Indian Journal of Radio & Space PhysicsVol. 22, February 1993, pp. 42-49

A comparison of aerosol size distributions obtained from bistatic lidar andlow-pressure impactor experiments at a coastal station*

K Parameswaran & G VijayakumarSpace Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022

Received 9 November 1992

Size distribution of aerosols in the atmospheric mixed region is studied using a bistatic CW lidarand a low-pressure impactor. Results obtained from these two experiments are compared. CW lidarobservations showed that the size index (assumingthe size distribution to foUowa power law)gener-aUylies in the range 3.5-5, whereas the size index obtained using the low-pressure impactor general-ly lies in the range3.7-4.2.

1 IntroductionThe characteristics of aerosols in the first few

hundred metres above the ground largely dependon the local sources and sinks as well as on theprevailing meteorological conditions. Their char-acteristics vary from place to place depending onthe topography and location (urban, rural, marine,etc.) of the observation site. These aerosols, whichare significantly influenced by various natural pro-cesses and biological (including anthropogenic)activities, in turn, affect the terrestrial biologicalactivity to a great extent. For example, during itsatmospheric cycle, the sea salt aerosols in a ma-rine environment play a pivotal role in the me-chanism of precipitation, in the reduction of ma-rine optical and infrared transmissivity, in theocean to air transport of bacteria, etc. Hence astudy of their physical and chemical characteris-tics is very important.

Different experimental methods are currently inuse for the study of aerosols. But no single exper-iment can provide information on all the aerosolparameters. Remote sensing methods are verypromising because of their operational conveni-ences as well as due to the fact that they do notdirectly affect the aerosol parameters being mea-sured. They can also be used to study the aerosolcharacteristics at different altitudes as well as atremote places. However, the remote sensingmethods depend upon certain reduction algo-rithms to deduce aerosol characteristics from

'"This paper was presented at the National Space ScienceSymposium held during 11-14 March 1992 at PhysicalResearch Laboratory, Ahmedabad 380 009.

measured quantities. On the other hand samplingmethods have the advantage of direct measure-ment of the desired parameters. It is often neces-sary and useful to compare the characteristics ofaerosols obtained by different methods to achieveconsistent results.

Lidar is a valuable tool for sensing remotely theatmospheric aerosols. When operated in bistaticmode!" it can provide information on scatteringas a function of scattering angle, which is a veryimportant input in the deduction-" of aerosolcharacteristics. Size distribution of aerosols in theatmospheric boundary layer can be studied fromthe angular scatter measurements. In deducing theaerosol characteristics from these measurements, itis necessary to assume the basic form of the sizedistribution function. Usually a power law-typedistribution is assumed for this purpose. Directcollection of particles by sampling can provide in-formation on aerosol size distribution. The sizeand number distribution of atmospheric aerosolsnear the surface employing this technique hasbeen the subject of a large number of investig-ations!". Various sampling methods? like impac-tiori'"!', filtration, and centrifugation are employ-ed for this purpose. At Trivandrum (8SN, 77°E),a coastal station, we have carried out investig-ations on aerosol size distribution using bistaticlidar and direct particle sampling methods, andcompared the results. In this paper, we presentthe results of this study.

2 Experimental set up and method of measure-ment

The bistatic CW lidar system consists of an ar-

PARAMESWARAN & VIJAYAKUMAR: AEROSOL SIZE DISTRIBUTION FROM UDAR & IMPACTOR 43

gon-ion laser transmitter and a transmission type(300 mm diam.) receiving telescope arranged inthe same horizontal plane separated by a fixeddistance of 380 m. The details of the lidar systemand principle of operation are described else-where". For conducting angular scatter observ-ations, the transmitter and receiver are scanned inelevation at convenient steps such that the scat-tered radiation from a fixed altitude, 190 m, is re-ceived for different scattering angles". Eventhough the range of possible scattering angles inthis case is 90°-180°, angles above 165° are notused to avoid effects of any spatial inhomogeneit-ies due to the large extent of the scattering vo-lume. The angularly scattered intensity is mea-sured for different scattering angles. Figure 1shows the typical variation of the received signalwith scattering angle for two days (23 Nov. 1989and 23 Apr. 1992). The vertical lines parallel toY-axis indicate the standard deviation of the sig-nal representing the fluctuating component. Thenormalized signals corrected for the variation inscattering volume and path-lengths, which form thebasic data for the present investigation, are ob-tamed" from these angular scatter signals.

