All 3 REMOTE SENSINO OF THE OPTICAL AM PHYSICAL DENSTIS OF fSoKEO A O RCLDmSIUI SRI INTERNATIONAL MUNLO

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NATION~AL BUftEAtU OF STANDARDS -1963-A

DISTRIBUTION UNLIMITED

REMOTE SENSINGOF THE OPTICALAND PHYSICAL DENSITIES

44P OF SMOKE, DUST,Cq AND WATER CLOUDS

mFinal Report

December 1982

By: Edward E. Uthe, DirectorRemote Sensing ProgramAtmospheric Science Center

Prepared for:

Army Research OfficePost Office Box 12211Research Triangle Park, North Carolina 27709Attn: Dr. Walter Flood, Director

Geosciences Division

ARO Contract DAAG-29-80-C-0g3

SRI Project 1335

-a CJTI i3SRI International A333 Ravenawood AvenueMenlo Park, California 94025(415) 859-6200TWX: 910-373-2046

or' Telex: 334 486

0a OO 0 0?

THE FINDINGS IN THIS REPORT ARE NOT TO BECONSTRUED AS AN OFFICIAL DEPARTMENT OFTHE ARMY POSITION, UNLESS SO DESIGNATEDBY OTHER AUTHORIZED DOCUMENTS.

i

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SZCURITY CLASSIFICATION OF THIS PAGE (lUhen Dle Enter*ed

REPORT DOCUMENTATION PAGE BEFORE MSLR IG ORMI. REPORT NUMBER 2. GOVY ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

REMOTE SENSING OF THE OPTICAL FinalAND PHYSICAL DENSITIES Oct 79 - Oct 82OF SMOKE, DUST, AND WATER CLOUDS. 6. PERFORMING GRI. REPORT NUMBER

SRI Proiect 13357. AyJTHOR(e) 4. CONTRACT OR GRANT NUMBER(e)

Edward E. Uthe, DirectorRemote Sensing Program oAtmospheric Science Center DAAG-29-80-C-003

9. PERFORMING ORGANIZATION NAME AND ADDRESS I0. PROGRAM 9EMMr. PROJECT. TASKAREA A WORK UNIT NUMBERS

SRI International333 Ravenswood AvenueMenlo Park California 94025

I. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

US. Army Research Office December 1982Post Office Box 12211 IS. NUMBER OF PAGES

Research Triangle Park. North Carolina 27709 3614. MONITORING AGENCY NAME & ADORESS(II different from Coltroling Office) 15. SECURITY CLASS. (of this report)

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Approved for public release; distribution unlimited.

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II. SUPPLEMENTARY NOTES

The findings in this report are not to be construed as an official Department of the Army position, unlessso designated by other authorized documents.

IS. KEY WORDS (Continue on reverse side if necese, and Identify by block number)

"11 LidarAerosol propertiesMultiwavelength observationsBackscatterTransmission

20. ATRAcr e"cmta m evm ad N eee-, mm! idtiy by bleek ambmb)

A new van-mounted four-wavelength lidar system designed for remote aerosol characterization has been constructedand tested. The most important design criteria established for the new systems was that the laser pulses should viewthe same path and be emitted in a minimal time period in order to reduce effects of time-variant aerosol distributionson multiple-wavelength observations. -

DO F=73 W3 TT s aur ' O, 1y ss oSOfLeTG Ulind8ECUMTY CLASSIFICATION OF THIS PAGE (Wnm Date Entered)

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Unclassified fn/ b * /Tm

SECURITY CLASSIFICATION OF THIS PAGE(US Dog& Zntwa' -

;','Energy at 033- and 1 .06-pttrwavelengtzs it transmitted into the atmosphere using a Nd:YAG laser; 3.8 #P istransmitted using a DF laser, and 10.6 Oins transmitted using a CCo laser. An optical arrangement provides fortransmitting the pulses along the same horizontal path, coaxial with a 12-inch-diameter Newtonian telescope.Wavelength-separation optics direct received energy to separate detectors for 053- and 1.06-wenergy and to acommon detector for 3.8- and 10.6-.91 energy. By deriving both 0.53- and 1.06-.m laser emissions from a singleflash lamp excitation, and by recording the four backscatter signatures within a single transient digitizer 2048-wordrecord, the four signatures can be collected within a 160-ps interval. Each four-wavelength lidar record is transferredto an LSI 11/2 microcomputer and written on nine-track magnetic tape, together with date and time information.

SRI conducted a field test of the four-wavelength lidar to evaluate system performance in an environment similar tothat of previous lidar operations at Army range facilities. The lidar van was located about 600 m from an 8-ft squarepassive reflector Both dust and smoke aerosols were generated near the midpoint of the lidar-to-target path. Thelidar successfullylecorded the time history of aerosol range-resolved backscatter at each of the four wavelengths.Target returns were used to evaluate time records of transmission at each of the four wavelengths. Results indicatedthat the lidar system is nearly ready for participation in Army field tests to further explore its use in remote aerosolcharacterization.

UnclafledSaCUMITY CLASSIFICATION OF THIS PAGE(Wh bDate 3flae

CONTENTS

LIST OF ILLUSTRATIONS................... . . .... ... .. .. . . .....

I STATEMENT OF THE PROBLEM......... ...... . ... .. .. .. ......

II FOUR-WAVELENGTH LIDAR SYSTEM. .................. 3

III SYSTEM TEST AND RESULTS ........................7

IV CONCLUSIONS ............................ 13

V LIST OF PUBLICATIONS ...................... 14

VI PARTICIPATING SCIENTIFIC PERSONNEL. .............. 15

APPENDICES

A LIDAR EVALUATION OF SMOKE AND DUST CLOUDS .. ............ 16

B PARTICLE SIZE EVALUATIONS USING MULTIWAVELENGTHEXTINCTION MEASUREMENTS. .................... 25

C AIRBORNE LIDAR MEASUREMENTS OF SMOKE PLUMEDISTRIBUTION, VERTICAL TRANSMISSION,AND PARTICLE SIZE. ....................... 32

or

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AviabilitId/Oii ilitiB1.0alanlo

ILLUSTRATIONS

1 Block Diagram of the Four-Wavelength Lidar System

for Remote Aerosol Measurement ....... ............... 4

2 Four-Wavelength Lidar System in 6-Meter-Long Van ... ...... 5

3 System Test Environment at Stanford University .... ....... 8

4 Four-Wavelength Lidar Backscatter Signaturefor Dust Generated Along 600 m PathBetween Lidar and Target ......... .................. 9

5 Detector Calibration Results for SRIFour-Wavelength Lidar ......... ................... 10

6 Four-Wavelength Lidar Backscatter Signaturesand Smoke Attenuation Derived from Target Returns

Using Preliminary Calibrations .... ............... . ... .11

7 Lidar Test Transmission History .... .............. ... 12

Iii

I STATEMENT OF THE PROBLEM

A major concern to developers of Army electrooptical systems is

system performance in the presence of battlefield obscurants such as

smoke, dust, and naturally occurring haze and clouds. To evaluate obscu-rant effects, a series of environmental tests are being conducted to pro-vide data on aerosol properties along paths viewed by optical instrumentsused to measure transmission, scattering, and path radiance for wave-length regions used by electrooptical systems.

SRI International (SRI) has been investigating the use of laser

radar (lidar) to measure optical and physical properties of obscurantsover extended areas--especially in the vertical--with spatial and tempo-ral resolution not possible with in-situ sensors. The first experiments

used single-wavelength lidar systems to measure variability of aerosol

concentration distributions along horizontal optical paths. Analysis ofbackscatter signatures in terms of absolute aerosol optical and physicaldensities was only partially successful. Use of a constant-reflectivitypassive target at the end of the lidar-observed path provided the means

to derive time-dependent transmissions over the lidar-to-target path.

However, the goal of the lidar program is to develop a method capable ofmaking single-ended measurements of optical or physical density so that

vertical and slant paths can be evaluated.

Absolute optical and physical-density analysis of backscatter sig-

natures was limited on previous studies because accurate correctionsfor attenuation and multiple-scattering could not be determined. Attenu-ation correction was applied by using a solution of the single-scattering

lidar equation that required assumptions on the backscatter-to-extinctionrelationship and on a boundary value of extinction along the observedpath. Results from these tests indicated that additional information on

the nature of the scattering medium was needed in order to derive usefulestimates of optical and physical densities from single-ended lidar mea-surements. In addition, information on the importance of multiple scat-tering would be required to assess the usefulness of the single-scattering

analysis approach and to formulate analysis methods that consider higherorders of scattering.

Upon sponsorship of the U.S. Army Research Office and U.S. Army

Atmospheric Sciences Laboratory, SRI assembled a 0.7- and lO.6-pnm dual-

wavelength lidar system and used the lidar to observe dust and smoke

aerosols during the DIRT I and Smoke Week II field programs (Appendix A).

Results from these tests showed that the wavelength dependence of back-scatter and attenuation provided information on particle size, and

therefore that smoke particles could be distinguished from larger-sizeddust particles.

1 m m ,,m m m I s _1 I

For dense smoke clouds, the longer wavelength was less affected byattenuation and multiple-scattering; a higher correlation was thereforeobtained between lidar-measured backscatter and extinction and aerosol

concentration terms. However, at both wavelengths, relatively largedata-point scatter and nonlinear relationships occurred in plots of path-integrated backscatter against optical depth (as derived from targetreturns). Theoretical considerations indicated that the scatter could bea result of changing particle sizes, and that observations in the 3.0-to 4.0-pm wavelength region might be less sensitive to particle-sizevariability. Because the extinction-to-mass-concentration ratio may beless sensitive to changes of particle size in this wavelength region,estimates of physical density from lidar backscatter signatures may bemore valid.

