained through the above-mentioned iterative procedure for k

equal to 21 and

57 sr are found at any height to fall within 10% of theprofiles ofA,z obtained for k 36 sr. As a con-sequence the error from neglecting the day-by-day

variability of k

in the application of the data-retrieval procedure does not exceed 10%. Further-more note that the error in considering the literature,k 36 sr, instead of the measured value of 40 sr,

does not exceed 2% in the lowest 100 m and is well

below 1% throughout most of the PBL.

3. Data Set and Results

Measurements performed during nine distinct winternights 20, 21, 27, 28, 30, and 31 January and 10, 11,and 18 February 1997, covering different weatherregimes, were considered in the period from 20 Jan-uary through 20 February 1997. Approximately 20radiosondes, some of which were borne on captiveballoons, were launched during the campaign to co-incide with lidar operation. The nine consideredmeasurements are those for which both lidar and

simultaneous radiosonde data are available. Thescheduled times for the reported lidar and radiosondemeasurements are listed in Table 1. Measurementswere performed mostly in clear-sky conditions, butthere were occasional overcast conditions during ob-servations. In Table 2 we report surface values oftemperature, pressure, and humidity as well as themeteorological conditions during the observation pe-riods the presence or absence of clouds. All thereported measurements were performed after sunsetand are characteristic of a boundary layer that isstably stratified.

A. Aerosol Backscattering

In Fig. 2 we illustrate the vertical profile of the aero-sol backscattering coefficient

A,723z

for the nine

nights of measurement. Reported profiles ofA,723z are obtained through the iterative procedureillustrated in Section 2, considering k 36 sr. Theterm A,723z instead of A,355z is reported anddiscussed because of the greater sensitivity of theformer parameter to aerosol microphysical changes.The nocturnal stable surface layer acts as a shield foraerosol transport at night. However, owing to aero-sol penetration in the mixed layer throughout the dayand its persistence in the residual layer at night,30 inthe absence of advection a clear change in aerosolconcentration appears evident at the top of the resid-ual layer, in the former entrainment zone, rather

than at the top of the stable surface layer. The re-sidual layer height ranges from 750 to 1100 m aboveground level for all nine selected nights of measure-ments and their magnitude depending on time of dayand local meteorological conditions.

In Fig. 3 we show a comparison between simulta-neous lidar and radiosonde data for 11 February; ra-diosonde data are expressed in terms of the verticalprofiles of potential temperature Tp and RH. The po-

Table 1. Scheduled Times for Lidar and Radiosonde Measurements for

Nine Selected Nights of Measurement

Datea

1997

LidarStarting and

Finishing TimeGMT

SondeLaunch Time

GMT

LidarAveragingtime, min

20 January 01:1702:06 01:50 4921 January 23:1223:42 23:38 3027 January 22:4423:25 23:10 4128 January 20:2621:07 20:15 41

30 January 19:0020:30 19:35 4131 January 17:0017:41 16:45 9010 February 02:2403:09 02:41 4511 February 18:0318:49 18:23 4618 February 19:3020:10 19:39 40

aDates refer to the day the lidar operation was started. Thismeans that in the case of 20 January and 10 February reporteddata, collected shortly after midnight, the measurements wereactually performed on 21 January and 11 February, respectively.

Table 2. Surface Values of Temperature, Pressure, and Humidity and

Weather Conditions for the Nine Selected Nights of Measurement

Date1997

Surface Values of

Weather ConditionsT C p mbar RH %

20 January 2.7 935.5 92 Clear sky21 January 2.4 939.5 97 Clear sky27 January 2.52 936.6 89 Partially cloudy28 January 3.7 938.2 99 Clear sky30 January 1.8 938.2 98 Clear sky31 January 4.2 934.2 88 Clear sky10 February 3.9 938.2 90 Partially cloudy11 February 4.3 934.2 95 Clear sky18 February 0.1 931.6 79 Clear sky



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tential temperature generally increases throughoutthe nocturnal stable boundary layer, acquiring an al-most constant value within the residual layer nearlyadiabatic. Within the entrainment zone the poten-tial temperature again increases with height to be-come slowly increasing or even decreasing in the freeatmosphere above.

