A global climatology of stratospheric OClO derived from GOMOS measurement

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Introduction
The discovery of the stratospheric ozone depletion in Antarctica by Farman et al. (1985) has led to numerous studies to understand the physico-chemical mechanisms involved in this recurrent phenomenon.It appears that the halogen species play an important role in the chemical cycles that lead to polar ozone depletion (Solomon et al., 1986).
Of these cycles, those involving active chlorine species (Cl, ClO, Cl 2 O 2 ) are among the most efficient (Salawitch et al., 1993).The presence of chlorine species in the atmosphere is mainly due to the emission of chlorofluorocarbons (CFC) at the ground.CFCs are chemically inert in the troposphere and are not soluble in water, which make them resistant to washout processes.Therefore, they are efficiently transported toward the stratosphere where they are photolyzed by UV radiation or oxidized to produce atomic chlorine and chlorine monoxide (ClO).One consequence of the accumulation of ClO in the stratosphere is the formation of chlorine dioxide (OClO) via one of the possible reactions between ClO and bromine dioxide (BrO): Thus, the detection of OClO in the stratosphere is a sign of chlorine activation.Also, nitrogen dioxide (NO 2 ) is of primary importance for polar ozone chemistry.Notably, in the presence of a third molecule (for example N 2 or O 2 ), NO 2 reduces the production of OClO by reacting with ClO or BrO to form inert chlorine and bromine reservoirs: and NO 3 during the night.Therefore in the permanent night of high latitudes regions, this reaction leads to the removal of almost all of NO 2 and NO 3 : this is the well-known denoxification process.Subsequently, Reactions (R4) and (R5) are very limited in the polar vortex and OClO can then be formed via the Reaction (R1).Consequently, OClO and NO 2 are expected to be anti-correlated in the polar regions.The OClO concentration is expected to remain constant during the night as the only sink of this species is the photolysis by the solar radiation.The absorption cross-section of OClO is characterized by strong differential structures in the near UV-range.This feature will also allow us to perform the retrieval of OClO.More details about stratospheric chemical processes involving chlorine and nitrogen species can be found in Solomon (1999).
Another important aspect of the OClO chemistry is its interactions with NO 2 which are not completely understood.A few studies have covered the subject (Riviere et al., 2003(Riviere et al., , 2004;;Berthet et al., 2007;T étard et al., 2009) but all have concluded that uncertainties in the understanding of these interactions persist.It is therefore critical to monitor simultaneously OClO and NO 2 .Although NO 2 monitoring in the stratosphere has been extensively tackled by the scientific community, OClO monitoring has been much less so.For example, we can mention the studies of the vertical column of OClO retrieved using ground-based instruments (Solomon et al., 1987;Miller et al., 1999) or using nadir-viewing satellite instruments: GOME (Wagner et al., 2002), SCIA-MACHY (Oetjen et al., 2011).Also, the vertical profiles of OClO concentration have been retrieved during nighttime using balloon-borne instruments: Absorption by the Minor components Ozone and Nox (AMON, Renard et al., 1997) and Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NO x (SALOMON, Renard et al., 2000).In this study, flights of these two instruments have been used to validate the OClO concentration profiles retrieved from the Global Ozone Monitoring by Occultation of Stars (GOMOS) measurements (Bertaux et al., 2010).Also, some satellite instruments are used to retrieve the vertical distributions of OClO using limb-scattered sunlight measurements: Optical Spectrograph and InfraRed Imager System (OSIRIS; Krecl et al., 2006) and SCIAMACHY (K ühl et al., 2008).Finally, the Introduction

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Full Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument is also able to retrieve OClO vertical profiles using its lunar occultation mode but no scientific studies about this have been published yet.GOMOS is the only satellite instrument able to perform nighttime measurements of the vertical distribution of OClO on a long term and on a global scale.Two previous studies about OClO retrieved using measurements from the GOMOS instrument have already been published: the first by Fussen et al. (2006)  In this paper, we present the method used to retrieve OClO from GOMOS measurements.The method is an evolution of that described in Fussen et al. (2006).A more careful statistical processing has been applied and the spectral window has been optimized.Firstly, a statistical analysis is applied on several co-located GOMOS transmittance measurements to construct an averaged transmittance measurement.Then, a DOAS process based on these averaged measurements is applied to compute slant column density (hereafter SCD) of OClO.Then, the study focuses on the validation of the OClO GOMOS products by inter-comparisons with those retrieved from two different balloon-borne measurements.Finally, global annual and monthly climatologies of OClO are presented and the anti-correlation between NO 2 and OClO is highlighted.
This article focuses on the importance and the power of the use of averaged measurements to detect small absorbers.

