Available online at www.sciencedirect.com

www.elsevier.com/locate/asr

Advances in Space Research 42 (2008) 1747–1764

SCIAMACHY limb measurements of NO2, BrO and OClO. Retrievalof vertical profiles: Algorithm, first results, sensitivity and

comparison studies

Sven Kühl a,*, Janis Pukite a, T. Deutschmann b, Ulrich Platt b, Thomas Wagner a,b

a Max Planck Institut für Chemie, D-55128, Mainz, Germanyb Institut für Umweltphysik, D-69120 Heidelberg, Germany

Received 31 October 2006; received in revised form 17 August 2007; accepted 17 October 2007

Abstract

The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) measures scattered sun light also inlimb viewing mode (i.e. tangential to Earth’s surface and its atmosphere), which allows determining vertical profiles of atmospheric tracegases. First results on the retrieval of NO2, BrO and OClO profiles from the SCIAMACHY Limb measurements are presented and com-pared to independent satellite and balloon borne observations.

For the profile retrieval we use a two step approach: First, Slant Column Densities (SCDs) of the respective absorber are determinedin the UV/VIS spectral range by Differential Optical Absorption Spectroscopy (DOAS). Second, inversion of the retrieved SCDs (asfunction of tangent height) yields vertical profiles of the trace gas concentration (as function of altitude). For that purpose, we eitherapply an optimal estimation method (constraining the inversion by a priori information) or invert the measurements by a least squaresapproach (independently from a priori). For both approaches box air mass factors calculated by the 3D full spherical radiative transfermodel TRACY-II are utilized as weighting functions.

The agreement of the retrieved profiles with balloon validation measurements as well as with other space borne measurements ofstratospheric trace gas profiles (e.g. by SMR on Odin) is investigated. For OClO, case studies on the sensitivity of the retrieved SCDson fitting parameters (e.g. wavelength range, polarisation correction) and the impact of the a priori on the retrieved profiles are per-formed. Also, the significance of the obtained dataset of concentration profiles of NO2, BrO and OClO for studies on stratospheric chem-istry is discussed.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: SCIAMACHY; Limb; Profile; Retrieval; Stratosphere; NO2; BrO; OClO

1. Introduction

Nitrogen and halogen compounds play a major role inatmospheric chemistry, e.g. as catalysts responsible forstratospheric ozone depletion. While NOx-chemistry hasa major impact on stratospheric ozone globally, chlorineand bromine species cause chemical destruction of ozonein polar spring, leading to the well-known ozone hole overAntarctica (Wennberg et al., 1994; Solomon, 1999). More-

0273-1177/$34.00 � 2007 COSPAR. Published by Elsevier Ltd. All rights resedoi:10.1016/j.asr.2007.10.022

* Corresponding author.E-mail address: skuehl@mpch-mainz.mpg.de (S. Kühl).

over, also at mid-latitudes and equatorial regions a nega-tive trend of the ozone column is observed, which isattributed to transport mechanisms and halogen chemistry(WMO, 2003).

Atmospheric chlorine and bromine is effectively stored inreservoirs by the reaction of the active (ozone destroying)compounds (ClO, BrO) with NO2 to the respective nitrate(ClONO2, BrONO2). Only during polar night, when NO2is removed from the stratosphere by denoxification and deni-trification, this equilibrium is shifted and active halogen(formed by heterogeneous reactions on polar stratosphericclouds) accumulates inside the polar vortex (Solomonet al., 1986). For bromine, this process can be monitored

rved.

mailto:skuehl@mpch-mainz.mpg.de

1748 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

by spectroscopy of BrO in the ultraviolet region. On the con-trary, ClO, which together with its dimer is the main activecompound for atmospheric chlorine, can only be measuredwith microwave spectroscopy (due to strong ozone absorp-tion for wavelengths below 310 nm). Therefore, OClO whoseabundance is linearly related to that of ClO during twilight(i.e., for solar zenith angles 690�), and which can be mea-sured with UV/VIS spectroscopy, is often used as an indica-tor for active chlorine (Solomon et al., 1987a; Schiller andWahner, 1996; Tornkvist et al., 2002).

Satellite measurements of the total columns of NO2, BrOand OClO by GOME (the predecessor of SCIAMACHY)and of the vertical profile of ClO (Waters et al., 1993; Santeeet al., 2003) have increased our knowledge of stratosphericand tropospheric chemistry (Wagner and Platt, 1998; Bur-rows et al., 1999; Wagner et al., 2001; van Roozendaelet al., 2006). Due to the long-term and global dataset(GOME provides measurements since 1995) inter-hemi-spheric and inter-annual differences in the activation of hal-ogens and their dependence on meteorological parameterscould be established (Wagner et al., 2001, 2002; Weberet al., 2003; Kühl et al., 2004a,b; Richter et al., 2005).

However, until the launch of SCIAMACHY, verticalprofiles of BrO and OClO have not been measured on along term and global scale. Compared to observations oftotal columns, the knowledge about the vertical distribu-tion of trace gases allows one to establish quantitative cor-relations between them (and to meteorological parameters)in a much more detailed manner and also on individualaltitude levels (instead of just the same location). Thus,limb measurements of scattered sunlight provide a newinsight in stratospheric chemistry and are valuable to inves-tigate still open questions.

Here we describe a fast and coherent two step algorithmto derive vertical profiles of NO2, BrO and OClO from theSCIAMACHY limb measurements. It allows to invert theretrieved SCDs either directly (applying a least squares fit)or to constrain the inversion by a priori knowledge. Theparting of the profile retrieval in two steps allows studyingthe impact of spectroscopy and radiative transfer simula-tion separately. Compared to previous SCIAMACHY limbretrievals of NO2 and BrO (Rozanov et al., 2005; Sioriset al., 2006) the combination of DOAS with inversion byeither optimal estimation or direct least square fitting is anovel approach. Also, it is the only algorithm for profileretrieval from SCIAMACHY limb measurements, forwhich calculations of a 3D full spherical Monte CarloRTM (Tracy II, see Deutschmann and Wagner, 2006;Wagner et al., 2007) are applied. First results are presentedand show a good agreement with expectations from atmo-spheric chemistry as well as with independent balloon andsatellite measurements.

2. Instrument

SCIAMACHY is (among MIPAS and GOMOS) one ofthe three instruments of the atmospheric chemistry mission

of the first European satellite constructed exclusively forenvironmental monitoring, ENVISAT-1. The satelliteoperates in a near polar Sun synchronous orbit with incli-nation from the equatorial plane of approximately 98.5�. Itperforms one orbit in 100 min with equator crossing timeof 10:00 in descending mode.

SCIAMACHY probes the atmosphere on the day sideof the Earth in alternating sequences of nadir and limbmeasurements. The limb scans are performed with approx-imately 3.3 km altitude steps from ground to approx.100 km height. Horizontally the atmosphere is scannedwith swaps across flying direction of 960 km at tangentpoint, consisting of 4 pixels with 240 km width each. Theinstantaneous field of view is 0.045� in elevation and 1.8�in azimuth, which corresponds to approximately 2.5 and110 km at the tangent point, respectively. SCIAMACHYis measuring in the UV-VIS-NIR spectral range from 240to 2380 nm, allowing the retrieval of a variety of trace gases(Bovensmann et al., 1999). In the UV/VIS, the spectral res-olution ranges between approximately 0.25–0.55 nm whichis sufficient to perform spectroscopy of ozone, NO2, BrOand OClO. With 14–15 orbits per day, global coverage isachieved in 6 days.

3. Profile retrieval

For the retrieval of vertical concentration profiles fromthe SCIAMACHY limb measurements, we apply a twostep approach: First, slant column densities (SCDs) ofthe respective atmospheric absorbers are retrieved by Dif-ferential Optical Absorption Spectroscopy (DOAS). Inthe second step, box air mass factors calculated by theMonte Carlo radiative transfer model ‘‘TRACY-II’’ areapplied as weighting functions to convert the SCDs (asfunction of tangent height) to vertical concentration pro-files (number densities as function of altitude).