The aerosols close to the surface are directlysampled using a low-pressure impactor (LPI). Thesystem (Andersen LPI Model 20-900) essentiallyconsists of 14 collection stages':' with 6 sizeranges below 1.0 ,urn. Each stage contains a num-ber of perforations with fixed diameters. The aircontaining aerosols is forced through these noz-zles at a very high velocity and allowed to im-pinge on a flat surface (collection substrates)where some or all of the particles above a givenmass (or diameter) are collected. The size of thenozzles determining the speed of the 'air at a flowrate decreases in subsequent stages from dimen-sions of several millimetres at the initial stages totenths of millimetres at the final stage. Each col-lecting stage has a sigmoidal characteristics: smallparticles can pass with no deposition, a certainsize range designated as cut-point for that stage iscollected with 50% probability and significantlylarger particles are collected completely. For agiven pressure drop, the velocity through the noz-zle will be fixed and hence the cut-point of thatstage also will be fixed. Thus each stage of theimpactor has a fixed cut-point for a given orificepressure drop, which is different for differentstages. These cut-points for different stages,shown in Table 1, range from 0.08 ,urn to 35 zzm(aerodynamic diameters) if the system is operatedat an orifice pressure drop of 114 Torr. Whenproperly cascaded, each stage collects particles

0.01

>~-t:lc

.~

'" 0.20•..CI 23Apr.1992"0...J

0.16

0.05

0.0 I.

0.03

0.02

0.12

0.08

0.0 I.

23 Nov.1989

80 100 11.0 160120

Scattering angle,degfig. 1 - Variation of angularly scattered intensity from a fixedaltitude (190 m) with scattering angle for 23 Nov. 1989 and

23 Apr. 1992.

having aerodynamic diameters between its cut-point and that of the previous stage. These parti-cles are collected on fibre glass substrates loadedin each of these stages. Mass of the particles col-lected in each size range is estimated by weighingthe substrates before and after the collection usingan electronic micro balance which can give themasses up to an accuracy of ± 5 ,ug. As the sys-

44 INDIAN J RADIO & SPACE PHYS, FEBRUARY 1993

Table I - Cut-points of different collection stages of the low-pressure impactor (LPI)

Stage Cut-point Size range MeanDpPaerodynamic (aerodynamic ,um-ATM

diameter diameters) of,urn particles collected

,urn0 35.0 > 35 > 35.1

I 21.7. 21.7-35 28.5

2 15.7 15.7-21.7 18.8

3 JO.5 10.5-15.7 13.2

4 6.6 6.6-10.5 8,7

5 3.3 3.3-6.6 5.1

6 2.0 2.0-3.3 2.8

7 1.4 1.4-2.0 1.8

t, 0.90 0.9-1.4 0.854

~ 0.52 0.52-0.9 0.176L.J 0.23 0.23-0.52 0.118

L4 0.11 0.11-0.23 0.077t, 0.08 0.08-0.11 0.056

4 <0.08 <0.052

DpP Product of aerodynamic diameter (Dp) and atmosphericpressure (P )

ATM Atmospheres

tern is operated at a constant orifice pressure, thevelocity of the particles in each stage is known,using which the Stokes diameter can be estimated.From the mass of the aerosol particles collectedin each size range thus obtained, the mass distrib-ution, dm(r)/dr (r being the particle radius), is ob-tained. The number-size distribution, dn(r)Klr, ofaerosols is estimated from the mass distributionas14•15

dn(r)dr

3 dv(r)4nr3 dr

3 dm(r)4nr3p dr ... (1)

where p and d v(r)/dr are respectively the meandensity" and volume distribution of the aerosolparticles.