Two other studies were recently performed at SRI to provide an eva-luation of multiple-wavelength lidar techniques for characterizing thescattering medium. One study (Appendix B) used a 14-wavelength transmis-someter to make extinction measurements of aerosols generated with par-ticles of different composition, size, and shape distributions. Thecollected data indicated that mean particle size smaller than 1-pm diame-ter could reasonably be estimated from measurements made using a single-laser lidar system operating at 0.53- and 10.6-pm wavelengths. Forlarger mean particle sizes, the extinction ratio is near one and longer-wavelength systems are required. The data indicated that a two-laserlidar operating at 10.6 and 0.53 pm could provide estimates of mean par-ticle size to diameters of at least 6 pm.

The second study (Appendix C) applied the results of the multiple-wavelength transmissometer study (discussed above) and two-wavelength(0.53 and 1.06 Pm) airborne lidar measurements to infer the mean particlesize of forest fire smokes from a remote distance. Although in-situverification data were not collected, the lidar particle-size resultsagreed well with previous in-plume sampling of combustion aerosols.

The objective of the program reported upon herein was to develop a3- to 4-pm wavelength lidar system to be used in conjunction with the

two-wavelength system used on earlier studies, and to evaluate the capa-bilities of the three-wavelength system for making quantitative measure-ments of aerosol optical and physical densities. However, subsequentanalysis of data collected during Smoke Week II (see Appendix A) showedthat the lidar observations of Army-generated smoke and dust clouds atdifferent wavelengths must be made along the same path--rather than sepa-rated paths as was necessary with the 0.7/1.06-pm wavelength system.

It was therefore determined that a new lidar system using coaxial

multiwavelength laser emissions would greatly facilitate the developmentof remote aerosol characterization techniques. This report describes thefour-wavelength lidar system that was designed and constructed and pre-sents some preliminary results obtained through observation of smoke and

dust aerosols.

2

II FOUR-WAVELENGTH LIDAR SYSTEM

The most important design criteria established for the multiwave-length lidar was that the laser pulses should view the same path and beemitted in a minimal time period in order to reduce effects of time-variant aerosol distributions on the multiwavelength observations.Wavelengths of 0.53, 1.06, 3.8, and 10.6 pm were chosen because they areimportant to current Department of Defense (DoD) electrooptical systems,and because a wide wavelength range is needed to analyze scattering datain terms of aerosol properties. Energy at 0.53 and 1.06 Pm is obtained

from a Nd:YAG laser, 3.8 pm from a deuterium fluoride (DF) laser, and10.6 pm from a CO2 laser. These lasers were available from previousstudies or were purchased on this project. The design utilized otherlidar components already available at SRI. SRI purchased a 7-m van sothat the lidar could be constructed as a mobile unit, which could easilybe transported to various field sites.

Figure 1 presents a block diagram of the lidar system; Figure 2 pre-sents external and internal views of the lidar van. The three lasers

and a 12-inch Newtonian telescope (coated for 10.6-pm wavelength) weremounted on a specially constructed optical table. Optics needed todirect the laser pulses coaxial with the telescope are contained in anenclosure constructed along one edge of the optical table. The currentdesign accommodates only horizontal or vertical measurements, but canbe modified to include scanning optics for slant-path measurements.

Each laser face has been fitted with a beam-splitter and pyroelectricdetector to provide zero time triggers to initiate data collection bythe digital data system, and to provide a signal proportional to laseremitted peak power. Because the data system uses only a single transientdigitizer (Biomation 8100), the Nd:YAG laser is pulsed twice to obtainboth 0.53- and 1.06-pm lidar signatures. The laser is pulsed twice witha single flashlamp excitation (double pulse mode) so that both laserpulses can be transmitted within a 50-ps interval. The 3.8-pm laser ispulsed about 50 ps following the final Nd:YAG pulse, and the 10.6-pmlaser is pulsed about 50 ps later. Thus, all four laser pulses areemitted within an approximately 160-ps interval to minimize effects oftime-variant aerosol distributions along the observed path.

Backscatter energy from the four-wavelength laser emissions is col-lected by a 12-inch Newtonian telescope and optically focused on a three-detector receiver package mounted on the telescope housing. A dichroicfilter directs the 3.8- and 10.6-pm wavelength energy to a HgCdTe liquid-nitorgen-cooled detector, while passing the 0.53- and 1.06 -pm wavelengthenergy to a second dichroic filter. The second filter directs 0.53-pm

energy on the surface of a silicon avalanche detector and the 1.06-pmenergy on the surface of a second silicon avalanche detector. Output of

3

,~TEM LS102 TP

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12BNWONIAIN

TRIGITER

SYSTM FR RMOT AEOLMPEAREMENTDE

0t

LOG 0AMPLIFIERS

(a) EXTERNAL VIEW SHOWING LASER PORT FOR HORIZONTAL MEASUREMENTS

(b) INTERNAL VIEW SHOWING LASER/OPTICS TABLE AND DATA RECORDING SYSTEM

FIGURE 2 FOUR-WAVELENGTH LIDAR SYSTEMIN 6-METER-LONG VAN

5

-- __

detectors is input to video logarithmic amplifiers each of which hasabout four orders of magnitude response. The time-dependent signalsare then digitized by a Biomation 8100 transient recorder.

A multiplextr combines the analog backscatter signatures for inputto the single transient digitizer. Each waveform consists of the outputfrom an energy monitor and lidar detector. The Biomation 8100 provides2048 8-bit words with sample intervals of 10, 20, or 50 ps (150-m rangeper ps). A clock-gate unit was designed and constructed to interruptthe Biomation's digitization process so that backscatter signatures fromall four wavelength pulses can be stored in a single 2048-word record.The digitization is controlled by a timing sequence that is initiated bylaser fire pulses, so that digitization is in progress when the energymonitor pulse from each laser is generated. Therefore, the energy moni-tor pulse and backscatter signature for each of the four wavelengths arerecorded within about 500 Biomation words of the complete 2048-wordrecord. At the conclusion of the digitization for the fourth laser fir-ing, the full Biomation record is transferred to an LSI 11/02 microcompu-ter using direct memory access (DMA). The computer formats the lidardata and date and time information and writes the record on a nine-track1600 bit/inch magnetic tape recorder. Recorded tapes can be read back bythe computer; values may be printed or graphed on a hardcopy terminal.Data evaluations using the data collection system thus facilitate fieldevaluation of recorded records.

6I

III SYSTEM TEST AND RESULTS

SRI conducted a test of the four-wavelength lidar to evaluate sys-tem performance in an environment similar to those experienced duringprevious lidar operations at Army range facilities (Appendix A). Thelidar van was located about 600 m from an 8-ft square passive reflectorconstructed on the field site, located at Stanford University (Figure 3).Both dust and smoke aerosols were generated near the midpoint of thelidar-to-target path. Four wavelength lidar observations of the genera-ted aerosol plumes were recorded on magnetic tape. Figure 4 presents anexample of a Biomation record as displayed on the oscilloscope monitor(shown in Figure 1) during a dust test. (The energy monitor pulses werenot yet incorporated into the Biomation record when these data were col-lected.) The larger ratio of dust-to-clear-air backscatter for longerwavelengths is expected, because dust (large-particle) scattering isnearly wavelength independent, but clear-air scattering is proportionalto the inverse fourth power of wavelength (assuming Rayleigh scattering).

Quantitative evaluation of lidar backscatter signatures requiresinformation relating observed signal (detector voltage or digitizercounts) to intensity of light incident on the detector. A preliminaryreceiver calibration for each of the four wavelengths was derived byplacing neutral density optical filters in front of the detectors andrelating reduction of lidar-observed target signals (Figure 4) to knownattenuation of the optical filters. Results of the detector calibrationfor each of the four wavelengths are shown in Figure 5.

Figure 6 presents an example of a computer display generated frombackscatter signatures recorded on magnetic tape during red smoke obser-vation. The dashed lines present clear-air backscatter signatures recor-ded before the start of the smoke test. #fter smoke detection, the targetreturns diminish because of two-way attenuation of the laser pulses asthey penetrate the smoke plume. The shapes of the target returns reflectthe laser pulse characteristics. The target returns in Figure 6 showthat highest resolution (i.e. shortest pulse length) is provided by the1.06-pm laser. The DF laser (3.8 pm) has the longest pulse length andtherefore the lowest range resolution.

Range-resolved backscatter increases between solid and dashed linesin Figure 6 are a result of increased scattering by smoke particles dis-tributed along the lidar-observed path. The effect of pulse length onthe observed smoke distribution is clearly evident and must be consideredin any multiple-wavelength analysis of plume backscatter data.

7

(a) LIDAR VAN AT TEST SITE

(b) U-ft SQUARE PASSIVE REFLECTOR 4600-mu from IldaI

FIGURE 3 SYSTEM TEST ENVIRONMENT AT STANFORD UNIVERSITY

TARGET TARGET TARGET TARGET0.53 Am 1.06 Mm 3.8Mm 10.6Mum

DUST DUST DUST DUST0.53ur Im .06 pm 3A #m 10.6 Am

E

0

1

cc

W

0.53,&m j 1.06Mum j 3.8Mm j 0.6Mmr

RANGE----

FIGURE 4 FOUR-WAVELENGTH LIDAR BACKSCATTER

SIGNATURE FOR DUST GENERATED ALONG600m PATH BETWEEN LIDAR AND TARGET

Stanford Field Site, 7 July 1982.

The target returns provide a means to evaluate obscurant transmission

along the lidar-to-target path quantitatively. Transmission values deter-

mined from a single firing of each laser are given in Figure 6. Figure 7

presents a time history of transmission derived from data collected dur-

ing a red-smoke test. These data indicate the optical properties of

smoke are nearly equal at 0.53 and 1.06 pm, with substantially less atte-

nuation at 3.8 pm and almost no attenuation at 10.6 pm. However, these

results are based on a preliminary calibration of the four-wavelengthlidar; further calibration is needed to confirm the results. More exten-

sive calibrations using optical filters of known attenuation are planned.

