measurements of stratospheric aerosols with a combined elastic-raman-backscatter lidar

Measurements of stratospheric aerosolswith a combined elastic–Raman-backscatter lidar

Michael R. Gross, Thomas J. McGee, Upendra N. Singh, and Patrick Kimvilakani

Improvements made to the NASA Goddard Space Flight Center Stratospheric Ozone Lidar system haveextended its atmospheric-aerosol-measuring capabilities. The methods by which aerosol-scatteringratio, aerosol backscatter, and aerosol extinction are simultaneously derived from lidar data are reported,and results obtained during several intercomparison campaigns at worldwide locations are shown. Theresults track the evolution of the Mt. Pinatubo aerosol cloud from 1991 to 1994 and report wavelength-dependence information for aerosol backscatter between 308 and 351 nm. Two analysis techniques, amore common inversion method and a combined elastic–Raman-backscatter approach, are also com-pared.Key words: Aerosol-scattering ratio, backscatter and extinction, elastic–Raman-backscatter lidar.

r 1995 Optical Society of America

1. Introduction

The eruption of the Philippine volcano Mt. Pinatuboin June 1991 injected large amounts of sulfur dioxidedirectly into the stratosphere. SO2 converts to sulfu-ric acid–water droplets in the stratosphere with atime constant of the order of 1 to 2 months.1 In thestrictest sense, these aerosols are the solid or liquidparticles, or both, plus the gas in which they aresuspended. In this case the gas is just the Earth’satmosphere. However, the definition of aerosols usedin this paper, as it is generally used in the atmo-spheric community,2 refers to only the solid or theliquid phase. These aerosols have since been nearlyuniformly dispersed over the entire globe,3,4 greatlyincreasing the particle concentration of aerosols inthe stratosphere. The problems associatedwithmak-ing accurate lidar measurements of ozone in thepresence of high aerosol loadings have been welldocumented.5–8 As a result of modifications made toNASA’s Stratospheric Ozone Lidar Trailer Experi-ment, making it possible to measure ozone in thepresence of large aerosol loadings,9 the system is nowalso capable of making improved measurements of

M. R. Gross and U. N. Singh are with the Hughes STX Corpo-ration, Lanham, Maryland 20706. T. J. McGee is with the NASAGoddard Space Flight Center, Code 916, Greenbelt, Maryland20771. P. Kimvilakani is with the IDEA Corporation, Beltsville,Maryland 20705.Received 8 September 1994; revised manuscript received 8 June

1995.0003-6935@95@306915-10$06.00@0.

r 1995 Optical Society of America.

both aerosol backscatter and aerosol extinction at 308and 351 nm. Other groups have already used theelastic–Raman-backscatter lidar technique to makeprofile measurements of atmospheric aerosols.10–14This method requires no a priori information concern-ing the relationship between aerosol extinction andaerosol backscatter, as is needed in the Klett inver-sion technique,15 an approach taken by many lidargroups when extracting aerosol information. Thispaper restates themethod and shows results obtainedduring several Upper Atmosphere Research Satellite1UARS2–Network for the Detection of StratosphericChange 1NDSC2 correlative measurement campaignsat various locations 1Maryland, California, France,New Zealand2.

2. Instrumentation

Before February 1992 the Goddard Space FlightCenter 1GSFC2 lidar operated strictly as an elastic-backscatter differential absorption lidar system, thechief measurable quantity being ozone. The on-lineozone wavelength at 308 nm was generated by twoXeCl excimer lasers, an oscillator and an amplifier,which when combined emitted a line-narrowed, low-divergence beam. The off-line wavelength near350 nm was generated with various Nd:YAG laser–excimer laser–hydrogen Raman-cell configura-tions.16,17 Since that time the system has been modi-fied, and it now uses two excimer lasers 1XeCl andXeF2 to emit the on-line and the reference wave-lengths at 308 and 351 nm, respectively. The re-ceiver system was also modified so that now not onlyis the elastic-backscatter return at the two laser

20 October 1995 @ Vol. 34, No. 30 @ APPLIED OPTICS 6915

wavelengths being collected, but also data are ob-tained from the inelastic Raman-scattering returnsfrom molecular nitrogen at 332 and 382 nm. Ontypical nights, Raman signals are detectable abovebackground levels to around 45 km. The 308- and351-nm returns are split into high- and low-sen-sitivity channels. The less-sensitive channels areelectronically gated on at approximately 10 km, andsignals are received to approximately 50 km whenbackground skylight begins to dominate the data.Therefore the system is capable of simultaneouslycollecting data at 308, 332, 351, and 382 nm in the10-to-45-km altitude range, the region of the atmo-sphere most affected by the Mt. Pinatubo eruption.

