atmospheric chemistry and physics on retrieval of lidar extinction

Atmos. Chem. Phys., 10, 2813–2824, 2010www.atmos-chem-phys.net/10/2813/2010/© Author(s) 2010. This work is distributed underthe Creative Commons Attribution 3.0 License.

AtmosphericChemistry

and Physics

On retrieval of lidar extinction profiles using Two-Stream andRaman techniques

I. S. Stachlewska1,2 and C. Ritter1

1Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, 14473 Potsdam, Germany2Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 7, 02-093 Warsaw, Poland

Received: 8 July 2009 – Published in Atmos. Chem. Phys. Discuss.: 28 September 2009Revised: 15 February 2010 – Accepted: 15 March 2010 – Published: 25 March 2010

Abstract. The Two-Stream technique employs simultaneousmeasurements performed by two elastic backscatter lidarspointing at each other to sample into the same atmosphere. Itallows for a direct retrieval of the extinction coefficient pro-file from the ratio of the two involved lidar signals. Duringa number of Alfred-Wegener-Institute (AWI) campaigns ded-icated to Arctic research, the AWI’s Polar 2 aircraft with theintegrated onboard nadir-pointing Airborne Mobile AerosolLidar (AMALi) was utilised. The aircraft flew over a vicin-ity of Ny Alesund on Svalbard, where the zenith-pointingKoldewey Aerosol Raman Lidar (KARL) has been located.This experimental approach gave the unique opportunity toretrieve the extinction profiles with a rarely used Two-Streamtechnique against a well established Raman technique. Bothmethods were applied to data obtained for clean Arctic con-ditions during the Arctic Study of Tropospheric clouds andRadiation (ASTAR 2004) campaign, and slightly pollutedArctic conditions during the Svalbard Experiment (SvalEx2005) campaign. Successful comparison of both evaluationtools in different measurement conditions demonstrates sen-sitivity and feasibility of the Two-Stream method to obtainparticle extinction and backscatter coefficients profiles with-out assumption of their relationship (lidar ratio). The methodhas the potential to serve as an extinction retrieval tool forKARL or AMALi simultaneous observations with the spaceborne CALIPSO lidar overpasses during the ASTAR 2007.

Correspondence to:I. S. Stachlewska([email protected])

1 Introduction

Retrieval of the particle microphysical parameters (particleeffective radius, index of refraction and size distribution),from lidar derived optical properties of particles in the at-mosphere (particle extinction and backscatter coefficient pro-files) consists of a mathematically ill-posed inversion prob-lem (Bockmann, 2001). The emergent efficiencies, typicallytaken from the Mie theory, act differently for extinction andbackscatter coefficients. Hence, any inversion of microphys-ical parameters fed with both coefficients calculated indepen-dently is performed more precisely, especially for the deter-mination of the particle size distribution (Muller et al., 1999;Veselovskii et al., 2002; Bockmann and Kirsche, 2006). Ob-taining information on the particle extinctionαpart(h) andbackscatterβpart(h) coefficients without the often used as-sumption of their relationship signifies a great step forward inthe interpretation of lidar data. Unfortunately, the commonlyused elastic backscatter lidar cannot alone provide completeinformation for the inversion of the microphysical parame-ters (two unknown coefficients in one equation describinga lidar return signal, Eq.1). The standard Klett-Fernald-Sasano approach for the evaluation of elastic backscatter li-dar data (Klett, 1981, 1985; Fernald, 1984; Sasano, 1985)requires knowledge or an assumption of the backscatter coef-ficient calibration valueβref and the lidar ratioB(h)=

αpart(h)βpart(h)

.The latter is usually not a very well known atmospheric prop-erty, as it varies significantly with the chemical compositionand size distribution of the aerosol particles present in theatmosphere (Ackermann, 1998).

If the elastic backscatter lidar is additionally equippedwith the Raman-shifted detection channels then an indepen-dently obtained extinction profile can be contributed towards

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2814 I. S. Stachlewska and C. Ritter: Two-Stream and Raman techniques

analytical solution for the retrieval (Ansmann et al., 1990,1992). The cross-section for the inelastic Raman scatteringof laser light with matter is almost three orders of magni-tude lower than the cross-section for the elastic Rayleigh/Miescattering. This results in significantly noise polluted signalsobtained from the Raman channels. The Raman signals areusually strongly averaged in time and range for further anal-yses, which can severely influence the results of the Ramanextinction coefficient retrieval (Pornsawad, 2008).

There is another raerly applied approach, which providesindependent information to the classic solution of the lidarproblem. The Two-Stream inversion, also referred to as thebipath or the double-ended lidar technique, requires two elas-tic backscatter lidars pointing at each other. This methodwas introduced in the 1980s (Kunz, 1987; Hughes and Paul-son, 1988), and later revised for application to ground basedlidars pointing at each other horizontally (Jorgensen et al.,1997); zenith-pointing ground based lidar and nadir-pointingairborne lidar (Stachlewska et al., 2005; Ritter et al., 2006)and also discussed for zenith-pointing ground based lidar andnadir-pointing space borne lidar (Cuesta and Flamant, 2004;Wang et al., 2007). The Two-Stream technique allows for thedirect retrieval of a range/height dependent extinction coef-ficient with the only assumption being that the atmosphere,sampled from opposite directions by the two lidars, is thesame. With this method the backscatter coefficient can alsobe obtained if any of the employed lidar instrumental con-stantsC or a backscatter reference valueβref at any givenheight in the interval covered by lidars’ simultaneous obser-vations is known.

In this paper we present a study dedicated to the di-rect comparison of the Two-Stream particle extinction andbackscatter coefficient profiles, and the lidar ratio profileswith the respective Raman retrievals. The Two-Streammethod was applied to data recorded during simultaneousmeasurements taken with the nadir-pointing Airborne Mo-bile Aerosol Lidar (AMALi), integrated onboard the AWIresearch aircraft Polar 2. The aircraft flew over the zenith-pointing Koldewey Aerosol Raman Lidar (KARL) based inNy Alesund on Svalbard. Both lidars and their configurationsduring the measurements are discussed in Appendices A andB. The Two-Streamβpart

TS (h), αpartTS (h) andBTS(h) profiles ob-

tained on 15 May 2004 and 19 May 2004 during the theArctic Study of Tropospheric clouds and Radiation (AS-TAR) campaign and on 14 April 2005 during the SvalbardExperiment (SvalEx) campaign are discussed. On each ofthese days the KARL performed inelastic Raman and elas-tic backscatter measurements for which the Ramanα

partRM(h),

βpartRM (h) andBRM(h) were retrieved. Good agreement of the

results obtained with the two evaluation techniques provesthe feasibility of the Two-Stream methodology for the ap-plication to the nadir-pointing, low altitude, airborne lidarmeasurements.

