Lidar setup for daytime and nighttime probing ofstratospheric ozone and measurements in polar andequatorial regions

Wolfgang Steinbrecht, Karl W. Rothe, and Herbert Walther

Measurements of atmospheric ozone at altitudes from 5 to 50 km using the lidar (DIAL) differentialabsorption technique are described. A XeCl excimer laser with a repetition rate of up to 50 Hz and awavelength of 308 nm was used for the measurements. The reference line was produced by the stimulatedRaman effect in a hydrogen gas cell, leading to a wavelength of 353 nm. The time-resolved measurement ofthe backscattered light allows one to evaluate the ozone concentration as a function of the altitude. Excep-tionally good agreement between the lidar data and those measured with chemical balloonsondes was

obtained. A narrow-band laser and a receiving system, including a set of three Fabry-Perot filters, also allowdaytime measurements. Thus investigations in polar regions are possible even in summer periods. Measure-ments performed in arctic and equatorial regions with the lidar system installed on board the research vesselPolarstern are reported.

1. Introduction

In this paper we report on measurements of strato-spheric ozone using the lidar (DIAL) differential ab-sorption technique' that were made in recent years.Ozone is one of the most important stratospheric tracegases that have been widely discussed in the recentyears since the discovery that anthropogenic pollutionof the atmosphere, e.g., by fluorochloromethanes, hasled to changes in the ozone layer.2 The discussion ofozone reduction reached its climax when the ozonehole in the Antarctic was observed. 3 -6

The maximum concentration of ozone is reached atan altitude of 20-25 km, being -500 ,g/m 3. The DIALlaser method is suitable for ozone survey work since itallows height-resolved measurements. To allow day-time measurements too, our earlier setup7 8 has beenmodified and considerably improved. In this paperthe new setup is described and the measurements fortesting the instrument are discussed.

H. Walther is with Max Planck Institute for Quantum Optics, D-8046 Garching, Federal Republic of Germany; the other authors arewith University of Munich, Physics Section, D-8046 Garching, Fed-eral Republic of Germany.

Received 9 September 1988.0003-6935/89/173616-09$2.00/0.© 1989 Optical Society of America.

II. Measuring Principle

The differential absorption technique is based onthe use of at least two different laser lines havingdifferent absorption cross sections for the gas understudy: one line must show large absorption, whereasthe other one has to remain unaffected.1 It is desirablethat the two lines be close to each other so that thebackscattering processes used to return a small frac-tion of the laser light to the receiver do not differ toomuch thus making additional corrections unnecessary.Since pulsed lasers are used the measurement of thebackscattered photons as a function of time can beconverted to distance information. The comparisonof the backscattered intensities at both wavelengthsfrom two adjacent altitudes yields the differential ab-sorption of the gas under study (if no corrections fordifferent backscatter cross sections for the wave-lengths used are necessary). The only further infor-mation required concerns the absorption cross sec-tions. With respect to ozone, the high-resolution dataof Bass and Paur9 are available, with the temperaturedependence being taken into account. These data aremore precise (2% or better) than previous measure-ments10 especially around 308 nm.

Lasers have been used several times in the past todetermine the ozone concentration in the stratosphere.However, since the laser systems used were not power-ful, the measurements were restricted to the lowerstratosphere.11-'4 Uchino et al.13 have already used apowerful XeCl excimer laser at 308 nm; however, they

3616 APPLIED OPTICS / Vol. 28, No. 17 / 1 September 1989

did not have a suitable reference wavelength. Themeasurements therefore were not highly accurate.This problem was circumvented in our setup by usingthe stimulated Raman effect.7

Many practical applications of the DIAL methodinvolve dye lasers, which allow the two wavelengths tobe chosen close to each other.15 In our measurements,however, the use of Raman shifted lines was moreadvantageous and much simpler; as a consequence thetwo wavelengths are separated by a larger amount.The wavelength dependences of the Rayleigh and Miebackscattering mechanisms therefore have to be con-sidered when the data are evaluated.