Transmission values evaluated from target returns can be used to develop

and validate transmission values derived from the range-resolved back-

scatter data, so that measurements can be made along vertical and slant

' paths.

(.m mm m m m m m m mmm m~i m mm -9

100

WAVELENGTH FIT

(Jm) (counts/dB) I,

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FIGURE 5 DETECTOR CALIBRATION RESULTS FOR SRI FOUR-WAVELENGTH LIDAR

10

160

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ZnCI SMOKE

140

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120

100

0

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SMOKE TARGET4 PLUME I

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FIGURE 6 FOUR-WAVELENGTH LIDAR BACKSCATTER SIGNATURESAND SMOKE ATTENUATION DERIVED FROM TARGETRETURNS USING PRELIMINARY CALIBRATIONS

Stanford Field Site, a July 1962

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122

IV CONCLUSIONS

A four-wavelength lidar system has been constructed for use in deve-loping and validating lidar techniques for remote evaluation of obscurentproperties. The new system provides means for making nearly simultaneousmultiwavelength observations over nearly identical atmospheric paths--arequirement established on previous lidar studies. The system uses 0.53,1.06, 3.8, and 10.6 pzn laser wavelengths, which are the wavelengths mostused by DoD electrooptical system designers. The lidar should thereforebe useful in evaluating atmospheric effects during field testing of vari-ous electrooptical systems, and the wide wavelength range should providethe means to characterize aerosol properties (such as particle size dis-tribution) as an aid to determining attenuation and multiple scatteringterms. These optical effects must be accounted for before useful aerosoldensities can be evaluated from lidar backscatter measurements.

The four-wavelength lidar was tested by observing smoke and dustaerosols generated at a field site at Stanford University. Althoughonly preliminary calibrations were available, reasonable transmission

results were obtained, indicating that the four-wavelength system isnearly ready for participation in more extensive tests. SRI plans toconduct more extensive calibration experiments and participate in futurerange testing of environmental effects on electrooptical systems such asthe DIRT, Smoke Week, and SNOW series.

13

V LIST OF PUBLICATIONS

The items listed below were published or presented during the courseof this project:

" Uthe, E.E., 1981: "Lidar Evaluation of Smoke and Dust Clouds,"Applied Optics, Vol. 20, pp. 1503-1510.*

* Uthe, E.E., 1982: "ALPHA-1/ALARM Airborne Lidar Systems andMeasurements," Proceedings of the Workshop on Optical and LaserRemote Sensing, Monterey, California, sponsored by the ArmyResearch Office (February).

* Uthe, E E., 1981: "Lidar Evaluation of Smoke and Dust Clouds,"Proceedings of the Smoke/Obscurants Symposium V, Harry Diamond

Laboratories, Adelphi, Maryland, sponsored by the Office of theProject Manager, Smoke and Obscurants (April).

Appendix A of this report

14

* -. = . ... a fa !. *-,-# u

VI PARTICIPATING SCIENTIFIC PERSONNEL

SRI staff members who participated in the research effort are listedbelow:

" Edward E. Uthe, Director, Remote Sensing Program

" Warren B. Johnson, Director, Atmospheric Science Center

* Norm B. Nielsen, Field Engineering Representative

* Jan van der Laan, Senior Research Engineer

* Stephen A. DeLateur, Research Engineer

" John M. Livingston, Research Meteorologist.

None of the project staff earned advanced degrees during the course ofthe project.

15

Appendix A

LIDAR EVALUATION OF SMOKE AND DUST CLOUDS

OI

i

*Anled ontics, V'ol. 20, pp. 1503-1510 (1961).I

i .. ..... _ i i

Lidar evaluation of smoke and dust clouds

Edward E. UIhs

Lidar provides the means to evaluate quantitatively the spatial and temporal variability of smoke and dustclouds as they are transported downwind from particulate sources. Quantitative evaluation of cloud opticaland physical densities from cloud backscatter is complicated by effects from particle size, shape, and compo-sition and by attenuation and multiple scattering for dense clouds. Examples are presented that review useof the lidar technique to provide useful evaluations of smoke and dust clouds.

I. Introductiom 1L U. da Evaluation of Cloud DistlbtonsLaser radiation is effectively scattered by particulate The atmosphere is typically a highly variable medi-

material suspended in the atmosphere and can be um, causing complex transport and diffusion of par-transmitted over long atmospheric distances as short- ticulate clouds downwind of their sources. Becauseduration high-energy pulses directed along well-colli- conventional measurement methods cannot adequatelymated beams. Using these properties, researchers have determine the spatial and temporal variability of par-developed laser radar systems to observe clear-air at- ticle concentration distributions over extended atmo-mospheric structure over extended areas with high spheric regions, establishing limiting effects of partic-resolution in space and time. 12 For atmospheric con- ulate clouds on various electrooptical systems is diffi-ditions of relatively high visibility, the effects of aerosol cult. Lidar provides a means to derive quantitativeattenuation and multiple scattering are slight, and information on particulate cloud distributions andrange-resolved backscatter observed by the lidar tech- optical properties over remote distances using instru-nique has been equated to the distribution of particulate mentation located at a single convenient location. Forconcentrations along the propagation path. For at- example, Fig. 1 presents a lidar-derived vertical crossmospheres containing higher particulate concentra- section through a smoke plume that provides informa-tions, however, attenuation and multiple scattering may tion on the location and dimension of the plume and oncomplicate the relation of observed backscatter to the distribution of particulate concentrations within it.particle concentration distributions. Moreover, vari- Representative cross sections made at about half-ations of particle size, shape, and composition along the minute intervals provide data on both instantaneousobserved path introduce uncertainties in the interpre- and time-averaged cloud geometry. A second example,tation of the lidar signature. Nevertheless, lidar is the Fig. 2, illustrates the use of lidar during a weapon ringmost effective method available to derive information exercise. Information is provided on both the smokeon distant particle concentrations. This paper presents cloud generated at tht weapon site and the dust clouddata examples, collected during several field programs, generated at the impact area.that demonstrate the usefulness of lidar for remote The U.S. Army recently conducted a series of smokeevaluation of smoke and dust clouds, and dust tests using lidar and transmissometer instru-

mentation to derive time records of optical propertiesalong a near surface horizontal path that intersectedgenerated smoke and dust clouds, as illustrated in Fig.3. A passive reflector terminated the lidar path so thattransmission over the path could be evaluated fromlidar-observed target returns. During Smoke Week II(Eglin Air Force Base, 6-16 Nov. 1978), a 10.6-pmwavelength lidar was operated by the U.S. Army At-

The author ib with SRI International, Atmospheric Science Center, mospheric Science Laboratory (ASL) and a 0.7-,umMenlo Park, California 94025. wavelength lidar (Mark IX) was operated by SRI in the

Received 13 September 1980. manner shown in Fig. 3. Backscatter data from both0003-6935/8l/091503-08100.50/0. lidars were recorded and processed by the data systemc) 1981 Optical Society of America. of the Mark IX lidar to facilitate two-wavelength

I May 1981 1Vol. 20, No.9 / APPLIED OPTICS 1503

17

IO-

-m t

I~"

Fig. 1. Example of computer-generated vertical plume densityproffile. Lidar is located at lower left corner. The height and dis- - ,

tance swae is 75 m/div. Plume vertical concentrations (relative to - -clear air with a scale of 10 dB/div) are plotted at the lower left, and Fig. 2. Lkisr-derived dist m i crs section a aerosol structurethe horizontal position associated with each profile is plotted in the resulting from mortar firing and impact.

upper right.

. m ( ix. a -.,, M sa -I

OARA AL os , ,,

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Fig. 3. Experiment configuration of two-wavelength lidar obser-vations during Smoke Week II.

TRIAL 17 TRIAL 20 TRIAL 22

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21R

TRIAL 27

E O

FO 0

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VEHICULAR

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Io TIMEv

Fig. 4. Range/time inte nsity- modulat

ed displays depicting lidar-ob served structure alon g instrumented

path in tece by aerosol cods

generated during Smoke Week 11. lper record is from SRI 0.7-m wavelength lidar

and lower record is from ASL 10.8-pm wavelength lidar.

Target, cloud, and retroreflector fIR only) are indicated by the letters T, C, and R, respectively.

804 APPL D OPIC I V o. 20, No 9 1 aMy 198 1

18

CE T** IA 2-JR*R~ TRA 27 -

II

analysis of collected records. Figure 4 presents inten- transmissometer values agreed relatively well in thesity modulated range/time cross sections of cloud dis- sixteen trials analyzed except in most of those withtributions observed by the two-lidar system during phosphorus clouds; the density of those clouds variesseveral Smoke Week Il trials. These records provide greatly with their height above ground, and the lidarinformation on the space and time variability of aerosol and transmissometer paths were separated by -2 m indensity along the observed path. The data also indicate elevation. Nevertheless, the agreement obtained forthat the wavelength dependency of cloud optical most observations validated the lidar/target method ofproperties depends on aerosol type. measuring cloud attenuation. Although this method

requires a passive reflector at the end of the lidar path,I. Udar Evaluation of Cloud Optical Denuylly observations over a variety of horizontal paths can be

A. Attenuation Estimates Derived from Target Returns made by use of multiple reflectors, and observations forslant paths can be made if elevated targets are in-

Transmission of laser energy over a lidar-to-target stalled.path can be estimated from the magnitude of lidar-observed target returns. During the Smoke Week II B. Attenuation Estimates Derived from Cloudtests, visible and infrared lidars viewed paths nearly Bclkscatercoincident with paths viewed by conventional two- The major objective of the Smoke Week II lidar studyended transmissometers operated by the Dugway was to determine the feasibility of making attenuationProving Ground. Therefore, lidar-derived transmis- measurements for dense aerosol clouds by analyzingsions can be evaluated against transmissometer records lidar backscatter from the cloud. By this method,in the manner shown in Fig. 5 for three Smoke Week II measurements could be made along a variety of slanttrials (HE dust is generated from high-explosive events paths through the cloud at different downwind dis-conducted near the earth's surface). The lidar and tances from the emission source.

