Homogenization of the Observatoire de Haute Provence ECC ozonesonde data record: comparison with lidar and satellite observations

. The Observatoire de Haute Provence (OHP) weekly Electrochemical Concentration Cell (ECC) ozonesonde data have been homogenized for the time period 1991-2021 according to the recommendations of the Ozonesonde Data Quality Assessment (O3S-DQA) panel. The assessment of the ECC homogenization beneﬁt has been carried out using comparisons with other ozone measuring ground based instruments also measuring ozone at the same station (lidar, surface measurements) and with collocated satellite observations of the O 3 vertical proﬁle by Microwave Limb Sounder (MLS). The major differences 5 between uncorrected and homogenized ECC data are related to a change of ozonesonde type in 1997, removal of the pressure dependency of the ECC background current and correction of internal pump temperature. The original 3-4 ppbv positive bias between ECC and lidar in the troposphere is corrected with the homogenization. The ECC 30-years trends of the seasonally adjusted ozone concentrations are also signiﬁcantly improved in both the troposphere and the stratosphere after the ECC homogenization, as shown by the ECC/lidar or ECC/surface ozone trend comparisons. A -0.29%-0.19% per year negative 10 trend of the normalization factor (N T ) calculated using independent measurements of the total ozone column (TOC) at OHP


Introduction
Stratospheric ozone recovery is expected due to a decreasedecreasing atmospheric amounts of ozone depleting substances.
Trends of ozone in the upper troposphere, lower and mid stratosphere however show latitudinal and seasonal variabilities which depend on (i) dynamical variability of the atmosphere, (ii) the temperature dependence of stratospheric ozone photochemistry, (iii) the increase of tropospheric ozone precursors in the upper troposphere (Szelag et al., 2020;Cohen et al., 2018;Thompson 25 et al., 2021). A large number of validation and intercomparison studies of free tropospheric and lower stratospheric ozone use balloon borne Electrochemical Concentration Cells (ECC) as reference (Tarasick et al., 2021). At Observatoire de Haute Provence (OHP), stratospheric and free tropospheric ozone monitoring is carried out since the mid-1980s with ozonesonde and lidar observations. The OHP station, located at 44°N,6°E, is one of the few long-term measuring stations for vertical ozone profiles in Southern Europe. This station allows the characterization of (i) the impact of ozone sources observed in one of the hot 30 spots of highthe tropospheric ozone column amounts observed by satellite (Richards et al., 2013) and (ii) the effects of climate variability on mid-latitudes total column ozone (Zhang et al., 2015;Petkov et al., 2014). Improvement and homogenization of the OHP ozone ECC observations have been achieved from 1991 to 2021 using the recent ozonesonde data quality assessment (O3S-DQA) panel recommendations (Smit et al., 2012;Smit and Thompson, 2021). An extensive use of lidar measurements both at tropospheric and stratospheric altitudes together with co-located satellite observations obtained during the OHP ECC 35 soundings has allowed the quantification of the ozone measurement improvement achieved with this homogenization of the ECC ozonesondes. Sections 2 and 3 summarize the corrections made to the ozonesonde measurements and the methodology for assessing its benefit. Section 4 presents and discusses the results of the different instrumental comparisons and the changes obtained in terms of interannual ozone variation at different altitudes between 0.7 and 30 km ASL.
2 Description of the ozonesonde homogenization 40 A total number of 1412 ECC ozonesondes have been launched at OHP since 1991 when Brewer-Mast regular soundings have been replaced by ECC sondes following the preparation instructions of Komhyr (1986) just after a lidar/ozonesonde intercomparison campaign held at OHP in 1989 (Beekmann et al., 1994). Ozonesondes are launched once a week generally near 9 UT but 40 soundings have been made during the night either for lidar/ozonesonde comparison or for detection of long range transport of polar ozone streamers forecasted by chemical transport models. The ozone partial pressure P O3 measured in 45 mPa by the ECC can be obtained from the electrochemical current I measured in µA, the background current I b measured in the preparation laboratory with