The single-scattering lidar equation is usually writ-ten as (see Refs. 1, 7, or 14):

M(R,X) = eFM 0 (X)ARfl(R,X)T(r,X)1/R 2, (1)

where M(R,X) is the number of photons backscatteredfrom a layer of depth AR at a distance R, is thedetection efficiency of the system, F is the area of thereceiving mirror, and Mo(X) is the number of photonsemitted per laser pulse at wavelength ; the termO(R,X) = n(R)(d(X)/dQ), describes the backscatter-ing by a medium of density n and differential back-scattering cross section (da(X)/dQ)X. In Eq. (1) r des-ignates the atmospheric transmission for the laserwavelengths used:

r(RX) = exp{-2 J [ax2(r,X) + as(rX)]dr},

where ag(r,X) is the absorption coefficient of the tracegas of interest, and a,2(r,X) is the attenuation coeffi-cient due to Rayleigh and Mie scattering. The absorp-tion coefficient is given by the product of the numberdensity and absorption cross section: ag(R,X) =N(R)W(X). The differential absorption technique isbased on the use of two wavelengths X, and 2 withdifferent absorption cross sections [(Xl) #- U(X2)] forthe gas under study. The gas concentration in a rangeinterval [R1,R2] with R2 = R, + AR is given by

N = 2AR[a(X2) -a(X1)]}-'Jn[M(R1,X1)/M(R,,)]

- n[M(R2 X1)/M(R 2 ,A2)]1 + B + A. (2)

To derive this formula, an infinitely short duration forthe laser pulse was assumed, corresponding to an infi-nite range resolution of the system. This assumptionis fulfilled in the actual setup since the laser pulseshave a duration of only 15 ns, corresponding to a rangeinterval of 2.25 m. For details see Refs. 1 and 15. Thecorrection terms B and A take the wavelength depen-dence of backscattering (B) and attenuation (A) byMie and Rayleigh scattering into account:

B = n[3(R 2 ,X1)/#(R 2 ,X2)] - ln[#(R1,X1 )/fl(R 1,X2 )],

A = -2AR[a(R*,X 1) - a(R*,X2 )1,

where R* is a distance in the interval [R1,R2], the exactvalue depending on the variation of the backscatteringsignal between R, and R2 .15

In many practical applications the two wavelengthsare very close together, in which case B and A can be

neglected; see Refs. 1, 15, and also 16 for a detaileddiscussion. For the measurements presented in thispaper, however, the two wavelengths are separated byat least 30 nm. Calculation of the ozone concentrationtherefore requires a knowledge of the ozone absorptioncross sections and of the correction terms B and A.

The main contribution to backscattering from thestratosphere in the UV spectral region results fromRayleigh scattering. A smaller but noticeable compo-nent in backscattering from altitudes of up to 30 kmcomes from Mie scattering, mainly caused by the dustoriginating from the eruption of the El Chichon volca-no in Spring 1982. This aerosol content is highlyvariable. In comparison with Rayleigh scattering,however, its wavelength dependence is much smaller.The necessary corrections for Mie scattering are there-fore small and can often be neglected. For optimumaccuracy, however, the aerosol contribution is deter-mined in our evaluation scheme by subtracting theRayleigh signal from the total backscatter intensity.The Mie contribution usually shows up in the plots ofthe total backscattering signal versus distance as smalllocalized peaks. This contribution can therefore easi-ly be identified, and it is done for the reference linefirst, the result then being extrapolated for the absorp-tion line and corrected. The Rayleigh background canbe derived with sufficient accuracy from known pro-files of the atmospheric pressure and temperature.For this purpose balloonsonde data or the CIRA refer-ence atmosphere'7 can be used. The change of theabsorption cross section of ozone as a function of tem-perature9 was corrected in the evaluation of the ozonedata.

The DIAL technique compares the relative magni-tude of the backscatter intensity at two adjacent rangecells for two wavelengths. This largely excludes in-strumental drift during the measurement. Many oth-er techniques for ozone monitoring suffer from instru-mental drift and therefore need special corrections.

Ill. Experimental Setup

The lidar setup featured a commercial XeCl excimerlaser system (Lambda Physik EMG 150TMSC) havinga pulse energy of 150 mJ, repetition rates of up to 50Hz, and a line width of less than 10 pm. The emittedwavelength (308 nm) coincides with the wing of astrong absorption band (Hartley-Huggins band) ofozone. The second wavelength, used as the referenceline, is generated by stimulated Raman scattering in ahigh pressure gas cell. The Raman medium was meth-ane or hydrogen, resulting in wavelengths of 338 and353 nm, respectively. Because both Raman shiftedwavelengths are only insignificantly absorbed byozone, their backscatter intensities from the upperstratosphere exceed that of the original XeCl laser line.