TIAL IT 15151 to TRIIAL IIImC 5U016 Inslm~US I...,I 'I 55Sf

S102 02, 0

o - I r..- .0-'

tO J 10 1

to' to-, -L~. 1

0 1 2 3 4 0 2 4 6 a to 12 0 0.5 1,0 1 5 2.0 2.5 30

.1 VISIBLE LIDAR AND TRANMMISSOMETER TRANSMISSION RECORDS

0 102 '

E 10 10 -0

~ 0 I 2 3 4 0 2 4 6 5 10 121 10 0$I01 . 5

Nb INFARED LIDARl AND TRANSMISSOMETER RECORDS

30-- I ? V0

0 2 3 4 0 2 4 6 a 10 12 0 05 10 t5 20 25 30

EVN IM -rnu EVENT TIME - n..nul. EVENT TIME - mlII VIILE AND INFRARED PATH-INTEGRATED ECESCATTER

Fig. 5.. Comparison or transmissions derived rrom lidar target returna and tranamiasometer recordIs ror (s) visible, (hi infrared systems, and(ci path-integre d backacatter for three Smoke Week || trial.

1 May 1901 / Vol. 20. No. 9 / APPLIED OPTICS 1506

19

I3.5II I

Ill (h)I,

0.3pm LIEAM 10,A.im LIDAR2.5 U "o.a - .11 U B .oesa a

0

210 .1

2.03 TRIAL 27 VEHICULAR DUST ....

.. ... ..o. .. 1 .4

.........r0.5

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 98 1A - 0.7 p.) -- k. 8 (A = 10.6 M.1 - km

INTEGRATED NORMALIZED BACKSCATTER

200 i . I I . 1 1 _ I I I I . . I - . I

lo) (j)

100M

10 I Fig. 6. Lidar-derived relationshipbetween cloud optical depth andpath-integrated backscatter (a)

.,b ,-..... O T,--. , and (b) and comparison of lidarSB.' , eackacatter-derived transmissions

0 2 4 6 8 10 12 0 2 4 6 8 10 12 with transmissometer records (c)EVENT TIME - I n EVENT TIME - .ftt and (d).

For each of the sixteen smoke or dust events analyzed, the time-dependent transmission record that is com-the path-integrated backscatter from the cloud nor- pared with the Dugway transmissometer record in Figs.malized to clear-air backscatter was evaluated as a 6(c) and (d). The lidar backscatter data reproduce thefuction of event time. Figure 5(c) presents the visible general features of the transmissometer trace. Theseand infrared path-integrated backscatter for trials 17, results indicate that useful information on cloud at-20, and 26. The results show that the wavelength de- tenuation can be derived from cloud backscatter data,pendence of attenuation and backscatter is a good in- providing that the optical depth-to-integrated back-dicator of aerosol type. Plots of optical depth (U = scatter expression remains valid for a wide range of dust-lnT, T = transmission) evaluated from the target re- aerosols. In practice, a vertically scanning lidar couldturns and path-integrated backscatter evaluated from observe a passive reflector terminating the near-surfacethe cloud returns typically showed that these parame- path to derive the calibration data necessary to interpretters are related in a nonlinear manner as shown, for slant-path transmissions.example, in Figs. 6(a) and (b) for vehicular dust (trial Another example of deriving cloud attenuation data27). The greatest data scatter between these parame- from cloud backscatter data is shown in Fig. 7 for aters usually occurred for smoke particles. At larger phosphorus event. The optical depth and integratedvalues of optical depth, the integrated backscatter backscatter data derived at the visible wavelength in-normally becomes independent of optical depth pri- dicated that valid transmissions could be evaluatedmanly because beckscatter from the far side of the cloud from backscatter date only for optical depths of <1.0.is not observed, being attenuated by particles in the near However, as shown in Fig. 7(a), infrared path-integratedside of the cloud. For several phosphorus events, in- backscatter is a relatively good indicator of visible op-

• tegrated backscatter increased with increasing optical tical depths for clouds extending to optical depths of 6.0.depth but decreased with further increases in optical Using the expressions shown in Fig. 7(a) and the in-depth. Therefore, both data scatter and nonuniqueness frared backscatter data, a time-dependent trace ofof the optical depth-to-backscatter relationship must visible transmission was derived and compared withbe addressed as limitations to applying the lidar tech- visible transmissions evaluated from target returns [Fig.nique for single-ended measurements of cloud attenu- 7(b)] and with visible tranamissometer data [Fig. 7(c)].ation. The discrepancy with the tranamissometer data was

The nonlinear expressions in Figs. 6(a) and (b) and expected because the visible lidar and transmissometerpath-integrated backscatter values were used to derive paths were separated by 2 m in elevation, and phos-

1506 APPLIED O PTICS / Vol. 20, No. 9 / M y198 1

20

E 5 j30 I

i .4

A.* . ~ - .. SI20

.4 1*0 0 5

0I.) 0(d)

0)01 02 0304 OS06 0708 0 0? 04 06 08 10INTEGRATED NORMALIZE b. RACKSCATTER. INTEGRATED NORMALIZED BACKSCATTER.

S102r 102

- 10 10,0 2r

10 io101 AAE IA AG M00

0 5 10 15 20 0 5 10 15 20EVENT TIME - mnuvoes EVENT TIME - nwfo Fig. 7. Derivationsa of optical depth-

integrated backscatter expresaiona and-102, VISISCI YgAkSI IE0EA 102 compariaona of tranamisaiona derived

- IA OAKWCTM from lidar target, lidar backacatter, andto #, 1 tranamiaaometer metho~s. Expreasiona

~(~% ~ - zderived are (a) between visible optical

a and (d) between infrared optical depthZ Z and infrared integrated backacatter.

"I, - 1005R S.CAWICOE. fls. Comparisons of backscatter-derive,"(C)01i tranamiasiona and those derived frm.-.

0 5 10 15 20 0 5 10 Is Zo lidar target and transmissometer reo'4EVENT TIME - -. Io., EVENT TIME - m,,num are shown in (b), (c), (a), and (G,.

phorus cloud density varied greatly with its height aboveground.

Figures 7(d), (e), and (f) show the derivation of in-frared transmissions from infrared backacatter data andcomparisons with results obtained from the lidar targetmethod and with the transmissometer record. Thesedata indicate that the lO.6-,Um wavelength can provide0000o~useful estimates of both visible and infrared transmis- ,

sion through dense smoke clouds.

IV. Lidw Evaluation of Cloud Type ot

The results of the Smoke Week 11 study indicate that 0

wavelength dependence of cloud attenuation and 1

backscatter provide information on cloud type. Dust 0 o 6

clouds could easily be distinguished from smoke clouds-. as large wavelength dependence indicates small particle

s izes of smoke aerosols, whereas small wavelength de- A 00 impndence indicates larger particle sizes of dust aerosols. n A.

Other examples of the use of lidar to evaluate cloud type .- ..*

are presented below.Figure 8 presents data derived from backscatter sig- Fig. 8. Plume returns plotted against opacities derived from near-

natures recorded while the lidar was observing black and far-side clear-air returns.

I MaY 1981 / VOL. 20. No. 9 / APPLIED OPTICS 1507

21

plume particle composition. Therefore, as shown in-------a ..-.... -.-- Fig. 8, the ratio of plume backscatter to plume attenu-

ation evaluated from lidar backscatter signatures8, 3oS. identifies white smoke from black smoke. This method

~. . Iof cloud identification is based on absorption of light by---- *-- ------------ -- I; the scattering particles. Particle backscatter is strongly

-- affected by particle absorption, whereas particle ex-"-.- , , e CO9 tinction is only weakly affected by particle absorp-

.. fica, , ,¢° o; ,Y" tion.. ,Figure 9 illustrates another method of cloud identi-............. 1 . . .1 fication. The receiver of the 0.7-jam wavelength Mark

o RaALLIL- -L~ZED IE-VER ",

-- IX lidar system is easily polarized by placing a polar-ization filter over the receiver aperture. Because the

... E... . -. ---.. , emitted laser energy is linearly polarized, the depolar-ization of cloud backscatter can be evaluated by rotating

. . . 'the polarization filter to observe the cross- and paral-lel-polarization components. Polarization of the inci-

Fig. 9. Parallel- and crosa-polarization backscattr observations of dent light is maintained for scattering from sphericaldust and water clouds made with the Mark IX lidar. particles but is destroyed for scattering from irregularly

shaped particles. Therefore, as shown in Fig. 9, thedifference between cross and parallel components of

I I1 I l~ backscatter is greater for water clouds (factor of 6.8)than for dust clouds (factor of 4). These data indicate

01 ._- s ' , .....L -AT~dSthat lidar can be used to distinguish between water anddust clouds from remote distances. Other investigators

1.0 _ _ _ *" a. have demonstrated the use of polarized lidar systemss."* for cloud studies. 4,5

V. Lidar Evaluation of Cloud Physical Density-..... .- " \ ,,Relationships between the optical and physical pa-

-0" '\, rameters of clouds typically depend on particle size,

\ \shape, and composition distributions. Therefore, in-o 0. \ ferences of cloud physical density from cloud back-

L E....- scatter or attenuation measurements are subject to_ uncertainties introduced by unknown properties of thex scattering particles. Recent research efforts have been

m 1directed to the development of optical techniques for2' I aerosol density analysis that minimize uncertainties

[3 L 7, o.o° 2 introduced by particle characteristics. For example,0.01 - I I 1,' I Fig. 10 presents measured and computed values of the

0AEENT 1 -p. extinction-to-mass concentration ratio as a function ofW-VL. I r. -1 wavelength for fractions of fly ash aerosols of different

0.5, 2.-5, 5-10, .od - 10 I i.l-V ,rl ftio0... sizes.6 Computed values agree well with measuredI .Bad 0. lo ....... ,,10,,,o, ,,,,, values for visible and 3.39-Mm (He-Ne laser) wave-

ily 'Io Gi diftliitiol. lengths. These data indicate that an optimum wave-Fig. 10. Theoretical and observed dependence of the extinction- length exists for relating aerosol extinction measure-to-mass ratio of fly ash aerosols on particle size and wavelength of the ments to aerosol mass concentration independent of

light source, particle size uncertainties.