In Fig. 3 the potential temperature profile reveals aresidual layer in the 100700-m vertical region char-

acterized by almost constant values ofTp in the rangeof 298300 K. A stable layer is present below 100 m,displaying a limited temperature inversion 297300K. Above the residual layer is the entrainment zone,characterized by a strong increase in Tp dTpdz 0.05Km and extending from 700 to 1000 m. Furtherabove is the free atmosphere characterized by almostconstant values of potential temperature 311312 K.

In Fig. 3 the potential temperature profile appears tobe anticorrelated with RH as well as with the aerosolbackscattering, the aerosol layer top at 1050 m be-ing associated with a maximum in potential tempera-ture and a minimum in RH. The profile of A,723z

shows the presence of two aerosol stratifications lo-cated in the NBL: a lower layer, extending through-out the residual layer to as far as 700 m, and an upperlayer within the entrainment zone to as far as 1000m. The transition between the two layers, occurringaround 700 m, is associated with a minimum in poten-tial temperature and a maximum in RH.

The upper aerosol layer displays three distinct re-gions characterized by different vertical gradients ofA,723z 700750, 750900, and 900 1050 m. Asimilar stratified structure is present in both the po-tential temperature and the RH profiles. These strat-

ified structures in the aerosol, the potential

temperature, and the RH profiles are probably pro-duced by wind direction changes with height31,32 as aresult of which air masses from different source re-gions and characterized by different aerosol contentare advected at different altitudes.

B. Aerosol Dimensional Characteristics

Aerosol average size characteristics can be deter-mined from either simultaneous measurements ofA,723z and A,355z or simultaneous measure-ments ofA,355z and A,355z. The first method isbased on a comparison between experimental andtheoretical values of the backscattering ratioA,723zA,355z. Assuming that the scattering ofaerosols in the PBL follows the Mie-scattering theory,the theoretical values for A,723zA,355z can beexpressed in the form

A,723z

A,355z

rmin

rmax

r2Qback723, r, m723dNrdrdr

rmin

rmax

r2Qback355, r, m355dNrdrdr

,

(8)

Fig. 2. Vertical profiles ofA,723z, as obtained through the iter-

ative procedure with k 36 sr for the nine selected nights ofmeasurement.

Fig. 3. Vertical profiles of A,723z for 11 February 1997 as ob-tained through the iterative procedure k

36 sr, bold curve and

simultaneous profiles of potential temperature open trianglesand RH solid dots.



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gate the dependence of A,723zA,355z on the re-fractive index and the size distribution parametersr1, 1, and 2. In particular, the sensitivity of thetheoretical value of A,723zA,355z on the distri-bution widths for the accumulation particle and the

Aitken particle modes is illustrated in Figs. 4b and4c, respectively: Changes in 2 between 1.2 and 2produce a variability ofA,723zA,355z not exceed-ing 25% throughout the investigated size range 0.1 r2 2 m, while similar changes of1 lead to a larger

variability ofA,723zA,355z. However, values of1 have been found to be characterized by a variabil-ity not exceeding 5% in the case of continentalboundary-layer aerosols.42 Such variability pro-duces effects smaller than 10% on A,723zA,355z.

The Aitken mode radius is expected to change as afunction of RH. The growth factor rr0RH definesthe ratio between the radius r of the particle at theRH and the radius r0 of the dry particle. Values ofrr0RH were taken from Hanel and Bullrich.43 A

variability of RH in the 2095% range was consid-ered in the computation. For dry-particle radii inthe 0.010.1-m interval, values ofrr0 are found to

range from 1.0004 for RH 20% to 1.64 for RH 95%. Mie-scattering computations show that thevariation ofr1 of a factor of 1.64 produces a variabilityof A,723zA,355z not exceeding 10% in the sizerange of 0.1 r2 2 m.