This inversion is performed either by optimal estimation(see e.g. Rodgers, 2000) or by an a priori independent LSQfitting (Menke, 1999). In the following, these two steps andthe two different inversion approaches are described indetail. A similar two step method (DOAS retrieval of SCDscombined with inversion by optimal estimation) has beenreported for the retrieval of O3 and NO2 vertical profilesfrom measurements by the OSIRIS instrument on Odin(Haley et al., 2004). However, the direct, a priori indepen-dent inversion by a LSQ approach is innovative (Sioriset al., 2006 applied DOAS combined with the Twomey-Chahine method on SCIAMACHY limb spectra). An algo-rithm based on DOAS and an iterative LSQ fit for inver-sion has been proposed by McDade et al. (2002) forOSIRIS spectra, but has not been applied to measuredspectra yet.

3.1. DOAS

Based on the Lambert Beer law, optical densities of therespective trace gas absorptions are determined. Therefore,

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1749

the spectral absorptivity of the relevant species i.e. theirabsorption cross sections need to be known. The key con-cept of DOAS is to utilize not the absolute absorptionstructures but to separate them in a broadband and a dif-ferential cross section (Platt, 1994; Platt and Stutz, 2007).Many absorbers with only small optical densities like e.g.BrO and OClO could be determined by DOAS for the firsttime. As results, SCDs – the integrated concentration of theabsorber along the light path – are obtained (the opticaldensity of an absorber is the product of its absorption crosssection and its SCD).

In the retrieval algorithm, the difference of the measuredspectra to the Fraunhofer reference spectrum (containingno absorption of the considered trace gas) is minimizedby weighting the trace gas absorption cross sections rj withthe respective SCDs Sj. The filling in of the Fraunhoferlines (Grainer and Ring, 1962) is accounted for by a calcu-lated Ring spectrum (Bussemer, 1993), which is handled bythe fit just like the cross sections (Solomon et al., 1987b).Broadband absorption and scattering is modelled by apolynomial of degree 3–5. The difference between the loga-rithm of the measured spectrum I(k) on one side and thelogarithm of the reference spectrum I0(k) and the sum ofthe trace gas absorptions (plus the Ring effect) and thebroadband polynomial on the other side is minimized byapplying a least square fit over all wavelengths k:

Xk

ln IðkÞ � ðln I0ðkÞÞ �X

j

Sjrj �X

p

apkpÞ

�����

�����2

! 0 ð1Þ

For the DOAS analysis, different wavelength ranges areutilized that were found to be optimal for the respectivetrace gas (see e.g. Wagner, 1999; Wagner et al., 2001; Kühl,2005; Kühl et al., 2006 and Pukite et al., 2006): For OClOthe fit-window ranges from 363.5 to 391 nm, for BrO from338 to 357 nm, and for NO2 from 420 to 450 nm. As refer-ence spectrum, the spectrum measured at a tangent heightof 36 km is applied, where both OClO and BrO absorptioncan be regarded as negligible. For the NO2 retrieval the ref-erence spectrum is taken at a tangent height of 42 km.

Table 1Wavelengths utilized and trace gas absorptions cross sections and additionalSCIAMACHY limb spectra

Trace gas retrieval Fit window (nm) Included cross sections

NO2 420–450 O3, 223 K (Bogumil et al., 200NO2, 223 K (Bogumil et al., 2H2O, 273 K (HITRAN)O4 (Greenblatt et al., 1990)

BrO 338–357 O3, 223 K (Bogumil et al., 200O3, 243 K (Bogumil et al., 200BrO, 223 K (Fleischmann et aNO2, 223 K (Bogumil et al., 2

OClO 363.5–391 O3, 223 K (Bogumil et al., 200NO2, 223 K (Bogumil et al., 2OClO (Kromminga et al., 200O4 (Greenblatt et al., 1990)

To account for possible intensity offsets due to instru-mental stray-light we include the inverse of the referencespectrum (1/I0), and for the BrO retrieval also k/I0; simi-larly, for correction of polarisation features, the eta andzeta spectra taken from SCIAMACHY calibration keydata are included. These additional spectra are includedin the fit in the same way as the trace gas absorption crosssections. Since the SCIAMACHY spectra related to oneparticular profile retrieval are measured in between approx.1 min, the wavelength-pixel relation for the reference (I0)and measured spectrum (I) are identical (likewise also forthe corresponding 1/I0 and k/I0). On the contrary, the fitis allowed to perform a wavelength shift of the cross sec-tions (see e.g. Stutz and Platt, 1996) relative to these spec-tra. A squeeze of the spectra is not allowed. Table 1summarizes the fit parameters for the different DOASretrievals.

Fig. 1 shows examples of DOAS analyses for OClO,BrO and NO2, each for one single SCIAMACHY limbspectrum. One SCIAMACHY limb state consists ofapprox. 30 discrete tangent height steps, with four spectrameasured at one particular tangent height (see Section 2).Thus, approx. 120 DOAS analyses like the ones shown inFig. 1 are needed to derive the SCDs of one single profile(currently, the results of the pixels at the same tangentheight are averaged; however, it is planned to utilize theirinformation separately, to account also for horizontalgradients).

In Fig. 2, the NO2 SCDs retrieved by the DOAS fit areshown as function of tangent height for selected limbstates of one example orbit. Although these are columndensities and not number densities, the figure alreadyreveals the differences in the abundance of NO2 due tophotochemistry and transport: For the equator, the pro-file peak is at higher altitudes than at mid-latitudes andthe NO2 abundance is also smaller in the lower strato-sphere. Note also the asymmetry between northern andsouthern hemisphere, reflecting the seasonal dependentconversion of the nitrogen reservoirs due to the solar dec-lination in mid of April.

spectra included for the DOAS analyses of NO2, BrO and OClO from

Additional spectra Reference tangent height (km)

3) Ring (Bussemer, 1993) 42–43003) 1/I0

3) Ring (Bussemer, 1993) 36–373) 1/I0, k/I0, eta, zetal., 2004)003)

3) Ring (Bussemer, 1993) 36–37003) 1/I0, k/I0, eta, zeta3)

Fig. 1. DOAS-Analysis of SCIAMACHY limb spectra for OClO (left), BrO (middle) and NO2 (right). The black curve is the absorption structure of therespective trace gas, scaled by the SCD retrieved in the least square fit according to Eq. (1). The red curve is the black curve plus the residual, i.e. noise andstructures that are not related to the spectra included in the fit. For OClO the figure is an example for a situation with high chlorine activation observed atlarge SZA (91�), also for BrO a spectrum measured at large SZA with high optical density for BrO is evaluated. For spectra with low abundances of BrOor OClO, their signature is not found that clearly. For NO2 the example is typical for an analysis between 25 and 30 km. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this paper.)

Fig. 2. NO2 SCDs as function of tangent height for orbit no. 11127 from16th of April, 2004. Shown are results of the NO2 DOAS retrieval forselected states at different latitudes, see legend. For the state at 72�N, theerror bars of the DOAS retrieval are given. The hollow triangles indicatethe errors of the retrieval for all states of this orbit.

1750 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

3.2. Inversion to profiles

To derive from the measured SCDs the desired trace gasprofile, the physical relation between these two quantitiesneeds to be established. For that purpose, radiative transferis simulated by the 3D full spherical Monte Carlo RTM‘‘TRACY-II’’ (Deutschmann and Wagner, 2006; Wagneret al., 2007). Applying the viewing geometry of theSCIAMACHY instrument for the particular observation,box air mass factors (AMF) are calculated that describethe impact of the trace gas concentration at all consideredaltitudes on the SCD measured for a certain tangent height.As result, a matrix Aij of calculated box AMFs relates thetrace gas concentrations (xj) at the different layers of theatmosphere to the measured SCDs (yi), where e is the errorof the measurement:

y1y2. . .

ym

0BBB@

1CCCA ¼

A11 A12 . . . A1nA21 A11 . . . A2n. . . . . . . . . . . .