3. Methodology

The lidar equation for signals from atmospheric elas-tic backscattering is given by

N11Z, ll2 5C13O1Z24

Z23bm1Z, ll2 1 ba1Z, ll24

3 exp522 e0

z

3am1Z, ll2 1 aa1Z, ll2

1 ag1Z, ll24dZ6, 112

where ll is the laser wavelength in nanometers 1nm2,N11Z, ll2 is the number of counts received in theelastic-backscatter channel; Z is the altitude 1in kilo-meters2, C1 is the system-calibration constant, whichincludes all terms that are not a function of Z; O1Z2 isthe overlap function between the laser beam and thefield of view of the receiver, bm1Z, ll2 is the molecular-backscatter coefficient 3in inverse kilometers timesinverse steradians 1km21 sr2124, ba1Z, ll2 is the aerosol-backscatter coefficient 1in km21 sr212, am1Z, ll2 is themolecular 1Rayleigh2-extinction coefficient 3in inversekilometers 1km2124; aa1Z, ll2 is the aerosol-extinctioncoefficient 1in km212; and ag1Z, ll2 is the absorptioncoefficient 1in km212 for all other atmospheric gases.For the case of ll 5 351 nm, the gaseous-absorptionterm becomes small with respect to the molecular andaerosol terms and can be neglected. This is not thecase for ll 5 308 nm as ozone absorption becomes veryimportant.The lidar equation for signals from atmospheric

Raman scattering is distinctly different from that forelastic scattering because it does not contain a termfor aerosol backscattering. It is given by

N21Z, ll, lr2 5C23O1Z24

Z2bR1Z, ll, lr2exp52 e

0

z

3am1Z, ll2

1 am1Z, lr2 1 aa1Z, ll2 1 aa1Z, lr2

1 ag1Z, ll2 1 ag1Z, lr24dz6 , 122

where lr is the Raman-scattering wavelength 1in nm2;N21Z, ll, lr2 is the number of counts received in theRaman channel; C2 is the Raman lidar calibration

6916 APPLIED OPTICS @ Vol. 34, No. 30 @ 20 October 1995

constant, which includes all terms that are not afunction of Z; O1Z2 is again the overlap functionbetween the laser beam and the field of view of thereceiver, and br1Z, ll, lr2 is the nitrogen Raman-backscatter coefficient, which is related to the differen-tial Raman-backscatter cross section 1in square kilo-meters2 and the molecular number density 1in inversecubic kilometers2. The extinction and absorptionterms are now a function of the laser wavelength ll forthe upward path length and the Ramanwavelength lrfor the downward path. Again gaseous absorptioncan be neglected for the 351- and 382-nm cases;however, ozone absorption is important at 308 nm,and at 332 nm it contributes approximately 2% to thetotal extinction.The quantity typically used to described atmo-

spheric aerosols as a function of altitude is theaerosol-scattering ratio 1ASR2, defined as 1 1

ba1Z, ll2@bm1Z, ll2, a unitless quantity.12,18 In regionswith low aerosol concentration, ba1Z, ll2 9 bm1Z, ll2,and the ASR approaches unity. This quantity can beobtained easily from a combination of Eqs. 112 and 122 ifproper normalization is used. Let us first considerthe ll 5 351 nm channel and its Raman-shiftedwavelength for molecular nitrogen at lr 5 382 nm.In this case only the molecular and aerosol terms arekept. The ratio of 351-nm to 382-nm counts yields