The use of stimulated Raman radiation proved to beadvantageous since it allows the two wavelengths to beemitted simultaneously into the same field of view.The measurement error is therefore reduced in rela-tion to other lidar systems, where the laser is tunedfrom one wavelength to the other by rotating a prism or

1 September 1989 / Vol. 28, No. 17 / APPLIED OPTICS 3617

Fig. 1. Scheme of the experi-mental setup. The transmitter isshown on the right and the receiv-

er on the left.

a grating inside the laser cavity, excluding simulta-neous measurements at two wavelengths. The Ramanmethod, in addition, also simplifies the setup and thealignment procedure, and ensures high pulse energy.The pulse energy conversion of the hydrogen Ramancell was adjusted to -40%. The cell pressure wasbetween 1.7 bar for hydrogen and -20 bar for methane.The latter was used only for the measurements on thesummit of the Zugspitze in the German Alps.

Figure 1 shows the arrangement schematically. Thelaser beam is focused into the Raman cell, generatingthe collinear reference beam with a similar divergence.Before the laser beams are sent into the atmosphere,their divergence is further reduced by using a beamexpander. The optical receiver consists of a collectingmirror with a diameter of 60 cm and a focal length of2.4 m. Its field of view-adjustable by an iris-is twicethat of the laser light divergence. A dichroic filter (D)separates the two spectral components in the back-scattered intensity with minimum losses. Interfer-ence filters (IF) and three-stage Fabry-Perot filters(FP) are used to reduce the bandwidth of the receivingsystem to -13 pm. The signals are detected by twophotomultipliers (PM, EMI 9893). The intensity isdetermined by photon counting. To avoid saturationof the photomultipliers by the signal backscatteredfrom low altitudes, a fast rotating mechanical chopper(CH) shuts the receiver system during and shortlyafter emission of a laser pulse.

A 512-channel fast memory unit is used to store thephoton counts, providing an altitude resolution of 200m. The signal processing is performed with a PDP 11/10 computer and the data are finally stored on floppydisks. The data storage procedure was usually thefollowing: backscattering signals of 1000 laser shotscorresponding to a measuring time of 40 s were firstaccumulated in the computer memory and then trans-

ferred to the floppy disk for later detailed evaluation(see Sec. IV).

The whole apparatus was installed in a laboratorycontainer designed for operation in both tropical andpolar regions. A predecessor of this system has beenused for several years to investigate the stratosphericozone profile at nighttime from the top of the Zugspit-ze.7,8

The operation of the system in polar regions duringthe summer periods implies daytime measurements.To make this possible, background radiation from thesky has to be reduced. This can be achieved by bothspatial and spectral filtering. Spatial narrowing canbe achieved rather easily just by reducing the field ofview of the receiver to about 0.2 mrad. Precise align-ment of all optical components is therefore necessary.

The reduction of the spectral width of the laser andreceiving system is not that easy. To get a sufficientlysmall bandwidth of the laser, an injection-locked XeClsystem was used, having a bandwidth of less than 10pm. The receiving system on the other hand wasequipped with interference filters as well as a two-stage and a three-stage Fabry-Perot system for the 308nm and 353 nm channels, respectively. (Daytimemeasurements have only been performed with the hy-drogen Raman cell.) The free spectral ranges of theFabry-Perot interferometers were -0.5 nm.

The three Fabry-Perot interferometers for the 353-nm channel have spacings of 120.3, 132.4, and 133.6Am. The spacers consist of three quartz disks whichare optically contacted to the mirror substrates. Thedifferences in distance guarantee that the interferenceorders of the interferometers do not overlap within asufficiently large spectral range: -50 nm for the com-bination of all three etalons. For only two of them(120.3 and 132.4 Am or 133.6 Mm) the overlap of thetransmission maxima occurred for a wavelength differ-

3618 APPLIED OPTICS / Vol. 28, No. 17 / 1 September 1989

ence of -5 nm. The Fabry-Perot mirrors are coatedwith dielectric multilayers with a reflectivity of 85%.