Am1 . . . . . . Amn

0BBB@

1CCCA

x1x2. . .

xn

0BBB@

1CCCAþ

e1e2. . .

en

0BBB@

1CCCA

ð2Þ

Fig. 3 shows an example for a calculation of box AMFs forNO2. They reveal that the observations are highly sensitivefor boxes that are close to the respective tangent height. Asan example, for a tangent height of 30 km the box at 30–33 km has an AMF of 65, the box at 33–36 km has anAMF of 28, etc., while the AMF for boxes situated below30 km is close to zero. This reflects the viewing geometryfor limb measurements: The light has to pass the layersabove the tangent height (at least) twice (from sun to atmo-sphere and then scattered to instrument), while the lightthat comes from layers below is only weakly affecting themeasurements. For altitudes around and below 15 km,the sensitivity is significantly reduced: For a tangent heightof 12 km the AMF for the box from 12 to 15 km is only 23,while the AMF for each box above is between 10 and 20.This reflects the increase in Rayleigh scattering for low alti-tudes due to the larger air density.

To retrieve the trace gas concentration profile x, Eq. (2)needs to be inverted. However, the problem is not exact(the retrieved SCDs can only be measured with the errore) and at the same time underdetermined (low sensitivityfor the troposphere) as well as over-determined (measure-ments at high altitudes may be contradictory within theuncertainty). Thus, for a given set of measured SCDs,many profiles would solve the equation (including alsounphysical ones like zigzag profiles or negative concentra-tion values). To overcome this difficulty, differentapproaches exist. One is to determine the profile by a leastsquares approach (Menke, 1999):

x̂d ¼ AT S�1e A� ��1

AT S�1e y ð3Þ

Fig. 3. Air mass factors at 435 nm (the middle of the NO2 fit window), calculated by the 3D full-spherical Monte Carlo RTM Tracy-II. Shown areabsolute values for 3 km boxes as a function of tangent height. For the atmospheric parameters like pressure, temperature and trace gas concentrations, adiscretization of 1 km is applied.

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1751

where Se is the measurement error covariance matrix.The inversion of the measurement vector y (i.e., the

SCDs for the different tangent heights) according to Eq.(3) produces reasonable results if measurements below15 km and above 42 km are excluded, i.e. for altitudes from15 to 42 km the problem can be treated as well-determinedby the least squares approach. However, this approach isnot practicable for wider altitude ranges (due to the lowsensitivity for altitudes below 15 km and the decreasing sig-nal to noise ratio for altitudes above 40 km).

Other approaches utilize the knowledge that exists aboutthe profile to be derived already before the measurement (e.g.climatology) as so called a priori information. In our case, theinversion is stabilized by an optimal estimation method(maximum a posteriori) based on Rodgers (2000). For theresulting profile, the retrieved SCDs, their error covariance,the a priori profile xa and its climatologic variability(described by its covariance Sa) are taken into account:

x ¼ xa þ SaAT ½ASaAT þ Sy ��1ðy� AxaÞ ð4Þ

One further result of the optimal estimation method are theso called averaging kernels. They determine the altituderange where the measurement provided information onthe profile: Large values (close to one) indicate a large im-pact of the measurement, for low values of the averagingkernel the retrieved profile approaches the a prioriinformation.

In our retrieval, the a priori information is taken fromvarious sources: For NO2, the climatology by Andersonet al. (1986) is applied. For BrO, there exist only a few inde-pendent observations of vertical profiles and for OClOeven less. Also, the abundance of these species, in particu-lar OClO, depends very strongly on meteorological condi-tions (i.e. temperatures below the threshold for PSC

formation). Therefore we apply as a priori a box profilewhich allows for retrieval of enhanced number densitiesat all altitudes, independently from any prior assumptionor modelling (the uncertainty of the a priori is set to100%). Alternatively, for BrO, profiles based on balloonmeasurements (Harder et al., 1998; Fitzenberger, 2000)are calculated for the corresponding SZA, and for OClOa modelled profile, calculated by assuming equilibriumbetween ClO, BrO and OClO (Solomon et al., 1987a), istaken as (latitude and SZA independent) a priori. ForNO2, the monthly and zonal means calculated byECHAM5/MESSy1 (Jöckel et al., 2006) are used as secondoption for the a priori. These alternative a priori informa-tion are applied only for comparison to assure that theretrieved profiles are not the result of the magnitude orshape of the a priori profile. In a case study we investigatedthe impact of the a priori profile on the retrieved OClOprofile (see Section 3.3.2).

For NO2, Fig. 4 shows an example of the inversion con-strained by a priori in comparison to the one by the leastsquares approach: The NO2 SCDs determined in the firststep of the retrieval (compare Fig. 2) are given in the leftpanel and the profile obtained from the particular inversionin the middle panel. In the right panel the averaging kernelsfor the retrieval with a priori are displayed, revealing theimpact of the measurement at the certain altitudes.

It can be seen that for a wide altitude range the retrievedprofiles (with and without a priori) are practically identical.Differences appear for altitudes below 15 and above 40 km,where also unphysical values (negative number densities)are retrieved for the inversion without a priori constraint,because this approach is not practicable at these altitudes(see above). For the retrieval with a priori, the profile atthese altitudes approaches the a priori profile, since there

Fig. 4. NO2 SCDs (left), retrieved vertical profile of number densities (middle panel) and averaging kernels for the retrieval with a priori (right). The upperpanel shows measured SCDs and retrieved profile for orbit no. 11127, state 9 (72�N) and the lower panel for state 16 (22�N) of the same orbit (compareFig. 2). Shown are as blue lines the measured SCDs and the profile retrieved by the least squares approach. The red lines indicate the profile retrieved byoptimal estimation and the SCDs corresponding to it, applying the calculated matrix of box AMFs. The green line is the a priori profile and the SCDscorresponding to it.

1752 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

the impact of the measurement is small, as can be seenwhen comparing with the respective averaging kernel. Notethat compared to Fig. 2, the SCDs displayed in Fig. 4 areoffset by the value of the a priori at the altitude of the ref-erence spectrum (36 km for BrO and OClO, 42 km forNO2). This is to account for possible abundances of therespective trace gas at this altitude (since the reference spec-trum is taken at this altitude, the corresponding SCD isequal to zero).

3.3. Sensitivity studies and error analysis

Due to the separation of the retrieval in two steps, thesensitivity and also the error contribution can be investi-gated for each step individually.

3.3.1. Retrieval of SCDs

In the first step, the statistical error of the SCD retrievalis determined from the magnitude of the residual (seeFig. 1) compared to the magnitude of the retrieved SCD(Stutz and Platt, 1996). For a review of error sourcesrelated to scattered light DOAS see Platt et al. (1997), theerrors occurring in limb measurements of scattered sunlightare summarized in Haley et al. (2004). Typical statisticalerrors of the NO2 SCD retrieval as applied in the algorithmdescribed here are shown in Fig. 2. Errors for the retrievalof BrO and OClO SCDs are generally larger, mainly due tothe lower optical densities of these absorbers (compareFig. 1). Also note that this calculation, which assumes thatthe retrieval residual consists of measurement noise only,does not include possible systematical retrieval errors

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1753

which may arise from interference (cross correlations) ofspectra, false wavelength calibrations, errors in the abso-lute value of the absorption cross sections, etc.

For OClO, the sensitivity of the SCD results on the fit-ting parameters wavelength range of the fitting windowand spectra accounting for stray light (1/I0, wl/I0) andpolarisation correction (eta, zeta) is shown for selected ret-rievals in Figs. 5 and 6. First, we compared the results ofthe OClO DOAS analyses for a selected orbit in a coldnorthern hemisphere winter (mid of January 2005) whenapplying four different wavelength ranges as fitting win-dow. Fig. 5 shows the retrieved OClO SCDs and standarddeviations as function of tangent height for a state at 57�Nwhere significant chlorine activation or large amounts ofOClO are observed (in agreement with stratospheric mete-orological conditions), and for a state at 20�N, where nosignificant OClO is observed, as expected. It can be seenthat a modification of the fit window by 15 pixel(�1.5 nm) on the long or short wavelength end alters theretrieved OClO SCD by approx. 5–10% or 0.5 to 2 * 10

13

Fig. 5. Comparison of retrieved OClO SCDs and corresponding standard devias fitting window: wl1 (363.5–391 nm), wl2 (365–391 nm), wl3 (362–391 nm)(57 �N) of orbit no. 15067 from 16th of January, 2005. Lower panel: limb sca

molec/cm2. The altitude of the peak and also the shapeof the retrieved SCD profile are very similar for all wave-length ranges.