N11Z, ll2

N21Z, ll, lr25 C

3bm1Z, ll2 1 ba1Z, ll24

br1Z, ll, lr2

3 exp52 e0

z

3am1Z, ll2 2 am1Z, lr2

1 aa1Z, ll2 2 aa1Z, lr24dZ6 , 132

where C 5 C1@C2 and O1Z2 is taken to be unity, a validassumption when the transmitted laser beam is en-tirely within the field of view of the receiver, which isthe case for the altitudes dealt with here. Thesystem-calibration constant C is computed at analtitude 1Z*2 known to be free of aerosols. For theGSFC lidar, C is taken as an average of the individualconstants computed every 150 m between 35 and45 km. This approach is valid if two assumptionsare made. The first is that aerosol concentrationsare a minimum in that region, a statement that isgenerally true. The second is that even when signalsfrom these altitudes are close to background lightlevels, a mean of the computed constants will con-verge to the correct calibration constant. This sec-ond assumption is valid only if the backgrounds of thereturned signals vary equally about the mean back-ground. Large fluctuations from the mean in onlyone direction will bias the calibration constant, whichdirectly relates to an error in the derived ASR profile.If the quantity 1bm 1 ba2@br is normalized to one inthis calibration region, then that quantity representsthe ASR, and once C is known, 1bm 1 ba2@br can besolved for in Eq. 132 at all altitudes. The ASR is then

given by

ASR 5N11Z, ll2

N21Z, ll, lr2

N21Z*, ll, lr2

N11Z*, ll2exp52 e

Z

Z*

3am1Z, ll2

2 am1Z, lr2 1 aa1Z, ll2 2 aa1Z, lr24dZ6 , 142

where Cwas replaced by

C 5N11Z*, ll2

N21Z*, ll, lr2exp5e

0

Z*

3am1Z, ll2 2 am1Z, lr2

1 aa1Z, ll2 2 aa1Z, lr24dZ6 . 152

The differential transmission terms between 351 and382 nm in Eqs. 142 and 152 are computed with modelatmospheric data from LOWTRAN,19 with both molecu-lar and aerosol terms being considered. This is avalid approach because even in regions of significantaerosol loading these transmission terms are domi-nated by the l24 dependence of molecular scattering.The effect of the differential transmission term in Eq.142 becomes greater with increasing altitude and con-tributes approximately 22% of the ASR at 25 km.The error in transmission at this altitude is estimatedto be less than 1%.To derive the ASR at 308 nm, the previous develop-

ment can be followed if ozone-absorption terms at 308and 332 nm are added. The GSFC lidar systemroutinelymeasures ozone concentrations fromapproxi-mately 10 to 50 km, using the nitrogen Raman chan-nels to compute ozone below approximately 30 km.Although the Raman-channel ozone analysis doesmake some assumptions and corrections for aerosolextinction, these adjustments, even during high-aerosol-loading conditions, change the ozone profileby less than 5%.9 Because the contribution of aero-sol extinction is orders of magnitude less than that ofaerosol backscatter in the 308-nm channel, this com-puted ozone profile can be used in the development ofthe 308-nm ASR. When available, coincident ozone-sonde data are also used. From the ozone-concentra-tion profile, ozone-absorption terms can be computedand the ASR at 308 nm can be derived. The uncer-tainty in the ASR at 308 nm is greater than theuncertainty in the ASR at 351 nm because of theadded error introduced through ozone absorption andbecause of the reduced signal strengths of the 308-and 332-nm channels for the GSFC lidar system.For a typical measurement made in 1992 with theGSFC lidar system, the statistical error in ASR at351 nm at the aerosol peak was typically 2–3%, and at308 nm the error was roughly 5%.Profiles of aerosol-backscatter coefficient ba 1km21

sr212 can easily be computed from ASR profiles ifancillary atmospheric-density data are available.In order to compute ba, the molecular-backscattercoefficient bmmust be known. bm is given by

bm1Z, l2 53

8psm1l2n1Z2, 162

where sm1l2 is the molecular-scattering cross sectionand n1Z2 is the atmospheric molecular number density.sm1l2 for each wavelength is obtained from Elter-man,20 and n1Z2 is obtained fromNationalMeteorologi-cal Center analysis data21 or from local radiosondes.Profiles of the aerosol-extinction coefficient can also

be computed from data acquired by the GSFC lidar,specifically from the 382-nm Raman channel.14,22After the natural logarithm is first taken and then Eq.122 is differentiated with respect to Z, the total aerosolextinction 1351 1 382 nm2 can be written as