In the future we plan to evaluate the data collected dur-ing the ASTAR 2007 by the zenith-pointing airborne AMALiand the satellite CALIPSO lidar (Winker et al., 2007) to per-form a feasibility study of this configuration.

2 Theoretical background

In the situation presented here the measuring scheme con-sisted of a zenith-pointing ground based lidar (denoted “K”),flown over at a heighthf by a nadir-pointing airborne li-dar (denoted “A”). Assuming that the same air column wasprobed when the airborne lidar overflew the ground basedlidar, both systems perceived this same air differently; theground based system with density decreasing with height andvice versa for the airborne instrument. This ensured math-ematically independent information content in both lidarequations. The elastic lidar equation (Kovalev and Eichinger,2004) describes the received signal as a function of the at-mospheric and system parameters. Assumptions of quasi-monochromatic coherent emitted laser light and instanta-neous elastic or inelastic scattering are taken into account,while processes of multiple scattering of light are neglected(discussed later). The lidar equation is usually used in a formof the range corrected signalS(h), obtained by multiplica-tion of the detected signal with the squared range vector. Theground based lidar equation can be written as in Eq. (1) andthe airborne lidar equation as in Eq. (2).

SK(h) = PK(h)h2= CKβ(h)T 2

[0,h](h) (1)

SA(h) = PA(h)(hf −h)2= CAβ(h)T 2

[hf ,h](h) (2)

Theh denotes the distance between lidar and target particlesor molecules. TheP(h) denotes then the intensity of the de-tected backscattered signal at a timet = 2h/c, with c beingthe speed of light. TheC is the lidar instrumental constant.Theβ(h) = βmol(h)+βpart(h) is the total backscatter coef-ficient, due to molecules and particles present at the heighth. The last termT describes the atmospheric transmittance(Eq. 3) between the ground based or airborne lidar and theheighth.

T (h,λ) = exp

(−

∫ h

h0

α(h,λ

)dh

)(3)

Theα(h) = αmolscat(h)+α

partscat(h)+αmol

abs(h)+αpartabs(h) is the to-

tal extinction coefficient dependent on the total number ofmolecules and particles scattering and/or absorbing the laserlight at the heighth.

In the Two-Stream approach the simultaneous equationsystem has four unknowns (two unknown lidar instrumentalconstantsCK andCA and unknownα andβ coefficients). Bydividing Eq. (2) by Eq. (1) the backscatter coefficient termsare eliminated and an expression for the height dependent ex-tinction coefficient can be calculated (Eq.4). Obtained this

Atmos. Chem. Phys., 10, 2813–2824, 2010 www.atmos-chem-phys.net/10/2813/2010/

I. S. Stachlewska and C. Ritter: Two-Stream and Raman techniques 2815

way,αTS(h) does not require any a priori assumption or fur-ther calibration and normalisation.

αTS(h) =1

4·

d

dh

(ln

(SA(h)

SK(h)

))(4)

After the retrieval of the extinction coefficient, the atmo-spheric transmittance can be obtained, and hence the ra-tio between both lidar instrumental constantsCA

CKis calcu-

lated by division of Eq. (2) by Eq. (1). The CA and CKcan be estimated from the Eqs. (1) and (2) if at any heightwithin the Two-Stream application range theβref or B(h) areknown. The assumption that exactly the same air parcels areprobed by both lidars during over-flights implies that onlyone representative profile ofα(h) andβ(h) exists. Thereforethe knowledge of any of the lidar instrumental constants al-lows for a direct calculation of the backscatter profile fromEqs. (1) or (2). Alternatively, the backscatter coefficient pro-files can be derived from the Two-Stream approach by mul-tiplying Eq. (1) by Eq. (2).

3 Lidar instrumental constants

Usually the lidar instrumental constantC is not known pre-cisely. For the standard elastic Klett-Fernald-Sasano and theinelastic Raman-Ansmann evaluation schemes it is redun-dant (height derivative of height independent variable). Anyexplicit calculations or measurements ofC are difficult andsuffer from considerable error contributions, mainly due tothe instabilities of the emitted laser energy, the transmissionof the optical elements, and the detection efficiency (changeswith applied voltage and surrounding temperature).

In our case, the lidar instrumental constants for the air-borneCA lidar and the ground basedCK lidar can be es-timated directly from the Eqs. (1) and (2) if at any heightwithin the Two-Stream application range additional informa-tion on the reference valueβref (e.g. known for aerosol freerange in tropopause) or the lidar ratioB(h) (e.g. knownBCifor Cirrus clouds, as in this study) is available. When any ofthe two lidars sense the whole troposphere, and the particleoptical depthτpart

sun (λ) is known (performed nearby sun pho-tometer measurement), the lidar constant of that system, e.g.CK can be derived by rewriting the Eq. (1) to Eq. (5). How-ever, Eq. (5) holds only for heightsh in the high troposphereor the stratosphere, were the particle extinction coefficientαpart can be neglected and the molecular extinction coeffi-cientαmol can usually be assumed or obtained from nearbybackscattering ratio radiosonde profiling.

SK(h) = CKβ(h)exp(−2τ

partsun

)exp

(−2∫ h

0αmoldz

)(5)

By estimatingβ(h) by βmol(h) derived from the density andtemperature profiles measured by the radiosonde, theCK can

be obtained as the mean value over all height incrementsi inthe high troposphere or the stratosphere (Eq.6).

CK =

⟨SK(i)

β(i)exp(−2τ

partsun

)exp

(−2∫ i

0 αmoldz) ⟩

i

(6)

For the purpose of this particular study theCK was estimatedseparately for each of the analysed days. It was estimatedin the aerosol-free range of the Arctic stratosphere between10–12 km (whereβ(h)≈βmol(h)) by using a standard Klett-Fernals-Sasano elastic inversion of the KARL’s data with thefollowing constrains:

– τpartKFS obtained from Klett-Fernals-Sasano particle ex-

tinction profile in the whole range where it was applied,must be within 10% of the sun photometer valueτ

partsun .

Note: Measurements were taken in the Arctic strato-sphere, where the range corrected lidar signals normallyfollow the density profile measured by a colocated ra-diosonde very accurately. The stratospheric particle ex-tinction can be neglected and the Eq. (5) holds true ifthe particle extinction in the lidar signal is expressedby the sun photometer’s particle optical depth measurednearby (for this reason we constrainτ

partKFS with τ

partsun ). If

so, knowledge ofβ[0,hov] andα[0,hov] in the overlapregion (hov denotes the overlap height) does not influ-ence the determination ofCK from the Eq. (5) becausewe express the integral from zero tohref in stratosphereby τ

partsun and we do not depend on the altitude where the

extinction takes place. For this kind of lidar calibrationour polar site conditions are more favorable comparedto a heavily polluted low latitude site. The knowledgeof a reasonably chosenβ(h) will practically limit theprecision for the retrieval of the lidar constant.

