Fluorescence of Atmospheric Aerosols and LidarImplicationsJerry Gelbwachs and Milton Birnbaum

The fluorescence of aerosols in the ambient atmosphere has been monitored in situ using cw argon ion laserexcitation in bands of 50 nm and 100 nm over the spectral region of 560-810 nm. The observed broadbandaerosol fluorescence may limit lidar (laser radar) determinations of pollutants. The limitation can beovercome by a method in which the aerosol fluorescence excited at two wavelengths is constant while themolecular signals differ. The effectiveness of the technique has been demonstrated by in situ measurementsof atmospheric nitrogen dioxide (NO2 ) in the presence of aerosols.

I. Introduction

The problems posed by atmospheric pollutionhave stimulated research and development aimed atacquiring new and improved methods for studyingand monitoring the concentration and distribution ofpollutants both in the ambient atmosphere and atthe source of pollution.", 2 A new technique basedupon molecular fluorescence has been developed bythis laboratory for in situ monitoring of ambientNO2.3 Our experimental observations disclosed thatthe presence of atmospheric aerosols resulted in aninterfering signal attributed to the fluorescence ofaerosols. Presented in this paper is the first obser-vation of fluorescence of aerosols in the ambient at-mosphere and the monitoring of this fluorescence inreal time.

Optical techniques employing lasers have a longhistory of application in studies of atmospheric aero-sols.

4 ' 5 For example, methods utilizing Mie scatter-ing have been used to measure the aerosol content inthe atmosphere.6 The chemical nature of atmo-spheric aerosols is discussed by Pierrand7 and Hidyand Friedlander.8 Altshuller2 and West9 reviewedthe techniques available for chemical identificationof aerosols. The fluorescence of aerosols may pro-vide a means for the chemical identification of ambi-ent aerosols.

Several authors have examined lidar (laser radar)techniques for remotely sensing atmospheric pollu-tant molecules.10"' Among the prominent opticalprocesses for pollutant detection are Raman scatter-ing and fluorescence. Atmospheric nitrogen, oxygen,water vapor, and pollutant species carbon dioxide

The authors are with The Aerospace Corporation, P.O. Box92957, Los Angeles, California 90009.

Received 26 March 1973.

(CO2) and sulfur dioxide (SO2) have been success-fully observed by Raman backscattering.' 2 "13 BothRaman and fluorescence interactions produce lightthat is shifted from the laser frequency by amountscharacteristic of the molecules. Broadband fluores-cence signals of aerosols may fall in the -detectionband for Raman scattering and fluorescence andthereby constitute a source of interference to lidardetermination based upon these interactions.

A two-wavelength excitation method to differen-tiate molecular fluorescence and Raman scatteringfrom continuum fluorescence is described. Utilizedare two pump wavelengths for which the aerosol flu-orescence is constant while the molecular signals dif-fer. The effectiveness of the method was demon-strated by in situ fluorescence measurements of at-mospheric NO2 in the presence of ambient aerosolfluorescence.

11. Monitoring of Aerosol Fluorescence

A. Apparatus

The apparatus employed for studying aerosols, de-picted in Fig. 1, is similar to that used for fluores-cence monitoring of ambient NO2 .3 Outside air isdrawn into a duct on the roof of the laboratory andpasses through a series of pipes with provision for fil-tering to the chamber. The arrangement permitssignals of filtered air and unfiltered air to be com-pared. The pressure in the chamber is monitored bya mercury manometer and kept near one atmosphere.The air then flows through the chamber at 4 /minbefore it is exhausted through a pump to the outside.

Excitation of fluorescence is provided by a cwargon ion laser oscillating on one of eight visiblelines: 458 nm, 466 nm, 473 nm, 476 nm, 488 nm,497 nm, 502 nm, and 515 nm. The beam is focusedby a 1-m fused-silica lens and passes through a blue-pass filter that absorbs longer wavelengths from theargon gaseous discharge. The beam enters the

2442 APPLIED OPTICS / Vol. 12, No. 10 / October 1973

signals. were not due apparatus fluorescence inducedby scattered laser light but were attributable toaerosols.

To obtain information about the spectral nature ofthe aerosol fluorescence and to monitor the fluores-cence, the following procedure was employed. Thecurrent from the PMT monitoring the Mie scatteringand the photoelectronic counts from the PMT thatviews the fluorescence were recorded. The air streamthen was diverted through the glass fritted filter (5Am pore-size) and signals were recorded from thefiltered air. The difference between signals with un-filtered versus filtered air was ascribed to tie aerosols.