an ozone removal filter after the sonde was exposed to ozone, the internal temperature of the air sample T i in K, the capture efficiency of the O 3 in the liquid phase α, the stoichiometry S of the O 3 to I 2 conversion and the ECC pump flow rate, φ p in cm 3 s −1 (Smit and Thompson, 2021). (1) 50 A major change in the sounding procedure occurred in 1997 when the Science Pump Corporation (SPC) ozonesonde was replaced by an EnSci ozonesonde while using a Sensing Solution Type (SST) of 1% (1% KI concentration and a full buffer concentration; (Smit and Thompson, 2021)). Using the instructions given by the O3S-DQA, the following corrections have been implemented before a new calculation of P O3 after homogenization usingin equation 1: -Change of α and its pressure dependency before 1996 when 2.5 cm 3 of KI solution was used in the cell instead of the 55 recommended 3 cm 3 .
-Scaling of P O3 measured by the EnSci-SST 1% ozonesondes after March 1997 to P O3 from SPC-SST 1% ozonesonde observations made before March 1997 assuming that SPC-SST 1% is a better reference than EnSci-SST 1% (Deshler et al., 2017). This correction is larger than -10% at altitudes above 30 km and on the order of -4% in the troposphere.
-When I b > 0.1 µA (less than 6% of the data set), I b is replaced by 0.05 µA, the average of the measured background 60 current for our dataset, while the uncertainty of I b becomes 0.1 µA.
-The pressure dependency of the background current has been removed for the homogenized version since the O 2 concentration does not play a significant role in the residual current when ozone is removed (Thornton and Niazy, 1983;Vömel and Diaz, 2010).
-No vertical smoothing of the ozone partial pressure. Smoothing over 100 m was applied in the uncorrected data, i.e.
-Correction of measured T i to account for changes in the position of the thermistor and for differences with the true air sample temperature (the thermistor was taped to the pump before July 2007 and inserted in the pump hole since that time) -Correction of the pump flow rate to account for the humidification effect when using the bubble flowmeter to determine 70 the flowrate in the laboratory as part of the pre-flight preparation of the sonde.
-Two different correction tables of the pump flow rate efficiency correction tables at pressures below 100 hPa are now applied for EnSci (Komhyr95) and SPC (Komhyr86) ozonesonde. Only Komhyr86 was applied for the current to P O3 conversion of all the uncorrected data.
As the background current uncertainty is a significant contribution to the P O3 uncertainty in the upper troposphere (Van Malderen 75 et al., 2016), the comparison of I b used before and after homogenization is shown in Fig.1 (Smit et al., 2012) are used.
In July 2007, the radiosonde type switched from Vaisala RS80 to MODEM M10. The MODEM M10 measures the true GPS altitude with the pressure altitude retrieved from this measurement. No correction is applied to the RS80 pressure measurements and an offset of 0.5-1 hPa may exist in the stratospheric pressures above 20 km before 2007 (Tarasick et al., 2021;Stauffer et al., 2014).

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The homogenized minus uncorrected ECC partial pressures normalized to the homogenized ECC ozone partial pressure are shown in Fig.2. Significant overall negative differences (≤ -5%) are obtained (i) in the upper troposphere (8-12 km) because of the removal of I b pressure dependency and (ii) above 28 km after 1997 when taking into account the change to EnSci. Positive differences reaching 5% in the stratosphere are also observed for the SPC period before 1997 because of the positive corrections for the pump flow rate (+2%) and the ECC pump temperature (+3%) without any negative corrections in the stratosphere.

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The UV/visible SAOZ (Système d'Analyse par Observation Zénithale) and the UV Dobson spectrophotometer total ozone column (TOC) measurements are available at OHP. Dobson was used from 1991 to 2004 and SAOZ from 2004 up to now. The so-called normalization factor (N T ) is calculated as the ratio of the spectrophotometer TOC and the ECC TOC. Following the methodology used in Smit and Thompson (2021) or Stauffer et al. (2020), the TOC corresponding to the ECC soundings is calculated using the integration of the ozone concentrations up to 10 hPa or the burst altitude, provided it is higher than 25 km.