– τpartKFS obtained from Klett-Fernals-Sasano particle ex-

tinction profile in the range corresponding to the rangewhere the Two-Stream was applied should match to theτ

partTS obtained from the Two-Streamαpart

TS better than5%.

Note: We need to estimateCK to make the backscat-ter coefficient profile calculated with the Two-Streamtechnique independent from the backscatter coefficientprofile calculated by the Raman technique. We estimatethis lidar constant and its uncertainty from the elastic li-dar equation with several Klett solutions, which matchto all avaliable information. In the range of altitudeswhere the Two-Stream is applicable we know the ex-tinction coefficient profile but the lidar ratio in this rangedepends onβpart

KFS, which in turn depends on the assump-tions above that Two-Stream region.

– altitudes where Cirrus or sub-visible clouds were de-tected by KARL at about 9 km were treated with a rel-atively low and uniform lidar ratio ofBCi = 12 sr, a

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value obtained by the transmittance method (Chen et al.,2002) applied to cases in this study.

Note: Cirrus clouds show a large spread in lidar ratios(Sassen and Comstock, 2001; Gayet et al., 2006; Imm-ler et al., 2008) which are, among others, related to theirtemperature and altitude of occurrence, as these factorsdetermine the form of the ice crystals in the cloud.Chenet al. (2002) used the transmittance method (explainedtherein) to deriveBCi = 29± 12 sr over Taiwan.Re-ichardt et al.(2002) reported values between 7 sr and30 sr at 355 nm lidar wavelength for a subarctic site. TheHigh Spectral Resolution Lidar measurements suggest avalue around 28 sr as being appropriate for Arctic Cirrus(Eloranta et al., 2006). For cases presented in this paperwe used a lidar ratio of 12 sr if Cirrus cloud appearedin the 532 nm signal. However, this number holds trueonly for the limited samples given here and not for allCirrus clouds over NyAlesund.

– underestimated particle extinction from the ground tothe KARL’s completed overlap at a heighthov wasaproximated for all height steps as a constant valueequal to the value of the particle extinction obtainedwith the Two-Stream at a heighthov (i.e. αpart

KFS(0 : 1h :

hov) = αpartTS (hov)).

As the flight altitude does not necessarily take place in theaerosol free range it is difficult to estimate the referencebackscatter value. A reasonably well chosenβref can begiven for the groundbased system in the lower stratospherewhere the clear sky condition holds with good precision. Ifthe KARL’s βref is fixed in the lower stratosphere, the valueof the backscatter at the Two-Stream range depends on thelidar ratio above the aircraft. Hence, for our calculations anassumption on the lidar ratio was also made. Due to the factthat the Arctic atmosphere was relatively clear above the air-craft’s flight altitude any reasonably chosen lidar ratio for theremaining altitudes (10 sr<B<50 sr) did not significantly al-ter the standard elastic Klett-Fernald-Sasano solution. Sixdifferent profiles were obtained for minimal, average andmaximal values of the two quantitiesβref andB (each corre-sponding to a slightly different value ofCK). The final valueCK used for the retrieval of the Two-Streamβ(h) was cal-culated as a mean averaged over all height positionsi in thereference range and over all of the different standard inver-sion solutionsj as in Eq. (7). From the scattering ofCK(j)

around its mean value an uncertainty of 5% for the determi-nation of the lidar constant was obtained.

CK =

⟨⟨SK(i)

β(i) T 2[0,i]

⟩(i)

⟩(j)

(7)

4 Experimental results

The applicability of the Two-Stream method depends criti-cally on the constraint that both lidars probe into the same airmasses. The best matching periods for the over-flights werefound by correlating the measured signals using an a prioricriterion (based on the input signals which do not depend onthe results) for which profiles of both systems resembled inthe bast way a common state in the atmosphere (Ritter et al.,2006). This was done by constructing a correlation map be-tween both lidars’ data sets corresponding to the time of theover-flight. In principal other methods might be feasible aswell. For all data sets at timesti , tj the correlation coefficientwas calculated accordingly to Eq. (8) in which extinction cef-ficient was obtained with assumption of a constant lidar ratio(B = 30 sr) using standard Klett-Fernald-Sasano inversion.

CCi,j : = corr

⟨SA(h,ti) ·exp

(+2∫ hA

ov

hKov

α(h)dh

),

SK(h,tj ) ·exp

(−2∫ hA

ov

hKov

α(h)dh

)⟩(8)

ThehKov denotes the height beyond which the KARL’s over-

lap is completed (650 m) andhAov denotes the height were the

AMALi’s overlap is completed (235 m).The correlation coefficient was found a weak systematic

function on position. The aircraft flew several legs (gener-ally 3) over Ny Alesund along coastline/fjord. There wasalways a fixed position in space where for the airborne sys-tem the correlation was maximal, with only weak time de-pendence to the groundbased system. For the limited dataset at the coastal site the position was more important thanthe temporal evolution of the aerosol load. As our terrainwas not homogeneous the best matching result for the Two-Stream technique at the minimal distance between the foot-prints of the two lidars cannot be expected. An example ofa correlation map can be found inRitter et al.(2006). In themap’s structure, places (and times) where the data sets be-tween AMALi and KARL match each other better or worsewere sistematically found. Hence, the correlation map andthe comparability of the data show a physical sense and notjust casual fluctuations.

The choice ofB influenced the values of the correlationcoefficients but it did not have an effect on their relative min-ima and maxima. Moreover, if one lidar recorded clouds oraerosol layers at a different altitude to the other lidar (typicalsituation near the coastline) it was easily detected as a shiftin the data sets (very low correlation in these cases). Forall days under consideration the data sets heve been selectedfor evaluation at times that provided the absolute maximumof the correlation coefficient. As a result more range thanrequired by constraints of each lidar’s overlap was excludedfor calculation (different air masses directly above the stationand directly below the aircraft’s flight path).



Both data sets were averaged over 60 m in altitude. Theshortest possible temporal averaging providing sufficientSNR (above 10 for a single 60 m range bin is required) wasapplied: 10 mn for the ground based system and 8 mn forthe airborne lidar. TheαTS(h) profiles were derived di-rectly from Eq. (4) without any noise-treatment, as the ex-tinction coefficient retrieval is methematically an ill-posedproblem and even slight noise filtering can sevearly influencethe inversion result (Pornsawad, 2008). Smoothing was ap-plied only to already calculatedαTS(h) by a running mean of300 m.