a1Z, ll2 1 a1Z, lr2 5d

dZ 3lnn1Z2

Z2N1Z24 2 am1Z, ll2

2 am1Z, lr2. 172

Note here that O1Z2 is again taken as unity andbr1Z, ll, lr2 has been written as nN21Z2*ds1ll, ll, p2@dV,where nN21Z2 is the molecular nitrogen number den-sity, which is proportional to n1Z2, the molecularnumber density, and ds@dV is the differential Raman-backscatter cross section. The aerosol-extinction co-efficient at the 351-nm wavelength is determinedfrom Eq. 172 by assuming a wavelength dependencebetween 351 and 382 nm. This is sometimes as-sumed to be of the power-law form, where aa isproportional to lx, with the exponent x taken asapproximately 21. Others have shown that an errorof only 61% results if this exponent is allowed to varybetween 0.8% and 1.2%.14 The 308-nm aerosol-extinction coefficient can also be computed if ancillaryozone-profile information is available. The GSFC-derived ozone profile cannot be used in this casebecause assumptions are made in the ozone deriva-tive concerning aerosol extinction. Again, the uncer-tainty in 308-nm aerosol extinction is larger than at351 nm because of added error introduced by ozoneabsorption and because of the reduced 332-nm signalrelative to the 382-nm signal. Because of the rela-tively small signal-to-noise values typically seen inthe 382- and 332-nm Raman channels, it is difficult tomake an accurate measurement of the aerosol-extinction-coefficient profile when background aero-sol conditions exist. This approach works best whena significant aerosol signature is present, as was thecase soon after the Mt. Pinatubo eruption.Another valuable aerosol quantity, which is com-

monly used in the lidar research field, is the lidarratio, defined as the ratio of aerosol-extinction coeffi-cient to aerosol-backscatter coefficient at a givenwavelength. The lidar ratio is a simplification of amore complicated relationship between extinctionand backscatter suggested by Klett,23 that is, ba1r2 5C1r2*3aa1r24k, where k is a constant and C1r2 ranges withaltitude. The lidar ratio is exactly the quantity1@C1r2 if k is defined as 1. When the elastic–Raman-backscatter lidar technique is used, lidar ratio profilescan be formed from measured profiles of the aerosol-extinction coefficient and the aerosol-backscatter coef-ficient. For the GSFC lidar system this can be doneat 308 and 351 nm. This quantity can be used to


locate regions of atmospheric-aerosol layering. It isa relative measure of aerosol-size distributions withrespect to height, with larger particles generallybeing represented by smaller lidar ratios.11,24 Duringconditions of low volcanic-aerosol loading, the strato-sphere is generally considered to be in a nearlyhomogeneous background state, having many moresmaller aerosol particles than the larger volcanicparticles.25 During such conditions, the lidar ratiocan realistically be assumed to be a constant withaltitude. This is typically not the case when highvolcanic-aerosol-loading conditions exist. When thestratosphere is contaminated with volcanic aerosols,a wide range of aerosol sizes can be seen, oftenseparated into defined layers.26 During those condi-tions, the lidar ratio, R1r2 5 1@C1r2, can be highlyvariable both in time and altitude.24,27,28 Becausethe derived lidar ratio profile depends on the derivedprofile of aerosol extinction, this too cannot typicallybe accurately measured during background aerosolconditions.A common method used in lidar studies to derive

aerosol quantities is the inversion technique, firstformally proposed by Klett.15 Before 1992 the GSFClidar system used Fernald’s29 variation of this tech-nique to measure theASR at 351 nm. The techniqueuses data from only one laser wavelength and as-sumes that the lidar ratio is known and is constantover the entire altitude range; however, a secondpaper by Klett23 relaxes this assumption, allowingdifferent backscatter-to-extinction ratios for variousaltitude regions 1see also Sasano et al.302. A region ofthe atmosphere for which aerosol backscatter andextinction are well known is also required. We areusing the more stable top-down inversion ap-proach,30,31 assuming that virtually no aerosols existabove 35 km. Using the Klett approach, aerosol-backscatter-coefficient profiles are directly obtainablefrom profiles ofASR, but aerosol-extinction-coefficientprofiles must be formulated by once again assuming alidar ratio. When stratospheric aerosol layering ef-fects are minimal and when the lidar ratio is wellknown, this analysis method is valid. When thestratosphere is inhomogeneous, this method tends tooverestimate theASR at altitudes where the assumedlidar ratio is underestimated and underestimatesASR at altitudes where the lidar ratio assumption ishigh. The primary advantage of this technique isthat the signal strength of the data typically used inthe inversion is very good. The favorable signal-to-noise ratio gives results with small statistical errors.