The halfwidth of the interference filter is 4 nm (fullwidth at half maximum) which is small in relation tothe resulting free spectral range of the three-etalonassembly. The interferometers were kept at a tem-perature of 20'C (stability better than 0.10) and weremounted in three different housings so that they couldbe pressure-tuned separately with SF6 (pressure -1bar) to match the laser wavelength. The pressure inthe interferometer housings was measured with a pie-zoresistive pressure gauge (Kistler type 4043 or 4061).The pressure had only to be readjusted every threeweeks. However, the laser wavelength was tuned tothe maximum of transmission of the interferometer setbefore each series of measurements. The overallbandwidth of the system was 15 pm (full width at halfmaximum).

The two Fabry-Perot interferometers for 308 nmhad spacings of 120.3 and 132.4 m. At this wave-length the background sky radiation is - one order ofmagnitude less than at 353 nm, and so it was notnecessary to suppress the background radiation fur-ther. The two-etalon setup gave a larger overall trans-mission at the laser wavelength; this resulted in asmaller statistical error for the signal backscattered at308 nm.

One important question is whether measurementswith the XeCl laser line and the other two laser linesare affected by trace constituents of the atmosphereother than ozone. The only stratospheric constituenthaving absorption in this wavelength region is SO2.The concentration of SO2 in the stratosphere is, how-ever, several orders of magnitude smaller than that ofozone so that no perturbation is possible.' 8

IV. Measurements and Evaluation Procedure

The first measurements with the XeCl-Raman lidarwere made from Zugspitze, the highest mountain in theGerman Alps, located -100 km south of Munich. Thecontainer with the lidar system was brought there inSept. 1982 and was in regular operation until July1984. Later the system was only occasionally used fortest measurements. At this time the lidar system didnot yet have the narrowband laser and the Fabry-Perotfilters; it could therefore only be used for nighttimemeasurements.

At the beginning of 1987 an improved version18 ofthe Zugspitze lidar was installed at HohenpeissenbergMeteorological Observatory of the Deutscher Wetter-dienst. The Deutscher Wetterdienst have been per-forming regular measurements with this instrumentsince mid-1987 in addition to the routine recording ofDobson and Brewer-Mast data. First results ob-tained with this instrument are given in Ref. 19.

After July 1985 the lidar system was removed fromthe Zugspitze and modified to make it suitable fordaytime measurements as well. The new and modi-fied version of the ozone lidar was then installed onboard the research vessel and icebreaker Polarstern atthe beginning of May 1987. Measurements were made

Fig. 2. Courses of the ozone lidar on board the icebreaker andresearch vessel Polarstern.

during a voyage from Bremerhaven to Tromso viaSpitzbergen (14 May-1 July, 1987). The course of thevessel during that period is shown in Fig. 2. Themeasuring periods are marked extrabold. The mainresults of the measurements are discussed in Sec. VII.

During another voyage of the Polarstern from Bre-merhaven to Rio Grande do Sul (Brazil) in Sept. andOct. 1987 a second series of measurements was made.Some of the results obtained with the lidar system onthe Zugspitze have already been published.7820 2 InSec. V-VII we summarize and highlight the main re-sults of all measurements performed so far.

To begin this discussion we briefly describe the eval-uation procedure of the backscattering data. Wementioned in Sec. III that during a measurement datawere stored on a floppy disk after averaging the signalsof 1000 shots. The actual evaluation procedure startswith a check of the consistency and steadiness of thesedata. Sometimes the backscattering signals were dis-turbed by small clouds passing through the laser beam,and so this first check was necessary. Bad data wereomitted from further evaluation. The rest of the datawere then averaged, usually in packages of 40,000 shots(measuring time -30 min).

From the time-averaged data the ratios of the loga-rithms ln[M(Ri,X,/M(R,X2)] Eq. (2)] were calculated.Before the concentration N was determined, these ra-tios were further smoothed by a running-mean proce-dure; thereby the data with an altitude resolution of200 m were further averaged so that, for example, at analtitude of 15 km the data resulting from an interval of

1 September 1989 / Vol. 28, No. 17 / APPLIED OPTICS 3619

1 km were averaged and at 35 km those from an alti-tude interval of 5 km; the range intervals for the aver-aging procedure were usually increased quadraticallywith the altitude.

From the smoothed ratios the ozone concentrationN is derived according to Eq. (2), where AR is adjustedto the varying range intervals according to the averag-ing procedure described above.

