Ozone monitoring with the GOMOS-ENVISAT experiment version 5

P. Keckhut, A. Hauchecorne, L. Blanot, K. Hocke, S. Godin-Beekmann, J.-L. Bertaux, G. Barrot, E. Kyrölä, A. van Gijsel, and A. Pazmino LATMOS-IPSL, CNRS/INSU, UMR 8190, UVSQ, UPMC, Verrières le Buisson, France ACRI-ST, Sophia Antipolis, France Institute of Applied Physics, University of Bern, Bern, Switzerland Oeschger Centre for Climate Chnage Research, University of Bern, Bern, Switzerland FMI, Helsinki, Finland RIVM, Bilthoven, The Netherlands


Introduction
Since the discovery of the Antarctic ozone hole by Farman et al. (1985) using a groundbased UV spectrometer, instruments onboard satellites provide observations on a global scale.The early ozone measurements from space with the Total Ozone Mapping Spectrometer (TOMS) confirmed the occurrence of seasonal polar ozone decreases, and provide in addition, the horizontal extension of the ozone hole.While the total column is well adapted to monitor the potential increase of the solar UV irradiance, the distribution of the modifications as a function of altitude is crucial to better understand the processes involved and the climate impact of the ozone changes (WMO, 2007).The two longest records of ozone profiles from space are available from two different types of instrument: the Solar Backscatter Ultraviolet (SBUV(/2)) satellite instruments and the Stratospheric Aerosol and Gas Experiment (SAGE I+II).Figures

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Full The SBUV/2 instrument is a scanning double monochromator measuring backscattered solar radiation in 12 discrete wavelength bands ranging from 252.0 to 339.8 nm.The data are available as partial column ozone layers of around 3 km thick.Retrievals are very sensitive to the a priori profile (Bhartia et al., 2004) and successive spectrometers cross-adjustments require comparisons with other data sources (Petropavlovskikh et al., 2005;Nazaryan and McCormick, 2005;Fioletov et al., 2006;Terao and Logan, 2006) to provide ozone series on a consistent scale using the full sequence of NOAA satellites (Stolarski and Frith, 2006).
The SAGE measurement technique (McCormick et al., 1989) is based on solar occultation, with ozone profile measurements obtained at sunrise and sunset on each of 14 orbits per day.This technique provides relatively high vertical resolution (∼1 km) and very small long-term drifts resulting from instrument calibration.However, spatial sampling is limited, and it takes approximately one month to sample the latitude range 60 • N to 60 • S.
Biases between the SAGE II and some SBUV instruments are reported (Nazaryan and McCormick, 2005;Fioletov et al., 2006;Terao and Logan, 2006).These biases, of several percent, are the largest in the upper stratosphere and will contribute to differences in trends derived from SAGE II and SBUV data.However, both series now better converge about long-term trend estimates and show significant declines of around 10-15% (over 17 years) through 1995 when averaged over 60  advantages of any occultation method is the self calibration process (Bertaux et al., 2004;Kyr öl ä et al., 2004) and the use of stars allows to increase the suitable number of occultations with time of measurement that is reduced.For all these reasons, GOMOS is a good candidate for long-term ozone 4-D distribution monitoring from space.After a few years of operation, the long-term evolution of the GOMOS version 5 of the data is investigated.Comparisons with ground-based ozone series at mid-latitude have been performed showing that potential bias can exist during the last period when the detector capabilities have decreased.