4. Results

The NASA GSFC lidar system first observed thePinatubo cloud in September 1991 while operating atNASA GSFC in Greenbelt, Maryland 139.9 °N,283.3 °E2. The system had been deployed at the JetPropulsion Laboratory’s Table Mountain Observa-tory 1JPL-TMO, 34.4 °N, 242.3 °E2 for engineeringtests during June– July 1991, and had just began


operation at GSFC in September. From September1991 to January 1992, the GSFC lidar system ac-quired data at GSFC when conditions permitted.Figure 1 shows monthly average profiles of ASR at353 nm for that time period and a pre-Pinatubo ASRprofile taken in July at JPL-TMO. In each case thealtitude resolution is 0.5 km, and the number of dailyprofiles used to form each monthly average is noted inthe figure legend. Daily ASR profiles are typicallycomputed from sums of 1,000,000 shots of raw lidardata, which represents 4 hrs of averaging. The July1991 profile, although taken after the Pinatubo erup-tion, was acquired before the aerosol cloud had reachedthe northern midlatitudes. Since the system wasonly acquiring elastic-backscatter data at that time,the profiles in Fig. 1 were computed with the inver-sion technique. Because of system constraints atthat time, ASR data were only available above 15 km.The assumed lidar ratio was taken as 20, a bestestimate from recently published results.11,24 Theaerosol peak can be seen to decrease in magnitudeand fall in altitude over time, most likely representingthe gradual fall out of larger aerosol particles. Theerror bars for these curves show the standard devia-tion of the data, indicating the spread of the dataduring the particular month.Between January 1992 and February 1992, changes

were made to the GSFC lidar system that increasedits measuring capabilities, as was discussed above.During February and March 1992, the instrumentwas again deployed to JPL-TMO in Wrightwood,

Fig. 1. Average monthly ASR profiles at 351 nm derived fromGSFC lidar data acquired at JPL-TMO and at GSFC during 1991after the Mt. Pinatubo volcanic eruption.

California, as part of an UARS and NDSC intercom-parison and validation campaign. Quality lidar datawere acquired on 19 nights during this period. Oneof the curves in Fig. 2 shows the average ASR profileat 351 nm for the entire February–March 1992 cam-paign. The altitude range for this ASR profile waslimited to above 20 km because of system constraintsset by the lidar’s mechanical shutter. Although loca-tions differ from Fig. 1 to Fig. 2, the location latitudesare relatively the same, and averages are taken overat least an entire month in each case. Comparisonsbetween plots shown in Figs. 1 and 2 should befeasible, as each profile roughly represents the aver-age ASR profile for northern midlatitudes.3,4 TheASR peak has continued to fall in magnitude and withaltitude, again showing continued fall out of thelarger volcanic-aerosol particles. The ASR curvesshown in Fig. 2 are averages of daily profiles thatwere computed with the elastic–Raman-backscatterlidar technique described above. The final averagedaltitude resolution for each case is 0.5 km. The errorbars shown for each curve represent the variance ofthe data over the averaging period.Later in 1992 the GSFC system was again de-

ployed, this time to southern France 143.9 °N, 5.7 °E2at the Observatoire de Haute Provence 1OHP2 to beused again in a UARS–NDSC intercomparison andvalidation exercise. During July and August 1992,the GSFC lidar acquired 25 quality nights of datawhile it was operated at OHP. The results are alsoincluded in Fig. 2, which shows an average profile of

Fig. 2. Average ASR profiles at 351 nm derived from GSFC lidardata acquired at various NDSC sites 1JPL-TMO; OHP, France;Lauder, New Zealand2 and at GSFC from 1992 to 1994.

ASR at 351 nm for July and August 1992. Althoughthis is yet another new location, comparisons betweenASR profiles measured at all three locations should beplausible because the latitude of observation is nearlythe same and averages are taken over at least aone-month interval.3,4From France the lidar trailer was immediately

transported to yet another UARS–NDSC intercom-parison site in Lauder, NewZealand 145.0 °S, 169.7 °E2.The system yielded quality data on 16 nights duringNovember and December 1992. An average profileof ASR at 351 nm for this time, computed with theelastic–Raman-backscatter lidar technique, is alsoshown in Fig. 2.During 1993 through February 1994, the system

acquired data at NASA–GSFC when conditions per-mitted. Two average profiles of ASR at 351 nm fromthis time are also shown in Fig. 2. The first is anaverage of five profiles acquired in April 1993. Thesecond is an average of all data, 18 measurements,acquired during the period August 1993 throughJanuary 1994. The relatively small error bars shownon the August 1993– January 1994 profile indicatethat the aerosol conditions changed little during thesemonths. Because of systemmodifications, data acqui-sition was only possible during April and during thelast 5 months of 1993.All GSFC lidar data since January 1992 have been