The sampler (LPI) is operated few metres ( - 5m) above the ground (so as to avoid surface ef-facts) for - 40 h. This sampling time is long enoughto collect measurable samples and also smallenough to avoid any saturation effect leading to par-ticle bouncing. This sampling time is arrived at bytrials satisfying these criteria. The collection subs-trates are properly pre-conditioned, and desiccated

before and after the collection to eliminate the effectdue to direct condensation of atmospheric watervapour.

3 Results

3.1 Aerosol size distribution from angular scatter observationsThe method of obtaining aerosol size index

from the angular scatter observations, presentedin detail elsewhere by Parameswaran et ai.4•12, es-sentially assumes that the air molecules in the at-mosphere are isotropic and do not scatter any in-tensity in the orthogonal direction, when the inci-dent beam is linearly polarized with the electricvector orientation along the scattering plane(which is true for the lidar configuration used forthe present study). In this case for a given scatter-ing angle 0, the ratio of differential angular scat-tering .cross-section of aerosols aa( 0), to the nor-malized signal kf. 0), can be written as"

aa( 0) = aaO _ n", am( 0)kf. 0) ko n; kf. 0)

... (2)

where am( 0) is the differential angular scatteringcross-section of molecules for the scattering angle0, aaO is the differential angular scattering cross-section of aerosols for 0= 90°, ~) is the normal-ized signal for 0= 90°, and nm/ na is the ratio ofthe molecular number density to aerosol numberdensity at the altitude (190 m in the present case)of scattering volume. The scattering angle 0 ischosen such that the altitude of scattering volumeis the same as that for 0= 90° (if there is any dif-ference, this factor is to be accounted'). If aa( 0)/kf. 0) is plotted against am( 0)/ kf. 0) for different va-lues of 0, the resulting curve will be close to astraight line if values of aa( 0) used are appropriatefor the aerosols present in the scattering volume,and slope of this line will be nm/ na at 190 m. Thisproperty of Eq. (2) is used to evaluate the aerosolsize index (v) by making mass plots of the two ra-tios for different scattering angles and estimatingthe best-fit straight line. Different combinations ofaerosol size distribution (power law indices) andrefractive index (m) are used for this purpose.The differential angular scattering cross-sectionfor each scattering angle evaluated for differentcombinations of aerosol size distributions (powerlaw size index in the range 2-6) and refractive in-dex (in the range 1.33-1.6) employing Mie theoryis used to calculate the ratio aa( 0)/ kf. 0). For eachcombination of v and m, the cross correlation co-efficient (R) is evaluated. When the assumed com-

PARAMESWARAN & VIJAYAKUMAR: AEROSOL SIZE DISTRIBUTION FROM LIDAR & IMPACTOR 45

bination (of aerosol parameters) is close to thereal size distribution and refractive index of theaerosols present in the scattering volume, thecorrelation will be maximum (negative) and theintercept C will be close to the observed ratio ofthe differential angular scattering cross-section ofaerosols to the normalized signal at ()= 900

• Tojudge the fitness of the intercept a ratio ~ is de-fined such that

~ = (o.c/~) - C)/(Oall/ ~)) ... (3)

The combination of size distribution and refrac-tive index for which the correlation coefficient (R)is highly negative and I~I is minimum is taken asthe one consistent with our observations. Follow-ing this procedure, the values of (v and m) for theobservations on 23 Nov. 1989 and 23 Apr. 1992are found to be (5 and 1.5) and (4.0 and 1.65) re-spectively. Using the above method, the aerosolsize index is obtained on days on which angularscatter observations are conducted using the bis-tatic lidar for different days in the period 1985-92. These are presented in Table 2. The values ofv lie in the range 3.5-5. The most probable valueof v is found to be 4.5, which is the same as re-ported by Parameswaran et al.4

3.2 Size distribution from impactor experimentCollection of particles using LPI is made on fi-

bre glass substrates. The substrate which is to beloaded in each of the impactor stages is identified