Only for ‘‘wl3’’, the fit window from 362 to 391 nm, astrong discrepancy is observed for altitudes from 22 to37 km. At the same altitudes the error for this fit windowshows enhanced values. A more detailed investigationreveals that this is caused by interference of the absorptionspectra of OClO with ozone, whose absorption becomesmore important for the shorter wavelengths that areincluded in this fit window (ozone shows an absorptionpeak at 359,6 nm which extends until �363 nm).

For the other fitting windows, the SCDs and also thecorresponding retrieval error are of comparable size, whichindicates that further reducing the fit window on either side(relative to the one from 363.5 to 391 nm, see Table 1) doesnot further reduce the interference with ozone nor theretrieval error. On the other side, an extension of the fitwindow to longer wavelengths (beyond 391 nm) is notapplicable due to the strong Ca+ Fraunhofer lines (with

ation as function of tangent height for different wavelength ranges appliedand wl4 (363.5 to 389.5 nm). Upper panel: limb scanning sequence no. 7nning sequence no. 12 (20�N) of the same orbit.

1754 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

the peak of the first one located at 393.37 nm). The fit win-dow ‘‘wl1’’ (363.5–391 nm) also shows the smallest retrievalerror when considering the retrievals for all limb states ofone orbit. Since this wavelength range is also applied inour nadir retrievals for OClO, we choose this fit windowas our standard limb retrieval.

In Fig. 6 we compared five different retrieval settingsregarding the spectra for correction of stray light andpolarisation features (see figure caption). Again, retrievalresults are compared for a case with large OClO SCDsand one with no chlorine activation, both from one orbitat January 17, 2005. It can be seen that, compared to thedifferences in the above study regarding the fit window,the results depend very strongly on the respective retrievalset: While the retrieved SCDs are very similar for the sets‘‘1ez’’ and ‘‘1ezl’’, OClO SCDs for the sets ‘‘1’’ and ‘‘1e’’are 1–5 * 10

13 molec/cm2 (or 10–25%) smaller and larger,

Fig. 6. Comparison of retrieved OClO SCDs and corresponding standard devstray light correction spectra included in the fit: 1 (only 1/I0 included), 1e (1/correction spectra included). Shown are in the upper panel retrieval results for lno. 15081 from 17th of January, 2005.

respectively. However, the altitude of the peak and alsothe shape of the SCD profile are very similar for all retrie-val sets shown in Fig. 6, except for the set with no correc-tion spectra (set ‘‘no’’).

This set also has the largest retrieval error, while thestandard deviations of the SCD retrieval for the other setsare of comparable size, with the retrieval error being small-est for the set ‘‘1ezl’’. This set, which includes all polarisa-tion and stray light correction spectra, is also the set whichshows the most consistent retrievals results globally and forwhich the scatter of the OClO SCDs for states with nochlorine activation is closest to zero (compare Fig. 6, lowerpanel). Moreover, the retrieved OClO profiles are in goodagreement with independent observations and also modelresults (see Section 4). In contrast, the retrieval with theset ‘‘1e’’ shows unrealistically large SCDs for altitudesbelow 30 km (in particular for the low latitude example)

iation as function of tangent height for different sets of polarisation andI0 and eta), 1ez (1/I0, eta and zeta), 1ezl (1/I0, eta, zeta and k/I0), no (noimb state no. 7 (57�N) and in the lower panel for state no. 16 (8�S) of orbit

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1755

and the set ‘‘1ez’’ results in negative SCDs for altitudesbelow 15 km. For these reasons we choose the set ‘‘1ezl’’as our standard retrieval for OClO.

This sensitivity study confirms the importance of aproper correction for polarisation features. This is essentialfor DOAS retrievals because the calibration of SCIAM-ACHY data revealed short-comings, mainly due to unreal-istic values of the polarisation fractions q and u asdetermined by the PMD measurements (see Tilstra andStammes, 2005). Therefore the spectra from the SCIAM-ACHY calibration key data (eta, zeta) are applied for acorrection of polarisation features in the fit.

The sensitivity on retrieval parameters is discussed herein detail for OClO, since this is the first publication onOClO profiles from SCIAMACHY spectra. For sensitivitystudies on the retrieval of NO2 and BrO profiles see Roza-nov et al. (2005) and Sioris et al. (2006).

3.3.2. Inversion to profiles

In the second step, the inversion to profiles, furtheruncertainties arise through the statistical error of theAMF calculation (1–2%), the unknown temperature andpressure profile (�2%) and the uncertainty in altitudepointing (1–10%). Errors given in brackets were deter-mined in case studies. For the correction of the knownpointing error of ENVISAT (von Clarmann et al., 2003),we apply the monthly mean tangent height offset statisticsas provided by von Savigny et al. (2005). The impact ofclouds (high cirrus, PSCs) on the calculated AMFs andon the retrieved profiles has been investigated in case stud-ies. It was found that clouds have a strong effect on the alti-tudes below and similar to the cloud top height, but not for

Fig. 7. Typical Averaging Kernels for the retrieval of NO2 (left panel), BrOobservations.

altitudes above, in good agreement with previous studies(Haley et al., 2004; Sioris et al., 2006). In the current algo-rithm version, no clouds are assumed for all AMF calcula-tions. The inclusion of a cloud detection algorithm isplanned in a coming update of the retrieval scheme (seeoutlook section). Refraction is not included in the RTM,but its effect on altitudes above 12 km was found to be neg-ligible in case studies (see also Sioris et al., 2006). TheMonte Carlo RTM Tracy II takes into account multiplescattering and full spherical geometry, thereby providingan optimal description of limb observations (Oikarinenet al., 1999; Loughman et al., 2004).

For determining the sensitivity of the measurement atthe different altitudes, i.e. the impact of the observationson the retrieved profile, it is essential to consider the aver-aging kernel of the retrieval. Fig. 7 shows typical averagingkernels of the two step approach when combined with apriori. They reveal that information about the vertical pro-file can be obtained for NO2 in the altitude range from 12–40 km, for BrO from 15–36 km and for OClO from 15–36 km.

For OClO, Figs. 8 and 9 show the inversion of theretrieved SCDs to number density profiles for two exam-ples: Fig. 8 is an example for a measurement inside thenorthern hemisphere polar vortex with substantial activa-tion of chlorine and Fig. 9 is typical for a retrieval outsidethe polar vortex. Also, the corresponding averaging ker-nels, measurement response profile and spread are plottedin panels c and d.

In Fig. 8b, both retrieval approaches are in good agree-ment and reveal large OClO number densities for altitudesbetween 20 and 15 km. At 12 km, the inversion without a

(middle) and OClO (right) vertical profiles from the SCIAMACHY limb

Fig. 8. Examples for OClO Profile retrievals. (Panel a) Measured SCDs and standard deviation in blue, SCDs of the a priori in green and SCDs of theretrieved profile in red. (Panel b) Profile obtained by inversion constrained by a priori (optimal estimation) in red, profile obtained by direct (LSQ)inversion in blue and a priori profile in green. (Panel c) Averaging kernels and measurement response profile. (Panel d): Spread. (Panel e) Retrieval withalternative a priori. (Panel f) Retrieval with modified a priori (2 · multiplied relative to (panel e). Shown are results for state no. 5 (65�N) of orbit no. 15066from January 16th, 2005. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

1756 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

priori constraint results in unrealistically large values, indi-cating that the measurement noise at 12 km is too large toretrieve a reasonable value (compare the measurementresponse profile in panel c). Between 36 and 15 km, themagnitude of the retrieved number densities and also thealtitude of the peak are in good accord with expectationsand also independent (not correlated) observations, seealso Section 4.

Fig. 9b shows that no significant OClO is retrieved forthe example outside the polar vortex by both inversionapproaches (considering the 1r error bars). For all alti-tudes the retrieved SCD – and thus also the correspondingnumber density – scatters around zero (with a retrievalerror of

Fig. 9. same as Fig. 8, but for state no. 8 (51�N) of orbit no. 15066. (Panel e) Retrieval with alternative a priori. (Panel f) Retrieval with modified a priori(shifted up by 3 km relative to panel e).