The GOMOS instrument
The GOMOS instrument has been embarked on the European platform ENVISAT (EN-Vironment SATellite)  and with an inclination of 98.55 • .The GOMOS mission ended on April 2012 when contact with ENVISAT was lost.During its ten years of operation, GOMOS carried out about 860 000 occultations.The instrument operation is based on a grating spectrometer operating in stellar occultation mode: at each tangent altitude z and each wavelength λ, the transmittance T (λ, z) is obtained by dividing the stellar radiance attenuated through the atmosphere S(λ, z) by the reference star radiance S 0 (λ) measured outside the atmosphere.Thus, these measurements are self-calibrated and this is one of the main advantages of the method.Another advantage is related to the important number of light sources available (about 180 stars are used) that allows a global coverage in about 3 days (according to the mission baseline scenario).Nevertheless, some drawbacks of the stellar occultation method have to be mentioned.The selected stars have different magnitude, temperature and spectra with a clear impact on the signal-to-noise ratio of the measured spectra and therefore on the retrieval error budget.Moreover, the scintillation of stars has to be considered (Bertaux et al., 1988).It is due to small-scale vertical temperature fluctuations of the atmosphere.To overcome this problem, two fast photometers (with 1 kHz sampling rate) were installed in parallel to the spectrometers.However, it appears that the correction of the scintillation is right when the star setting is vertical (close to the orbital plane) but imperfect for oblique occultations (Sofieva et al., 2009).In this case, residual scintillation persists due to atmospheric inhomogeneous horizontal structures.Figure 1 shows an example of transmittance spectra obtained from oblique (top panel) and vertical (middle panel) occultations.The presence of residual scintillations is clearly visible in the spectra from oblique occultation whereas scintillation is almost perfectly removed in the spectra from vertical occultation.The vertical resolution is 1.7 km for vertical occultations and better for oblique ones.
GOMOS is in fact made of four spectrometers: the two first in the UV-visible spectral range (from 250 to 675 nm with a spectral resolution of 0.8 nm), the third in the 756 to 773 nm range (with 0.13 nm resolution) and the last in the near infrared (926 to 952 nm with a spectral resolution of 0.13 nm).The main goal of GOMOS is the global, long term Introduction

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Full monitoring of stratospheric and mesospheric ozone.Furthermore, the large wavelength range and the spectral resolutions allows the retrieval of other species like NO 2 , NO 3 , H 2 O, O 2 , OClO and aerosol extinction (Bertaux et al., 2010).The following section explains the method used to retrieve OClO.

The retrieval of OClO
The starting point of most occultation retrieval algorithm is the well-known Beer-Lambert law.It describes how the signal is affected by the presence of atmospheric absorbers along the line of sight at tangent altitude z: where σ i is the extinction cross-section and N i is the slant column density of each atmospheric absorbers in the spectral range selected.A detailed overview of the GO-MOS operational algorithm can be find in Kyr öl ä et al. (2010b).Most of the expected species can be directly retrieved from single measurements but the combination of the weak signal-to-noise ratio of a single GOMOS measurement and the small optical thickness of OClO forces us to combine several single measurements in order to detect it.The retrieval of OClO from GOMOS measurements requires two steps.The first is the calculation of an averaged transmittance spectra and the second is the inversion process of these spectra.
As written previously, the first step is required to increase the signal-to-noise ratio so that the retrieval of OClO can be carried out in the second step of the process.The idea is to use several co-located (spatially and temporally) GOMOS measurements to build a new measurement.This is possible because each measurements consists of a transmittance and is consequently independent of the selected star.This averaged measurement is supposed to be representative of one particular location for a given Introduction