Table 2 - Values of size index obtained from CW lidar experi-ment

Date21 Mar. 1985

10 Apr. 1985

9 Aug. 1985

2 Sep. 1985

9Sep.1985

4 Oct. 1989

9 Nov. 1989

23 Nov. 1989

16 Jan. 1990

14 Feb. 1990

16 Aug. 1990

11 Oct. 1990

14 Nov. 1990

23 Apr. 1992

Size index (v)

4.5

4.5

5.0

4.5

4.5

3.5

4.5-5.0

4.5-5.0

4.54.5

3.5-4.0

4.0

4.5

4.0

and kept in separate Petri dishes having identifica-tion marks. These substrates are pre-conditionedby first keeping them in a hot air oven (at lOO°C)for about 1 h and then desiccating until they at-tain the room temperatur.e. The initial mass ofeach substrate is then measured using a microbal-ance with an accuracy of ± 5 J.lg. These are thenloaded at the respective stages of the impactorand taken to the sampling site ( - 5 m above thesurface). The pump is switched on and the pres-sure at the critical orifice quickly adjusted to 114Torr, which gives a flow rate of 3 litres of air perminute through different stages of the impactor.The sampling is continued for about 40 handthen terminated. The collection substrates arecarefully removed from each stage of the impac-tor and are kept in the respective dishes. Thesecollected substrates are desiccated for about 24 hbefore the final weighing. Substracting the initialmass of each substrate from the final mass, themass of aerosol particles collected in each sizerange defined by the impactor cut-points are ob-tained. The particle size referred here is the aer-odynamic diameter, which is defined as the di-ameter of a unit density sphere which would be-have in the impactor in the same way as the parti-cle (i.e., the diameter of a unit density sphere withthe same terminal settling velocity as the particle).

From the mass of aerosol particles collected ineach stage, the cumulative percentage mass foraerosols smaller than a particular size (CPMS) isestimated. Typical plot of CPMS as a function ofeffective cut off aerodynamic diameter on a log-probability graph for two samples (April 1991and March 1992) is shown in Fig. 2. The massmedian diameter is the particle size for which theCPMS is 50%. This is - 3.4 J.lm for April 1991and - 8 J.lm for March 1992. This shows that50% of total mass collected is below 3.4 J.lm sizein April 1991 whereas it is below 8 f.J.min March1992. The CPMS curve is more or less linear forApril 1991 but it shows small undulations forMarch 1992.

The Stokes diameter of a particle is defined asthe diameter of the sphere with same density asthe particle which would behave in the impactorin the same way as the particle (i.e., the diameterof a sphere of the same density as the particlewith the same terminal settling velocity). From theaerodynamic diameters the Stokes diameter whichis closer to the physical diameter of the particlecan be calculated if the actual density of the aero-sol particles is known. This also requires the aer-osols to be homogeneous as to density. This is a

46

E::J."'I.....

OJ

~ 10·0E","0

~ 1.0E",c>.-g 0.1I.....OJ

<0.01

INDIAN J RADIO & SPACE PHYS, FEBRUARY 1993

100(1) APRIL 1991(.2) MARCH 1992

o 50 100(PHS

Fig. 2 - Cumulative size distribution curve for aerosols ob-tained using the low-pressure impactor during April 1991 and

March 1992.

fairly valid assumption for natural atmosphericaerosols which can be taken to be a homogeneousmixture of particles produced from one or moresources. As the chemical composition of the aero-sol particles is not known, using the available in-formation on particle composition 17 near the sur-face at a coastal station like Trivandrum, themean density of the particles can be approximat-ed to 2.5 g cm - 3 (Ref. 18). This value of p is usedin our calculations.