The refractive index for the Aitken particle mode isexpected to change as a function of RH, following35

nf n0 rr0

RH

3

nw1 rr0RH3

, (10)where nf is the complex refractive index of the parti-cle at RH, n0 is the complex refractive index of the dryparticle, nw is the complex refractive index of water,

and rr0RH is the growth factor at RH. Mie-scattering computations show that the variation ofthe aerosol refractive index nf, associated with a vari-ability of RH in the 2095% range, produces a changein A,723zA,355z not exceeding 10% in the sizedomain 0.1 r2 2 m.

In Fig. 5 we illustrate simultaneous measurementsofA,723z and A,355z for 30 January 1997 togetherwith the corresponding profile of the ratio A,723zA,355z. This ratio ranges from approximately0.05 at the PBL aerosol layer top to 0.22 at 200 mabove the station level. Computations illustrated inFig. 4a indicate that such values of A,723z

A,355z are consistent with aerosol particles charac-terized by an accumulation mode ranging from 0.1to 0.3 m. Furthermore values ofA,723zA,355zdecreasing with height are indicative of aerosol sizesreducing as a function of height. In Fig. 6 we reportthe measured vertical profiles of A,723zA,355zfor eight of the nine selected nights of measurements,as obtained from simultaneous or almost simulta-neous measurements of A,723z and A,355z. On10 February 1997 it was not possible to retrieve the

vertical profile ofA,723zA,355z because of par-tially cloudy weather conditions. Some profiles

present no data for less than 400 m because of thedetectors overloading for the lidar signals at 355 nmbelow this height. All profiles of A,723zA,355zdisplay values in the 0.020.7 range, with larger val-ues at lower levels and values decreasing with height,with the only exception of 27 January. Such valuesofA,723zA,355z are consistent with aerosol par-ticles characterized by an accumulation mode rang-ing from 0.1 to 0.8 m; seven of the nine reported

Fig. 5. Simultaneous measurements ofA,723z and A,355z for

30 January 1997 and corresponding profile of the ratio A,723zA,355z.

Fig. 6. Vertical profiles ofA,723zA,355z.



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profiles of A,723zA,355z display values lowerthan 0.3, corresponding to values of r2 smaller than0.4 m.

Assuming that the computed values of A,723zA,355z in Fig. 4a display a linear dependence onthe accumulation mode radius r2 within the size do-main of 0.10.8 m, the measurement error ofA,723zA,355z produces an uncertainty on the re-trieved estimates of r2 not exceeding 30%.

In Table 3 we report the height-integrated values ofthe backscattering coefficient IB, RH, and aerosolsize for the nine reported nights of measurements.The vertical integration has been performed up to theresidual layer top. High values of IB appear to beassociated with large values of RH. High values ofIB may be the result of either the growth of particlesin the accumulation mode, as was probably the casefor 27 January and 11 February characterized bylarge values ofr2 in the range of 0.20.8 m, or thenucleation of new particles in the Aitken particle

mode, as occurred on all remaining days when smallvalues ofr2 were observed.Dubinsky et al.44 found an approximate linear re-

lationship between the aerosol backscatter coefficientat 514 nm, A,514z, and the liquid water contentwAz for different cloud droplet size distributions:

wAz cA,514z. (11)

In particular, for submicrometer water dropletclouds, they found a conversion factor ofc 5 g m2

sr. For an approximate analysis, Eq. 11 can beextended to hydroscopic aerosol particles to estimate

the liquid water content of PBL aerosols from thereported measurements of A,723z. In Fig. 7 wereport the vertical profiles of aerosol liquid watercontent wAz for the nine considered nights of mea-surements, as obtained through application of Eq.11. Values of the column liquid water content ofNBL aerosol WA were obtained by vertically integrat-ing the profiles of wAz up to zh. As expected, val-ues of WA Table 3 are found to be larger incoincidence with larger values of the height-integrated RH.