The Rayleigh extinctionαmol(h) and backscatterβmol(h)

profiles due to the existence of molecules in the probed at-mosphere were calculated from the temperature and pres-sure profiles measured by daily radiosonde launches at theKoldewey Station in NyAlesund. These were substracted(Rayleigh calibration) from the totalα(h) andβ(h) profilesto obtain the particleαpart(h) andβpart(h) profiles. The samesubstraction procedure was applied to the Two-Stream, theRaman, and the Klett-Fernald-Sasano approach.

The absorption contribution at 532 nm (mostly due toozone) is negligible in the Arctic troposphere and thus wasnot considered in this analysis.

The particle optical depth calculated with the Two-Streamapproach, i.e.τpart

TS , was obtained by integration of theαpartTS

profiles over the range interval avaliable for the Two-Streamapplication, which was estimated using the correlation algo-rithm applied prior to the evaluation (Eq.8). Informationon the particle optical depth of the whole atmosphereτ

partsun

at 532 nm was obtained from almost simultaneous measure-ments using the multi-channel spectrophotometer SP1A-14(instrument described in Appendix C). The tropospheric par-ticle optical depthτpart

KFS was integrated fromαpartKFS over the

avaliable range of the KARL’s standard elastic Klett-Fernald-Sasano retrieval.

We used the inter comparisons of the particle optical depthobtained within the Two-Stream rangeτpart

TS (lower tropo-

sphere), the sun photometer’s particle optical depthτpartsun

(whole atmosphere), and the ground based lidar particle op-tical depthτpart

KFS (almost whole troposphere) for the determi-nation of the lidar constantCK . Such comparisons are moreappropriately applied to situations with as little as possiblecloud contamination. As the sun photometer and the zenith-pointing lidar were aimed at different directions in the Arc-tic atmosphere (low sun elevations) we cannot assume thatthere were time intervals where lidar and sun photometer si-multaneously saw no cloud. Hence, a cloud screening wasdone for both instruments separately (in the case of the sunphotometer the data with low particle optical depthτ

partsun and

high Angstrom exponentA were chosen). The comparisonwas done for the clearest time intervals, i.e. the lowest Cirruscontamination in the upper troposphere. In this case a 10%error ofτpart

sun = 0.1 gives rise to an error of approximately 2%in the aerosol extinction.

Fig. 1. The Two-Stream retrievals (solid) ploted with Raman (dashed) profiles for 532 nmon 15 May during the ASTAR 2004 campaign.figure

29

Fig. 1. The Two-Stream retrievals (solid) ploted with Raman(dashed) profiles for 532 nm on 15 May during the ASTAR 2004campaign.

The ground based lidar instrumental constantCK wasestimated in the aerosol-free calibration range between 10and 12 km with constraint on the best agreement of theτ

partKFS, τ

partTS andτ

partsun . For 15 May 2004 the best agreement

was obtained for a reference valueβpartref =0.03(±0.05) ·βmol

(backscattering ratio(βmol+βpart)/βmol

= 1.03) resulting ina lidar constantCK = 1.65(±0.1)×1014 mV m3 sr. For thesame conditions on 19 May 2004 significantly higher valueCK = 2.03(±0.1)×1014 mV m3 sr was obtained (due to im-plementation of a new flashlamp in the KARL’s hardwareand an increase of temperature in the laser room). For thedata of 14 April 2005 the best agreement was obtained forβ

partref =0.02(±0.05) · βmol (backscattering ratio of 1.02) re-

sulting in a lidar constantCK = 1.65(±0.1)×1014 mV m3 sr.

With knownCK andαpartTS (h) profiles we obtainedβpart

TS (h)

profiles directly from Eq. (1) for each of the three days. Theairborne lidar instrumental constantCA was obtained fromEq. (2). TheCA = 1.43(±0.1)×1013 mV m3 sr was calcu-lated on 15 May 2004 and 19 May 2004. On 14 April 2005it was CA = 3.8(±0.1)×1013 mV m3 sr (due to a setting of50 V higher PMT voltage).

The Two-StreamαpartTS (h), β

partTS (h) andBTS(h) profiles at

532 nm were compared to the respective profiles derived us-ing a standard method for the Raman retrieval (Ansmannet al., 1990, 1992) applied to the KARL’s signals measuredover NyAlesund. The Ramanαpart

RM(h) profiles were derivedfrom the inelastic scattering lidar equation for 607 nm, andRamanβ

partRM (h) profiles were calculated from the ratio of

532 nm and 607 nm signals with the mentionedβpartref , for con-

sistency. The Raman evaluation was performed with 20 mnintegrations in time to assure sufficient SNR and (similarly



Fig. 2. The Two-Stream retrievals (solid) ploted with Raman (dashed) profiles for 532 nmon 19 May during the ASTAR 2004 campaign.

30

Fig. 2. The Two-Stream retrievals (solid) ploted with Raman(dashed) profiles for 532 nm on 19 May during the ASTAR 2004campaign.

to the Two-Stream case) only retrievedαpartRM(h) profiles were

smoothed by a running mean of 300 m. The 10 mn attenu-ated backscatter measurements used for the Two-Stream re-trieval were centered within the 20 mn Raman data acquisi-tion sequence for 19 May 2004 and 14 April 2005. Due tothe warm up of the KARL lidar a shifted mean (09:55–10:15)was used for the Raman evaluation on 15 May 2004 (Fig. 1),compared to a center of 10:00 for the Two-Stream. The bestmatching 20 mn interval available for the Raman compari-son was then applied. For the particle extinction retrievalwith the Raman method theAngstrom exponentA was pre-defined. TheAngstrom exponent was derived from the sunand star photometer measurements over almost a decade atthe observational site at NyAlesund, taking into account abroad variation depending on season and condition (e.g. oc-currence of Arctic Haze, background aerosol, etc.) and gaveon averageA= −1 for May (Herber et al., 2002). As the sub-visible Cirrus were frequently detected in the lidar profiles athigh altitudes at this site a slightly lower valueA= −1.2 waschosen for all Raman retrievals discussed in this paper. Thisassumption results in the error of the retrieved Raman parti-cle extinction being less than 2%. However, this error mightbe larger in the ranges directly below/in an inversion layerwhere the size, shape and possibly even the chemical com-position of the scattering particles may drastically changewith altitude. The Two-Stream approach is free of this errorsource as it directly utilises two elastic signals and thereforethe Two-Stream extinction profile is retrieved independentlyof the choice of theAngstrom exponent.