The cut off value of 0rP (the product of aero-dynamic diameter, 0P' and the atmospheric pres-sure, P, in which the LPI is operated incorporat-ing the correction for slip factot) correspondingto one atmospheric pressure (P~ 1 atm.) is fur-nished by the manufacturers for each LPI stage,using which the mean value of OpP (average ofthe cut off OpP for a particular stage. and that ofits previous stage) is obtained. This mean value of0fP for each stage, presented in the last columno Table 1, is divided by particle density (p) toobtain the equivalent aerodynamic term DeP, andthe Stokes diameter term (OsP) is estimated usingthe impactor calibration chart (shown in Fig. 3)provided by manufacturers for the critical orificepressure of 114 Torr. As the low pressure stagesLI to L; are operated at 0.15 atm., DsP of thesestages obtained from the chart is divided by 0.15to get the true Stokes diameters (Os). But for theother stages (0 to 7) which are operated at P= Iatm., the mean Stokes diameters are the same as

10c > ,

~ 1.0

<~0.-

Q.

a 0.1

0.011 , ,""""-;1 , ,.) , . Ii , "Ii. "1~0.0001 0.001 0.01 0.1 1.0

De P,j.Jm-ATM

Fig. 3 - Conversion chart for Stokes diameter from aerody-namic diameter.

the DsP values of the respective stages. DividingDs by 2, the Stokes radius, which is more closerto the geometrical radius of the particle (hereinaf-ter will be referred to as the particle size), is esti-mated. Out of the 14 stages of the LPI excludingthe first (0) and last (4) terminal stages, the re-maining 12 size ranges (stages 1 to ~), which areused in our present study, have mean Stokes radii(in .urn) of 11.7, 7.7, 5.4, 3.5, 2.0, 1.0, 0.67, 0.31,0.22, 0.11, 0.073 and 0.033 respectively. In thesame way the Stokes radius ranges (dr and dlogr)for each size range, which are necessary for ob-taining the mass-size distributions and number-size distributions, are also evaluated for eachstage of LPI.

From the mass of aerosol particles collected ineach stage, dm: r), the mass distribution with parti-cle size is obtained by plotting dm:r)/d log r versesr. Figure 4 shows the mass distribution with parti-cle size during April 1991 and March 1992 forwhich the CPMS is shown in Fig. 2. The verticallines parallel to Y-axis represent the error due touncertainty in weighing the substrates (i.e., ± 5 .ugin dm). As the LPI is operated at a constant flowrate of 3 litres/min for 40 h during each sampl-ing cycle, the values of d m: r) /d log r for each stageis the mass of aerosol particles contained in theradius interval rand r+ dr, contained in 7200 li-tres of air. The mass distribution per unit volumeof air can then be obtained by dividing dm:r)/drwith this volume of air. This is marked on theright hand side axis of Fig. 4.

The mass distribution shows a clear minimumaround 2 .urn in April 1991 whereas the minimumaround 2 .urn in March 1992 is rather broad. Ex-cept for two minor peaks (at r = 0.1 .urn and r = 3.urn) the mass distribution remains almost con-stant with r in March 1992. The general form ofthe mass distribution curve for April 1991 is al-most similar to that obtained by Khemani et al.19

PARAMESWARAN & VIJAYAKUMAR: AEROSOL SIZE DISTRIBUTION FROM LlDAR & IMPACfOR 47

2.4

2.0

1.6

1.2

"" 0.8E.:""0"D<,

KARCH 1992~ 0.4E"D

0.3

0.2

0.1

300

200

"""E100 g'

•...0\o

'"60 ~•...50 .g

40

3020

10

0.01 100.1Particle radius, um

Fig. 4 - Typical mass distribution of aerosols obtained usingthe low-pressure impactor during April 1991 and March

1992.

for Poona, whereas that in March 1992 shows de-viations. It may also be noted that the absolutevalue of dm(r)/dlogr in April 1991 is 5 to 10times more than that during March 1992.