Method
The occultation technique was developed several decades ago (Hays and Roble, 1968).It is based on a reference stellar spectrum firstly measured when the star can be seen above the atmosphere.When the line of sight crosses the atmosphere, the light is observed through the atmosphere, and the spectrum is modified by absorption, scattering and refraction.Spectra from 248 to 952 nm are spread over several charge coupled devices (CCD) to cover the full spectrum.The UV domain can be used to retrieve ozone.The detailed layout of the GOMOS instrument is already described (Bertaux et al., 2001(Bertaux et al., , 2010)).The main derived quantity is the transmission along the line of sight that is calculated by dividing the star spectra measured outside and through the atmosphere.The main benefit of this method is based on its independence of the instrumental characteristics and the ratio provides a measurement self-calibrated and free of calibration coefficients.The challenge of the pointing system consists in keeping the star image in the center of the slit with a good stability.One of the main characteristics of the star occultation technique concerns the conditions of the measurement that can differ from one occultation to the next.The overall quality of the measurements depends on the signal level and also the contrast of the signal compared with Introduction

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Full the background light.The quality of the measurements is thus depending on different conditions defined by the amount of scattering solar light.Dark conditions provide the lowest noise level and the most accurate measurements.To minimize noise the CCD temperature has been reduced.For ozone, the UV and visible domain, that provide the largest absorption bands, are the most appropriate part of the spectrum.To gain a good spatial coverage a wide variety of stars are used.However, for signal to noise ratio issues, only the 70 brightest stars were selected, as described in Hauchecorne et al. (2005).Some of them provide a strong UV spectrum favorable for ozone absorption retrieval and some of them exhibit a smaller signal.During the ESA-ENVISAT commissioning phase, a large program of coincident ground-based measurements using lidar, ozonosonde, and microwave profilers has been put in place and which is still being continued.All these data permitted to validate the GOMOS v4.02 ozone profiles and evaluate bias for the different conditions.The star temperature and magnitude determine the signal strength of the observed UV spectrum.The hot and bright stars provide the most favourable conditions for ozone retrieval.Because ozone is quite variable, the collocation is always an issue.Meijer et al. (2004) conducted an intensive program of validation using a very large number of coincident measurements mainly from NDACC (Network for the Detection of Atmospheric Composition Changes) and found the best compromise between spatio-temporal differences and the number of profiles to provide an adequate confidence of 800 km and 20 h.When data are partitioned according to the limb illumination conditions, and compared with ground-based measurements, Meijer et al. (2004), found for bright limb conditions, large negative bias of 18-33%.
The bias remains when hot stars are used even if it is reduced.Results improve for twilight and even more for dark conditions with bias smaller than 10% and down to 2.5% for some altitude levels.Star magnitude and temperature (related to spectrum shape) seems to be less critical conditions on the data quality of the retrieval.This study highlighted the large influence of the limb background level on the retrieval.Introduction

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Noise estimate and evolution
One important step in the GOMOS data processing is the removal of the background light, coming from the solar light scattered by the atmosphere, and the internal noise of the CCD (dark charge) to extract the star signal.In dark limb conditions, the background light is assumed to be negligible.In the GOmos PRototype code prototype (GOPR) version 6.0, used in the present operational GOMOS-ESA version 5 processing, the CCD dark charge (DC) is estimated using a measurement performed by pointing the GOMOS telescope in a direction without any star in the field of view, called "dark sky area" (DSA), in the sky.On each orbit a dark sky area measurement is planned around the equator and is used to correct all dark limb occultations performed during the same orbit.The noise of a CCD is increasing exponentially with temperature and doubles every 6-8 • C. In order to take into account this temperature effect, the temperature of the CCD is measured.On GOMOS this measurement is made with a digitization step of 0.4 • C, corresponding to a change of about 4% in dark charge.This digitization effect limits the accuracy of the correction to be applied, especially for high CCD noise.
During the GOMOS life the average dark charge of the CCD presented a quasilinear increase starting from very low values in 2002 as shown in Fig. 1.The main reason for this increase is the appearance of "hot pixels" on the CCD with a definitively increased noise after particle precipitations occurring mainly in the South Atlantic Anomaly.Another reason is the increase of the CCD temperature from −6 to 0 • C between 2002 and 2009.The evolution of the CCD dark charge and CCD temperature can be followed in the GOMOS Monthly Reports available on: http: //earth.esa.int/pcs/envisat/gomos/reports/monthly/.
Consequently the inaccuracy of the dark charge correction is also increasing proportionally to the dark charge increase and has an increasing effect on the evaluation of the star spectrum particularly in the UV for faint stars.In the example shown in Fig. 2, the estimated background light after dark charge correction in the upper band of the Introduction