obtained in the combined elastic–Raman-backscatterconfiguration; therefore aerosol information for thistime can be extracted at both 351 and 308 nm. Alldata shown in Fig. 2 is at 351 nm. The 308-nm datahave been left out for simplicity, and because the308-nm profiles have approximately twice the uncer-tainty of the 351-nm profiles as a result of errorsintroduced by ozone absorption at 308 and 332 nmand of reduced signal strengths. Figure 3 showscoincident 308- and 351-nm data for 28 July 1992taken at OHP, France. The uncertainties shownrepresent only the statistical error in the data. Theozone profile for that night from the GSFC lidar wasused to compute ozone absorption needed in develop-ing the 308-nmASR curve. Because molecular back-scatter varies with wavelengths as l24 and aerosolbackscatter typically varies with wavelength11,32 inthe range from l20.5 to l22.5, the magnitude of theASR at 308 nm is less than the ASR at 351 nmthroughout the aerosol cloud. The 308-nm ASRcurve was limited on this night to above 16 kmbecause of system constraints. A detailed depen-dence of aerosol backscatter on wavelength couldtheoretically be calculated from these data. As hasbeen shown elsewhere,32,33 this dependence typicallyvaries with both time and altitude. For Fig. 3, theaerosol-backscatter wavelength dependence between308 and 351 nm at the aerosol peak 117–21 km2 isl21.46. Taking into account all uncertainties, includ-ing those introduced in the modeled molecular andaerosol extinction, the measured ozone extinction,and the statistical error of the data, the standarddeviation in wavelength dependence is approximately


60.25. Wavelength dependencies between 308 and351 nm for average aerosol-backscatter profiles takenthroughout 1992, 1993, and 1994 at various locationsare shown in Table 1. The wavelength dependencefor each case was computed within the main aerosollayer, and the peakASR altitude is noted in the table.The statistical error in each case is approximately60.25. It should be noted that the derived wave-length dependence for aerosol backscatter between308 and 351 nm is very sensitive to normalization ofthe ASR curves. A 2% error in either the 308- or the351-nm ASR profile relates to a 25% error in wave-length dependence. Given this information, the er-ror in wavelength dependence is probably closer to60.50.Aerosol extinction can also be computed with the

GSFC lidar elastic–Raman-backscatter data. Aver-age aerosol-extinction profiles at 351 nm with 0.5-kmresolution are shown in Fig. 4 for measurements

Fig. 3. ASR profiles at 351 and 308 nm derived from GSFC lidardata acquired at OHP, France, on 28 July 1992.

Table 1. Wavelength Dependencies of Aerosol Backscatter between 308and 351 nm at the Altitude of Maximum ASR for Several Locations during

1992, 1993, and 1994

Date l DependenceASR Peak

Altitude 1km2 Location

2–3@92 21.90 23.0 JPL-TMO, Calif.7–8@92 21.96 20.0 OHP, France

11–12@92 22.18 18.5 Lauder, New Zealand4@93 21.46 19.5 GSFC, Md.

8@93–1@94 21.50 18.0 GSFC, Md.


made in 1992, 1993, and 1994. The variance of theseprofiles over the averaging period is quite large,especially below approximately 15 km. The featuresseen below the tropopause are probably the result ofcirrus cloud layers. The standard deviation of thedata from above 15 km to approximately 25 km isgenerally 60.003 km21 for each case. As statedabove, the measurable quantity is actually the sum ofaerosol extinction at 351 and 382 nm. An aerosol-extinction wavelength dependence must be assumedto determine aerosol extinction at only 351 nm. Forthe profile shown in Fig. 4, this value was assumed tobe l20.6, taken from research done by Ferrare.11Aerosol extinction at 308 nm can also be obtained,given ancillary ozone information. An aerosol-extinction wavelength-dependence study between 308and 351 nm is not feasible here because a l20.6

wavelength dependencewas already assumed in deriv-ing 308- and 351-nm extinction.When the elastic–Raman-backscatter lidar ap-

proach is used, aerosol backscatter and aerosol extinc-tion can both be computed, therefore profiles of thelidar ratio, R1r2 5 aa1Z, ll2@ba1Z, ll2, can be derivedfrom measurable quantities. Figure 5 shows aver-age lidar ratio profiles for measurements made from1992 to 1994 at OHP, France, at Lauder, New Zea-land, and at GSFC, Maryland. The variance for the1992 average profiles is approximately 5 sr at theaerosol peak, growing to approximately 10 sr 5 km