Aerosol average size characteristics can also be de-

termined by comparing experimental and theoretical values of the extinction-to-backscattering ratiok355z. Results from this approach can be com-pared with those derived from measurements ofA,723zA,355z. Theoretical values of theextinction-to-backscattering ratio k

can be computed

through application of the Mie-scattering theory bynumerically integrating

kzA,zA,z

rmin

rmax

r2Qext, r, mdNrdrdr

rmin

rmax

r2Qback, r, mdNrdrdr

,

(12)

where Qext, r, m is the Mie extinction efficiencyand all other parameters already defined in Eq. 8.

As in the case of Eq. 8, numerical integration isperformed over 2000 intervals of constant logarith-mic width, with rmin 0.001 m and rmax 20 m,assuming that the bimodal lognormal size distribu-tion holds. By means of a sensitivity study, k355z

was found to depend poorly on the size distributionparameters r1, 1, and 2 and to depend strongly onr2. This result supports the use of measurements ofk355z to retrieve r2.

Theoretical values ofk355 and k723 have been com-puted as a function of the mean radius for the accu-mulation mode r2 and are illustrated in Fig. 8. Notethat the overall variability of k

in the 355723-nm

spectral range does not exceed 15% in the size rangefrom 0.1 to 1 m. This theoretical result supportsthe hypothesis performed in Section 2 concerning thepoor wavelength variability of k

.

Table 3. Height-Integrated Values of A,723z, RHz, r2z, and wAz

for nine Selected Nights of Measurements

Date1997

Height-Integrated Values of

A,723 sr1 RH % r2 m wA g m

2

20 January 0.00065 52 0.1 0.003321 January 0.00122 57 0.1 0.006127 January 0.00452 68 0.4 0.022628 January 0.00312 80 0.1 0.015630 January 0.00229 59 0.1 0.0115

31 January 0.00763 67 0.1 0.038210 February 0.00934 61 Not available 0.046711 February 0.00395 67 0.45 0.019818 February 0.00070 36 0.1 0.003518 February 0.00091 42 0.1 0.0045

Fig. 7. Vertical profiles of aerosol liquid water content wAz for

the nine selected nights of measurement.



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Measurements of k355z, obtained from simulta-neous lidar measurements of A,355z and A,355zthrough the procedure defined in Section 2, were per-formed on six nights during the measurement cam-paign 31 January and 3, 10, 11, 17, and 18

February. All profiles of k355z display values inthe 198-sr range with larger values usually found atlower levels in four of the six cases. Such values ofk355z are consistent with aerosol particles charac-terized by an accumulation mode radius r2 in the0.11-m range. The mean vertical profile ofk355z, temporally averaged over six nights, displays

values not exceeding 70 sr, which are consistent withvalues ofr2 smaller than 0.4 m.

Measurements of k355z on three of the six nightsare among measurements of A,723zA,355z thatwere also performed on eight nights 31 January and11 and 18 February. Figure 9 illustrates measuredprofiles of k355z on these three nights. When ex-

perimental and theoretical values of k355z for 31January 97 are compared, it turns out that measured

values ofk355z, decreasing with height from 60 sr at450 m to 5 sr at 750 m, are consistent with aerosolparticles characterized by an accumulation mode r2decreasing from 0.3 m to less than 0.1 m. In thesame altitude range measured values of A,723zA,355z were found to decrease from 0.25 to 0.1,corresponding to values ofr2 decreasing between thesame size limits from 0.3 to 0.1 m. On 11 Febru-ary 1997 measured values of k355z were found todecrease from 95 sr at 420 m to 60 sr at 750 m and toincrease again to 75 sr at 1000 m. These values are

consistent with aerosol particles characterized by anaccumulation mode radius r2 decreasing from 0.8 to0.3 m and increasing again to 0.5 m. In the samealtitude range, values of A,723zA,355z werefound to decrease from 0.7 at 400 m to a value of 0.2at 750 m and to increase again to 0.4 at 1050 m,corresponding to values of r2 decreasing within ap-proximately the same size limits from 0.7 to 0.2 mand again 0.5 m. For 18 February 1997 the alti-tude range at which measurements of k355z areavailable is only partially superimposed on the ver-tical region where A,723zA,355z is measured

700 800 m. At these heights, values ofk355z donot exceed 30 sr, being consistent with values of r2around 0.1 m and with measured values ofA,723zA,355z, not exceeding 0.1.