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Full moment.For the selection of the GOMOS measurements used to build the averaged measurements, we have used the following steps: selection of stars: stars with effective temperatures greater than 4100 K to ensure a sufficient UV flux and stars with magnitudes lower than 2 to have acceptable photon fluxes.These criteria lead to a selection of 44 stars.
temporal resolution: one month or one year.The time period is September 2002 to the end of 2011.The choice of this temporal bin size is influenced by the latitudinal resolution hereafter.
latitude bands: for the annual climatologies, we have used 10 • latitude bands and for the monthly time series, we have used 20 • latitude bands for the regions located between −30 • and +30 • and 30 • latitude bands elsewhere to ensure sufficient numbers of measurements in each data set.
illumination conditions: only dark limb and straylight condition measurements are used.Dark limb condition corresponds to measurements made when the solar zenith angle is higher than 120 • .These measurements are supposed to be used without restriction.Measurements are said to be in straylight conditions if the instrument is illuminated by light coming from the scattering of solar light.Straylight measurements are considered as of good quality (more details can be found in the GOMOS product handbook at http://envisat.esa.int/handbooks/gomos/toc.htm).
the GOMOS data used here are obtained with the version 5 of the ESA level 1 operational processor.
all the selected spectra are linearly interpolated on a common vertical grid of tangent altitudes from 0 to 60 km with 1 km step.
Figure 2 shows the latitudinal distribution of the GOMOS data used in our study.A total number of about 162 000 GOMOS measurements has been processed.While both polar regions are well covered during the winters and much less outside these Introduction

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Full During the first months of 2005 (until June), almost no measurements are available due to an instrumental breakdown.However, some measurements in the polar regions could be carried out (particularly in January).
For each bin, we have built a data set: T 1 λ i , z j , . . ., T n λ i , z j where z j are the 61 tangent altitude levels, λ i are the wavelengths and n is the number of GOMOS measurements in the bin.Once all the data sets are built, they should be statistically analyzed to guarantee the homogeneity of the bin data sets and subsequently, to ensure the representativity of the averaged measurement that will be derived from this data set.For each data set, statistical detections of multimodal distribution are performed by fitting the distribution with theoretical models.Figure 3 shows examples of GOMOS transmittance distributions for a monomodal and a bimodal case.The bimodality observed here in the high latitude band is due to the fact that some measurements are inside the polar vortex and others are outside.In fact, transmittances inside the polar vortex are greater than those outside the polar vortex because the decrease in NO 2 concentrations in the vortex area (denoxification) is not sufficiently offset by the increase of OClO concentrations.All the cases of bimodal distributions observed in our study are due to measurements done inside and outside the polar vortex.In this case, both modes are studied separately and, after visual inspection, we have selected the one that represents the high latitude band, in other words the one inside the polar vortex with greater transmittances (the other modes will be analyzed in the frame of some case studies).Then, an outlier detection is performed for each dataset using a jackknife method.Finally, for each tangent altitude and each wavelength, the weighted median transmittance is calculated.A weighted median calculation starts by sorting the transmittance values in increasing order, and rearranging the associated weights in the same fashion.The cumulative distribution of these weights is subsequently evaluated.The weighted median is then the transmittance value correponding to the 50 % level of this cumulative weight distribution.The uncertainty associated with the weighted median is calculated as the weighted median absolute deviation (WMAD).It is the median Introduction