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The residual ozone above 10 hPa or the burst altitude has been calculated using the monthly mean climatology of McPeters and Labow (2012) at pressures smaller than 10 hPa. The homogenized data are not normalized with this normalization factor which is only used as a quality flag. based on the OHP stratospheric lidar up to 40 km and satellite observations above (Robbins et al. 89). The residual ozone is also multiplied by the ratio between the climatological O 3 concentration at burst level and the measured ECC O 3 concentrations in the 100-m layer below the burst level so that only the relative dependence of the monthly 105 climatology with altitude is taken into account in this calculation.
The homogenization procedure also includes a retrieval of the uncertainty on the ozone partial pressure at each vertical level.
The detailed description of the uncertainty calculation is given in Smit and Thompson (2021). All the error terms have been included in our calculation except the bias due the sensor time response and the pressure uncertainty.The median value of the relative erroruncertainty on the ozone concentration measured by the ECC is on the order of 6-7% in the stratosphere and

Data and homogenization assessment
In this work, the benefit from homogenization of the ECC ozonesonde timeseries is assessed by comparison of homogenized and non homogenized ECC ozone concentrations with other ozone measurements carried out at OHP. First, these comparisons 115 are made as a function of altitude using either Ultraviolet DIfferential Absorption Lidar (UV-DIAL) or Microwave Limb Sounder (MLS) satellite observations in the stratosphere (Froidevaux et al., 2008). Two UV-DIAL have been operated at OHP.
The first one, LiO3St, is optimized for stratospheric O 3 profiling between 10 and 50 km ASL using an absorbed wavelength at 308 nm and a reference wavelength at 355 nm (Godin-Beekmann et al., 2002). The second one, LiO3Tr, is optimized for tropospheric ozone monitoring between 2.5 and 14 km ASL using the 289 nm and 316 nm wavelength pair (Ancellet and 120 Beekmann, 1997). Regular nighttime measurements (2-4 per week) have been made with LiO3St since 1985 and with LiO3Tr since 1990. The LiO3Tr is most accurate in the 6 km to 10 km altitude range with the smallest lidar systematic erroruncertainty (< 8%) due to the mismatch of the overlap function mismatch between the two wavelengths at ranges below 4 km and due to the background signal correction of the photomultipliers (PMT) nonlinear response above 10 km (Ancellet and Ravetta, 2003).
The LiO3St best accuracy (≤5%) is generally in the 15-40 km altitude range when ozone concentrations are large enough 125 to minimize lidar systematic errors and when signal-to-noise (SNR) ratio is maximum (Nair et al., 2011). hPa with a vertical resolution on the order of 2 km at these levels. The overpass criteria is ±5deg latitude, ±8deg longitude, and all MLS profiles meeting this distance criteria within one day of the sonde are averaged to make the comparison with the ozonesonde. Although the spatio-temporal differences between ECC soundings and satellite overpasses will be greater 140 using these criteria, we obtain many more comparisons than by restricting ourselves to nighttime soundings. The ECC/satellite comparison will then be complementary of the ECC/lidar difference analysis. For the sake of a more complete discussion of the two types of comparisons made in the stratosphere, we also considered a lidar data set of 366 profiles from 2005 to 2021 with less restrictive measurement time difference with the ECC launches (< 12h). Such a criterion is valid as long as the rapid O3 variations typically encountered below 18 km are not present. Thirdly, the benefit of homogenization on long term ozone trends for several altitude ranges in the troposphere and the stratosphere has been studied using all the lidar and ECC measurements made at OHP. The lidar monitoring period is indeed 155 as long as the ozonesonde data set, and includes the major ozonesonde preparation or ozonesonde type changes in 1997, 2004 and 2007. Only simple linear trends of the ozone concentrations corrected for the mean seasonal variation at OHP will be considered in this study for the assessment of the homogenization. The trend uncertainties are calculated using the 95% confidence limit of the slope of the linear regression assuming that the residuals are not correlated for weekly (ECC) or 2/3 per week (lidar) observations.. A more comprehensive trend analysis for the OHP would need either multiple linear regression 160 model as described in Nair et al. (2013) or Thompson et al. (2021) for the stratosphere or statistical regularization method as described in Chang et al. (2020)  4 Results and discussion

Normalization Factor Trend
The time evolution of the normalization factor N T is plotted in Fig. 4 for the uncorrected and homogenized OHP ECC sondes.