Fig. 1. Fluorescence apparatus for monitoring atmospheric aero-sols and NO2 .

chamber through a window at Brewster's angle andtwo apertures in the arm (Fig. 1) designed to reducethe scattered laser light level in the chamber. Iden-tical facilities are provided for the light as it existsfrom the chamber.

The fluorescence is viewed through a side portparallel to the direction of the linearly polarizedlaser beam to minimize the laser scattered light inthe viewing direction. A collecting lens focuses itthrough a narrow slit in the image plane. The slitreduces light scattered toward the photodetectorfrom locations other than along the line of fluores-cence. In addition, the reduction in acceptanceangle diminishes fluorescence reaching the photo-multiplier tube (PMT) that is generated when scat-tered excitation light impinges upon the filters thatdefine the fluorescence detection band. The band-pass of these filters is 0.56-0.61 Mm, 0.61-0.66 ,mm,0.65-0.75 Am, and 0.70-0.81 Mm. (Efforts at ob-taining detailed spectra data with a doublemonochronometer were unsuccessful due to lack ofsensitivity.) After the fluorescence passes throughthe slit, it is imaged onto the photocathode of acooled (-20'C) PMT (EMI 9659QAM) with an ex-tended S-20 response. To further decrease the darkcounts to the order of 2-3 sec-', the effective cath-ode area is reduced to approximately 1 cm2 by amagnetic lens. The output pulses of the PMT areamplified and fed into a discriminator-counter to ef-fect photon-counting detection. In a typical mea-surement, counts are accumulated for intervals be-tween 5-50 sec. A PMT mounted above the cham-ber monitors the Mie scattering from the aerosols.

Experiments designed to generate fluorescencesignals due to scattered laser light in the apparatusyielded negative results. For example, with a non-fluorescing gas (nitrogen) in the chamber, the Miesignals at the side PMT were adjusted to equal thoseunder heavy aerosol loading. No increase in level offluorescence was recorded. It is later shown that thedaily ratio of aerosol fluorescence to Mie scatteringvaried by factors of approximately two presumablydue to diurnal weather changes. From these ob-servations it was concluded that the fluorescence

B. Experimental Results

Data were acquired mostly during the morninghours when high aerosol and NO2 levels prevailed.The laboratory is located in El Segundo, California,5 km east of the Pacific Ocean and 3 km southwestof Los Angeles International Airport. The data ofthe morning of 6 October 1972, shown in Fig. 2, illus-trate the aerosol fluorescence on a smoggy day. Ex-citation at 488 nm was provided at a 1.0-W level.Aerosol and NO2 fluorescence 3 were monitored inthree spectral bands: 0.61-0.66 m, 0.65-0.75 Am,and 0.70-0.81 Mm. Large Raman signals from atmo-spheric oxygen and nitrogen reduced the sensitivityat wavelength bands displaced from the excitationby less than 2400 cm-l. Therefore, monitoring wasnot performed for these wavelengths. The ordinatein Fig. 2 is the concentration of NO2 in parts perhundred million (pphm) that would produce anequivalent number of counts in the fluorescenceband. Stating the aerosol signals in these units pro-vides a direct measure of the interference of aerosolsin NO2 detection, or vice versa.

The aerosol fluorescence in each spectral band isseen to track the Mie signals at the excitation wave-length. On this day and on each of the days of the

E 200

- 100

IO

-- 30

10

I I I

MIE -488.0nm(DIFFERENT ORDINATE)

061- 0.66,m

X/ NO2 -

0.70 -081 plm \ 6575 um

6 OCT. 1972

I t ..IA ..A~~~~~:08:30AM 10:00 11:30

PACIFIC STANDARD TIME

Fig. 2. Temporal variations of atmospheric aerosols and NO2

monitored by fluorescence in three spectral bands excited at 488nm.

October 1973 / Vol. 12, No. 10 / APPLIED OPTICS 2443

51:00

SPECTRAL FLUORESCENCEBANDS PRECISION

I 0.56 - 0.61 ±im 10%I 0.61- 0.66izm + 10%I 0.65 - 0.75jm ± 15 %1E 0.70- 0.81 m ±20%

- 488.0 nmEXCITATION

I

I

/I,

,/I

I/

./a

//if

/'a.