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Full of the absolute values of the difference between each transmittance values and the weighted median.This statistical study ensures that the calculated averaged spectra are representative of the considered region.Figure 1 shows transmittance spectra in the 250 to 690 nm range for a single oblique occultation, a single vertical occultation and an averaged occultation.We can note that the signs of residual scintillation disappear in the averaged spectra (bottom panel) and that residual scintillations are almost perfectly corrected in the vertical occultation spectra (center panel) whereas some remaining scintillations persist in the oblique spectra (top panel).
The second step consists of a Differential Optical Absorption Spectroscopy (DOAS) algorithm applied to averaged transmittance spectra to retrieve all the chemical species involved in the absorption of the radiation in the wavelength range used.The DOAS technique has been reviewed by Platt et al. (1979).The principle of this method is quite simple: it is based on the idea that the transmittance consists of two components, one varying slowly with the wavelength and the other rapidly varying.In the spectral range used, the slowly varying component T s (λ) reflects the Rayleigh scattering, aerosol extinction and the slowly varying part of the gaseous absorptions while the second component represents the rapidly varying part of gaseous absorptions.T s (λ) is obtained by fitting the measured transmittance T (λ) by a second order polynomial in the wavelength window selected.Then, the experimental differential transmittance dT (λ) is simply expressed as the difference between T (λ) and T s (λ) while the modeled differential transmittance M(λ) is expressed as: where N gas j and δ σ gas j correspond respectively to the SCD and the differential crosssections of the absorber gas in the wavelength interval.δ σ gas j is the difference between the absorption cross-section of gas j and a first order polynomial fitting this cross-section.Using Eq. ( 2), a nonlinear least-square minimization between the Figures

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Full modeled and the experimental differential transmittances weighted by the measurement errors is applied for each tangent altitude to obtain the SCD of each species contributing to the absorption in the selected wavelength range and the retrieval errors (extracted from the jacobian matrix).We have studied 4 spectral ranges: 355-381 nm, 355-390 nm, 390-425 nm and 355-425 nm.For these 4 fitting windows, we have calculated at each tangent altitude z t the reduced chi-square χ 2 : In this expression, n is the number of pixels in the fitting window, d f is the degree of freedom (n minus the number of parameters used in the minimization process) and is the error associated to the averaged differential transmittance.For the fitting windows selected, the reduced chi-square are between about 0.1 and 1.5 (an example is presented in the top left panel in Fig. 4).On the basis of this example, the best chi-square (χ 2 ≈ 1) is obtained with the interval 355-381 nm for all the altitude range.Moreover, the analysis of the mean of the residual (i.e. the difference between observed and modeled transmittances) at each tangent altitude shows that below 25 km there is almost no differences but above, the mean residual is lower when the 355-381 nm wavelength range is used (bottom left panel in Fig. 4).Furthermore, residuals at each tangent altitude (right panels in Fig. 4) are clearly smaller below 390 nm.The same study has been done using several other averaged occultations and the conclusions are always identical: we have therefore selected the 355-381 nm spectral window.In this spectral range, we have to consider the absorptions of OClO, NO 2 and also O 3 even if its cross-section is very small in this range.Thus, these 3 species are fitted simultaneously.In Fig. 5, we show an example of OClO SCDs in the Antarctic region (in blue), in the Arctic region (in black) and in the equatorial region (in red) for the year 2003.For OClO, the retrieval errors are generally better than 50 %.We can already notice several specific structures that will be discussed in more detail in the following sections.In particular, we can observe the presence of an "OClO layer" in the middle stratosphere Figures in the equatorial regions and the expected chlorine activation in the lower stratosphere in both polar regions with a higher amplitude in the south.
The following section presents a preliminary validation of the OClO GOMOS product by comparisons with products retrieved using balloon-borne instruments.