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The major changes in the ozonesonde supplier or the ozonesonde preparation procedure shown in Fig. 2 are also reported in Fig.   4. The uncorrected N T time evolution shows that the dispersion of points is larger before the switch to MODEM ozonesonde in 2007, but more striking is the significant negative trend of -0.29±0.04%-0.19±0.03% per year which is as large asnot negligible compared to the reported O 3 trends in the troposphere (Gaudel et al., 2018). The homogenized N T does not exhibit a significant trend (0.00±0.04%/year)(0.02±0.03%/year), indicating the strong benefit of the homogenization. However the 180 average normalization factor for the whole data record is not equal to 1 but shows a likely -5%increases from 1.019 to 1.037, corresponding to a -3.7% bias of the ECC TOC compared to the OHP spectrophotometer. This may be partly due to the calculation of residual ozone above the burst altitude and partly to a possible bias in the stratosphere. The calculation of the residual ozone which representsaccounts for 7-10% of the ECC TOC leads to an uncertainty of about 1% of the TOC according to Witte et al. (2018). depends strongly on the last ozone concentrations measured before the balloon burst. An underestimation 185 of these concentrations on the order of 10% (e.g. due to freezing or evaporation of the ozonesonde solution) would lead to an 9 underestimation of the order of 2% of the TOC. A negative bias of about 3% in the stratosphere is still necessary to explain an average normalization factor of 1.051.037.

Nighttime Ozonesonde/Lidar comparison
The ozone concentration vertical profiles of ECC ozonesondes launched within 2 hours of the LiO3Tr observations have 190 been divided into six 1.5-km vertical layers between 3 and 12 km. The relative differences between the ECC and lidar O 3 concentration are calculated for each 1.5 km vertical bin. The mean of the relative difference and its uncertainty are then calculated for the 40 profiles, the time distribution of which is shown in Fig.3. The uncertainty of the mean difference in a 1.5-km vertical interval for a single O 3 profile is based on mean absolute errorsuncertainties (systematic and statistical) of both lidar and ECC measurements (see section 2 and 3) at each recorded altitude in the corresponding 1.5-km vertical interval.