From the values of dm(r) for each LPI stage,dm(r)/dr is estimated. From this the number-sizedistribution, dn:r)/dr, is obtained employing Eq.(1). Figure 5 shows these distributions for differ-ent months during the period 1990-92. Eachcurve in this figure represents the average sizedistribution of near surface aerosols spread over aperiod of 8 to 10 days in that month as thesampling time is spread over this period. In gener-al, the number of particles decreases with increasein particle size, approximately following a powerlaw. The size index corresponding to each ofthese distributions is estimated by least-squaremethod considering all the points in the sizerange 0.03-11 p.m which is almost the same asthat assumed for obtaining size index from angu-lar scatter observations of CW lidar'-. The valuesof the size index for different months obtained us-ing LPI are presented in Table 3. The size distrib-ution curves also show small undulations (markedby vertical arrows in Fig. 5) indicating the pres-ence of one or more modes in addition to thegeneral power law behaviour. The values of r onwhich the mode appears in different months is al-so presented in Table 3. In all the months except

Table 3 - Aerosol size index, preferred modes and surface num-ber density observed from the analysis of LPl data

Month Size index Modes Surface aerosol#ffi number density

X 104cm-3

August 1990 4.05 ±0.08 0.7 8.75

November 1990 4.22±0.21 0.7 8.64

February 1991 3.86±0.1l 0.7 8.26

March 1991 3.97±0.24 0.1,3.5 3.14

April 1991 3.97±0.04 0.7 4.94

July 1991 3.83±0.07 0.7 4.43

November 1991 3.80±0.14 0.7,3.5 5.96

December 1991 3.80±0.21 11.6

March 1992 3.91 ±0.12 0.1,3.5 1.51

March 1991 and March 1992, this mode general-ly appears around 0.7 p.m. In April 1991 and De-cember 1991 the mode is rather weak. DuringMarch 1991 and March 1992 more than onemode appear-one close to 0.1 p.m and the otherclose to 3.5 p.m. By integrating the size distribu-tion curve between the limits 0.03 p.m and 11.7p.m, the total number of aerosol particles sampledis estimated. The numbet density of aerosols nearthe surface, obtained by dividing this total numberof aerosols with the volume of air sampled ineach sampling cycle, is also presented in Table 3.The near surface aerosol number density is maxi-mum during winter months and minimum duringMarch to July. Khemani et al'", from their impac-tor measurements at Poona, reported a similarfeature-the total mass of particles collected be-ing maximum in winter months.

4 DiscussionTables 2 and 3 provide a direct comparison of

the size index obtained using remote sensing (lid-ar) and direct sampling (LPI) methods. The valuesof the size index obtained by CW lidar experi-ment for different days during the period 1985 to1992 lie generally in the range 3.5-5. These va-lues are obtained from the angular scatter mea-surements conducted during a time interval of -30 min at an altitude of - 190 m. Similarly thevalues of size index obtained from direct samplingtechniques during the period 1990-92 lie in therange 3.7-4.2.

The CW lidar measures the size distribution ofaerosols remotely at an altitude of - 190 m. Thesize distribution is obtained from the angularlyscattered intensity, assuming horizontal stratifica-tion (within a few hundreds of metres) of spheri-cal aerosol particles. The basic form of size dis-

48 INDIAN J RADIO & SPACE PHYS, FEBRUARY 1993

210

-410

MAR. 1991 NOV. 1991AUG. 1990

210

E NOV. 1991 APR. 1991=:!, DEe. 1991;;;-....'E=:!, -4.. 10'--u

"L.

C-u

210

FEB. 1991 JUl. 1991 MAR. 1992

-410

-110

0.01 0.1 1.0 10 100 0.1 1.0 10 100 0.1 1.0 10 100Par+lcl e radius, urn

Fig. 5 - Size distribution of aerosols from the low-pressure impactor experiment for different months during the periodAugust 1990-March 1992.

tribution is assumed to be of power law type. Thesize range of aerosols which contributes for scat-tering in the lidar wavelength is between 0.1 .urnand 5.um. Even though the inversion assumesaerosol size distribution from 0'.02 .urn toJfl ps»,aerosols in the size range 0.1-5 .urn will be mainlyaffecting the estimated size distribution. As thecomplete measurement process takes only half anhour it can be considered to be almost an instan-taneous measurement. The LPI measurementsgive the average size distribution close to the sur-face over a period of - 40 h spread over 8 to 10days. Particles are assumed to be homogeneoushaving a constant density of - 2.5 g em - 3. Thus,the involved assumptions and uncertainties in thetwo measurements are quite different. Withinthese limitations the general agreement between

the size indices obtained from these two tech-niques is quite satisfactory. The assumption onthe general form of the size distribution (powerlaw) for CW lidar measurements seems to bequite reasonable as indicated by the LPI measure-ments.