7/

0.5 0.6 0.7WAVELENGTH, ,lm

|5 OCT 1972

nIm /

I , I

stant and equaled twice the ratio recorded on 27 Oc-tober. On 6, 10, and 11 October, the weather wasdry and clear skies prevailed while on the last day itwas cloudy and humid. Under conditions of highrelative humidity the increase in the Mie scatteringrelative to aerosol fluorescence may be due to thegrowth of aerosol particles by accretion of water. 7

On two mornings fluorescence was detected in the0.70-0.81-gm band while excitation was provided at458 nm, 488 nm, and 515 nm. The relative aerosolfluorescence intensity was 1.3:1.0:1.8, respectively.The corresponding fluorescence intensity for NO2was 2.0:1.0:0.4. Both sets of measurements haveuncertainties of 420%. Improvement in the abilityto discriminate aerosol fluorescence from NO2 fluo-rescence is therefore possible by selection of appro-priate laser wavelengths.0.8-

Fig. 3. Low resolution spectrum of atmospheric aerosol fluores-cence.

four-week period in October and a two-week periodin April-May 1972 over which aerosols and NO2were monitored, the temporal variations of the aero-sol signals closely followed those of the NO2 levels,which suggests that the mechanisms active in the at-mosphere for the generation of NO2 may be in partresponsible for aerosol buildup. As a rule, it was ob-served that aerosol signals were in excess of NO2 flu-orescence for the excitation wavelengths and detec-tion bands employed. The extent to which theaerosol signals dominated the NO2 returns was de-pendent upon the wavelength of excitation and thewavelength band selected to monitor the fluores-cence. It ranged from one to seven times the NO2signals. Only on the morning of 30 October 1972 didthe aerosol signals dip beneath the level of fluores-cence from NO2. A Santa Ana condition occurredon this day, which is characterized by dry desert airblowing over the Los Angeles basin. Under similarweather conditions Hidy and Friedlander have re-corded minimum aerosol concentrations.8

On 5 October 1972, a low resolution spectrum ofaerosol fluorescence excited at 488 nm (Fig. 3) wasobtained by averaging the signals in four spectralbands over a 4-h period. The broadband aerosolfluorescence shows a dropoff at longer wavelengths.Compared to the NO2 spectrum, much more of theaerosol spectral content is concentrated at the shorterwavelengths as illustrated in Fig. 2. Between 9:30and 10:00 a.m. the NO2 level was 25 pphm. Theaerosol signals recorded in the wavelength bandnearest the laser wavelength (0.61-0.66 gim) cor-responded to 170 pphm, while the signals monitoredin a band furthest from excitation wavelength (0.70-0.81 gim) were equivalent to only 45 pphm.

On the mornings of 6, 10, 11, and 27 October, theaerosols were excited at 488 nm and detected at0.70-0.81 gm. The ratio of aerosol fluorescence toMie scattering on the first three mornings was con-

2444 APPLIED OPTICS / Vol. 12, No. 10 / October 1973

C. Identification of Aerpsols by FluorescenceThere is little doubt that if the identification of

aerosols by fluorescence were realizable, it wouldoffer distinct advantages, which include (1) identifi-cation of the chemical nature of aerosols and (2)real-time determinations. Detection and identifica-tion of aerosol emissions from stacks may prove fea-sible. The aerosol concentration may be sufficientto allow high resolution (<1 nm) fluorescence mea-surements, thereby permitting the identification ofthe fluorescing species. If one type of aerosolpredominates, such as cigarette smoke or pollen, alow resolution spectrum, i.e., several large band-widths, may be sufficient to provide monitoring ofthis aerosol. If a fixed relationship exists among thesignals in the various spectral bands, the identifica-tion and quantification of this component mightprove realizable.

Ill. Remote Detection in the Presence of Aerosols

A. Interference of Aerosol FluorescenceOur findings show that in the ambient atmo-

sphere, NO2 and aerosol fluorescence constitutesources of mutual interference. Aerosol fluorescencemay seriously impair remote sensing by fluorescenceor Raman backscattering. By comparison of thestrengths of the aerosol fluorescence with Ramanscattering from nitrogen both excited at 488 nm, anestimate of the interference for remote Raman mea-surements of SO2, nitric oxide (NO), and carbonmonoxide (CO) can be made.'4 Consider a band ofdetection 4 nm wide displaced from the laser wave-length by the energy of the appropriate Raman ac-tive mode. Under the condition of severe aerosolloading our data reveal that fluorescence from aero-sols contributes signals equivalent to 600 ppm ofSO2 , 6000 ppm of NO, and 3000 ppm of CO. Onclear days aerosol fluorescence levels have been re-corded that are a factor of 100 less than levels moni-tored during heavy smog. Interference estimates forclear days consequently are reduced by this factor.