Figure 11 presents extinction-to-volume concentra-tion measurements as a function of mean particle size

and white smoke plumes. A combustion smoke gen- for both visible and 3.39-Mm wavelengths. (Sauter di-erator designed for training of smoke inspectors was ameter is defined by the ratio of the third and secondused to produce spherical particles. The chemical moment of the particle size distribution.) Data werecomposition of the fuel determined whether black or collected for four sizes of fly ash fractions, three sizes ofwhite smoke was emitted. Plume opacity (1.0 - silica fractions, and an iron oxide fraction of one size.6

transmission) measurements were derived from the Although these particles vary greatly in size, shape, andlidar signature by comparing clear-air returns from the composition, the data presented in Fig. 11 indicate thatnear and far side of the plume return.3 The opacity a lidar extinction measurement at 3.39 um provides ameasurements are not strongly a function of plume good estimate of aerosol volume concentration for allparticle composition. The plume returns, normalized aerosol types used in this experiment.to the near-side clear-air returns, depend strongly on To investigate the use of lidar for quantitatively

1508 APPLED OPTICS / Vol. 20. No. 9 / I May 1981

22

o OSe/ VL ComnluonsSWS... o ,., / Lidar provides the means to evaluate quantitatively

the spatial and temporal variability of smoke and dustI SILICA

3 .. o s, clouds. However, attenuation, multiple scattering, and/ unknown particle characteristics can introduce uncer-

/ tainties in the interpretation of the lidar backscatter° / signature. For dense smoke clouds, longer wavelength

o, systems are better suited because of lesser attenuation/ o .and multiple scattering.

/ The capability of lidar to make remote evaluations/ of cloud type, optical density, and physical density has

...... /been demonstrated. However, additional informationa, A , or assumptions on the characteristics of the scattering

particles generally are needed, and cloud density eval-

-,, ..... .... ---- uations, therefore, are subject to uncertainties regardingS.. [0-S. 0 0' particle characteristics.

o / ,The data examples presented in this paper were col-lected with ground-based mobile lidar systems, but

oo, S aircraft-mounted systems offer substantial advantagesin many applications. Recently, an airborne lidar

Fig. 11. Extinction-to-volume concentration ratios a a function of system (ALPHA-i: Airborne Lidar Plume and Hazeparticle size for visible and infrared (3.39-pm) wavelengths. Analyzer) was used to track aerosol plumes to long

downwind distances from their sources and to observethe distribution of aerosol layers over large regionalareas.5 An example of plume tracking is presented in

evaluating the emission rate of fugitive particulate Fig. 13 for a plume that was subvisible only a few kilo-sources, an experiment was conducted with a control- meters downwind of the source. The ALPHA-1 systemlable particle emission source. 7 The lidar made cross- operates simultaneously at two wavelengths (1.06 andplume observations similar to those shown in Fig. 1 at-500 m downwind of the smoke source. In addition tothe lidar observations, wind speed and direction wererecorded at the lidar site. Cross-plume backscattervalues were integrated and adjusted by wind speed anddirection to provide an estimate of smoke emission rate.The smoke generator was used at three emission rates, 9 k m ONIND Of PLNT

and for each rate -10-20 plume cross sections were re-corded. Figure 12 shows that the resulting time-aver-aged cross-plume backscatter values were linearly re-lated to the source emission rate. These results indicatethe potential of lidar for monitoring smoke plume par-tide concentrations.

COLORADO RIVER

5h mmOF PLMIT

II I T i [ I III- j ,6o --

OIst RELEASEUt5105 ~ oI FA 0VE SURFA.CE l0 IF 0-1 sISOC

PARIA RIVER COLORADO RIVER MARBI.E CANYON

,o t 02/ 1

.?It.

, I i il I 'll l I I

V2.1,.C111 A.11..1 CORRAD RIVERR 0 0

LOAR 5l5 - SIND 1OA ROO T? OAF INTEGRATED CROSS 1-o VR.. o 0 ,o M .. o f. Fig. 13. Airborne lidar observations of the cross-plume structure

N. 12. Particulate emission rate evaluated from lidar plotted as of a subvisible power plant plume at different downwind distancesa function of emission rate evaluated from particle feeder, from the plant (1.06-sum lidar wavelength).

I May 1981 / Vol. 20, No. 9 / APPLED OPTICS 1509

23

0.53 pm) to provide additional information needed toestimate aerosol optical and physical parameters.Other multiple wavelength systems are currently underconsideration for extending the lidar technique so thatit can be used more widely in the evaluation of aerosolclouds.

The Smoke Week II lidar experiment was funded bythe U.S. Army Research Office, Geosciences Division,with additional support provided by the U.S. ArmyAtmospheric Sciences Laboratory. Other data wereobtained from projects with the U.S. EnvironmentalProtection Agency, U.S. Air Force, and the ElectricPower Research Institute.

Reftoncos1. R. T. H. Collis and P. B. Russell in Laser Monitoring of the At-

mosphere, E. D. Hinkley, Ed. (Springer, New York, 1976), p. 71.2. E. E. Uthe, Proc. Soc. Photo-Opt. Instrum. Eng. 142,67 (1978).3. C. S. Cook, G. W. Bethke, and W. D. Conner, Appl. Opt. II, 1742

(1972).4. S. R. Pal and A. 1. Carswell, Appl. Opt. 12, 1530 (1973).5. K. Sassen, J. Appl. Meteorol. 17,73 (1978).6. E. E. Uthe, APCA J. 30,382 (1980).7. E. E. Uthe, C. E. Lapple, C. L. Withsm, and R. L. Mancuso, Third

Symposium on Fugitive Emissions Measurement and Control,EPA Report 600/7-79-182 (1979), pp. 431-442.

8. E E Uthe, N. R Nielsn, and W. Jimison, Bull Am. MeteoroL. Soc.61.1035 (1980).

1610 AP.ED OPTCS / Vol. 20, NO. 9 / Msy INI

24

Appendix B

PARTICLE SIZE EVALUATIONSUSING MULTIWAVELENGTH EXTINCTION MEASUREMENTS

A.li, 4 Oftics, Vol. 21, pp. 454-459 (1982).

I IIII __________L_____m_________________

Reprinted from APPLIED OPTICS. Vol. 21. page 454. February 1, 1982Copyright Q 1982 by the Optical Society of America and reprinted by permission of the copyright owner

Particle size evaluations using multlwavelength

extinction measurements

Edward E. UU*

As the firat phase of a program to develop a lidar method for remote evaluation of mean particle size of sta.tionary source emissions, a data base was experimentally collected consisting of multiple-wavelength extinc-tion coefficients and mean particle sizes of generated aerosols. Extinction data were collected using multi-wavelength (14) transmissometers and a 10-m long aerosol tunnel facility. Generated aerosols consisted of

five size fractions of fly ash, three size fractions of silica, and single-size fractions of six other types of particu-late material. Particle size evaluations were made by multistage impactor and by air permeability (Fisher)analysis of packed powder. The data base indicates that mean particle size smaller than I -lm diem couldbe estimated usefully from aerosol extinction measurements using a single-laser lidar system operating at1.06- and 0.53-pm wavelengths. For larger mean particle sizes the extinction ratio is near unity, and longerwavelength systems are required. The data indicate that a two-laser lidar operating at 10.6 and 0.53 Atmcould provide estimates of mean particle size to diameters of at least 6 um.

1. ittroduction The experimental study reported in this paper em-Characteristics of particulate emissions from sta- ployed a 14-wavelength transmissometer to demon-

tionary sources are generally determined by in-stack strate that multiwavelength extinction measurements

sampling methods that provide information at only a can be interpreted in terms of mean particle size of ob-single location within the stack and therefore require served aerosols. The study represents the first phasetraversal of the stack to obtain a representative sample. of a program designed to apply lidar techniques for re-Such methods are difficult to apply without affecting mote evaluation of the mean particle size of stationaryparticle concentrations or size distributions and have source emissions.no potential for remote measurement applications forenforcement of particulate emission standards. N. Expelfmmfetl Facllles and Procedures

White-light cross-plume transmissometers are widely A. Transmissometersused to monitor stack emissions. Instrumented in-stack measurements can be used for abiding by plume Multiple-wavelength transmission measurementsopacity or visual standards. In addition, plume opacity were made using instrumentation readily available fromcan be extended to remote off-site observations using other projects. Wavelengths and additionallidar techniques.'- 3 transmissometer information are provided in Table I.

It may be possible to apply transmissometer and lidar Included were a multiple-wavelength (0.39-1.64-Mm)opacity techniques for providing additional character- sunphotometer, an IR photometer (3.9 pm), and IRization of stationary source emissions. For example, He-Ne (3.39-pm) and carbon dioxide (10.6-pm) con-recent experimental results indicate that an IR tinuous wave lasers. The photometers employedtransmissometer can be used to monitor particulate wideband photodiode detectors operating in the pho-mass emissions.4 Other studies have indicated that tovoltaic mode with wavelength response determinedmultiple-wavelength systems can provide information by narrowband (-10-nm) filters. A zirconium arc lampon particle size distributions.- .6 (100 W) was used as a broadband light source. A liq-

uid-nitrogen-cooled HgCdTe detector was employedin the He-Ne and C02 laser receiver which alternatelyobserved each wavelength by a chopper arrangement.