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1757

the value at 12 km changes in both cases, which indicatesthat this altitude is already impacted by the a priori, ingood agreement with the measurement response profile(see panel c). Similarly, for the case outside of the polarvortex, Fig. 9 e and f reveal that there is practically noinfluence of the a priori on the retrieved number densityfor all altitudes inside the retrieval range (also when thealtitude of the a prior profile peak is shifted up by 3 km).

The vertical resolution of the profile in the retrieval rangeis approx. 2.5 to 3 km (determined by calculating the spreadof the Averaging Kernels, see Figs. 8d and 9d). In this range,the complete relative retrieval error (including also system-atic errors) for the OClO profile is estimated to be 50–100%; for the BrO profile, the error is estimated to be 20–100% and for the NO2 profile, the error is estimated to be10–50%. Note that in this estimate the lower value indicatesthe typical error for number densities close to the peak and

the large value gives the maximum error for low absolute val-ues of the number density and unfavourable measurementgeometry like e.g. large SZA. For very low number densitiesthe error might even be larger than 100%. Therefore it may bemore reasonable to give an estimate for the absolute error,which is estimated to be 3 * 10

6 molec/cm3 for OClO,5 * 10

6 molec/cm3 for BrO and 5 * 107 molec/cm3 for NO2

for small absolute number densities and SZAs below 80degrees, and 2 * 10

7 molec/cm3 for OClO, 2 * 107 molec/

cm3 for BrO and 3 * 108 molec/cm3 for NO2 for larger SZAs

and large number densities.

4. First results and comparison to independent measurements

The retrieved profiles have been tested for reasonabilityand for all studies they were found to be in accord withexpectations from atmospheric chemistry (concerning the

1758 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

magnitude of the derived number densities, their latitudinaland seasonal dependence, the altitude of the profile peak,etc.). As an example, the global evolution of ozone, NO2,BrO and OClO during the southern hemisphere polar win-ter is described below. Moreover, the results for NO2 andBrO have been compared to SCIAMACHY limb retrievalsfrom other institutes like e.g. the IFE Bremen (Rozanovet al., 2005), and a good qualitative and quantitative agree-ment was found. Also, comparisons with correlated bal-loon measurements performed exclusively for thevalidation of SCIAMACHY observations were made:For NO2, a few profiles have been validated by balloonmeasurements and it was found that the profiles show agood agreement (Butz et al., 2006). Also for BrO, compar-isons to balloon measurements (Dorf et al., 2006) havebeen performed; examples are shown in Fig. 10.

Fig. 10. BrO balloon measurements performed for SCIAMCHY valida-tion for the 9th of October, 2003 near Aire sur l’Adour, France (top) andfor the 24th of March, 2004 near Kiruna, Sweden (bottom). The balloonobservations (red) are performed at the same day and correlated to theSCIAMACHY measurements (black) by trajectory calculations. Also, aphotochemical correction to match the SZA of the SCIAMACHYobservations has been applied (Dorf et al., 2005). (For interpretation ofthe references to color in this figure legend, the reader is referred to theweb version of this paper.)

It can be seen that the BrO profiles agree within theerror bars (1r) for both cases (while the difference is smallerfor the mid-latitude case). There is a tendency for largervalues in the SCIAMACHY BrO profiles (also for thosecomparisons not shown). This can be explained by the dif-ferent BrO absorption cross sections utilized in the retri-evals: For the balloon profile retrieval, the cross sectionmeasured by Wahner et al. (1987) is used, while for theSCIAMACHY retrieval we apply the ones from Fleisch-mann et al. (2004). The latter is approx. 10–15% smaller,which leads to larger SCDs and thus also larger numberdensities. Therefore the agreement with the balloon obser-vation is in fact better than it appears on the figure. Asmentioned above, the BrO and NO2 profiles are also ingood accord with the ones from the IFE Bremen, alsoretrieved from SCIAMACHY spectra (Rozanov et al.,2005). In contrast, we do not observe the high concentra-tions of BrO in the lower Stratosphere as reported by Sioriset al., 2006. Again, this is in good agreement with otherobservations (Sinnhuber et al., 2005; Sheode et al., 2006).

In contrast to BrO and NO2, there are no reports on ret-rievals of OClO profiles from SCIAMACHY limb spectraso far. However, for OSIRIS observations, Krecl et al.(2006) demonstrated the successful retrieval of OClO verti-cal profiles for selected orbits in the southern hemisphere inSeptember 2002. Furthermore, there are no correlated bal-loon measurements of OClO profiles so far and a valida-tion is therefore not possible. However, the retrievedvalues for the number density are in good accord withmodelled values and comparable previous balloon mea-surements (Pommerau and Piquard, 1994; Fitzenberger,2000), and also the height of the profile maximum agrees.

Fig. 11 shows results for OClO profile retrievals forselected days in the Arctic winters 2005 and 2006, in com-parison to balloon measurements performed in the Arcticwinters 1992, 1999 and 2000. While the abundance ofOClO depends very strongly on the degree of chlorine acti-

Fig. 11. Vertical OClO profiles measured by SCIAMACHY for selecteddays in the Arctic winters 2005 and 2006 (see legend) compared to earlierballoon measurements (Pommerau and Piquard, 1994; Fitzenberger,2000).

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1759

vation for the particular time and region, it can be seen thatthe shape and magnitude of the profiles agree very good.Only for altitudes below 15 km there is a tendency to largervalues for the SCIAMACHY profiles compared to the bal-loon measurements. This can be explained as a result of theviewing geometry of satellite limb sounders: Due to thelong light-path the retrieval for one particular tangentheight will be impacted not only by absorptions close tothe tangent point but by all absorptions occurring alongthe line of sight.

In case of horizontal gradients (which occur e.g. at thepolar vortex edge) this will alter the retrieved profile as fol-lows: Positive gradients of the considered trace gas fromthe tangent point (TP) to the direction of the instrument(as appear for OClO in the northern hemisphere due toits dependence on the SZA) will lead to larger concentra-tions and lower altitudes of the peak. Vice versa, a negative

Fig. 12. OClO number density for 16th of January 2005 as retrieved from SCIAfunction of altitude and latitude), values are averaged in longitude. Bottom: G

gradient (from TP towards instrument position) will lead tolower concentrations and higher altitudes of the peak. Amethod to overcome this retrieval shortcoming by includ-ing the measurement(s) performed directly before theobservation to be retrieved in a so called 2D retrievalapproach has been described recently (Pukite et al.,2007). Since there are no correlated measurements of OClOprofiles so far, and also the SCIAMACHY measurementsinhibit a statistical and a systematic error, a validation islacking but necessary. Also, the diurnal effect is until nownot corrected for (see below).

As an example for a global and height resolved retrievalof OClO number densities, Fig. 12 shows a latitudinal crosssection of OClO profiles and the corresponding map of theglobal OClO number densities at 18 km, both for the 16thJanuary, 2005. It can be seen that chlorine activated airmasses (i.e., large abundances of OClO) are observed for

MACHY limb spectra. Top: Latitudinal cross section (number density aslobal cross section at 18 km altitude.

1760 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

regions close to the wintertime north pole only. A compar-ison to meteorological parameters (T and PV, not shown)reveals that the chlorine activated air masses are foundinside the polar vortex and that stratospheric temperatureswere below the threshold for PSC formation.

Since OClO is an indicator for ClO it is also informativeto compare the OClO number density derived fromSCIAMACHY spectra to the ones for ClO from micro-wave spectroscopy (e.g. by MLS on AURA or SMR onOdin). We did this for several days with large and moderatechlorine activation in January 2003 and 2005. Some exam-ples of this comparison exercise are shown in Fig. 13.