The Two-Stream profiles (solid lines) and the Raman pro-files (dashed lines) for three days under consideration arepresented in Figs.1, 2 and3.

Fig. 3. The Two-Stream retrievals (solid) ploted with Raman (dashed) profiles for 532 nmon 14 April during the SvalEx 2005 campaign.

31

Fig. 3. The Two-Stream retrievals (solid) ploted with Raman(dashed) profiles for 532 nm on 14 April during the SvalEx 2005campaign.

For 15 May 2004 (Fig.1) the Two-Stream method was ap-plied to data recorded around 10:00 UT at a height intervalbetween 635 and 2435 m. Two aerosol layers of strongly en-hanced particle extinction and lidar ratio, indiscernible in theparticle backscatter, are clearly visible in the Two-Stream aswell as in the Raman retrievals. The upper layer has a max-imum at 1800 m in the Two-Streamαpart

TS (h) profile, while

in the RamanαpartRM(h) profile this maximum is 100 m lower.

The lower layer has a maximum at around 900 m for bothretrievals with higher particle extinction values in the Two-Stream profile. The particle optical depth calculated fromthe Two-Stream intervalτpart

TS sums up to 0.064, mainly dueto contribution from the mentioned layers. The sun pho-tometer particle optical depthτpart

sun measured at 10:00 UT inNy Alesund was 0.095 for 532 nm. The radiosonde ascentat 11:00 UT in NyAlesund confirmed the existence of twolayers. One at the altitude of 1200 m was characterised bythe temperature gradient of1Tinv = 0.014◦C m−1 and thehumidity gradient of1RHinv = 0.264% m−1). The secondlayer was found at 1800 m with1Tinv = 0.01◦C m−1 and1RHinv = 0.525% m−1. During this measurement AMALiand KARL recorded volume depolarisation below 5% at532 nm. The calculations of backward trajectories performedwith the NOAA Hysplit Model (Draxler and Rolph, 2003)suggest, that the air remained isolated in the Arctic for atleast six days. Apart from these two humid layers the air wasvery clean with particle extinction background values around1.5× 10−5 m−1 and a lidar ratio around 20 sr. These val-ues are characteristic for the clean Arctic air (Stachlewska,2006a; Hofmann et al., 2009).

The data from around 09:35 UT on 19 May 2004 wereanalysed within a height interval of 815 and 2075 m (Fig.2).The obtained retrievals coincided well displaying only small



deviations at around 1800 m. Here again a layer of enhancedparticle extinction and lidar ratio around 80 sr is visible. Onthis day even lower values of the particle backscatter wereobserved. Apart from the layer, the air appears clean, simi-larly to the previous day. Theτpart

TS was 0.02 and measured

at 10:00 UT on that dayτpartsun was 0.11 for 532 nm. The re-

diosonde released at 11:00 UT recorded a layer at 1800 mwith 1Tinv=0.012◦C m−1 and1RHinv=0.14% m−1. Bothlidars again measured very low volume depolarisation at532 nm. According to backward trajectories, the air streakedthe coastline of north-western Sibiria three days prior to itsarrival in Ny Alesund.

On 14 April 2005 at around 14:45 UT (Fig.3) the heightinterval between 660 and 2640 m shows strongly enhancedβpart(h), αpart(h) andB(h) profiles (if compered with bothprevious days). The lowest values of the particle extinctionare around 0.3×10−4 m−1 corresponding to the maximum ofthe measured values on 19 May 2004. Likewise, the valuesof the backscatter are much higher than previously varyingbetween 1–2.2×10−6 m−1 sr−1. The lidar ratio varies around34 sr sugesting slightly polluted Arctic atmosphere. Theτ

partTS

sums up to 0.076 and the sun photometer measurement at14:45 UT recordedτpart

sun of 0.084 for 532 nm. At the time cor-responding to the evaluation there was no evidence of Cirrusor sub-visible clouds in the upper troposphere in the KARL’ssignals. Neither KARL nor AMALi recorded significant de-polarisation signature at 532 nm. The radiosonde ascent at12:30 UT showed a very weak inversion layer at 700 m with1Tinv=0.005◦C m−1 and1RHinv=0.066% m−1, and no ev-idence of other layers up to the tropopause. The decreasein temperature and relative humidity droping from 50% at700 m to its minimum of 22% at about 2000 m and risingagain to reach 28% at 2500 m, showed similarities with theparticle extinction profile. The backward trajectories indi-cated uniform transport straight form the industrial part ofSiberia, a typical precursor for an Arctic Haze event.

4.1 Error analysis

The error analysis for the Two-StreamαpartTS (h) andβ

partTS (h)

profiles were performed according to error propagation. TheSNR was determined for each lidar signalP(h) with con-sideration of a height independent electronic noiseµ anda photon noise for the calculation of a height dependent errorE(h) = λ

√P(h)+µ. The electronic noiseµ was estimated

out of the background corrected raw data at the range whereno laser light influenced the signals, i.e. for KARL at an al-titude interval between 60 and 120 km, and for AMALi ina pretrigger range of 400 m width.

Useful procedures for estimatingλ are described inLiuet al. (2006). In the data presented here the values forλ

were estimated from altitude intervals with constant aerosolload where variations in the lidar profiles on a scale of indi-vidual height increments were assumed to be caused purelyby noise. For KARL this was in the lower stratosphere and

Fig. 4. The signal to noise ratio of AMALi and KARL lidars’ raw data with the Two-Stream particleextinction and backscatter error values for 15 May 2004 (dashed), 19 May 2004 (solid) and 14 April2005 (dotted). Values refer to 10 mn and 60 m averaging at 532 nm.

32

Fig. 4. The signal to noise ratio of AMALi and KARL lidars’ rawdata with the Two-Stream particle extinction and backscatter errorvalues for 15 May 2004 (dashed), 19 May 2004 (solid) and 14 April2005 (dotted). Values refer to 10 mn and 60 m averaging at 532 nm.

for AMALi in the layers which showed the least height de-pendent particle contamination in theαpart

TS (h) andβpartTS (h)

profiles. This caused an overestimation of the noise in theairborne lidar signals when the variability of aerosol in thechosen range interval was present and, hence, a lower limitof the SNR of the airborne lidar was considered.

The molecular extinction and backscatter coefficientsαmol(h) andβmol(h) necessary for the Rayleigh calibrationwere calculated from radiosondes profiling. The error in theair density was estimated to be at most 2% for the time differ-ence within two hours with respect to the Two-Stream calcu-lation time. With this assumption approximatlely 10% of theerrors in the particle extinction and backscatter coefficientsαpart(h) and βpart(h) for both the Two-Stream and Ramanapproach are caused by possible air density fluctuations.