Comparisons with other instruments
Since the nighttime OClO SCD from GOMOS averaged measurements is a new product, it is not easy to validate it.In this validation exercise, we have excluded O 3 and NO 2 SCDs retrieved using the present algorithm because the spectral window used is not the most appropriate one to perform the retrieval of these species, considered here as interfering species.Nonetheless, as a first analysis of the quality of our products, we can compare NO 2 retrieved from GOMOS averaged measurements and the operational GOMOS data.This has been done previously and published in T étard et al. (2009).The results (not reproduced here) indicate that there is a very good agreement between both products (typically, relative differences are between −10 and 10 %) and that our method seems to work properly.
The second step consists in comparing the GOMOS products with those from other instruments.The only nighttime OClO concentration profiles available for comparisons are those retrieved from the AMON and SALOMON balloon-borne instruments.One should keep in mind that these comparisons are done between spatially and temporally localized measurements and a composite of several measurements localized in a latitude band.AMON and SALOMON instruments performed nighttime remote sensing measurements using respectively the stellar and the lunar occultation method.They provided slant column densities of OClO.However, since the observation geometries of GOMOS and of balloon-borne instruments are different, it is not appropriate to compare directly the slant column densities but instead should compare the OClO vertical profiles.To do so, we had to perform spatial inversion of the SCDs.For GOMOS, this has been done by using an onion peeling method.The atmosphere is divided into Figures concentric layers that are assumed to be homogeneous.Using the matrix formalism, the problem consists of solving the following linear system: where the matrix elements of N are the OClO SCDs at each tangent altitude, K is the kernel matrix (a triangular square matrix) and n is the OClO vertical profiles matrix.
The GOMOS spatial inversion is a well-conditioned problem and is easy to solve by using standard method.
AMON is a UV-visible spectrometer that uses the stellar occultation method.The flight used took place in March 2003 in Kiruna, northern Sweden (latitude 67 • 53 N, longitude 21 • 05 E).We have used the measurements from the occultation of Sirius (α Canis Majoris) and Alnilam ( Orionis).Both stars emit enough UV radiation to detect OClO (effective temperatures are respectively 11 000 and 30 000 K).
The second balloon-borne instrument used is SALOMON (Renard et al., 2000).It is a UV-visible spectrometer that uses the lunar occultation method.The flight occurred in January 2006 in Kiruna and the measurements were done during the balloon ascent inside the polar vortex.The OClO concentration profile from the SALOMON measurements has been studied by comparisons with the results of the model described in Berthet et al. (2007).In particular, they have shown that OClO product from SALOMON is in an acceptable agreement with results from chemistry transport model calculations.
For the inter-comparisons, the GOMOS measurements used to build the averaged measurements were chosen located between 60 • and 75 • and took place in a 20 days window centered on the flight date.Since the GOMOS vertical resolution is close to the AMON and SALOMON resolutions, no smoothing procedure was required and direct comparisons could be done.the AMON ( -ORI) profiles but is higher for the AMON (α-CMA) profile: 5.3 × 10 7 cm −3 , 5 × 10 7 cm −3 and 1 × 10 8 cm −3 respectively.The differences in concentration could be explained by asserting that the OClO concentrations can vary according to the area of the vortex observed.A secondary maximum is observed in the three profiles at almost the same altitudes: 26 km for GOMOS and AMON ( -ORI) and 26.5 km for AMON (α-CMA).Here again, the concentration values reached are almost identical for the GOMOS and the AMON ( -ORI) profiles (respectively 1.6 × 10 7 and 1.8 × 10 7 cm −3 ) whereas the AMON (α-CMA) profile exhibits a higher value: 2.5 × 10 7 cm −3 .Overall, the OClO profile from the GOMOS averaged measurement is closer to the product of AMON when -ORI is occulted: for the entire altitude range, the AMON ( -ORI) profiles are well within the GOMOS error bars.
Concerning the comparisons with SALOMON (right panel of Fig. 6), the agreement is moderate.The first maximum is reached at about 20 km for SALOMON and at 18 km for GOMOS.This difference may not be significant because of the vertical resolution of SALOMON for this product (2 km).However, the maximum of OClO concentration measured by SALOMON is twice the one retrieved by GOMOS (1.2 × 10 8 and 6.1 × 10 7 cm −3 respectively).Moreover, above 28 km, the SALOMON concentrations increase strongly reaching a value of 1.3 × 10 8 cm −3 at 32 km whereas the OClO concentrations from GOMOS are always decreasing.In summary, the OClO product from GOMOS averaged measurements compares very well with that from AMON measurements and slightly less well with that of SA-LOMON.Overall, the conclusion is that our OClO retrieval is of sufficient quality for scientific use.Introduction