For the evaluation of N the values of A and B of Eq.(2) are needed. The value B was generally neglected inthe evaluation of the data taken during the two voy-ages of the Polarstern but not for the measurementsperformed from the Zugspitze, since there the Miescattering due to the El Chichon dust required correc-tions. In this case the Mie contribution of B and A wasderived as described in Sec. II by subtracting the Ray-leigh background from the Mie peak. The Rayleighpart of value A was derived from balloonsonde dataobtained during the measurement periods or by usingthe CIRA reference atmosphere.'6 The latter wasdone for altitudes where balloon data were not avail-able (above 30 km) or for most of the measurementsfrom the Zugspitze. It turns out that the correctionsfor altitudes above 25 km are mostly negligible. Dur-ing the voyages of the Polarstern radio balloonsondeswere regularly launched so that pressure and tempera-ture distributions for the evaluation of the data wereavailable. The change of the absorption cross sectionas a function of the temperature was considered in theevaluation of the data.

V. Intercomparison Between Ozone Lidar and ChemicalOzonesondes

The ozone profiles measured with the lidar on theZugspitze agree quite well with measurements per-formed with chemical ozone sondes, launched fromHohenpeissenberg -30 km from the Zugspitze. ThereBrewer-Mast electrochemical sondes have beenlaunched on a routine basis two or three times a weeksince 1967. (For a detailed description of the sondessee Ref. 22.) The sondes are carefully prepared, a factreflected in the low correction factors. They reach amaximum height of 35 km. The accuracy is -5% anddecreases with altitude in the stratosphere. Severalcomparisons with other ozone sondes were performedin the past,23 confirming the reliability of the dataobtained.

Owing to the inherent differences between the lidarand the electrochemical techniques, a comparison ofthe results obtained by the two techniques is far fromtrivial. First, the two locations where the measure-ments were performed are -30 km apart. Second, andmore important, the lidar probes vertically, whereasthe balloonsondes do not. Strong winds can causeballoons to drift more than 100 km during their ascent.Third, lidar data were usually averaged for a wholenight and therefore show a mean profile, whereas theballoonsondes give the momentary value.

The lidar profiles of each night were compared withthe balloon launches performed closest to them intime. The most frequent situation was a launch the

next morning, but some balloons were also launchedlater in the day. There were even two balloonlaunches performed at midnight to get a good compari-son. In those cases of close coincidence in time, excep-tionally good agreement of the profiles was found; seeRefs. 7, 8 and especially Ref. 21.

Summarizing, it can be stated that in general shortertime intervals between the two measurements resultedin better agreement. It was only for a few measure-ments that the discrepancy was significant. In thiscase, however, there was a large time interval betweenthe two measurements. On the other hand the balloonsondes basically no longer work well for heights >30km. A discrepancy between lidar and sonde profilestherefore has to be expected at these altitudes.

It should also be mentioned here that further com-parison was recently made between the data obtainedwith the XeCl Raman lidar at Hohenpeissenberg Me-teorological Observatory and those of Brewer-Mastsondes. Despite the fact that the time difference be-tween the measurements with the two methods was upto 10 h good agreement could be demonstrated.'9

Results of the Nighttime Measurements with the Lidar onthe Zugspitze

First, we discuss the statistical error of the backscat-tering signal. With a laser repetition rate of 50 Hz thevariance of the intensity which is backscattered froman altitude of 40 km is typically 10% when the signal isaveraged for a range interval of 1 km and a measuringtime of 20 min. For measurements at these high alti-tudes the signals from larger range intervals are usual-,ly averaged. If this is done for an interval of 3.5 km thevariance of the resulting ozone concentration is 2% fora 3-h measurement; a variance of 5% can be obtainedfor a measuring time of 0.5 h.

It should be mentioned here that the backscatteredintensity observed during the Zugspitze measure-ments was -1OX > that obtained when the setup wastested at lower altitudes in Garching near Munich.The gas constituents and the aerosols at lower alti-tudes considerably diminish the emitted and backscat-tered intensities.

The variability of the ozone concentration above 30km is relatively small because photochemical reactionsdominate the ozone concentration in the upper strato-sphere. However, the ozone content in the lowerstratosphere is highly variable, due to transport mech-anisms taking place mainly at low altitudes. As anexample Fig. 3 shows all ozone profiles measured inSept. 1983. Each profile was obtained by averagingthe signal during a whole night. The statistical errorsare small at all altitudes so that the variations of theprofiles from day to day correspond to real changes.