The author is with SRI International. Atmospheric Science Center, A photopic (eye-response) transmissometer was avail-Menlo Park, California 94025 able from a previous study. Field of view of each

Received 5 September 1981. tranamissometer receiver was minimized (typicall: 2*)0003-6635/82/030459-06801.00/0. to reduce effects of forward scattering on transmissionV 1962 Optical Society of America. measurements.

44 APIED OPTICS/ Vol. 21. No. 3 /1 Feruary 1962

26

4.

Table L Trlausarkr SeeelU.alau

Wavelength Bandwidth(nm) (nm) Source Detector

390 20 Zirconium arc lamp Silicon photodiode450 19.6 Zirconium arc lamp Silicon photodiode514 18.6 Zirconium arc lamp Silicon photodiode613 7.8 Zirconium arc lamp Silicon photodiode670 11.3 Zirconium arc lamp Silicon photodiode870 12.6 Zirconium arc lamp Silicon photodiode930 14. Zirconium arc lamp Silicon photodiode

1045 18.8 Zirconium arc lamp Silicon photodiode1240 10.7 Zirconium arc lamp Germanium photodiode1640 15. Zirconium arc lamp Germanium photodiode3390 <0.01 He-Ne laser HgCdTe3900 156. Zirconium arc lamp PbSe photodiode

10600 <0.01 CO2 laser HgCdTePhotopic Eye response Tungsten lamp Silicon photodiode

B. Calibration and Data Recording :0.5 dB over the 0-100% range. Therefore, the in-

An Esterline Angus model PD-2064 data logger was struments were assumed linear in all subsequent dataused to sample transmissometer output voltages and reduction and analysis.write digital values on magnetic tape. Voltage levels C. Aerosol Tunnelat 0 and 100% transmission were determined before eachdata collection run from data for clear air paths and The transmissometers were mounted across a 10-mcomplete blockage of the source energy. Transmisso- long aerosol tunnel that was designed with open endsmeter response was evaluated with neutral density All- to facilitate transmissometer and lidar experiments.ters, and all channels were found to be linear within Figure 1 presents a diagram of the experimental facility.

Particulate material was metered out by a grooved diskfeeder with a powder feed rate controlled by disk rota-

10. T. 110 tion speed. The material was dispersed via a nozzle

"POL CA R ITZZLIoperated at sonic velocity into a 164-rn 3 (5800-ft3)/minairflow generated by a centrifugal blower. Dust-ladenair was gradually decelerated into an expanding duct-

DUST 1190O work and allowed to enter a plenum chamber beneath00- the aerosol sighting tunnel. Two nozzles located tthe

-. chamber ends generated aerosol-laden air curtains toI"fOW carry away any outside air blown into the tunnel and

0 C , I....... thus establish a fixed boundary for the dust-laden air"Como in the sighting tunnel. A perforated plate in the centerMIN... AH of the chamber provided for uniform distribution of air

into the tunnel without significant particle loss. The

Fig. 1. Disgram of experimental facility. air moved from the center section toward the aerosol

TOMlL f t Prepas;ee of Te Armeal

Cascade Impactor StandardMm Median Geometric Sauter Diame

Type Diameter (#m) Deviation- Impactor Fisre

AP

0- 10-#m fly ash 3.7 2.1 2.8 4.15-10-am fly ash 5.1 2.0 3.9 5.82.5-5-pm fly ash 2.3 1.5 2.1 3.20-2.5-pm fly ash 1.6 2.1 1.1 1.6Superfine fly ash 0.85 1.7 0.75 0.795-06m silica 0.7-2.0 2.4-5.0 0.4-0.8 0.8410-pm silica 1.3 3.3 0.6 1.4515.pam silica 2.3-7.0 2.94.0 1.0-1.9 2.1Iron oxide 0.9-2.0 5-12 0.08-0.2 0.73Ammonium chloride 0.4 3.8 0.2 -Zinc chloride 0.5 1.5 0.4 -Titanium oxide 0.3 2.0 0.2 -Aluminum 2.0 1.3. 1.9 -Rosin -12 -40 0.003 -

* Standard geometric deviation was determined from the best-fit log-normal size distribution.From Fisher subsieve size analysis of bulk powders before sereolization.

I Februry 15 / Vol. 21. No. 3 APPLWUE OPTICS 451

27

curtains and out the exhaust stacks. The tunnel pre-viously has been used on several transmissometer andlidar development and evaluation projects4.7.8; furtherdetails of its design and operation are available.9

D. Test AerosolsTable II lists types of aerosols that were generated for

observation by the multiple-wavelength transmisso- II "i I I 1 1% 1meters. The ground silica, fly ash, iron oxide, alumi- 2 2 i 2 2 a % A 0 Inum, and titanium dioxide were all dispersed from thedisk feeder as dry powders. Ammonium chlorideaerosol was generated by mixing the vapors from solu- "tions of ammonium hydroxide and hydrochloric acid. l -

Zinc chloride aerosol was obtained from commercially 0: n . . . .

available smoke candles. Rosin smoke was made byheating rosin untilit fumed and sweeping the vapor into. a 0 M 0the chamber with a cold airstream, resulting in con- : .densation of rosin particles. .

.9 •.4 . , .9. .• - 0 * . . .4

The dust-laden air was sampled for particle size in theductwork just before the air entered the tunnel. Linear . . . . . . t-t -sampling velocity was matched to airflow in the duct to -d"Ad

approximate isokinetic sampling conditions. To con-dition the sampling system with equilibrium tempera- . -, , :. ;.

ture and humidity, a continuous airstream was drawnfrom the ductwork through the sampler for long periods * - -

of time before particle size sampling was initiated. The : 3 a X9conditioning air was filtered to remove particulate i 8 I 82material and routed to a multistage piezoelectric cas- -,, It 4 .4 14 •.cade impactor manufactured by California Measure- ...ments, Inc. Particulate samples were taken by running -unfiltered air from the ductwork directly through the . * . ..4 .4.4* . - *0

impactor. Aerosol concentration data were obtained a -28;5 'dfrom open-faced filter samples taken through the same..sampling port that was used to collect the impactor , ., ,•, ,.• , ,

samples. 5 J 1 . d i - = 'I : 2 -!The impactor data provide information on the mass

median diameter of the particle size distribution and ., ,,o . .44 -. . .the effective standard geometric deviation (widthofa t 1£ I St , S A U Z Xlog-normal particle size distribution). Sauter diameters -

(volume-to-surface mean diameters)'° can be obtained 1% 9 1 9 I . . . .from these results assuming a log-normal particle size 03- 218 : -. 3distribution. Sauter diameters also were determineddirectly by applying air permeability (Fisher subsieve . 4 a -. 6 W9size) analysis to packed powder. Particle size results f............................averaged over several measurements are presented inTable II.

AIII Expehtwfltal Results . %. -

A. Wavelength Dependence of Extinction h - .Magnetic tape records of digitized vdltages produced a J L. n 1 - S j : I 1

•-,. from the transmissometer array were reduced in termsof aerosol extinction coefficients a assuming linear re-sponse of the transmissometers and from voltage levelscorresponding to 0 and 100% transmission T and a --ilL InT, where the path length L - 10 m. No cor-rection was applied for multiple-scattering effects.Neutral density filters of known transmission of -,50%

456 AP. OPTICS / VO. 21, No. 3 / I Felrma1962

28

were periodically used to check transmissometer re- - ' T-s"s " !,e " ....o] TIM MW MAJACTIA tJlml

sponse. Typically, data collection runs were conducted a • *o a.. a. - -. --*with three arbitrary levels of aerosol concentration to a ao. o 0 ._. 5s - A

avoid evaluating extinction coefficients from either very a a . shigh or low transmission values. o

Typical wavelength-dependent extinction coefficients *aare given in Table III for each of the test aerosols listed 0 ,in Table II. Figure 2 presents a plot of extinction 10

coefficients as a function of wavelength; data are derived - , ,

from several experimental runs using different types ofparticulate material. Coefficients derived from the ophotopic response transmissometer are plotted at awavelength of 0.55 pm, although the plotted data indi- " .. . .cate a slightly longer effective wavelength of 0.58 pm. WAVSELIG. -For these data iron oxide particulates had the largestdiameters (Fisher analysis of 0.73-/pm diam) and ex- Fig. 2. Extinction as a function of wavelength for various smokeshibited less wavelength dependency than the other of different particle sizes (particle Sauter diameter determined fromparticulates. Zinc chloride, ammonium chloride, and impactor measurements).rosin had progressively smaller particle sizes accom-panied by progressively stronger dependence of theextinction coefficient on wavelength.

Figure 3 presents extinction-to-mass (t/M) concen- ° 0 ...stration ratios as a function of wavelength derived from 00000ooo o atransmission data for fly ash aerosols. The mass con-centration was derived from the photopic transmisso- r o n o n o o °meter readings and previously determined extinction- I .. " *

to-mass concentration ratios evaluated from time- 1:averaged data collected with this instrument.9 The 9 Isss, U 'data presented in Fig. 3 show that the extinction-to- INS

mass ratio is strongly dependent on particle size at short 0565 ;1,

wavelengths but is nearly independent of particle size _ _ _"__ _ _at long IR wavelengths. Previous data derived from the I. . .photopic and 3.39-pum transmissometers provided s 1 , a ,oW0,

similar results, while a theoretical analysis indicated I --

that alM would significantly increase with particle sizeat the 10.6-pm wavelength.4 A wavelength region with Fig. 3. Extinction.to-mass concentration as a function of wavelengthalM independent of particle characteristics is impor- for various particle size fractions of fly ash (particle Sauter diametertant for development of a method to monitor particulate determined from impactor measurements).

mass concentrations. The results presented in Figs. 2and 3 indicate that for particles smaller than 1 m, the ,,_,_. . . . .greatest wavelength dependence of extinction is atlonger wavelengths, and for particles 1-31pm in size, the Io , IL. Igreatest wavelength dependence is in the 1-4-pm a *wavelength range.