Errors caused by neglecting the absorbtion due to tracegases and multiple scattering were not concidered.

Figure4 shows the result of the error analysis for the Two-Stream cases, where the SNR at 532 nm channel for bothlidars and the corresponding errors of the Two-Stream ex-tinction 1α

partTS (h) and backscatter1β

partTS (h) are given. The

higher SNR for the AMALi on 14 April 2005 was causedby higher PMT voltage for the aquisition on that day. The1α

partTS (h) do not show pronounced height dependence. In

the Two-Stream approach both lidar signals have an oppo-site gradient of the SNR, which is a clear advantage overevaluation schemes with only one lidar. For the investi-gated cases the error of the Two-Stream particle extinctionis below 1α

partTS =2×10−6 m−1. Figure 5 gives an estima-

tion of SNR at KARL’s 607 nm N2 channel, and errors ofthe Raman retrieved extinction1α

partRM(h) and backscatter



Fig. 5. The signal to noise ratio of the KARL’s 607 nm N2 Raman channel at 20 mn temporal and 60 mspatial averaging. The particle extinction and backscatter error values for 15 May 2004 (dashed), 19 May2004 (solid) and 14 April 2005 (dotted) according to the standard Raman evaluation method.

33

Fig. 5. The signal to noise ratio of the KARL’s 607 nm N2 Ramanchannel at 20 mn temporal and 60 m spatial averaging. The particleextinction and backscatter error values for 15 May 2004 (dashed),19 May 2004 (solid) and 14 April 2005 (dotted) according to thestandard Raman evaluation method.

1βpartRM (h). As the SNR declines significantly with altitude

the error increases accordingly. In the case of the data pre-sented here, the error of the Raman particle extinction1α

partRM

is almost 10 times higher than the corresponding Two-Streamapproach error1α

partTS .

The error of the Two-Stream backscatter1βpartTS (h) de-

pends almost entirely on the error of the established lidarinstrumental constants. According to applied constrains theCK was obtained with 5% uncertainty which resulted inan accuracy of the backscatter coefficient below1β

partTS =

2×10−7 m−1 sr−1. Any possible errors in the determinationof the extinction do not affect the backscatter retrieval. Incontrary to the1α

partTS (h), the 1β

partTS (h) does not decrease

with increasing signal strength. This is due to the fact thatan uncertainty of the range corrected lidar signalS is pro-portional to 1S

√S

. As long as the error in the range corrected

lidar signal1S is almost propotional to the root of the signal,there is no dependence on the error of1β

partTS (h). The error

of the particle backscatter coefficient profiles obtained within the Two-Stream and the Raman evaluation are similar foranalysed data, as the boundary conditionβref, necessary inboth cases, determines this uncertainty.

A thorough error analysis is recommended when applyingthe Two-Stream method for another important reason. Whentwo poorly matching lidar returns are divided by each otherthe Two-Stream algorithm can produce a physically unrealis-tic oscillation in theαpart

TS (h) profile. With simple error anal-ysis this problem can be easily addressed. If an amplitude ofthe mentioned artificial oscillations exceeds a value expectedfrom the error analysis both employed lidar signals obviouslydo not contain the same atmospheric signal.

5 Discussion

For our calculations the assumption of negligible multiplescattering effect was made. For the AMALi (large beam di-vergence and field of view; see Appendix A;Stachlewska etal., 2004, 2009) the distance from lidar flight altitude to theground was so short for a nadir-pointing configuration (lessthan 2.7 km) that instrumental effect is negligible. Hence,only multiple scattering due to very dense aerosol load, thickclouds or fog would have had any impact. The multiple scat-tering effect calculated for the case of a mixed-phase cloudsystem (Stachlewska et al., 2006b; Gayet et al., 2007) usingthe multiple scattering model (Eloranta, 1998) showed thatfor such a relatively short distance the effect of multiple scat-tering is relevant only beyond the particle extinction thresh-old of 0.65× 10−3 m−1. In the profiles retrieved here theextinction was at least 2 orders of magnitude lower, and ne-glecting the multiple scattering for the AMALi observationswas justified. For the KARL, neglecting the multiple scatter-ing effect was justified by the instrument and measurementgeometries (see Appendix B;Hofmann et al., 2009). The op-tical depth for midlevel clouds where the multiple scatteringoccurs is about 0.5, far above the values considered in thisstudy. Hence, it was decided that any influence of multiplescattering for the KARL could also be neglected.

The applicability of the Two-Stream method depends criti-cally on the constraint that both lidars probe into the same airmasses. This constraint was often not fulfilled at the Kold-ewey Station, where the KARL is installed. It is located nearthe coastline of Kongsfjord, an area rich in local meteoro-logical phenomenon due to the cliffy orography of Svalbard.Additionally, due to their relative movement AMALi andKARL always detected different air masses directly abovethe station and below the aircraft. Hence, for the Two-Streamcalculations more height steps were excluded than requiredonly by a constraint on each lidars’ overlap. The retrievalwas strongly dependent on each lidars’ SNR level. The datamust be evaluated to obtain as high as possible SNR for asshort as possible spatial and temporal averaging. In our caseSNR of at least 10 for a single 60 m range bin was required(about 10 mn integration time). The optimal configurationfor the Two-Stream method employs two lidars with a similarSNR (as the particle extinction coefficient retrieval dependson both lidar signals equivalently). To find the most consis-tent data sets a correlation method was successfully applied.

The τpartsun obtained from almost simultaneous measure-

ments with a sun photometer could only approximately becompared with theτpart

TS obtained from the Two-Stream parti-cle extinction profiles, due to the short range of the latter re-trieval. Care was also taken while comparingτ

partsun with τ

partKFS

obtained from the standard elastic Klett-Fernald-Sasano in-version applied to KARL’s data. The zenith-pointing KARLgenerally probed different air to that of the sun photometer,which measured only at low elevations above the horizon(about 29◦ at our polar site). In the direct comparison of



the τpartKFS with the τ

partsun the latter value can only be used as

basic information. Additionally, during both campaigns theKARL underestimated signals in the lowermost troposphere(high overlap), which resulted in the underestimation of theτ

partKFS. Hence, the lidar constantCK , necessary to derive di-

rectly the Two-StreamβpartTS (h), was obtained using mainly

the constraint of theτpartKFS matching theτpart

TS . At lower lat-itudes more constraints could have been taken into account,so that an improvement in the retrieval for such cases can beexpected.