The influence of transport mechanisms shows upclearly in Fig. 3. Measurements started in the night of7-8 Sept. (first line) and continued, weather permit-ting, through the night of 29 Sept. The abscissa is thealtitude, increasing from right to left, while the ordi-nate represents the ozone concentration. The datashow very nicely that interesting information on the

3620 APPLIED OPTICS / Vol. 28, No. 17 / 1 September 1989

-1982 1983 1984

.2 80 . . . . . .~60-

WE

20

Fig.4. ON D J F M A M J J A S O N D J F M A M J J Amonth

Fig. 4. Ozone concentration measured at an altitude of 40 km withthe lidar system on the Zugspitze in the period Oct. 1982-July 1984.

The monthly averages are plotted.

20 30 40Altitude [km]

Fig. 3. 3-D representation of the temporal evolution of the ozoneconcentration during Sept. 1983 (1 ,ug/m3 corresponds to 1.25 X 1010

molecules/cm 3 ).

dynamics of the stratosphere can be extracted frommeasurements. It is obvious that a double maximumstructure built up at an altitude of 20 km in the period27-29 Sept., and it is probable that this was due to agravity wave in the stratosphere.

For further results see also Refs.7,8,17, and 21). Inthe following we present the longterm trends of themeasurements. As is obvious from Fig. 3, there is astrong variation of the ozone concentration below 30km due to transport processes in the stratosphere.Observation over a longer time period shows that sea-sonal changes also show up, occurring regularly in dif-ferent years. Figure 4 shows the monthly averages forthe 40-km altitude for the period in which our lidarsystem on the Zugspitze performed regular measure-ments. This altitude is of special interest since anozone reduction of -20% within the next twenty yearshas been predicted for this altitude (see, for example,Ref. 2). At the same time an ozone increase of aboutthe same amount is expected in the troposphere andlower stratosphere. This change is mainly due to tem-perature-dependent nitrogen oxide reactions. Botheffects together leave the total ozone content nearlyunchanged. Since the variability (daily and seasonal)of the ozone concentration at 40 km is much less thanat lower altitudes, the chance of seeing a variation ismuch larger in the upper part of the stratosphere. Thedata plotted in Fig. 4 show a seasonal variation with anincrease in the summer months, as expected owing tothe increased production of ozone. The productionrate essentially determines the ozone concentration atthese high altitudes. The seasonal variation amountsto -30% of the ozone concentration. It is therefore notpossible to see a predicted variation of -'1% per year.

E

0'

ISI

35 k~~1001 ~. x

J F M A M J J A S O N D Jmonth

Fig. 5. Seasonal variation of the ozone concentration. The datameasured in the same month of subsequent years are averaged for analmost biennial period (Oct. 1982-July 1984). The seasonal varia-tions can clearly be seen; they show opposite behavior at low (21 km)and high altitudes (30 km and above). The daily variability of themeasurements is much larger at altitudes below 30 km than above.

To achieve this aim, a much longer observation time isrequired.

The seasonal variation at lower altitudes is shown inFig. 5. The figure shows the monthly averages, includ-ing the same month of the subsequent year for analmost biennial period. It should be noted that thedaily variability of the data is much larger at altitudesbelow 30 km than above. The corresponding valueshave not been evaluated, but their variability is quiteobvious from the data presented in Fig. 3.

The few data presented show clearly the main ad-vantage of the lidar measurements (especially obviousfrom Fig. 3): besides the long-term trend the lidarmethod allows one to study the short-term variationsin a much more detailed way than the Brewer-Mastsonde. In addition, data at altitudes >30 km are avail-able.

VIl. Daytime Measurements in Polar Regions

The new and modified version of the ozone lidar wasinstalled on board the research vessel and icebreakerPolarstern at the beginning of May 1987. Measure-

1 September 1989 Vol. 28, No. 17 / APPLIED OPTICS 3621

-40kmx' 1& ~

ments were made during the voyage from Bremerha-ven to Troms6 via Spitzbergen (14 May-1 July, 1987).The course of the vessel during that period is shown inFig. 2. Whenever the weather allowed, measurementswere made. The measuring periods are marked extra-bold in Fig. 2. The main results of the measurementsare now briefly discussed.

In the first part of the voyage the ship travelled fastfrom 60' N-80' N. A decreasing ozone concentra-tion was observed at all altitudes with increasing geo-graphic latitude (see Fig. 6 and Ref. 24 for compari-son). The decrease is especially strong for lower alti-tudes (see, for example, results for 15 km). In thesecond part of the voyage the ship stayed in a regionaround 750 N. During this period no significant sys-tematic changes of the ozone concentration werefound.