- 4

=1

'r-cZ

t= 3

U, 2

I- I

1.C,__1 0

In many cases it is not necessary to monitor the fullwidth of the Raman spectrum. Discriminationagainst aerosol fluorescence can be enhanced by con-fining the detection spectral region to include only-the narrow Q branch of the Raman mode.'0 Opticalwidths of several tenths of a nanometer can then beemployed. An order of magnitude reduction in in-terference should be realized by this procedure.

Since aerosol fluorescence measurement durationswere tens of seconds, the estimates of interferenceare independent of lifetime.' Aerosol fluorescencelifetimes were not determined because of lack of in-strument sensitivity. For remote sensing, the timerequired to monitor typical range intervals of tens ofmeters is approximately 0.1 gsec. Aerosol fluores-cence recorded in this range interval would be only afraction of the total fluorescence if the lifetime werein excess of 0.1 gisec. Until aerosol fluorescence life-times are established, our interference estimatesmust be regarded as a upper limit.

In addition to interference for Raman determina-tions, our data demonstrate that aerosol fluorescencemay exceed the fluorescence of 1 ppm of NO 2. Dueto the continuum nature of the NO2 fluorescence'5

an improvement in discrimination against aerosolfluorescence is unlikely by use of narrow opticalbandwidths. In most locations ambient SO2, NO,and NO2 concentrations rarely exceed 1 ppm whileCO levels are less than 100 ppm.' 6 If aerosol fluo-rescence of the magnitude similar to that observed inLos Angeles is present in these environs, remoteRaman detection of ambient SO2, NO, and CO, andfluorescence monitoring of NO2 may be severely hin-dered. To date, lidar-Raman measurements ofatmospheric molecules and pollutants have not dis-closed aerosol fluorescence.13,17,18,19 Ambientaerosols were not present in sufficient quantity togenerate interfering signals during these measure-ments. In a polluted ambient atmosphere we expectthat remote measurements with sensitivities com-parable to those reported by Hirschfeld et al.' 3 willshow the fluorescence of aerosols.

B. Two-Wavelength Method

A two-wavelength excitation scheme is proposed todistinguish molecular fluorescence and' Ramanscattering from aerosol fluorescence. It is similar tothe method outlined by Schotland, where the laser istuned on and off an absorption line of the molecule. 2 0

The relative strength of the returns at the laser wave-lengths for the above technique is related to the con-centration of the species. Our method consists oflocating a pair of excitation wavelengths for which (1)the aerosol fluorescence is approximately constantwhile (2) the fluorescence signals from the moleculehave a large difference. To distinguish Ramanscattering from broadband aerosol fluorescence, theRaman returns are monitored in conjunction withsignals received when the laser wavelength is shiftedso that the Stokes wavelength no longer falls withinthe detection band. The aerosol signals recorded in

the monitoring band are required to remain un-changed.

In the monitoring band, let C be the total numberof photoelectronic counts; M, the counts due to ei-ther fluorescence or Raman scattering from the mol-ecule of interest; B, the counts from backgroundsources such as sky light, artificial lights, etc.; D, thedark counts; and A, the aerosol fluorescence counts.Subscripts 1 and 2 refer to quantities excited at XIand 2, respectively. The photoelectronic countscan be expressed as

C1 = M + A + B + D,

C2 = M 2 + A 2 + B2 + D2 -

(1)

(2)

The same photomultiplier tube detects the signalsexcited at XI and at 2 so that D = D2 e D. Thedetection band is fixed; therefore equal backgroundcounts are expected; i.e., B, = B2 = B. Condition(1) imposes that A = A 2 A, while condition (2)requires that M 2 = 3Ml e3 #M, where 3 is a con-stant of proportionality less than unity. Performingthese substitutions, we see that

C1 = M + (A + B + D),

C2 = M + (A + B + D).

The signal in the presence of aerosols is

SA = C1 - C2 = (1 - )M.

(3)

(4)

(5)

The noise NA in the photon-counting mode of lightdetection can be written as

NA = [2(A + B + D) + (1 + fl)M]*2. (6)

The corresponding signal-to-noiseence of aerosols then is

ratio in the pres-

SA/NA = (1 - fl)M[2(A + B + D)+ (1 + [)M] 12- (7)

10 - A/M

Ir /30.5 0.38 - B 1000 1.0

J _ B 10>w6 7 3.0

QU 2 -- -

>

C

Z 00 5 10 ' 15 20

Q (S/N) FOR SINGLE-WAVELENGTH LIDAR,AEROSOL FLUORESCENCE NEGLECTED

Fig. 4. Signal-to-noise ratio of a two-wavelength method for mo-lecular detection in the presence of aerosol fluorescence comparedto that of a single-wavelength method with aerosol fluorescenceneglected. The ratio of molecular signals excited at the twowavelengths is j# while B is the background counts and AIM is

the ratio of aerosol fluorescence to molecular signal.