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Full CCD (top left of the figure) deviates from 0 in the UV (CCD columns 1-500).A value near 0 is expected in dark limb.Similarly the star spectrum (Fig. 2 bottom left) deviates also from 0 in the UV while a value near 0 is expected at the end of the occultation corresponding to a very low tangent altitude.This bad correction of the UV part of the star spectrum may have some effects on the star spectrum estimate and then in the ozone retrieval itself because it is interpreted by the spectral inversion routine as a lack of ozone.
In GOPR version 7.0ab an improved analysis in preparation for the next ESA 6 version is used to estimate the dark charge.In addition to the preceding noise analysis using DSA, another step was added to improve the noise residual using the two side CCDs.The UV flux is estimated to be 0 at lower altitude in upper and lower bands of the CCD and the CCD temperature is estimated assuming that the measured signal is equal to the dark charge.After this adjustment, the star spectrum is close to 0 in the UV (Fig. 2 bottom right part of the figure).The impact on the spectrum in the UV domain is very clear in this new version.

Impact of the noise on ozone retrieval
When data are processed with the DSA noise extraction method in GOMOS-ESA v5 data, the level 1b ozone line density profiles show sometimes a strong negative bias with the largest amplitude above 50 km (Fig. 3 left).These bad retrievals occur mainly when noise is large (twilight and bright conditions) and the signal is low.When the level of the star signal is comparable to the noise, an error in the background estimate has a strong impact on the retrieval.The estimated transmission can then be too large or too small.However, negative transmissions are not allowed for physical reasons and so for this reason, the distribution of bias is not symmetric.We suspect that it is due to the incorrect dark charge correction at shortest wavelengths as explained in the previous section.In these cases when the 240-280 nm section of the spectrum is discarded in the retrieval (corresponding pixels are flagged), the retrieved vertical profiles appear to be less biased (Fig. 3  While the reduced spectral range eliminated most of the biased profiles, it increases the noise in the upper part of the ozone profile where ozone variability can be monitored too much (Fig. 3b).The method of improving the ozone retrieval by reducing the spectral window in the UV domain systematically is a strong limitation because not all star occultations were exhibiting biased ozone profiles.As confirmed in the next section, the bias is not distributed uniformly but rather large biases are reported for some conditions.Those bad retrievals can be visually identified as bad retrievals and removed manually from data series for any inter-annual or climatologic studies.Kyr öl ä et al. ( 2006) have identified a black list of some stars (18 out of 70) that appear to be suspicious because they provide some bad ozone retrieval while other seem to be more reliable.A new alternative GOPR V7.0ab processing including both the DSA background noise estimates and the new baseline noise correction, described in Sect.2.2, has been proposed for inclusion in the systematic new ozone GOMOS processing.As shown in Sect.2.2 the impact on the Dark Charge estimate in the UV range is quite important and improves the star spectrum.The retrieval with such a new correction show that biased ozone profiles are eliminated for occultations done under "bad conditions" and that the quality of occultations with high signal that were unbiased with the GOPR V6.0cf algorithm are processed exhibiting similar results (Fig. 4).In the framework of the Network of Detection of Atmospheric Composition Changes (Kurylo and Salomon, 1990), some instruments providing ozone profiles are and have been running systematically over decades.Many satellite experiment validations and long-term investigations were performed giving large confidences and those instruments have participated in regular cross-validation exercises (Keckhut et al., 2004).

Data comparisons
While mid-latitude regions exhibit the best compromise for comparisons in providing the best and largest amount of collocations for dark conditions, the complementary lidar and microwave instruments from the Alpine stations have been used for comparisons with GOMOS profiles on a long-term basis.