The Two-StreamαpartTS (h), βpart

TS (h) andBTS(h) profiles de-rived from the 532 nm elastic signals of AMALi and KARLwere compared with corresponding profiles derived for theKARL’s Raman returns. The Raman evaluation was donewith a minimum of 20 mn integration time to assure suffi-cient SNR, while the Two-Stream retrievals were obtainedwith roughly 10 mn averages. The error analysis showed,that the Ramanαpart

RM(h) retrievals are obtained with highererrors. Profiles obtained with both techniques (Figs.1, 2,and3) agree well, with the mentioned uncertainties (Figs.4and 5) and deviate only in the layers of high particle ex-tinction values. While for 15 May 2004 one might spec-ulate about a slight height shift between the profiles, suchbehaviour was not observed on other days. Therefore, weaddress these deviations to real variations of the atmosphereduring the longer intergration of the Raman-shifted lidar pro-files.

For both ASTAR 2004 days lidars recorded mainly clearair with the background particle extinction coefficient around1.5× 10−5 m−1 and the lidar ratio of 20 sr, which are val-ues characteristic of the clean arctic summertime condi-tions. Generally during ASTAR 2004 campaign extremelylow contaminations were observed (Engvall et al., 2008).Theτ

partsun averaged for the whole campaign was around 0.08

for the 532 nm, which must be at least partially addressedto the existence of Cirrus and sub-visible clouds in the up-per troposphere. With both methods we obtained humid lay-ers, which can be characterised by enhancedαpart(h) values,hardly visible in theβpart(h) profiles. This once again under-lines the necessity to determine backscatter and extinction in-dependently of each other from the lidar measurements. Thelayers retrieved on both days at an altitude of about 1800 m(Figs.1 and2) matched the inversion layers measured by theradiosonde. Although the radiosonde launches took place upto two hours after the over-flights, we assume that the inver-sion layers could not change significantly during this timeperiod as there were prolonged stable weather conditions onthese days (Dornbrack et al., 2010).

The enhanced relative humidity indicates existence ofmore wet particles in the layers than outside of it. The tem-perature below−8◦C suggests supercooled conditions ratherthan ice formation, which is supported by the low depolariza-tion ratios indicating spherical particles. The calculations ofbackward trajectories give no evidence of possible anthro-

pogenic pollution, which could contain absorbing particles.If the layers were dominated by spherical particles which arecomparable or much larger than the interacting wavelengthsize (532 nm) it should show as an enhancement in both ex-tinction and backscatter profile. We hypothesize that only anincreased number of rather small spherical particles (effec-tive radius of about 200 nm) could give an enhanced particleextinction accompanied by a small particle backscatter andlarge lidar ratios, as seen in the layers at 900 m and 1800 m inFig. 1 and at 1800 m in Fig.2. Accordingly toGarrett et al.(2004) the smallest measured droplet sizes are much largerthan 500 nm, so this study illustrates particles rather thandroplets. We argue that these layers were mainly composedof very small spherical supercooled water particles, not un-usual in the pristine conditions of the Arctic region (Pintoet al., 2001; Treffeisen at al., 2007). However, heaving noin-situ measurements within, or outside these layers makesit difficult to discriminate between water and non absorbingcloud activated aerosol, which might consist of a sulfate corewith a water shell (similar index of refraction). Hence, thehigh lidar ratios reported here may also indicate the exis-tence of submicron aerosol particles activated in the humidenvironment.

Backward trajectories for 15 May 2004 suggest that theair under consideration remained isolated in the Arctic for atleast six days and no anthropogenic pollutants could havebeen mixed into these layers. The backward trajectoriesof 19 May 2004 show the air mass passing briefly throughnon-Polar Regions as the air streaked the coastline of north-western Europe and Sibiria three days prior to its arrival inNy Alesund. Due to the low extinction of this air, significantparticle loads could not have been taken up and no anthro-pogenic pollutants were mixed into it. Therefore, a local ori-gin of such humid layers over the Koldewey Station seemsmore likely than an advection phenomenon. Nearby moun-tains with an altitude of around 1 km cause local meteorolog-ical disturbances at the site, which seemed to be a source forformation of these layers (Dornbrack et al., 2010) .

On 14 April 2005 a very local source of contaminationswith CO, NO2 and SO2 from the coal mining village in Bar-entsburg on Svalbard was possible. However, the backwardtrajectories indicate a uniform long-range transport over themid-continental area of Yenisey and Lena Delta where sootparticles could be expected in the anthropogenically con-taminated air. This, together with the enhanced values ofparticle extinction and backscatter coefficients accompaniedby the slowly varying vertical lidar ratio around 34 sr (sig-nificantly higher than values typical for clean Arctic air ofaround 20 sr), the low humidity of 25–35%, and the low vol-ume depolarisation (below 10%) indicate the occurrence ofa weak Arctic Haze event rether than a local contaminations.

The difference of about 50% between the Two-Stream andRaman retrievals of the extinction and backscatter profilesobserved below about 1 km in Fig.3 on 14 April 2005 couldnot be caused by an overlap misalignment of the KARL, as



it displayed more scattering. Neither was it a multiple scat-tering effect in the AMALi signal because the observed opti-cal depth was too low for multiple scattering to set in, andbackscatter and extinction were affected in the same way.Both methods retrieve a lidar ratio close to 40 sr below 1 km,which is close to the value for Arctic Haze measured at ourhigh latitude station (Ritter et al., 2004); a value lower thanthe one found for Arctic Haze at lower latitudes (Muller etal., 2007). Altitudes below 1 km in NyAlesund are generallyin a regime directly influenced by the local topography (e.g.in this study the radiosonde launched at 11:00 UT recorded apronounced temperature inversion around 0.93 km altitude).Hence, it seems that below 1 km more local and more vari-able conditions were probed by the lidars whereas above1 km the conditions were more stable. In both regimes thelidar ratio was however similar.

6 Conclusions

The Two-Stream method is an interesting evaluation tool forcombined lidar observations. By means of the Two-Streamalgorithm, (i) the two un-calibrated backscatter lidar signals,(ii) the reference backscatter coefficient value at any arbi-trary altitude and (iii) the profiles of the molecular extinc-tion and backscatter characterising the Rayleigh atmosphereare enough to retrieve the particle extinction and backscattercoefficient profiles and lidar ratio profile without ambiguity.Unlike for the standard elastic Klett-Fernals-Sasano inver-sion algorithm, which affects the gradient of the backscat-ter profile with incorrectly chosen lidar ratioB(h) and valueof the scattering at the reference altitudeβref, for the Two-Stream the choise of theβref causes only a bias on the re-trieved backscatter profile. If theβref is not avaliable, theβTS(h) solution can be obtained directly from the lidar equa-tion if at least one of the lidar instrumental constantsC isestimated in any aerosol-free range.