October 1973 / Vol. 12, No. 10 / APPLIED OPTICS 2445

Similarly, the signal-to-noise ratio for single-wave-length excitation can be obtained when fluorescencefrom aerosols has been neglected. The total countsin this case are

C = M + B + D,where

S = M

(8)

(9)

and

Table I. NO2 Fluorescence Intensity Excited by cwArgon Ion Laser

Excitation Wavelength(nm)

458 473 488 497 502 515

Fluorescence intensity(0.70-0.81 Am)(arbitrary units) 2.0 1.1 1.0 1.0 0.5 0.4

N = [2(B + D) + M11".

For the single-wavelength excitation aerosol-free casethe signal-to-noise ratio is

(S/N) = M/[2(B + D) + M]112. (11)

In most lidars the dark counts that occur while thereceiver channel is open are small compared to thecounts that are of atmospheric origin and can be ne-glected. Therefore, the ratio of signal to noise as de-termined by the two-wavelength method comparedwith single-wavelength excitation where aerosolshave not been considered is

(SA/NA)/(S/N) = (1 - f3)/1[2A+ 2B + (1 + [3)M]/(2B + M)j112. (12)

Signal-to-noise ratios for single-wavelength lidarthat neglects aerosol interference may be trans-formed with the assistance of Fig. 4 to signal-to-noiseratios that include aerosols. Plotted in Fig. 4 is (SA/NA) vs (S/N) for [ = 0.5, D = 0, and two values ofbackground radiation; B = 10 counts and 1000counts. Curves are shown for A/M = 0.3, 1.0, 3.0,and 10 for each value of B. The curves are relativelyinsensitive to background levels; an increase of twoorders of magnitude in B is reflected by barely a fac-tor of 2 in (SA/NA) for a given (S/N). For A/M lessthan 0.1 aerosol interference can effectively be ne-glected and still attain 10% precision with single-wavelength lidar. When A/M exceeds 10, accuratemeasurements are no longer practical, even employ-ing the two-wavelength scheme. For intermediatevalues of A/M where precise pollutant determinationis precluded by single-wavelength lidar measure-ments, detection in the presence of aerosols is feasi-ble by the two-wavelength method.

C. Experimental Determination of NO2 in Presenceof Aerosols

To demonstrate the proposed approach, fluores-cence monitoring of NO2 in its natural aerosol sur-roundings was undertaken with the in situ apparatusdescribed in Sec. II. Data on NO 2 absorption andfluorescence can be found in the literature.2 1-26

To determine an appropriate pair of excitation wave-lengths, fluorescence measurements were performed.A fixed concentration of NO2 was obtained by flowingnitrogen over a NO 2 permeation tube. The wave-lengths of the argon ion laser were varied and thefluorescence signals in a 0.70-0.81-gm band weremeasured. From the results of this experiment (sum-

Table II. Determination of Atmospheric NO2 in the Presenceof Ambient Aerosols by the Two-Wavelength Method

NO 2 measuredTime (a.m.) NO2-measured by by fluorescence

on 26 October two-wavelength with air stream1972 method (pphm) filtered (pphm)

8:30 2.0 4 2.0 3.6 4 0.59:00 5.4 i 2.3 4.9 dt 0.59:45 8.0 4 2.5 7.0 at 0.6

10:15 8.3 4- 2.4 7.1 i 0.610:45 4.3 4 2.3 5.6 b 0.511:30 7.9 4 2.3 7.1 i 0.6

marized in Table I) the wavelengths of 497 nm and502 nm were.selected. A 5-nm wavelength increaseis accompanied by a 50% decrease in NO 2 fluorescencefor equal intensities of excitation at 497 nm and at 502nm.