The figures reveal a good qualitative agreement: airmasses with comparable degrees of activated chlorine areobserved at the same places and altitudes. Note that thediscrepancy between the maps for 17th–19th January ofOClO and 19th January for ClO can be explained by east-ward transport (due to the counter-clockwise rotation ofthe polar vortex). Similarly, for the OClO map from 23–25 January, the activated air masses above northeast Can-ada are only observed for the 25th (and thus can not beseen in the ClO observation by Odin for the 23rd). For aquantitative comparison it is essential to consider theimpact of the SZA on the measurement. This is importantfor all photochemical species, like NO2, but even more forBrO and OClO, the latter having a photochemical lifetimeof a few seconds.

The possibility to correct the profile retrieval from limbmeasurements by modelling their diurnal cycle has beendescribed recently: Mc Linden et al. (2006) reported thatfor BrO profiles the retrieval error due to neglecting the diur-nal effect is�100% at a SZA of 88� and an altitude of 15 km.For higher altitudes this error is considerably smaller (e.g.�25% at a SZA of 88� and an altitude of 20 km). Also forlower SZAs the impact of the diurnal effect on the absoluteretrieval error is decreasing significantly. However, it shouldbe noted that also at the altitudes of the OClO profile peak(approx. 17–21 km) there is a retrieval error due to the diur-nal effect which in our estimation will range between 10% and50% (depending on the SZA).

Apart from the related difficulties in the profile retrieval,the diurnal effect offers also possibilities for research: sinceOClO and BrO profiles are simultaneously measured bySCIAMACHY and ClO profiles are available by e.g. theMLS on AURA, the combination of these observationswill give a further insight into questions related to strato-spheric ozone chemistry, in particular when combined withmodel simulations that can study the effect of different pho-tolysis rates and extract budgets.

As an example for the application capabilities of theSCIAMACHY limb measurements in atmospheric sci-ences, Fig. 14 shows latitudinal cross sections (number den-sities as function of altitude and latitude) of NO2, BrO andOClO for two selected orbits in end of August and end ofSeptember, 2004. The observed evolution of the trace gasabundances is a good example of the variety of issues onstratospheric chemistry that can be investigated with the

SCIAMACHY limb measurements: denoxification anddenitrification in polar winter, replenishing of nitrogen oxi-des in polar spring, and activation of chlorine and brominein winter and deactivation in spring followed by substantialdepletion of ozone (for 60–70�S) at the altitudes where thehalogens were activated.

These observations are in good accord with indepen-dent, already existing observations (Dessler et al., 1996;Santee et al., 1996, 2003; Randall et al., 2002; Sinnhuberet al., 2002) but will also contribute to new insights instratospheric chemistry: By analyzing the whole data setof SCIAMACHY, interactions like these can be monitoredfor a long time scale (SCIAMACHY measures limb spectrasince August 2002 until present) and globally. Since long-term, vertically resolved global observations of OClO andBrO do not exist so far, these observations will enhanceour understanding of stratospheric chemistry.

5. Conclusion and outlook

A two step algorithm for the retrieval of stratospherictrace gas profiles from SCIAMACHY limb measurementswas described. It is possible to perform the profile retrie-val either with or without constraining the inversion by apriori knowledge. Results for NO2, BrO and OClO werepresented and compared to balloon validation measure-ments or satellite observations. The inversion by the leastsquares approach (independently from a priori) results inreasonable profiles for altitudes between 15 and 40 km.For wider altitude ranges, it is necessary to apply a prioriprofiles.

For all comparison and validation studies performed, agood agreement to independent observations was found.However, the number of correlated measurements is small(for OClO none exist so far). After further validation andsensitivity studies, the complete record of SCIAMACHYlimb spectra (since October 2002 until present) will be eval-uated for NO2, BrO and OClO profiles. The obtained dataset of vertical profiles will be applied for studies on strato-spheric chemistry, like inter-annual and inter-hemisphericcomparisons (variability and evolution of the magnitudeand profile peak height), correlations to stratospheric tem-peratures and other stratospheric trace gases, and compar-ison to results from established model simulations.

Although the presented initial results and agreementswith independent observations are very encouraging,improvements and modifications are planned that will beimplemented in future algorithm updates. For instance,the retrieval of clouds from the limb measurements willbe added, since the minimum retrieval altitude is deter-mined by the occurrence of clouds in the lower strato-sphere/upper troposphere. Another advantage might bethe enhancement of the handling of the temperature depen-dency of the absorption cross sections.

For BrO and OClO it may be possible to improve theprofile retrieval if the diurnal cycle is taken into account.This is in particular important for limb measurements close

Fig. 13. OClO number density at 18 km retrieved from SCIAMACHY limb observations in comparison to the ClO mixing ratio measured by SMR (SubMillimetre Radiometer) on Odin at 19 km for selected days in January, 2003. For details on the ClO observations see (Urban et al., 2004). Displayed arevalues for the northern hemisphere in polar projection.

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1761

Fig. 14. Latitudinal cross sections (number density as function of altitude and latitude) of nitrogen dioxide, bromine oxide and chlorine dioxide forselected orbits from 24.08.2004 and 30.09.2004. For altitudes from 10 to 15 km the retrieved profile consists mainly of the a-priori information (see Fig. 5).Note also that the latitudinal extension of the two orbits differs.

1762 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

to the terminator, where the abundance of BrO and OClOis strongly depending on the SZA. In this respect, it will beuseful to complete the daytime OClO profiles derived fromlimb observations with the night-time OClO profiles fromSCIAMACHY occultation measurements. Moreover,together with the BrO profiles (and additional ClO profilefrom microwave spectroscopy), these observations give afull picture of stratospheric halogen chemistry and will cer-tainly give new insight into current open questions (Cantyet al., 2005).

Acknowledgements

We thank M. Dorf (IUP, Universität Heidelberg) forproviding the balloon borne BrO validation measurementsfor SCIAMACHY, and J. Urban (Chalmers University,Göteborg) for providing the Odin-SMR ClO observations.

The Deutsche Forschungsgemeinschaft (DFG) is grate-fully acknowledged for financial support.

References

Anderson, G.P., Chetwynd, J.H., Clough, S.A., Shettle, E.P., Kneizys,F.X. AFGL Atmospheric Constituent Profiles (0–120 km), AFGL-TR-86-0110, Environmental Research Papers, 954, 1986.

Bogumil, K., Orphal, J., Homann, T., Voigt, S., Spietz, P., Fleischmann,O.C., Vogel, A., Hartmann, M., Bovensmann, H., Frerik, J., Burrows,J.P. Measurements of molecular absorption spectra with the SCIAM-ACHY pre-flight model: instrument characterization and referencedata for atmospheric remote-sensing in the 230–2380 nm region. J.Photochem. Photobiol. A 157, 167–184, 2003.

Bovensmann, H., Burrows, J.P., Buchwitz, M., Frerick, J., Noel, S.,Rozanov, V.V., Chance, K.V., Goede, A.P.H. SCIAMACHY:mission objectives and measurement modes. J. Atmos. Sci. 56,127–150, 1999.

Burrows, J.P., Weber, M., Buchwitz, M., et al. The Global OzoneMonitoring Experiment (GOME): mission concept and first scientificresults. J. Atmos. Sci. 56, 151–175, 1999.

Bussemer, M. Der Ring-Effekt: Ursachen und Einfluß auf die spektrosk-opische Messung stratosphärischer Spurenstoffe, Diploma thesis,University of Heidelberg, 1993.

Butz, A., Bösch, H., Camy-Peyret, C., et al. Inter-comparison ofstratospheric O3 and NO2 abundances retrieved from balloon borne

S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764 1763

direct sun observations and Envisat/SCIAMACHY limb measure-ments. Atmos. Chem. Phys. 6, 1293–1314, 2006.

Canty, T., Riviere, E.D., Salawitch, R.J., et al. Nighttime OClO in thewinter Arctic vortex, J. Geophys. Res., 110, D01301, doi:10.1029/2004JD005035, 2005.

Dessler, A.E., Kawa, S.R., Douglass, A.R., Considine, D.B., Kumer, J.B.,Roche, A.E., Mergenthaler, J.L., Waters, J.W., Russell III, J.M., Gille,J.C. A test of partitioning between ClO and ClONO2 using simulta-neous UARS measurements of ClO, NO2, and ClONO2. J. Geophys.Res. 101, 2515–12521, 1996.

Deutschmann, T., Wagner, T. TRACY-II Manual. Universität Heidel-berg, 2006.