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Annual climatology
As specified previously, we use 10 • latitude bins for the annual climatologies.For each altitude, the distribution of OClO SCDs with latitude are fitted by one or two lorentzian function(s), taking into account the retrieval errors.
In Fig. 7, we present the isopleths of the latitude-altitude OClO slant column densities (log scales) for years 2003 to 2011.The altitude range is limited from 15 to 45 km.Outside this range, either the retrieval does not succeed or the uncertainties of the retrieval are too large.The 9 latitude-altitude maps of OClO SCDs presented in Fig. 7 show approximately the same structure.In the lower stratosphere, we can observe high values in the polar regions, reaching about 10 16 cm −2 in the southern hemisphere and generally slightly less in the north (about 10 15 cm −2 ).These high values are expected (Eq.R1) and are an indication of the chlorine activation that occurs in spring (in the northern hemisphere) and in fall (in the southern hemisphere).At higher altitude in the polar regions, OClO SCDs decrease abruptly for all years.This decrease can be explained by the scarcity of ClO at these altitudes.This is well observed by the Microwave Limb Sounder (MLS) instrument onboard the satellite platform AURA (see plots available at http://mls.jpl.nasa.gov).
In the equatorial region, an OClO layer is present all year between about 30 to 35 km.The latitudinal extent as well as the maxima of the layer is somewhat variable from one year to another.The fact remains that the maxima are about a few 10 14 cm −2 .The location of the maxima is approximately around the equator.Note that the year 2011 exhibits the stronger OClO equatorial layer (the maxima is nearly 10 15 cm −2 ).The OClO equatorial stratospheric layer was first discovered by Fussen et al. (2006) for the year 2003.A possible explanation for the presence of OClO at these altitudes can be the low pressure at these levels which make the 3 body reactions between ClO/BrO and NO 2 (Reactions R4 and R5) less effective.Thus, the Reaction (R1) between ClO and BrO become dominant and explain the presence of OClO.Moreover, the satellite instrument 3525 Figures

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Full MLS confirms the presence of a ClO layer in the upper stratosphere in the equatorial region.One may notice the more extended (in terms of altitude) OClO layers for the years 2008 and 2011.Finally, the OClO SCDs decrease progressively above this layer.
The other latitude regions are characterized by weak OClO values.Consequently, in the following, our attention will be focused on equatorial and polar regions.

Monthly climatology of OClO
In Fig. 8, we show the time series of OClO SCDs in three latitude bands: the equatorial band (10 • S-10 • N, middle panel), the north polar band (60 • N-90 • N, top panel) and the south polar band (60 • S-90 • S, bottom panel).In the equatorial region, the missing data in 2005 are due to a GOMOS failure.The discontinuity observed in the two polar time series are due to the spatial coverage of GOMOS at these altitudes (cf.Fig. 2).
Regarding the northern latitude band, we can clearly observe an annual increase of OClO followed by a decrease in the lower stratosphere (below about 25 km).The increase begins in December and reaches a maximum of about 5 × 10 15 cm −2 to 3 × 10 16 cm −2 .The corresponding OClO concentrations (obtained after the spatial inversion described in Sect.4) are 5 × 10 7 to 9 × 10 7 cm −3 .This is in good agreement with the values previously observed with other instruments (summarized in Table 1).
Regarding the altitude of the maximum of OClO concentrations, the values obtained using GOMOS measurements (about 17 km) are generally lower than those found by the balloon-borne instruments.The high OClO values are the sign of chlorine activation in the winter polar vortex.Note that the year 2011 shows strong OClO values in the lower stratosphere.Indeed, the ozone depletion in the Arctic winter 2010/2011 has been one of the most important of the last decade (Kuttippurath et al., 2012).This feature is also observed in the corresponding annual isopleth (Fig. 7). Figure 9 shows the anti-correlation plot between OClO and NO 2 concentrations (from the GOMOS operational algorithm version 5) in the Arctic region at 17 km.This altitude has been chosen as an example because it corresponds to the altitude of the maximum of OClO detected by GOMOS.This choice has no qualitative impact on the following discussion.3526 Figures