Comparison with Bern microwave
The ground-based microwave radiometer GROMOS is located at Bern ( 46• 57 N, 7 • 26 E) in Switzerland and observes the middle atmosphere at an elevation angle of 40 • in north-east direction (Peter, 1997).GROMOS measures the vertical ozone distribution from 20 to 70 km altitude under almost all weather conditions during day and night since 1994.The instrument is a triple switched 142.175 GHz total power radiometer and contributes primary data to NDACC.The continuous time series of ozone profiles from GROMOS are regularly used for satellite validations and ozone trend studies (Steinbrecht et al., 2009;Dumitru et al., 2006).The intercomparison of ozone profiles from ENVISAT/GOMOS and GROMOS is similarly performed as the intercomparison of ozone profiles from ENVISAT/GOMOS and the ground-based microwave radiometer SOMORA at Payerne which was described by Hocke et al. (2007).The high-resolution ozone profiles of ENVISAT/GOMOS are adjusted by averaging kernel smoothing to the vertical resolution (about 10 km) of GRO-MOS.The coincident profile pairs of ENVISAT/GOMOS and GROMOS have a time difference ∆t<1 h and a horizontal distance of the sounding volumes d <800 km.The relative difference profile of ozone concentration from ENVISAT/GOMOS and GRO-MOS is given by: Introduction

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Full Figure 5 shows the time series of ∆O 3 at altitudes h=) 26,31,36,41,46, and 51 km for the time interval 2002 to 2006.The number n of coincident profile pairs (blue dots) is between 778 and 784 as denoted in the title of the viewgraphs of Fig. 5.The mean biases are ranging between −0.1% and 10.9%.The best agreement is achieved at altitudes from 36 to 41 km.The running mean shows a gradual increase of EN-VISAT/GOMOS differences with too small amounts of ozone due to some individual differences of 100% that occur more frequently more obvious in the stratopause region (lower panels of Fig. 5).In 2006, positive biases (20%) are reported at 26 km, a smaller bias over the 31-41 km range, and a larger negative bias over the 46-51 km range (20%).The comparisons reveal that the biases are not systematic but are partitioned in two classes of profiles with one fully unreliable.This conducts to a bimodal distribution of the differences with increasing occurrence of biased profiles.The class of GOMOS profiles presenting a large negative difference with GROMOS microwave data is clearly related with the class of profiles with a strong negative bias shown in Fig. 3.