B. Evaluation of Particle Size from Extinction DataSeveral authors have addressed the problem of esti- "

mating particle size distribution n(a) from multi-wavelength extinction or scattering observations 4 osai(X). i-1 Basically, this involves solving a linear matrix o / ,

equation of the form

o(X,) - E K(X,,aj)n(o,) + E,. ,i"

where the matrix K is derived from theoretical consid- a,erations, and the E are measurement errors. Formalsolution techniques are not required to demonstrate o 0 0 4 5 5

that mean particle size a can be estimated from aO), P,CLIA ,A-IS. .U- -although calculaing a from n(aj) would optimize usageof the information content provided by the measure- Fig. 4. Ratio of extinction at 10.6- and 0.51-am wavelengths as sments (iX,), function of mean (Sauter) diameter for fly ash aerosols.

1 Febnay 192 / Vol. 21, No. 3 / APPLIED OPTICS 45?

29

i

For this study we have chosen to relate the ratio of Figure 5 presents a plot of the ratio of extinctionextinction values observed at laser or near-laser wave- coefficients at 1.045- and 0.514-pm wavelengths as alengths to independently evaluated particle size data. function of Sauter mean diameter (impactor) derivedFigure 4 presents a plot of the ratio of extinction coef- from observations of generated aerosols with particleficients for fly ash aerosols observed at 10.6- and sizes <1 pm. These data indicate that this extinction0.51-pm wavelengths as a function of Sauter mean di- ratio can be used as an indicator of particle size. Theameter. These data show that the extinction ratio is greater data scatter about the regression line than forlinearly related to the Sauter mean diameter evaluated the fly ash results shown in Fig. 4 probably results fromfrom either the impactor or Fisher permeability anal- particle composition and shape effects. The dataysis. Particle diameters evaluated by the two methods presented in Fig. 5 indicate that a single laser system,used in this study are in approximate agreement for Nd:YAG (1.06 jm) and its first harmonic (0.53 pm),particles smaller than 1 pm but disagree for larger would be useful to evaluate particles of sizes <1 pm.particles with the difference directly proportional to The extinction ratio Or.045/60.514 tends to converge toparticle size. This disagreement in particle size mea- a value of 1.2 for particle sizes >1 im (see Table III flysurement between the two methods probably reflects ash values) and therefore is not a good indicator ofan error in estimating the log-normal distribution curve particle size for larger particles. The results of thisand subsequently calculating the standard geometric study show that extinction ratios of less than unity aredeviation. The Sauter mean diameter calculated from useful for estimating mean particle size. Extinctionthe impactor measurement is dependent on the stan- ratios near unity indicate larger particles and requiredard geometric deviation and the mass median diame- longer wavelength extinction measurements for particleter. The disagreement may also reflect the possibility size analysis.that large particle bounce in the impactor indicated alower than actual mass median diameter. These dataindicate that CO2 and neodymium harmonic laserwavelengths could be used for remote evaluation of IV. Conclusionsmean particle size of fly ash emissions from stationary An experimental study has been conducted to collectsources, a data base consisting of multiple-wavelength extinction

coefficients and mean particle diameters of generatedaerosols representative of stationary source emissions.The data base indicates that the ratio of extinctioncoefficients can be used as a measure of mean particlesize, providing the ratio of long wavelength to shortwavelength extinction is less than unity. When the

, I , , ,ratio is near unity, larger particles are present and stilllonger wavelength observations are needed.

0* ,- ,u... - The results of this study indicate that a lidar systemS...o,.,"~'~ /,' using a single laser generating energy at 1.06- and

WOW .o, / . 0.53-pm wavelengths can provide estimates of mean,.,, ot0 #2, particle size for mean particle diameters smaller thanG7 - 1.0 pm. Required lidar extinction measurements of

stationary source emissions can be made by observationof clear-air backscattering on the far side of the plumeparticulate scattering. Therefore the results of this

lo le study indicate the feasibility of using a single-laser lidar°e ,system to make remote measurements of mean particle

.. e :, size of stationary source emissions having <1-pm mean"P particle diam. For emissions with mean particle di-

ameters from I to 6 pm a two-laser system could be used

03 for remote measurement of mean particle *size.

This study was funded by the U.S. Environmentalo, o, Protection Agency under contract 68-02-3267. William

D. Conner, the contract technical monitor, provided. 02 ,, * A A , , helpful discussion on the experimental approach.

. , . a,-, Clyde L. Witham and Robert W. Gates were responsiblef•xrocts DIM"A. on.- for aerosol generation and characterization. William

Fig. 5. Ratio of extinction at 1.045- and 0.514-prn wavelengths s D. Dyer and Jan van der Lann implemented thea function of mean (Sauter) diameter (impactor measurements) for transmissometer instrumentation; William D. Dyer also

submicron particles of different compositions, conducted the measurement program.

4M APPLIED OPTICS / Vol. 21. No. 3 /1 Fenuwy 1982

30

Relerenc..7. C. R. Lapple and E, E, Utlie. Remote Sensing of PARticulate StackEmissions, AiChE Symp. Set. 72.181 (1978).

I1. W. E. Evans, "Development of Lidar Stack Effluent Opacity 8.E. E. Utlie and W. E. Evans, "Lidar Calibration and PerformanceMeasuring System," NTIS PB 233-136/AS (U.S. Govt. Printing Evaluation," Environmental Protection Agency Final ReportOffice, Springfield, Va., 1967). contract 68-01-4137 (Task 4) (SRI International, Menlo Park.

2. C. S. Cook,.G. W. Beth~ke,sand W. D. Conmer, Appi. Opt 11, 1742 Calif., 1979).(1972). 9. E. E. Uthe and C. E. Lapple, "Stuidy of Lase fackacatter by

3. W. D. Conner and N. White, Comparative Study of Plume Particulates in Stack Emissions," Environmental ProtectionOpacity Measurement Methods. Environmental Protection Agency Final Report contract CPA 70-173, PB 212530(1972).Agency report EPA-600/2-80-OD1 (1980). 10. C. EK Lapple, "Particle Size Analysis and Analyseis." Chem Eng..

4. E. K. Uthe, Air Pollut. Control Assoc. J. 30,382 (1980). 75. No. 11, 149 (20 May 1968).5. K. 1.Cashdollar, C. K. I"e, and J. M. Singer, AppL. Opt. IS, 1763 it. S. Twomney and H. B. Howell, Appl. Opt. 6. 21256(1967).

(1979). 12. M. D. King, D. M5. Byrne, B. M5. Herman, and J1. A, Reagan, J.6. P. C. Ariessohn, S. A. Self, and R. H. Eustia, AppL. Opt. 19.3775 Atmos. Sci. 35. 2153 (1978).

(1980). 13. G. Yamamoto and M. Tanaka, AppI. Opt. 8447 (1969).

31

i EI I Ii I l I,

Appendix C

AIRBORNE LIDAR MEASUREMENTSOF SMOKE PLUME DISTRIBUTION,

VERTICAL TRANSMISSION, AND PARTICLE SIZE*

Aplied Optics, Vol. 21, pp. 460-463 (1982).

_ _ _ _ __ _ A|

Reprinted from APPLIED OPTICS, Vol. 21, page 460, February 1, 1982Copyright © 1982 by the Optical Society of America and reprinted by permission of the copyright owner

Airborne lidar measurements of smoke plume distribution,

vertical transmission, and particle size

Edward E. Uthe, Bruce M. Mouley, and Norman B. Nielsen

Observations were made of a dense smoke plume downwind from a forest fire using the A.PHA-1 two-wave-length downward-looking airborne lidar system. Facsimile displays derived from lidar signatures depictplume dimensions, boundary layer height, and underlying terrain elevation. Surface returns are interpretedin terms of vertical transmission as function of cross-plume distance. Results show significantly greaterplume attenuation at 0,

53-,um wavelength than at 1.06-Mm, indicating -0.1-um mean particle diameters or

the presence of gaseous constituents that absorb the visible radiation. These results demonstrate the poten-tial of multiple-wavelength airborne lidar for quantitative analysis of atmospheric particulate and gaseousconstituents.

1. ALPHA-1 Airborne Lidar can be operated at a pulse rate of 10/sec, backscatterAn airborne lidar system has been designed, con- signatures at both wavelengths resulting from each

structed, and applied to environmental studies for the pulse can be digitized to provide 2048 values with range

Electric Power Research Institute. The ALPHA-1 resolution of 1.5 m. The fine vertical and horizontal

(Airborne Lidar Plume and Haze Analyzer) is a down- resolution data are plotted in pictorial form and are

ward-looking system flown aboard the SRI Queen Air, available in real time [Fig. l(b)] for directing the dataa twin-engine research aircraft with specialized navi- collection. The signatures are also recorded on nine-gational instrumentation and facilities for lidar appli- track magnetic tape for later analysis of atmospheric

cations. External and internal views of the ALPHA- I behavior and aerosol physical and optical properties.aircraft are shown in Fig. 1. The lidar simultaneously System details and data examples have been presented

transmits laser pulses at two wavelengths and receives elsewhere.' 2

range-resolved backscattered energy from the atmo- 11. Plume and Ground Obeervallonesphere and reflected energy from the earth's surface.The near IR wavelength (1.06,pm) is very sensitive to During a flight test of the ALPHA-1 observationsdetection of low density (subvisible) particulate pollu- were made downwind of a forest fire located near thetion because of the larger ratio of aerosol-to-molecular California coast. Visually, the fire appeared to bescattering at longer wavelengths. The shorter wave- contained within a small area; the resulting smokelength (0.53 pm) is used for deriving visibility related plume towered over the source but was transportedinformation and for applying two-wavelength analysis downwind at lower altitudes.of backscatter signatures in terms of other aerosol pa- Three cross-plume flights were made with therameters (e.g., extinction and concentration). ALPHA-1 system. The initial pass was made -2 km

The ALPHA-1 data system uses dual microprocessors downwind from the source with the lidar receiver sys-for data recording at high collection rates and real-time tems operating at full gain. This resulted in observa-processing for height/distance pictorial (facsimile) tion of low-density background aerosol distributions;display of atmospheric and terrain features. The lidar however, the receiver electronics were saturated by

surface returns which typically exceed the backgroundaerosol returns by 4 orders of magnitude. The secondpass across the plume was made at about the samedownwind distance but with reduced receiver gain sothat the variability of surface returns could be observed

The authors are with SRI International Atmospheric Science and recorded. The third pass was made with less sen-Center, Menlo Park, California 94025. sitive lidar receivers but was flown over the densest part

Received 25 September 1981. of the plume- 500 m from the source.

oo03-6936/82/030460-04$01.00/o. Figure 2 is a facsimile plot of the IR signatures (log-01982 Optical Society of America. arithmic values) collected during the first ALPHA-i

460 APPLED OPTICS / Vol. 21. No. 3 1 Februry 1982

33

-..- -

I- I I

&ACKCATTE.