The Two-Stream method was successful in the inversionof the AMALi and the KARL data at our polar site. The ex-tinction coefficients were retrieved more accurately than withthe Raman technique. Acurate retrievals were obtained forboth clear and polluted atmospheric conditions. However,in unstable meteorological conditions, the critical constrainwas the demand that both instruments probe the very sameair to avoid artificial and meaningless extinction values. Inprofiles retrieved with the Two-Stream and the Raman tech-niques layers of enhanced values inαpart(h) profiles, indis-cernible in theβpart(h) profiles, but corresponding to veryhigh B(h) were found in the two cases of the ASTAR cam-paign. We interpreted them as layers of small spherical wetparticles, of a non-antropogenic and local topography relatedorigin. During the SvalEx campaign case such layers didnot appear. Here strongly enhanced profiles ofαpart(h) andβpart(h) accompanied by an almost constant and high valueof B(h) were interpred as a weak Arctic Haze event of long

range advection of aged small particles of antropogenic ori-gin from lower latitudes.

The fact that one of the involved instruments in the Two-Stream method must be air/space borne and eye-safe makesthis approach quite expensive to be used on a everyday ba-sis. However, during ASTAR 2007 ample application of thistechnique to the airborne lidar, ground based lidar and spaceborne lidar data, especially for the investigations of the latewinter’s Arctic Haze conditions, was made. For applicationof the Two-Stream approach to combined ground based li-dar and satellite lidar measurements the evaluation schemewould have more factors to concern. For example frequencyand time of the satellite overpasses, obtaining sufficient SNR,multiple scattering effect due to clouds and far range of satel-lite lidar would need to be considered. However, for theTwo-Stream comparison it would be viable to utilise it ona quasi-homogeneous terrain with as little topographical in-fluence as possible to allow more averaging for satellite li-dar (e.g. from an aircraft flying over a free ocean along thesatellite path). An experiment dedicated to the Two-Streamvalidation and analysis of the CALIPSO nadir-pointing li-dar, frequently overflying the Arctic regions including vicin-ity of the Koldeway Station, was already performed duringvarious AWI campaigns (e.g. ASTAR 2007) were a zenith-pointing ground based KARL lidar and a zenith-pointing air-borne AMALi lidar were used. In the future we plan to workon these data sets.

Appendix A

The Airborne Mobile Aerosol Lidar (AMALi)

The Airborne Mobile Aerosol Lidar (AMALi) developed atAWI is a small portable backscatter lidar designed for re-mote, simultaneous, high resolution detection of vertical andtemporal extent of tropospheric aerosol load and depolar-ization (Stachlewska et al., 2004, 2009). In this study theversion of the AMALi based on the Nd:Yag laser operat-ing with 15 Hz repetition rate at 1064 nm and 532 nm withpulse energy of 60 mJ and 120 mJ, respectively was used. Asa reciever a 10.2 cm parabolic off-axis mirror with FOV of3.1 mrad was employed. The eye-safety at distances greaterthan 2.5 km off the system was assured by using a large laserbeam divergence of 2.6 mrad. The nadir-pointing airbornemeasurements were limited to the near range by the eye-safety constrains and the maximum flight altitude of 3 km forthe installation onboard a Dornier Do 288 aircraft (the AWIPolar 2 aircraft). Length of retrieved profiles varied between2.5–2.7 km depending on flight altitude and taking into ac-count 235 m losses due to overlap. This limitation allowedneglecting the multiple scattering effect due to large FOVand large laser beam divergence. For the Two-Stream calcu-lations discussed in this paper the 532 nm signals averagedover 8 mn with 60 m range resolution for 90 kt aircraft’sspeed over ground were used.



Appendix B

The Koldewey Aerosol Raman Lidar (KARL)

The developed at AWI Koldewey Aerosol Raman Lidar(KARL) is a ground based system integrated at the KoldeweyStation in NyAlesund, Spitsbergen (78.9◦ N, 11.9◦ E) serv-ing for detection of tropospheric aerosols and water vapour(Ritter et al., 2004, 2008). The version of the KARL usedfor this study employed the Nd:Yag laser operating with30 Hz repetition rate at 355 nm, 532 nm and 1064 nm, eachwith energy around 2 W. The recieving system had two mir-rors; 10.8 cm diameter with FOV of 2.25 mrad for nearrange (from 650 m to 6 km) and 30 cm diameter and FOVof 0.83 mrad for far range (from 2 km to lower stratosphere)measurements. Detection was provided at the IR, VIS, UVelastic backscatter channels, VIS depolarisation, and Raman-shifted wavelengths for nitrogen at 387 nm and 607 nm andfor water vapour at 407 nm and 660 nm. For the applica-tion to the Two-Stream the 532 nm elastic data with standardaveraging over 10 mn and 60 m ranging from overlap up to15 km were used. The inelastic signals at 607 nm were aver-aged over 20 mn and 300 m.

Appendix C

The multi-channel spectrophotometer

The multi-channel spectrophotometer SP1A-14 manufac-tured by Dr. Schulz & Partner, Buckow, Germany has a mea-surement range covering UV, VIS and IR light spectrum,where 8 channels are selected accordingly to the WMO/1983recommendation and VDI 3786/10/3 recommendation (368,412, 500, 600, 675, 778, 862, 1024 nm) and 10 are additional(353, 389, 450, 532, 760, 911, 946, 967, 1045, 1064 nm).A full measuring cycle, i.e. collecting and storing the dataof 18 channels and calling up the next cycle, is taken within8 s. Calibration is performed with artifical radiation sourcesat the optical laboratory using the Leiterer calibrating method(Leiterer et al., 1985) or during a field experiment using theLangley-extrapolation method (Shaw, 1983). The latter onemust be performed in a case of cloud absence along the opti-cal path and extremely low variations in the planetary bound-ary layer, which are often occuring during very clear air con-ditions in the Polar Regions. For this study we used the532 nm channel for the particle optical depth comparisons.

Acknowledgements.The authors would like to thank the anony-mous referees and Adrain Webb who helped us to improve thismanuscript with their valuable comments and contributions. Weacknowledge the NOAA Air Resources Laboratory (ARL) for theprovision of the HYSPLIT transport and dispersion model used inthis publication.

Edited by: A. Minikin

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http://opus.kobv.de/ubp/volltexte/2006/698/

http://opus.kobv.de/ubp/volltexte/2006/698/

http://www.atmos-chem-phys-discuss.net/9/18745/2009/


atmospheric chemistry and physics on retrieval of lidar extinction

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