4. Summary

Lidar measurements of the aerosol backscatteringcoefficient A,z in the NBL have been obtainedfrom elastic lidar signals at 355 and 723.37 nm

through the application of an iterative analysis pro-cedure based on the assumption of a constant value ofk

z within the NBL.Aerosol average size characteristics have been ob-

tained from measurements of both A,723z andA,355z and A,355z and A,355z. The mean ra-dius for the accumulation mode of a bimodal lognor-mal size distribution is retrieved by comparingmeasured and theoretical values ofA,723zA,355zand k355z with theoretical values for these twoquantities obtained through application of the Mie-scattering theory. In general, measured profiles ofA,723zA,355z and k355z are consistent with

aerosol particles characterized by an accumulationmode radius smaller than 0.4 m. A sensitivityanalysis has been performed to estimate the retrievalmethod dependence on the aerosol refractive indexand the size distribution parameters as well as on themeasurement error. Aerosol data have been alsopresented in terms of liquid water content.

Furthermore, to illustrate the typical behavior oflidar measurements as a function of meteorologicalparameters, for a selected case study 11 February1997 lidar measurements ofA,723z have been com-pared with simultaneous radiosonde data expressed

Fig. 8. Theoretical values of k355 and k723 as a function of themean radius for the accumulation mode r2 1 2 1.6.

Fig. 9. Vertical profiles of the aerosol extinction-to-backscattering ratio at 355 nm, k355z, for three nights during themeasurement campaign.



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in terms of potential temperature Tp and RH. Thepotential temperature profile appears to be anticor-related with both the RH and the aerosol backscat-tering, the aerosol layer top being associated with amaximum in Tp and a minimum in RH.

Appendix A: Experimental Setup

Measurements reported here were performed by thelidar system in Potenza Southern Italy, 4036N,

1544E, 820 m above sea level. The instrumentwas developed around a Nd:YAG laser source oper-ating on both second and third harmonics 532 and355 nm. The 532-nm beam is used to pump a dyelaser tunable within the 690730-nm spectral region.

Aerosol measurements reported in the present paperwere obtained from elastic backscatter lidar echoes at723.37 and at 355 nm and from anelastic backscatterlidar echoes at the molecular nitrogen Raman-shiftedwavelength 387 nm.

The receiver consists of a vertically pointing Cas-segrain telescope 0.5-m-diameter primary mirror;5-m combined focal length. Spectral selection wasperformed with monochromators placed in the tele-

scope focal plane and the selected radiation was de-tected by means of cooled photomultipliers. A moredetailed description of the lidar system in Potenza,together with a block diagram of the experimentalsetup, is reported in Ref. 28.

Measurements at 723.37 nm have been acquiredwith a vertical resolution of 3 m, whereas measure-ments at 355 nm have been characterized by a ver-tical resolution of 30 m. Because of the applicationof the photon-counting technique, measurementswere limited to nighttime. Lidar averaging timeranged from 30 to 90 min and are in the 4050-minrange in eight of the ten reported cases Table 1.

During the measurement campaign, radiosondeswere launched to coincide with lidar operation in or-der to obtain simultaneous profiles of the meteorolog-ical parameters of interest. These radiosondes Vaisala Model RS 80 provided measurements ofpressure, temperature, and RH with an accuracy of0.2 mbar, 0.2 C, and 1%, respectively. Radiosonde

vertical resolution depends on the balloon ascentspeed and on the sonde data transmission rate. Forthe current measurements the vertical resolutionranged between 40 and 250 m. Our radiosonde sys-tem does not provide wind speed measurements, soinformation concerning the Richardson number andthe depth of the turbulent layer cannot be obtained.

The authors thank Antonio Anastasio for technicalsupport. This research was supported by a contri-bution from Istituto Nazionale per la Fisica dellaMateria, through the project Tecniche ottiche inno-

vative per il monitoraggio ambientale e piani di tu-tela e risanamento and partially supported by theEuropean Union.

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