(a) EXTERIOR VIEW Fig. 2. Cross-plume structure derived from ALPHA-i plume

backscatter at 1.06- m wavelength (aircraft pass 1).

(W1 INTERIOR VIEW SHORING AIIPA-I I.ECT5EICS.

DIEPLAV UNIT. AND OPERATOS

Fig. 1. ALPHA-I aircraft. o.aa..

flight across the smoke plume. The plume base coin-cides with the top of a haze layer -600 m above thesurface. Haze layer tops normally indicate temperature 'maninversions that restrict vertical transport of atmosphericconstitutents.1 Particulate material from the fire ,- 2 -

penetrated the inversion and was transported down-wind above the inversion level. The most buoyant partof the plume extended to -1900 mn above the surface.

Il. Transmission Analysis and InterprealltonFigure 3 shows plume cross sections derived from IR 1 so

and visible backacatter sigratures recorded during thesecond pass of ALPHA-1 across the plume. Greaterplume attenuation of the visible energy than that ex- toperienced by the IR energy is evident by the absence ofplume visible backscatter following penetration by thelaser pulse of the denser plume elemer a. Plume a __ 0-

- -. transmissions in the vertical were evaulated by nor- .----- 0s 3 smalizing to 100% the surface returns in the absence of , . Iplume backscatter and assuming constant surface re- 0 1O too ve00 4 500W

flectivity and laser transmitted peak power for each

laser firing made along the cross-plume path. Lidar Fig. 3. Cross-plume structure derived from ALPHA-I plumeresponse information required for quantitative analysis backscatter and vertical transmissions derived from surfsce returns

of backscatter signatures was derived using standard at 1.06- and 0.53-O m wavelengths (aircraft pass 2).

1 Fewruwy 1982 / Vol. 21, No. 3 / APPUED OPTICS 461

34

calibration techniques with natural density filters of , , I ,known attenuation. Vertical transmission for one-waypassage of the laser energy through the smoke plume are o, ,- ,. -plotted in Fig. 3. The results show minimum plume . ,,transmissions of~50% at 1.06 pm and --6% at 0.53 pm. ,.1 POTThe strong wavelength dependence of attenuation .. oo.,.

suggests submicron particle sizes, although strong ab- , ,Vy

sorption in the visible and weak absorption in the IR byplume constituents could also explain the observa-tions. P"s

Figure 4 shows IR and visible wavelength facsimile . 7records and transmission analysis derived from data ,collected during the third pass across the plume. This : a" ,pass was made nearer to the source (-500 m) over the fldensest part of the plume, as determined visually. Y °" 0>Greater plume density, increased vertical dimensions, 1and reduced horizontal dimensions nearer to the source .

1 0&2

0 I I l I I

0 11 0.2 CL 0.4 0° * ,7 0.

Fig. 5. Relationship of extinction ratio at 1.045- and 0.514-jamwavelengths and mean particle diameter derived from laboratory

__-- --- results.4

are clear in Fig. 4. Because the green wavelength sur-face returns were below the receiver noise level, mini-mum transmission were probably <5%. Minimum IR

jI,,.,transmissions were -15%. Lower visible transmissionscould have been measured by removing the receiver gainj j t reduction; however, the surface returns for clean air

N regions would have saturated the receiver system. A, . greater receiver dynamic ranhe is needed for ground

S--* return experiments over dense aerosol plumes.4 Analysis of the ALPHA-I two-wavelength topo-

S. .graphic returns shows that the smoke plume attenuatedthe green laser energy significantly more than the nearIR laser energy. One possible explanation is that the

o scattering particles were sufficiently small in size tointroduce the observed wavelength dependence. Figure

'I i5 shows laboratory data r~lating the ratio of light ex-- tinction at 1.045 and 0.514 pm to particle diameter. 4

These are near the 1.06- and 0.53-mm wavelengths of theALPHA-I system and indicate that the observed dif-

20 - ferences in extinction may be explained by small par-ticle sizes. The ratio of optical depths (u - -InT) at1.06- and 0.53-pm wavelengths derived from the plumetransmission T data in Fig. 3 are betwen 0.2 and 0.3.Therefore, particle diameters smaller than 0.1 pm are

5 indicated by the relationship of Fig. 5. Verification0 00 200 3o 400 500 40 data were not collected on this experiment; however,

T WAS" previous in-plume sampling of combustion aerosols has

Fig. 4. Cross-plume structure derived from ALPHA-I plume found mean particle diameters in this size range. Meanbackscatter and vertical transmissions derived from surface returns particle sizes of combustion smokes have been measured

at 1.06- and 0.53-.om wavelengths (aircraft pass 3). as 0.002-0.3-pm diam,5 the wide size range possibly

442 APq..ED OPTICS / Vol. 21, No. 3 / I Fabruwy 1982

35

resulting from particle agglomeration near the source. usually be derived from plume backscatter. 12 Coordi-MacArthur6 found most particles from brush fires to nated airborne lidar and in-plume sampling is neededhave diameters near 0.1; this was later verified by Vines for validation of plume aerosol and chemical propertieset at.7 However, recent measurements by Stith et al.8 derived from multiple-wavelength lidar signatureindicate a 0.3-,m volume-to-surface area mean diam. analysis.downwind of burning forest slash. Cashdollar et al.9

found mean particle sizes of 0.1 um from wood fires. The data presented in this paper were collectedTherefore, the particle size derived from the ALPHA-1 during a flight-test of the ALPHA-I airborne lidarmeasurements agrees well with previous data. system for the Electric Power Research Institute. Data

Another possible explanation of the greater plume analysis was supported by SRI International.attenuation at the visible than at the near IR wave- Refeeslength is absorption by plume constituents. For ex- 1. E. E. Uthe, N. B. Nielsen, and W. L. Jimison, Bull. Am. Meterol.

ample, NO 2 is a product of combustion and has greater Soc. 61,10:35 (1980).absorption at 0.53 ,m than at 1.06 Am. Therefore, the 2. E. E. Uthe, W. L. Jimison, and N. B. Nielsen, "Development ofwavelength-dependent attenuation observed by an Airborne Lidar for Characterizing Particle Distribution in theALPHA-1 could be (at least partially) a result of the Atmosphere," Electric Power Research Institute Rep. EA-1538presence of NO2 or other constituents with similar op- (1980).

tical properties. Differential absorption lidar (DIAL) 3. W. Viezee and J. Oblanas, J. Appi. Meterol. 8, 369 (1969).

systems have been developed that use closely spaced 4. E. E. Uthe, "Particle size evaluation using multiwsvelength ex-

wavelengths located in strong and weak absorption tinction measurements," Appl. Opt., 21,000 (1982).

spectra of gas species for remote measurement of gas 5. G. M. Byram and G. M. Jimison, U.S. Dept. of AgricultureTechnical Bulletin (1948) p. 954.

concentrations. 10, 11 The two-wavelength ALPHA-1 6. D. A. MacArthur, Aust. For. 30,274 (1966).measurements may be similarly interpreted, although 7. R. G. Vines, L. Gibson, A. B. Hatch, N. K. King, D. A. MacArthur,wide wavelength spacing is used. and R. J. Taylor, Division of Applied Chemistry Technical Paper

1 (Commonwealth Scientific and Industrial Reseach Organiza-IV. ConcluIsOnm tion, Australia, 1971).

Previous studies have shown that the airborne lidar 8. J. L. Stith, L. F. Radke, and P. V. Hobbs, Atmos. Environ. 1l, 73technique is perhaps the best method available for (1981).analysis of plume transport and diffusion downwind of 9. K. L. Cashdollar, C. K. Lee, and J. M. Singer, Appl. Opt. 1S, 1763particulate sources. The results presented in this paper (1979).

show that plume transmissions for vertical paths can be 10. R. T. H. Collis and P. B. Russell, in Laser Monitoring of the At-mosphere, E. D. Hinkley, Ed. (Springer, New York. 1976).evaluated from topographic lidar returns observed with 11. J. G. Hawley, L. D. Fletcher, G. F. Wallace, and H. Heron, "A

airborne Uidar systems. Moreover, multiple-wavelength Mobile Differential Absorption Lidar (DIAL) for Range Resolvedtransmissions can be interpreted in terms of informa- Measurements of SO 2, 03. and NO2," in Extended Abstractstion on plume gas and aerosol concentrations and on Tenth International Laser Radar Conference (Americanmean particle size. Further information on aerosol Meteorolical Society, Boston, 1980).properties and particle concentration distributions can 12. E. E. Uthe, Appl. Opt. 20,1503 (1981).

36