It should be mentioned here that at least in normalweather conditions the motion of the vessel did notaffect the measurements: the roundtrip time of thelight was still short enough, even for measurements upto altitudes of 40 km, so that the pointing of the laserbeam and the receiving telescope were not separated.

The errors in Figs. 6 and 7 are statistical errors.They are strongly influenced by the background countrate due to daylight. As discussed above, the data areaveraged over range intervals increasing quadraticallywith the altitude. (At 15 km range interval 1 km; at 30km range interval 4 km.) Owing to the backgroundcount rate the error for a 5-h measurement at 30 km isabout six times as large as that obtained in the night-time. Measurements at 15 km result in about thesame statistical errors in the daytime and nighttime.

In the polar regions the maximum of the ozone con-centration is much lower than at our latitudes; mea-surements up to 30 km are usually sufficient (the totalerror for a 5-h measurement is then -80g/M 3). How-ever, in the equatorial region, where the ozone concen-tration reaches its maximum at -30 km, longer averag-ing times are necessary (see Sec. VIII). Therenighttime measurements are also possible, givingmuch better accuracy.

The results show one of the big advantages of thelidar technique, namely, it gives height resolution ofthe ozone concentration. At an altitude of 15 kmozone increases from 750 N to 800 N. This increase isprobably due to the downward motion of ozone rich airfrom higher altitudes being caused by the low tempera-tures in the north-polar region marked by the ice-edgearound 770 N, which is where the ozone concentrationat lower altitudes is increasing (see also Ref. 25).

Figure 7 shows an example of a long-time ozonemeasurement (about 36 h) while the ship was anchoredalongside a large icefloe. It is the only daytime lidarsystem that allows such time-continuous height-re-solved measurement of the ozone concentration. Themeasurement shows an ozone decrease at heightsabove 21 km and an increase at heights below 15 km.The steady increase at 12 km indicates an influx ofozone-rich air from other regions.

During the winter 1987/88 the ozone lidar system

ozone concentration

I g/m 3]

300

200

300

200

500 -

400 -

300

400

300 -

2001-

.' --~~~~~~~~~~~~~~~~~~~~~~~~~~~ h~ ~ ~ ~ ~

-S- 4 II 'I '

t 1-f I~~~~~~~~

__ -I--

1-t / \A lo I \'

altitude

27.5km

25km

20km

15km

60'N 70'N 80CNlatitude

Fig. 6. Ozone concentration at different altitudes as a function ofgeographic latitude, measured during the polar voyage. The ozonedecrease with increasing latitude can clearly be seen. The errorsfollow from the variance of the backscattering data. The change ofthe error as a function of altitude can also be seen. Where no barsare given the errors are negligible. (Two measurements were per-

formed at -79 N.)

ozone concentration attitude

300 _ _ i_ _ _9_ _ ic~ f24km200 F

21 km

300

400 ~~~~~~~~~~~~18 km

3 00

300 .5k

2001.

12 km200 _; . .,.... __._

04 06 08 10 12 14 16 18 20 22 00 02 04 06 08time IGMT) June 21-June 22

Fig. 7. Dependence of the ozone concentration (5-h mean values) atdifferent altitudes on the time of day between June 21 and 22, 1987

at 75.3° N 10.60 W.

3622 APPLIED OPTICS / Vol. 28, No. 17 / 1 September 1989

[,Ug/M31 33km�

100 ._. _. .- .

was installed at Kiruna, Sweden (67.510 N, 20.160 E).The first results of the measurements will be publishedin Ref. 26. They show the changes of the ozone con-centration owing to atmospheric transport processes,also demonstrated by changes in the stratospherictemperature distribution measured simultaneously byradiosondes.

Vil. Ozone Measurements during the Atlantic Voyagefrom Bremerhaven to Rio Grande do Sul (Brazil)

As on the Arctic voyage strong variations of theozone profiles with geographic latitude were observed.In the equatorial region ozone profiles are shifted to-ward higher altitudes, ozone peak concentrations arehigher, and the width of the profiles is smaller thanthat which is observed at higher latitudes. Profilesmeasured around 300 S at the end of October aresimilar to profiles measured at 500 N at the end ofSeptember (see Fig. 8). Towards the equator theozone content decreases at heights below 23 km where-as it increases at higher altitudes. These results agreewith the general trend in earlier observations (see, forexample, Refs. 27 and 28). This behavior might becaused by two processes: strong solar irradiance in theequatorial regions produces much ozone at higher alti-tudes and, the high temperature at the earth's surfacecauses an upward motion of the ozone-poor air fromthe troposphere into the lower stratosphere (see alsoRef. 25).