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Full The correlation coefficient obtained is about −0.75 which confirms the anti-correlation between NO 2 and OClO.Indeed, during the permanent polar night, NO 2 is remove from the stratosphere because of Reactions (R4) and (R5).Therefore, the reaction of formation of OClO (Reaction R1) is more active.
For the south polar region (bottom panel in Fig. 8), only a few months of each year can be analyzed (from July to September).Typically, the time series shows high OClO values from July to September indicating the sign of chlorine activation.The OClO SCD maximum reached in the Antarctic regions (from 1.5 × 10 16 to 5.7 × 10 16 cm −2 ) are larger than those observed in the Arctic regions.This is well illustrated in Fig. 10 showing these OClO SCD maxima for winters 2003 to 2011.The maximum concentrations range from 7 × 10 7 cm −3 to 2 × 10 8 cm −3 .Unfortunately, we can not validate our values because of the lack of occultation measurements in the south pole.
The equatorial region (middle panel in Fig. 8) is characterized by an OClO layer between about 30 to 40 km for each year.There are too few values available in 2005 to discuss this year.In this layer, the maximum is located at about 35 km for the years 2003, 2006, 2007 and 2009 to 2011.For the years 2004 and 2008, the maximum is located slightly lower, around 32 km.This layer appears to be present during the entire year with only small variations.This is in good agreement with the results obtained by the MLS instrument which shows a quasi-constant ClO layer at these altitudes with a volume mixing ratio of 0.3 to 0.4 ppbv, corresponding to concentrations of 2 × 10 7 to Introduction

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Conclusions
In this article, we have presented an innovative method to retrieve small absorber concentration.This method is applied to the GOMOS measurements to detect OClO.It consists of the combination of several co-located GOMOS measurements to build one measurement with a higher signal-to-noise ratio.Then a specified algorithm is used to retrieve OClO slant column densities.
We have compared our results with those obtained using balloon-borne instruments AMON and SALOMON.It appears that our OClO product is in a very good agreement with the AMON product and compares slightly less well with SALOMON.
We have constructed climatologies of OClO based on annual binning.It appears clearly that the same structures emerge every year.First, in the polar regions, we systematically observed a strong increase of OClO in the lower stratosphere.This is related to the activation of the chlorine species in the polar vortex.Moreover, an OClO layer in the equatorial region is clearly detected each year in the middle stratosphere.This layer is the result of the reaction between BrO and ClO effectively detected at these altitudes by the MLS instrument.Time series of OClO SCDs have been built using averaged measurements representative of one month for three latitude bands corresponding to Arctic, Antarctic and equatorial regions.The period of these climatologies is from 2003 to 2012, covering the entire GOMOS mission.It appears that activation of chlorine species is well observed in the winter pole.Furthermore, the OClO layer in the equatorial middle stratosphere is detected during all the years studied and presents no particular variations.The anticorrelation between OClO and NO 2 concentrations is well observed.The next step of these studies will be the use of models to interpret quantitatively the results presented in this article.The method presented here is really promising and could be applied to other instrumental data.In addition, it can be used to detect other weak absorbers such as BrO.The climatology presented in this paper can be Introduction

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)
During the night, NO 2 reacts with nitrogen trioxide (NO 3 ) to form dinitrogen pentoxide (N 2 O 5 ).The nighttime formation of N 2 O 5 is responsible for the slow decrease of NO 2 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | has demonstrated the ability to retrieve OClO and has discovered the presence of an equatorial OClO layer in the upper stratosphere and the second by T étard et al. (2009) has illustrated the anticorrelation between OClO and NO 2 in the Arctic polar vortex only for winters 2003 to 2008.Nevertheless, no extended climatology of stratospheric OClO from GOMOS has yet been published.
in February 2002 on a heliosynchronous orbit at 800 km altitude Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | periods, the other latitudes are very well sampled during all years (except in 2005).
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 6
shows the OClO concentration profiles comparisons.The curves on the left concern the comparisons with AMON and show a satisfactory agreement.The maxima are reached at approximatively the same altitudes for the three profiles (19 km).The value of the maximum of OClO concentrations are almost identical for the GOMOS and 3523 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | considered as the first and only long-term nighttime OClO climatology that exists at presentDiscussion Paper | Discussion Paper | Discussion Paper | Solomon, S.: Stratospheric ozone depletion: A review of concepts and history

Table 1 .
Burrows, J. P., and Platt, U.: Continuous monitoring of the high and persistent chlorine activation during the Arctic winter 1999/2000 by Maximum of OClO concentrations previously detected by balloon-borne instruments during nighttime in the Arctic polar vortex.The altitude of the maximum is also indicated.