Fig. 4. Average aerosol-extinction-coefficient profiles at 351 nmderived from GSFC lidar data acquired at various NDSC sites1JPL-TMO; OHP, France; Lauder, New Zealand2 and at GSFC from1992 to 1994.

above and below the peak. For the 1993 and 1994profiles, the errors become 10 and 20 sr, respectively.The ratio is greatest at the edges of the Pinatubocloud and smaller near the cloud peak, where most ofthe larger aerosol particles are located. The rationear the aerosol peak, 15–20 km, can also be seen toincrease with time, which is most likely because of thegradual fall out of the larger volcanic-aerosol particles.The variations in lidar ratio below approximately14 km are probably due to clouds, the dynamics ofwhich are not discussed in this paper. From thesedata, it can be seen that the lidar ratio is not constantwith altitude, and over long integration times 1manymonths2, it is not constant with time.Acquiring data in both elastic- andRaman-backscat-

ter channels allows for a comparison between theinversion and the elastic–Raman-backscatter meth-ods. Figure 6 shows the ASR at 351 nm for twocases, one in July 1992 and one in April 1993. Twoprofiles of ASR, one computed with the inversiontechnique and another computed with the elastic–Raman-backscatter approach, are shown for eachday. The agreement in ASR on 20 July 1992 in Fig.6 is extremely good, within 10% throughout theprofile. A lidar ratio of 18 was assumed in invertingthe 351-nm channel for this day. This assumptionwas made after viewing the computed lidar ratioprofile, derived from coincident elastic–Raman-backscatter data, shown in Fig. 7. The agreement on20 July 1992 is good because the lidar ratio isrelatively constant throughout the altitudes of inter-

Fig. 5. Average lidar ratio profiles at 351 nm derived from GSFClidar data acquired at various NDSC sites 1OHP, France; Lauder,New Zealand2 and at GSFC from 1992 to 1994.

Fig. 6. Profiles of ASR computed with both the elastic–Raman-backscatter lidar approach and the inversion technique for 20 July1992 at OHP, France, and for 18April 1993 at GSFC.

Fig. 7. Lidar ratio profiles for 20 July 1992 at OHP, France, andfor 18April 1993 at GSFC.


est. In July 1992, the Pinatubo cloud has dispersedinto a single, nearly homogeneous aerosol layer.3,4This is not the case on 18 April 1993, where the lidarratio shows significant changes with altitude, espe-cially near 13 and 19 km. By April 1993, the Pina-tubo aerosols had fallen, with the most dense layerbeing between 13 and 19 km. The inverted ASRprofile for 18 April shown in Fig. 6 was derived with aconstant lidar ratio value of 60. Looking at thecalculated lidar ratio profile for 18April in Fig. 7, 60 isthe value that most represents the lidar ratio databetween 19 and 25 km. Although the two profilesagree well down to approximately 19 km, discrepan-cies between the two ASR profiles for 18 April areevident below 19 km because of the assumed constantlidar ratio. Figure 8 shows the effect of choosingdifferent constant lidar ratios. Values of 10, 20, 40,and 60, which are realistic atmospheric values,28 wereused to compute the ASR in the four inversion casesshown in Fig. 8. In the top-down inversion, thederived ASR profile is less sensitive to the assumedlidar ratio at the upper altitudes, 19–35 km. Differ-ences become more significant as the inversionprogresses downward. The inversion algorithm doeshave some sensitivity to the upper normalizationaltitude. If the algorithm is applied within the aero-sol region, low values of ASR are derived, and if it isapplied too high into the profile 1above 40 km2, someaerosol-free altitudes just below the aerosol regionlead the algorithm to overestimate the aerosol concen-

Fig. 8. Comparison of ASR profiles derived with the elastic–Raman-backscatter lidar approach versus the inversion technique.Inverted profiles created with input lidar ratio values of 10, 20, 40,and 60 are shown.