Because photochemical reactions strongly influencethe ozone concentration, a day-night variation of theozone profile is to be expected. In the mesosphere andupper stratosphere such a variation is well known (e.g.Ref. 29). To our knowledge no such variation has beenobserved up to heights of 30 km, since no technique hasallowed time-continuous, height-resolved observationof the ozone concentration with good precision in thisheight range. To overcome the problem of naturalvariations and geographical trends and to minimizethe statistical error, we averaged separately all day-time and all nighttime measurements. The mean pro-files obtained for daytime and nighttime ozone areshown in Fig. 9. Below 25 km ozone increases duringthe night, while above 25 km it increases during theday, whereas the total ozone from 15-38 km remainsnearly constant (244 t 8 DU for daytime and 241 1DU for nighttime profile). Despite the fact that thestandard deviations of the ozone concentration fromday to day are very large at these altitudes (-80 Mg/mi3

at 17 km and 40 Mug/M3 at 27 km), the observed trendalso shows up in most of the individual measurements.The observations thus indicate that there is a slightday-night variation in the height range from 15-35 km.It might be caused by an upward motion of the airbeing warmed up during the day; such a vertical trans-port process would leave the total ozone content un-changed.

IX. Conclusion

The Zugspitze lidar setup has been working success-fully since 1982. Its main features are high precision

0

400-

300-

200-

100 -

10 20 30 40

altitude km]

Fig. 8. Ozone profiles measured on the Atlantic voyage. The pro-files are averaged over all measurements taken in a 100 intervalaround the given latitudes. The total error of the measurements atan altitude of 30 km (for example) that follows from the variance of

the backscattering data is -20 pg/M3.

Eof

Mso0

ml.

N

ax

Ro

C0x"I

o0 L 0 10 20 30

attitude [km]40 50

Fig. 9. Mean daytime and mean nighttime profile on the Atlanticvoyage.

and lack of any drift. The XeCl-Raman lidar tech-nique therefore proved to be useful for detecting short-term and long-term trends in the concentration distri-bution of the stratospheric ozone layer.

The comparison of ozone profiles obtained by twodifferent methods (Brewer-Mast sondes and lidardemonstrates the high reliability as well as the distinctfeatures of both methods. In the height range from18-30 km nearly identical averages are found. Theuse of a new laser system and a narrowband detectionscheme made the system work reliably during the day-time as well as in extreme environmental conditions.

A pronounced decrease of the ozone concentrationwith latitude-especially at altitudes <25 km-wasfound on the polar voyage. During the equator voyagethe ozone concentration decreased with decreasing lat-itudes for heights <23 km and increased at heights >23km. The mean daytime and mean nighttime profilesshow a day-night variation. The averaged profilesshow that the ozone layer shifts toward higher alti-tudes, with the ozone maximum shifting from about 25km to 26.5 km, whereas the total ozone content re-mains nearly unchanged.

1 September 1989 / Vol. 28, No. 17 / APPLIED OPTICS 3623

20'N20-s

30-S

50N

The authors gratefully acknowledge the contribu-tion of J. Werner, who was in charge of the measure-ments on the Zugspitze. The intercomparison withchemical ozone sondes was the result of a cooperationwith Meteorologisches Observatorium Hohenpeissen-berg; we would like to thank W. Attmannspacher, K.Wege, and co-workers for their help. The polar mea-surements were performed in cooperation with Alfred-Wegener-Institut, Bremerhaven, from on board thePolarstern. We are especially indebted to E. Augsteinfor the possibility to participate in the Polarstern voy-age. We also thank S. El Naggar of the Alfred-We-gener-Institut for the help with the measurements.We thank the members of the mechanical and elec-tronic workshop of the Physics Section of the Universi-ty of Munich for their great personal effort in con-structing and setting up the equipment on theZugspitze as well as in constructing the new containerfor the Polarstern.

H. Walther is also with the Physics Section of theUniversity of Munich.

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