Comparisons with the OHP lidar
The ozone lidar measurements are performed using the DIAL method, which requires the emission of two laser wavelengths with different ozone absorption cross-sections.Range resolved measurements are obtained with the use of pulsed lasers.The spectral range of the laser beams is chosen in the ultraviolet where the ozone absorption is most efficient.In the case of the OHP lidar, the absorbed radiation is emitted by a XeCl excimer laser at 308 nm and the reference line is provided by the third harmonic of a Nd:Yag laser at 355 nm (Godin et al., 1989).signals is handled by separating each return signal into a low and a high-energy channel for the measurement of ozone in the low and middle-high stratosphere respectively.The ozone number density is computed from the differentiation of the signals.Two additional wavelengths corresponding to the first Stokes vibrational Raman scattering by atmospheric nitrogen of the laser beams are detected in order to retrieve ozone in case of high aerosol loading, (McGee et al., 1993).The lidar measurements are performed during the night and typically last several hours, which results in a spatial resolution of about 100 km, depending on atmospheric conditions.The vertical resolution has to be reduced as a function of altitude due to the decreasing signal-to-noise ratio.It ranges from several hundreds of meters in the lower range to several kilometers above 40 km.
The total accuracy ranges from about a few percent below 20 km to more than 10% above 45 km (Godin-Beekmann et al., 2003).The lidar in the present configuration has been operated routinely at OHP since 1994.First stratospheric ozone lidar measurements were performed with a simpler system at OHP in campaign mode in 1985 and 1986.Routine measurements were obtained with the simpler system from 1986 to 1993.The OHP ozone data have been compared to various satellite measurements such as SAGE II, MLS-UARS, GOME, GOMOS, MIPAS, SCIAMACHY (Meijer et al., 2004;Brinksma et al., 2006;Meijer et al., 2006;Cortesi et al., 2007;Iapaolo et al., 2007, Jiang et al., 2007;Rozanov et al., 2007;van Gijsel, 2009).
Comparisons between lidar and GOMOS ozone profiles have been performed with the similar methodology described previously and exhibit a variability of nearly ±20%.If these biases profiles exist for a wide range of latitude, long-term trend studies with the GOMOS data presently available will be less reliable.These differences are due to some bad retrieval cases corresponding to some star occultations exhibiting weak signals or large noise levels.These biases may be due to an imperfect dark charge correction of the CCD coupled with the regular dark charge increase.The distribution of ozone profiles exhibits a clear bimodal structure that allows a straightforward detection of bad retrievals.Resulting biases in the ozone series do not allow long-term ozone change estimated with the GOMOS-ESA V5 retrieval dataset if disregarding the data quality.One possibility to deal with this consists in removing bad retrievals or some occultations of some known dangerous stars.However, this will reduce the number of measurements.
A new dark charge correction algorithm has been implanted in GOPR V7.0ab.The preliminary comparisons with a subset of data show that the retrieval greatly improves the quality of GOMOS ozone profiles.The full comparison of this data version with a larger set of NDACC validation instruments will be done after the ESA V6 reprocessing with this new algorithm, which should take place in 2010.This work also pointed out the importance of long-term validation of satellite data with ground-based network as provide by NDACC.Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N-60 • S and altitudes of 35 to 50 km(WMO, 2007)  and a decline of up to 10% between 20 and 25 km altitude.These decreases did not continue with the same amplitude in the last decade.Solar occultations provide two different times of measurements at sunset and sunrise that frequently introduce instrumental differences and also atmospheric bias due to ozone photochemical diurnal cycle or indirectly temperature tides.One alternative method, able to provide both good vertical resolutions and sufficient horizontal sampling can be provided by stellar occultation technique.In March 2002 ESA has launched an environmental satellite called ENVISAT carrying the GOMOS spectrometer (Global Ozone Monitoring by Occultation of Stars).One of the main Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | right) while the overall noise of the measurement increases 14719 Discussion Paper | Discussion Paper | Discussion Paper | mainly above 70 km because the accuracy of the O 3 retrieval is mainly driven by the low UV part.
Version 5 of the ESA-GOMOS ozone data have been compared with two long data sets that have been compared with numerous other data sets.Two different techniques are used: lidar and microwaves.The GOMOS data selection was performed in a space box of ±10 • in latitude and ±20 • longitude.The brightest stars have been selected which correspond to the stars with an ID from 1 to 70. Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The reception system includes 4 telescopes of 50 cm diameter with optical fibres mounted in the focal plane in order to collect the laser light and a spectrometer to separate the various detected wavelengths.Photon-counting is used for the acquisition of the lidar signals.The large dynamic of the Discussion Paper | Discussion Paper | Discussion Paper | A clear negative bias in 2006 of 10-20% is observed in most profiles 40 km and less evident at 20 km.The bias profiles correspond mainly to the noisier spectra obtained during twilight and bright conditions. in GOMOS ozone ESA-V5 (corresponding to GOPR6.0cf) data when compared to the ground-based validated NDACC lidar at OHP and the microwave profiles operated at the Alpine station in the northern mid-latitude.While the first years of operation seem to reveal a good agreement between the ground-based and GO-MOS measurements, some large biases (10-20%) are observed in 2006 and after.

Fig. 1 .
Fig. 1.Dark charge time evolution given by the central band of the CCD (middle) used to acquire star signal and the two other bands of the same CDD in the upper and lower side dedicated to limb background estimate.Here signal corresponds to calibration mode when instrument is not pointing at any star called "dark sky area".