trations. The fact that the lidar ratio cannot bedetermined without ancillary data, and that this ratiomay change over time, can also lead to errors whenthe inversion technique is applied. When dynamicvolcanic-aerosol conditions exist andwhen these aero-sols separate into distinctive layers, which has some-times been the case since the Pinatubo eruption, asingle constant value for the lidar ratio will not bevalid. Even when the aerosols in the stratosphereare relatively homogeneous with altitude, elastic-backscatter lidar data alone are not sufficient.Ancillary data detailing the magnitude of the lidarratio must be available. The lidar ratio was alsoseen to change with time, further indicating thatalthough the inversion technique is effective in cer-tain instances, it is not the best technique duringtimes of dynamic aerosol changes. This is the situa-tion after a significant volcanic eruption like Pinatuboin 1991. The main advantage of the inversion tech-nique is that the elastic-backscatter channel used inthe analysis scheme is typically approximately 100times stronger than its Raman return, resulting insmaller statistical errors. The elastic–Raman-back-scatter approach is arguably the more accurate tech-nique to measure aerosols during volcanic conditions,especially with respect to systematic errors. Its onlydrawback is the reduced signal of the Raman data,which results in larger statistical uncertainties in thederived ASR, aerosol backscatter, aerosol extinction,and lidar ratio profiles. A systematic error resulting

Fig. 9. Aerosol-extinction-coefficient profiles at 351 nm for 20July 1992 at OHP, France. One profile was created with the382-nm Raman channel, and the other was computed with theinversion technique with an assumed constant lidar ratio.

from calibration, however, can sometimes bias thederived ASR profile. This can occur when signalcharacteristics are unfavorable, typically when signallevels are reduced because of atmospheric 1cloud2 orsystem constraints.A comparison of aerosol-extinction measurements

made with the elastic–Raman-backscatter approachversus the inversion technique further strengthensthe argument that the elastic–Raman technique pro-duces more reliable results, i.e., smaller systematicerrors. Figure 9 shows two aerosol extinction pro-files at 351 nm for 20 July 1992. One profile iscomputed with the elastic–Raman-backscatter ap-proach, the other uses the assumed lidar ratio andancillary atmospheric-density information to convertASR, computed with the inversion technique, to aero-sol extinction. Once again data taken when thesingle, constant lidar ratio assumption is valid are inmuch better agreement with the aerosol-extinctionprofile measured with the elastic–Raman-backscatterapproach. Likewise, as shown in Fig. 10 for 18 April1993, agreement is poor when the lidar ratio is notconstant with altitude. Of course during back-ground aerosol conditions, the Raman-channel ap-proach is less satisfactory because of small Ramansignal-to-noise ratios andminimal aerosol signatures.

5. Conclusion

We have described techniques that use the dualelastic–Raman-backscatter capability of the GSFC

Fig. 10. Aerosol-extinction-coefficient profiles at 351 nm for 18April 1993 at GSFC. One profile was created with the 382-nmRaman channel, and the other was computed with the inversiontechnique with an assumed constant lidar ratio.

lidar system to routinely measure aerosol propertiesin the stratosphere. Results obtained from 1991 to1994 are reported, characterizing various aerosolquantities at several worldwide sites and tracking thefallout of the Pinatubo aerosol cloud. A comparisonbetween the elastic–Raman-backscatter lidar analy-sis technique and the more common inversion tech-nique shows the shortcomings and advantages of eachwhen used in postvolcanic stratospheric conditions.We have shown that when dynamic conditions exist,the assumption of a constant lidar ratio over a rangeof altitudes, as is often used in the inversion ap-proach, is invalid. The elastic–Raman-backscattertechnique is a more accurate method to measure ASRand makes possible a means to measure aerosolextinction and lidar ratio accurately. The reducedsignal strength in the nitrogen Raman channel cancause larger uncertainties in the derived aerosolprofiles than would be expected from the inversionanalysis method; however, this is a statistical errorthat can be reduced by the use of system enhance-ments and averaging. The inversion approach, how-ever, suffers from systematic errors that are difficultto correct.

The authors would like to thank the staff at JetPropulsion Laboratory Table Mountain Observatoryin Wrightwood, California, the staff at the Observa-toire de Haute Provence at St. Michel Obervatoire,France, and the staff at the New Zealand NationalInstitute for Water and Air Research, Lauder, NewZealand, for their assistance in the three UARS@NDSC intercomparison campaigns. Thanks also tothe NASA Upper Atmosphere Research Program andthe UpperAtmosphere Research Satellite CorrelativeMeasurements Program for providing funding for theStratospheric Ozone Lidar Trailer Experiment project.Also thanks to Alternative Fluorocarbons Environ-mental Acceptability Study Inc. for providing travelfunds for some of the deployments.

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