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The statistical standard uncertainty of the overall mean difference is then retrieved assuming an independent error forthat the 40 comparisons are independent with uncorrelated uncertaintiestaken into account. The mean relative differences between the homogenized ECC and LiO3tr show an insignificant bias on the order of 1% for the altitude range 4.5 to 9 km considering the errorstatistical standard uncertainty on this difference which is on the order of ±2% (Fig. 5a). The mean relative differences between the uncorrected ECC concentration and LiO3tr however show a significant bias on the order of +4% in the same 200 altitude range. It is due to the errormay be explained by differences introduced by not correcting the O 3 partial pressure for EnSci-SST 1% and by using a pressure dependent background current subtraction. The comparison between the altitude dependence of the uncertaintyerror of the lidar measurement and that of the ECC measurement in the troposphere (Fig. 5b) shows that the ECC erroruncertainty remains in the range 7%-9%, while the lidar is less accurate (uncertainty > 9%) below 4.5 km and above 11 km. Below 4 km the significant bias of -4% between the homogenized ECC and the LiO3tr can be then  Fig.3. The means of the relative difference between ECC and LiO3St are then calculated for 8 vertical layers between 14 and 30 km using the geometric altitude for the ECC sondes, as the geopotential altitudes become significantly larger than lidar geometrical altitudes above 25 km. As for the previous comparison with LiO3tr, the uncertainty 215 of the mean difference between the two instruments is retrieved assuming an independent error for the 40 or 366 comparisons taken into account. For the shorter time difference, the mean relative differences between the homogenized ECC and LiO3St still show a significant bias on the order of -3% to -5% between the ECC and LiO3St above 17 km at altitudes between 18 km and 28 km with an error on the mean difference which is on the order of ±1.5% (Fig.6a). Near 15 km, this difference decreses to less than 1%. In contrast to the LiO3Tr comparison, the mean difference between the homogenized and uncorrected ECC measurements is small (≈2%), except above 28 km where the homogenized ECC concentrations are even lower than the lidar concentrations by -8%-10% (Fig.6a). For the period 2005-2021 and using a time difference less than 12 hours, the negative bias between the homogenized ECC and the lidar decreases down to -2% between 22 and 24 km, but remains as large as -7% above 28 km (Fig.6b). Note also that the mean uncorrected ECC and lidar difference is now slightly positive (+1%) for the 2005-2021 period in good agreement with the N T negative trend shown in Fig.4. Below 18 km, the -4% negative bias 225 between homogenized ECC and lidar (Fig.6b) should be interpreted by possible significant O 3 concentration changes within 12 hours in this altitude range. The time evolution of the relative difference of ECC and LiO3st ozone concentrations is shown in Fig.6c and Fig.6d for uncorrected and homogenized ECC, respectively. Many of the differences between uncorrected ECC and LiO3St are greater than +6% between 2007 and 2016, while there are some negative differences approaching -6% in 2006.
Homogenization improves the relative differences, now remaining between -5% and +5%, except in 2006 when the negative 230 bias decreases down to values smaller than -6%. The comparison between the altitude dependence of the error of the lidar measurement and that of the ECC measurement in the stratosphere (Fig. 76b) shows that the ECC error remains in the range 5.5%-6.5%, while the lidar is very accurate (error <2%) between 18 km and 30 km.
Considering stratospheric lidar observations are highly accurate above 28 kmin the stratosphere, frequent freezing or evaporation of the ozonesonde sensing solution may explain the ECC low bias relative to the lidar during nighttime soundings may 235 be an explanation for the lowest performances above 28 km of the ECC launched at OHP. The O 3 partial pressure error related to a pressure offset for the Vaisala RS80 period may be another reason for the large difference with LiO3St ozone values, but this error will be limited as it exists for only one third of the ECC sondes used for this comparison (25 MODEM and 15 RS80 radiosondes). When examining differences above 26 km between homogenized ECC and LiO3St for the MODEM and the RS80 subsets separately, there is indeed a larger negative bias of -4% to -10%-11% for the RS80 than -6% to -7%-9% for the 240 MODEM. We have also considered two subsets with ECC pump temperature T i at 30 km either higher or lower than 290 K.
The negative bias between the homogenized ECC and LiO3St O 3 concentrations above 26 km remains greater than decreases down to -3% for the high T i subset while it ranges from -5% to -7%-7% to -9% for the low T i subset. More investigations are needed to conclude that freezing or evaporation of the solution is indeed the major uncertaintycontributor to the negative bias of the ECC concentration measurements above 26 km.