Dorf, M., Bösch, H., Butz, A., et al. Balloon-borne stratospheric BrOmeasurements: comparison with Envisat/ SCIAMACHY BrO limbprofiles. Atmos. Chem. Phys. 6, 2483–2501, 2006.

Fitzenberger, R. Investigation of the Stratospheric Inorganic BromineBudget for 1996–2000: Balloon-Borne Measurements and ModelComparisons, Ph.D. Thesis, University of Heidelberg, Heidelberg,Germany, 2000.

Fleischmann, O.C., Hartmann, M., Burrows, J.P., Orphal, J. Newultraviolet absorption cross-sections of BrO at atmospheric tempera-tures measured by time-windowing Fourier transform spectroscopy. J.Photochem. Photobio. A Chemistry 168, 117–132, 2004.

Grainer, J.F., Ring, J. Anomalous Fraunhofer line profiles. Nature, 193,1962.

Greenblatt, G.D., Orlando, J.J., Burkholder, J.B., Ravishankara, A.R.Absorption measurements of oxygen between 330 and 1140 nm. J.Geophys. Res. 95, 18577–18582, 1990.

Harder, H., Camy-Peyret, C., Ferlemann, F., Fitzenberger, R., Hawat, T.,Osterkamp, H., Schneider, M., Perner, D., Platt, U., Vradelis, P.,Pfeilsticker, K. Stratospheric BrO profiles measured at differentlatitudes and seasons: atmospheric observations. Geophys. Res. Lett.25, 3843–3846, doi:10.1029/1998GL900026, 1998.

Haley, C.S., Brohede, S.M., Sioris, C.E., et al. Retrieval of stratosphericO3 and NO2 profiles from Odin Optical Spectrograph and InfraredImager System (OSIRIS) limb-scattered sunlight measurements. J.Geophys. Res. 109, D16303, doi:10.1029/2004JD004588, 2004.

Jöckel, P., Tost, H., Pozzer, A., et al. The atmospheric chemistry generalcirculation model ECHAM5/MESSy1: consistent simulation of ozonefrom the surface to the mesosphere. Atmos. Chem. Phys. 6, 5067–5104,2006.

Krecl, P., Haley, C.S., Stegman, J., Brohede, S.M., Berthet, G. Retrievingthe vertical distribution of stratospheric OClO from Odin/OSIRISlimb-scattered sunlight measurements. Atmos. Chem. Phys. 6, 1879–1894, 2006.

Kromminga, H., Orphal, J., Spietz, P., Voigt, S., Burrows, J.P. Newmeasurements of OClO absorption cross sections in the 325–435 nmregion and their temperature dependence between 213–293 K. J.Photochem. Photobiol. A.: Chemistry 157, 149–160, 2003.

Kühl, S., Wilms-Grabe, W., Beirle, S., Frankenberg, C., Grzegorski, M.,Hollwedel, J., Khokhar, F., Kraus, S., Platt, U., Sanghavi, S., vonFriedeburg, C., Wagner, T. Stratospheric chlorine activation in theArctic winters 1995/96 to 2001/02 derived from GOME OClOMeasurements. Adv. Space Res. 34, 798–803, 2004a.

Kühl, S., Dörnbrack, A., Wilms-Grabe, W., Sinnhuber, B.-M., Platt, U.,Wagner, T. Observational evidence of rapid chlorine activation bymountain waves above northern Scandinavia. J. Geophys. Res. 109,D22309, doi:10.1029/2004JD004797, 2004b.

Kühl, S. Quantifying Stratospheric chlorine chemistry by the satellitespectrometers GOME and SCIAMACHY, Ph.D. Thesis, UniversitätHeidelberg, 2005.

Kühl, S., Wilms-Grabe, W., Frankenberg, C., Grzegorski, M., Platt, U.,Wagner, T. Comparison of OClO nadir measurements from SCIAM-ACHY and GOME. Adv. Space Res. 37, 2247–2253, 2006.

Loughman, R.P., Griffioen, E., Oikarinen, L., Postylyakov, O.V., Roza-nov, A., Flittner, D.E., Rault, D.F. Comparison of radiative transfermodels for limb-viewing scattered sunlight measurements. J. Geophys.Res. 109, D06303, doi:10.1029/2003JD003854, 2004.

McDade, I.C., Strong, K., Haley, C.S., Stegman, J., Murtagh, D.P.,Llewellyn, E.J. A method for recovering stratospheric minor speciesdensities from the Odin/OSIRIS scattered-sunlight measurements.Can. J. Phys. 80 (4), 395–408, 2002.

Mc Linden, C.A., Haley, C.S., Sioris, C.E. Diurnal effects in limb scatterobservations. J. Geophys. Res. 111, D14302, doi:10.1029/2005JD006628, 2006.

Menke, W. Geophysical data analysis: discrete inverse theory. AcademicPress, 1999.

Oikarinen, L., Sivhola, E., Kyrölä, E. Multiple-scattering radiance inlimb-viewing geometry. J. Geophys. Res. 104 (D24), 31261–31274,1999.

Platt, U. Differential optical absorption spectroscopy (DOAS), in: Sigrist,M.W. (Ed.), Air Monitoring by Spectroscopic Techniques, ChemicalAnalysis Series, vol. 127. John Wiley, New York, 1994.

Platt, U., Stutz, J.. Differential Optical Absorption SpectroscopyPrinciplesand Applications. Springer, Berlin, 2007.

Platt, U., Marquard, L., Wagner, T., Perner, D. Corrections for zenithscattered light DOAS. Geophys. Res. Lett. 24, 1759–1762, 1997.

Pommerau, J.-P., Piquard, J. Observations of the vertical distribution ofstratospheric OClO. Geophys. Res. Lett. 21, 1231–1234, 1994.

Pukite, J., Kühl, S., Deutschmann, T., Wilms-Grabe, W., Friedeburg, C.,Platt, U., Wagner, T. Retrieval of stratospheric trace gases fromSCIAMACHY limb spectra, in: Proceedings of the first AtmosphericScience Conference, 8–12 May, ESA/ESRIN, Frascati, Italy, ESA SP-628, 2006.

Pukite, J., Kühl, S., Deutschmann, T., Platt, U., Wagner, T. Applicationof 3D MC RTM ‘‘TRACY-II’’ on SCIAMACHY limb measurementsin the UV/VIS spectral range, in: Proceedings of the 2nd ENVISATSymposium, Montreux, Switzerland, 2007.

Randall, C.E., Lumpe, J.D., Bevilacqua, R.M., Hoppel, K.W., Shettle,E.P., Rusch, D.W., Gordley, L.L., Kreher, K., Pfeilsticker, K., Boesch,H., Toon, G., Goutail, F., Pommereau, J-P. Validation of POAM IIINO2 measurements. J. Geophys. Res. 107, doi:10.1029/2001JD001520,2002.

Richter, A., Wittrock, F., Weber, M., Beirle, S., Kühl, S., Platt, U.,Wagner, T., Wilms- Grabe, W., Burrows, J.P. GOME observations ofstratospheric trace gas distributions during the splitting vortex event inthe Antarctic winter 2002 Part I: measurements. J. Atm. Sci. 62, 778–785, 2005.

Rodgers, C.D. Inverse methods for atmospheric sounding. Theory andpractice. World Scientific Publishing Co. Ltd., Singapore, 2000.

Rozanov, A., Bovensmann, H., Bracher, A., Hrechanyy, S., Rozanov, V.,Sinnhuber, M., Stroh, F., Burrows, J.P. NO2 and BrO vertical profileretrieval from SCIAMACHY limb measurements: sensitivity studies.Adv. Space Res. 36, 846–854, 2005.

Santee, M.L., Froidevaux, L., Manney, G.L., Read, W.G., Waters, J.,Chipperfield, M.P., Roche, A.E., Kumer, J.B., Merganthaler, J.,Russell III, J.M. Chlorine deactivation in the lower stratosphere polarregions during late winter: results from UARS. J. Geophys. Res. 101,18835–18860, 1996.