On 26 October 1972, ambient NO2 and aerosolswere monitored by fluorescence with excitation of 0.2W at 497 nm and at 502 nm. It was noted thataerosol signals were approximately equal at each ex-citation. Independently NO2 levels were monitoredby filtering the air stream.3 In Table II the concen-trations obtained by the two methods are compared.Aerosol signals constituted the largest source of in-terference. Fluorescence of aerosols was five timesgreater than the NO2 fluorescence and four times inexcess of the combined background and dark counts.The latter refer to the signals in the absence of NO2and aerosols. The measurements in the presence ofaerosols are four times less precise compared to aero-sol-free determination in accordance with the theoryin the preceding section. Within the experimentaluncertainty the NO 2 levels agree. Thus, fluores-cence detection in the presence of ambient aerosolshas been successfully demonstrated.

D. Raman Detection in the Presence of NO2

Detection of atmospheric pollutants by lidar-Raman is susceptible to interference from anothersource of broadband fluorescence. Fouche et al.point out that fluorescence from NO2, a pollutantspecies itself, may mask Raman signals from othermolecules.26 They suggest a highly dispersive detec-tor to discriminate against the fluorescence contin-

2446 APPLIED OPTICS / Vol. 12, No. 10 / October 1973

(10)

Table ll. Aerosol and NO2 Fluorescence (0.61-0.66,um) Excitedat 488 nm and at 497 nm

Fluorescence counts-25-sec interval(10% precision)

Time 488 nm (0.2 W) 497 nm (0.2 W)(a.m.)on 25 NO2 NO2

October Aero- [ conc. Aero- conc. 11972 sols L (pphm)J sols L (pphm) J

8:30 1817 202 (10) 1565 226 (11)9:00 1295 220 (11) 1395 230 (11)9:45 2580 390 (19) 2534 381 (18)

10:15 2898 443 (21) 2934 431 (21)10:45 2991 465 (22) 2557 432 (21)11:30 1205 192 (9) 1227 197 (9)

uum of NO2. The two-wavelength method may beemployed as an alternative method of discrimina-tion. The requirements on the excitation wave-lengths are now that (1) the aerosol fluorescence andNO2 fluorescence are constant while (2) the detectedRaman signals differ. One choice for A, and A2 is 488nm and 497 nm, respectively. Detection is performedat the Stokes wavelength corresponding to excita-tion at A. When pumped by either wavelength theNO2 fluorescence intensities are equal (see Table I)while the aerosol returns have been observed to beconstant (see Table I) thus fulfilling requirement(1). Since the wavelengths differ by more thantypical Raman widths (4 nm), the Raman signal atthe Stokes wavelength associated with excitation atA2 will not fall within the detection band. Condition(2) is then satisfied. The two-wavelength methodmay overcome interference to Raman detection ofmolecular pollutants from ambient NO2 fluorescenceas well as fluorescence of aerosols.

IV. Conclusions

Fluorescence of aerosols in the ambient atmo-sphere has been observed and monitored in fourbroad spectral bands extending from 0.56 gm to 0.81gim. Both aerosol and NO2 were concurrently moni-tored by fluorescence in real time. Fluorescence is apotentially useful means for aerosol identificationand monitoring.

Ambient aerosol fluorescence may hinder remotesensing of atmospheric pollutants. Under conditionsof severe aerosol loading, aerosols may generate sig-nals equivalent to 600 ppm of SO2 , 6000 ppm of NO,and 3000 ppm of CO as measured by Raman scatter-ing and 1 ppm of NO2 as determined by fluorescencewhen excited at 488 nm. To resolve the desired sig-

nals from aerosol fluorescence a two-wavelength ex-citation method has been proposed. The methodhas been demonstrated by in situ measurements.For utilization of pollutant detectability calcula-tions, i.e., signal-to-noise ratios, for single-wave-length lidar in which aerosol fluorescence has beenneglected, curves have been presented that trans-form these signal-to-noise ratios to ones applicablefor two-wavelength lidar determinations in the pres-ence of aerosols.

The expert technical help of Curtis L. Fincher hasbeen invaluable throughout this project. Armin W.Tucker and Alan Petersen have provided the calibra-tion of the equipment utilizing NO2 permeationtubes.

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Fincher, Opto-Electron. 4, 155 (1972).4. F. S. Harris, Jr., Lasers in Meteorology (Old Dominion Uni-

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21. T. C. Hall, Jr. and F. E. Blacet, J. Chem. Phys. 20, 1745(1952).

22. A. E. Douglas and K. P. Huber, Can. J. Phys. 43, 74 (1965).23. G. M. Myers, D. M. Silver, and F. Kaufman, J. Chem. Phys.

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(1969).25. L. F. Keyser, S. Z. Levine, and F. Kaufman, J. Chem. Phys.

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October 1973 / Vol. 12, No. 10 / APPLIED OPTICS 2447