The results obtained from these two experi-ments (though corresponding to different days)show that size distributions, in general, are quitesimilar at the two altitudes (5 m and 190 m). Thismeans that in this altitude range the aerosols arewell mixed and height of the mixing region in theearly night hours ( - 1900 hrs) is more than 190m. The altitude profiles of aerosols obtained fromCW lidar experiment indeed show that the heightof the mixing region is generally greater than 200m (Ref. 12). The value of size index obtained in

PARAMESWARAN & VIJAYAKUMAR: AEROSOL SIZE DISTRIBUTION FROM LIDAR & IMPACTOR 49

the present investigation also matches well withthe values reported by Pasceri and Friedlander",and Blifford and Ringer" for continental tropos-pheric aerosols at lower altitudes by direct sampl-ing technique. Goel et a[.2°. studied the size dis-tribution of aerosols in the size range 0.2-5.0 /-lmusing a four-stage impactor at Roorkee, and ob-served that the size index generally lies in therange 4-5. It may be noted that the present com-parison can be considered to be direct only forAugust 1990 and November 1990 and may be forMarch-April 1992. For these months there is a sa-tisfactory agreement between the size indices ob-tained from the two experiments. For the othermonths, LPI and lidar data do not correspond tothe same month. So in these cases the generalform of the size index obtained from these twoexperiments only can be compared. The values ofv obtained from LPI seems to be, in general,slightly on the lower side as compared to thoseobtained using CW lidar. This is quite reasonableas the number of large particles will be more nearthe surface compared to at 190 m, resulting in adecrease of the size index.

Even though the general form of size distribu-tion in Fig. 5 can be best approximated to a powerlaw, as noted earlier, it shows small undulations.Thus these distributions can also be representedas a combination of one or more log-normal dis-tributions or a combination of power law withlog-normal distributions. In such cases, the moderadius of these log-normal distributions will ap-pear as cusps in the resulting distribution. Theundulations in Fig. 4 can be attributed to this.The value of r for which the mode appears ineach month is tabulated in Table 3. From this it isseen that the mode around 0.7 /-lm appears to bepresent during all the months except duringMarch. This matches very well with the large par-ticle mode seen in the solar radiometer measure-ments " at Trivandrum. The small particle modeobserved in solar radiometer derived size distrib-utions (- 0.3 zzrn) is not seen in the LPI derivedsize distributions. This indicates that particlescausing the mode at 0.7 /-lm in solar radiometermeasurements are mainly confined to the atmos-pheric mixed region whereas the small particlemode would have been caused by particles in thefree troposphere above. During the month ofMarch the mode around 0.7 /-lm becomes weakwhereas a small particle mode at - 0.1 zzrnand alarge particle mode around 3.5 f..lID appear. Thisfeature is observed in March 1991 as well as inMarch 1992. The mode during March 1992 israther weak. Thus, the size distribution near the

surface shows variation with season. A larger database is required to quantify these seasonal var-iations. However, these finer details of size dis-tribution (presence of modes) cannot be delineat-ed from bistatic lidar observations.

AcknowledgementsThe authors thank Dr B V Krishna Murthy for

his valuable suggestions during the preparation ofthis manuscript and Dr K Krishna Moorthy andMr P Pradeepkumar for their excellent co-opera-ion in conducting the CW lidar experiment. Theyare also thankful to the Analytical and SpectroscopyDivision of VSSC for its valuable support in pre-conditioning and weighing the substrates.

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