Santee, M.L., Manney, G.L., Waters, J.W., Livesey, N.J. Variations andclimatology of ClO in the polar lower stratosphere from UARSMicrowave Limb Sounder measurements. J. Geophys. Res. 108, 454,doi:10.1029/2002JD003335, 2003.

Schiller, C., Wahner, A. Comment on Stratospheric OClO measurementsas a poor quantitative indicator of chlorine activation by J. Sessler,M.P. Chipperfield, J.A. Pyle, R. Toumi, Geophys. Res. Lett., 23, 1053–1054, 1996.

Sheode, N., Sinnhuber, B.-M., Rozanov, A., Burrows, J.P. Towards aclimatology of stratospheric bromine monoxide from SCIAMACHYlimb observations. Atmos. Chem. Phys. Discuss. 6, 6431–6466, 2006.

Sinnhuber, B.-M., Arlander, D.W., Bovensmann, H., et al. Comparisonof measurements and model calculations of stratospheric brominemonoxide. J. Geophys. Res. 107, doi:10.1029/2001JD000940, 2002.

Sinnhuber, B.-M., Rozanov, A., Sheode, N., Afe, O.T., Richter, A.,Sinnhuber, M., Wittrock, F., Burrows, J.P., Stiller, G.P., vonClarmann, T., Linden, A. Global observations of stratospheric

http://dx.doi.org/10.1029/1998GL900026http://dx.doi.org/10.1029/2004JD004588http://dx.doi.org/10.1029/2004JD004797http://dx.doi.org/10.1029/2003JD003854http://dx.doi.org/10.1029/2005JD006628http://dx.doi.org/10.1029/2005JD006628http://dx.doi.org/10.1029/2001JD001520http://dx.doi.org/10.1029/2001JD001520http://dx.doi.org/10.1029/2002JD003335http://dx.doi.org/10.1029/2001JD000940

1764 S. Kühl et al. / Advances in Space Research 42 (2008) 1747–1764

bromine monoxide from SCIAMACHY. Geophys. Res. Lett. 32,L20810, doi:10.1029/2005GL023839, 2005.

Sioris, C., Kovalenko, L.J., McLinden, C.A., et al. Latitudinal andvertical distribution of bromine monoxide in the lower stratospherefrom Scanning Imaging Absorption Spectrometer for AtmosphericChartography limb scattering measurements. J. Geophys. Res. 111,D14301, doi:10.1029/2005JD006479, 2006.

Solomon, S., Garcia, R.R., Rowland, F.S., Wuebbles, D.J. On thedepletion of Antarctic ozone. Nature 321, 755–758, 1986.

Solomon, S., Mount, G.H., Sanders, R.W., Schmeltekopf, A.L. VisibleSpectroscopy at McMurdo Station, Antarctica 2. Observations ofOClO. J. Geophys. Res. 92, 8329–8338, 1987a.

Solomon, S., Schmeltekopf, A.L., Sanders, R.W. On the interpretation ofzenith sky absorption measurements. J. Geophys. Res. 92, 8311–8319,1987b.

Solomon, S. Stratospheric ozone depletion: a review of concepts andhistory. Rev. Geophys. 37 (3), 275–316, 1999.

Stutz, J., Platt, U. Numerical analysis and estimation of the statisticalerror of differential optical absorption spectroscopy measurementswith least-squares methodes. Appl. Optics 35 (30), 6041–6053, 1996.

Tilstra, L.G., Stammes, P. Alternative polarisation retrieval for SCIAM-ACHY in the ultraviolet. Atmos. Chem. Phys. 5, 2009–2107, 2005.

Tornkvist, K.K., Arlander, D.W., Sinnhuber, B.-M. Ground-Based UVMeasurements of BrO and OClO over Ny-Alesund during Winter 1996and 1997 and Andoya during Winter 1998/99. J. Atm. Chem. 43, 75–106, 2002.

Urban, J., Lautie, N., Le Flochmoen, E., et al. The northern hemispherestratospheric vortex during the 2002–03 winter: subsidence, chlorineactivation and ozone loss observed by the Odin Sub-MillimetreRadiometer. Geophys. Res. Lett. 31, doi:10.1029/2003GL019089,2004.

van Roozendael, M., Loyola, D., Spurr, R., et al. Ten years of GOME/ERS-2 total ozone data-The new GOME data processor (GDP)version 4: 1.Algorithm description. J. Geophys. Res. 111, D14311,doi:10.1029/2005JD006375, 2006.

von Clarmann, T., Glatthor, N., Grabowski, U., et al. Retrieval oftemperature and tangent altitude pointing from limb emission spectrarecorded from space by the Michelson Interferometer for Passive

Atmospheric Sounding (MIPAS). J. Geophys. Res. 108 (D23), 4736,doi:10.1029/2003JD003602, 2003.

von Savigny, C., Kaiser, J.W., Bovensmann, H., Burrows, J.P., McDer-mid, I.S., Leblanc, T. Spatial and temporal characterization ofSCIAMACHY limb pointing errors during the first three years ofthe mission. Atmos. Chem. Phys. 5, 2593–2602, 2005.

Wagner, T., Platt, U. Satellite mapping of enhanced BrO concentrations inthe troposphere. Nature 395, 486–490, 1998.

Wagner, T. Satellite observations of Atmospheric Halogen oxides, Ph.D.Thesis, University of Heidelberg, 1999.

Wagner, T., Leue, C., Pfeilsticker, K., Platt, U. Monitoring of thestratospheric chlorine activation by Global Ozone Monitoring Exper-iment (GOME) OClO measurements in the austral and boreal winters1995 through 1999. J. Geophys. Res. 106, 4971–4986, 2001.

Wagner, T., Wittrock, F., Richter, A., Wenig, M., Burrows, J.P., Platt, U.Continuous monitoring of the high and persistent chlorine activationduring the Arctic winter 1999/2000 by the GOME instrument on ERS-2,J. Geophys. Res. 107 (D20), 8267, doi:10.1029/2001JD000466, 2002.

Wagner, T., Burrows, J.P., Deutschmann, T., et al. Comparison of Box-Air-Mass-Factors and Radiances for Multiple-Axis Differential Opti-cal Absorption Spectroscopy (MAX-DOAS) Geometries calculatedfrom different UV/visible Radiative Transfer Models. Atmos. Chem.Phys. 7, 1809–1833, 2007.

Wahner, A., Tyndall, G., Ravishankara, A. Absorption cross sections forOClO as a function of temperature in the wavelength range from 240–490 nm. J. Phys. Chem. 91, 2735–2738, 1987.

Waters, J.W., Froidevaux, L., Manney, G.L., Read, W.G., Elson, L.S.Lower stratospheric ClO and O3 in the 1992 southern hemispherewinter. Geophys. Res. Lett. 20, 1219–1222, 1993.

Weber, M., Dhomse, S., Wittrock, F., Richter, A., Sinnhuber, B.-M.,Burrows, J.P. Dynamical control of NH and SH winter/spring totalozone from GOME observations in 1995–2002. Geophys. Res. Lett. 30(1), doi:10.1029/2002GL016008, 2003.

Wennberg, P.O., Cohen, R.C., Stimpfle, R.M., et al. Removal ofstratospheric O3 by radicals: in situ measurements of OH, HO2, NO,NO2, ClO, and BrO. Science 266, 398–404, 1994.

WMO. World Meteorological Organization, Scientific Assessment ofOzone Depletion: 2002, Geneva, 2003.

http://dx.doi.org/10.1029/2005GL023839http://dx.doi.org/10.1029/2005JD006479http://dx.doi.org/10.1029/2003GL019089http://dx.doi.org/10.1029/2003GL019089http://dx.doi.org/10.1029/2005JD006375http://dx.doi.org/10.1029/2003JD003602http://dx.doi.org/10.1029/2001JD000466http://dx.doi.org/10.1029/2002GL016008

SCIAMACHY limb measurements of NO2, BrO and OClO. Retrieval of vertical profiles: Algorithm, first results, sensitivity and comparison studiesIntroductionInstrumentProfile retrievalDOASInversion to profilesSensitivity studies and error analysisRetrieval of SCDsInversion to profiles

First results and comparison to independent measurementsConclusion and outlookAcknowledgementsReferences