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The -2% to -4%-5% difference between LiO3St and the homogenized ECC in the altitude range 19-27 kmstratosphere even after homogenization is consistent with the mean normalization factor of 1.037 shown in Fig. 41.05 discussed in the previous section. Indeed, the means of the relative difference between LiO3St and ECC are no longer significant below 28 km when the ECC concentrations are multiplied by the normalization factor (black curve in Fig. 6a). Note however that such a correction is not recommended for the tropospheric ECC ozonesonde measurements (Smit and Thompson, 2021).  be present, but is considerably less prominent than the drop-off as observed at other measurement sites in Stauffer et al. (2020). The uncorrected ECC and OMI/OMPS biases range between -1% to +5% while it is between 0 and +3% for GOME.
Those differences are mostly negative and between -4%-3% and 1% after homogenization. The ECC minus satellite TOC 275 comparisontemporal evolution is consistent with the time distribution of the normalization factor shown in Fig.4. However TOC differences are close to zero between 2010 and 2016 using the satellite data, while a -3% bias is present using the OHP total ozone measurement. In this context, we mention that the expected bias between GOME and SAOZ is between -3% to +1% (Hendrick et al., 2011). 22-years trend of ozone mixing ratio associated with this inter-annual variation, the latter is deseasonalised by subtracting 285 from the surface mixing ratios the monthly averages calculated over the 22 years of data. This removes a major source of O 3 mixing ratio intra-annual variability which is on the order of 20 ppbv. The trends of the ozone mixing ratio and their 95% confidence interval estimates are calculated using the regression lines across all the available deseasonalised mixing ratios. A weak negative trend on the order of -1.3±0.9 ppbv/decade is obtained for the uncorrected ECC deseasonalised mixing ratio (called ozone anomalies hereafter) and this trend changes very little (-1.1±0.7 ppbv/decade) after homogenization of the ECC 290 (Fig.10). The ECC negative ozone trends compare very well with those obtained from surface measurements using either all the O 3 daily means between 1998 and 2021 (-1.3±0.2 ppbv/decade) or only the hourly means for ECC launching times (-1.1±0.6 ppbv/decade). The small difference between the trend calculated for all the surface daily means available and the trend using only the ECC launching times shows that the sensitivity of the trend magnitude to the sampling by ECC is not so large. The negligible difference between the uncorrected ECC trend and the homogenized ECC trend near the surface is mainly due to surface measurements (Fig.10), therefore in better agreement with the homogenized ECC inter-annual variations.

Tropospheric trend
In this section, the inter-annual variation of homogenized and uncorrected ECC ozone are compared in the free troposphere for three layers of 2-km thickness at 5 km, 7 km and just below the dynamical tropopause taken at 2 PV units (Fig.11). The three layers were selected in order to compare the ozone trends of the ECC sondes with those of the LiO3tr lidar. The mean 305 altitude Z tp of the dynamical tropopause is calculated using ECMWF meteorological analysis with 1 o horizontal resolution and 137 vertical levels. The mean value of Z tp is 10.5 km at OHP (10 km in winter and 11.5 km in summer), so the upper layer approximately corresponds to the 8 km to 10 km altitude range. As for the surface trend retrieval, the mean ozone concentrations of the layers are deseasonalised before calculating the trends of mixing ratios from the regression lines across all the 2-km ozone mixing ratio averages available in the 30-year database. The uncorrected ECC trends are always positive 310 and significant and they increase with altitude, with the largest value (4.4±0.8 ppbv/decade) in the layer below the tropopause (Table 1). The lidar measurements also show significant positive trends for the 3 layers but with smaller values, e.g. 3.1±0.9 ppbv/decade below the tropopause. The lidar trends are in better agreement with the trends calculated using the homogenized values, e.g. 3.2±0.8 ppbv/decade below the tropopause. Although the lidar and homogenized ECC yearly average of ozone anomalies are not similar from year to year considering the sampling differences, the main decennial changes are seen by both 315 instruments above 6 km, namely the sign change of the anomalies between the period 2000-2010 (positive) and 2010-2020 (negative) (Fig.11). Overall the homogenization greatly improved the ECC tropospheric trend retrieval with smaller and more realistic values.   anomalies between 1991 and 2020, O3 trends in ppbv/year and their uncertainties with a 95% confidence are also shown in each panel.

Stratospheric trend
Here the interannual variation of homonogenized and uncorrected ECC ozone are compared in the stratosphere for three layers 320 of 2-km thickness at 19 km, 25 km and 29 km (Fig.12). The three layers were selected to be able to compare the ozone trends of the ECC sondes with those of the LiO3St lidar. The methodology developed for the surface and tropospheric ozone trends has been applied on the ozone concentrations given in molecules.cm −3 which is the primary unit used by the LiO3St for the ozone retrieval (Leblanc et al., 2016). The uncorrected ECC trends shown in Table 2 .12). Such differences in the range of the yearly ozone anomalies are related to a different sampling for ECC and lidar 330 profiling, with the lidar providing more than twice as much ozone profiles than the sondes. The homogenization nevertheless greatly improved the stratospheric 30-years trend assessment with a better agreement with the lidar trend analysis, the latter being recognized as very accurate in the stratosphere above 18 km (Nair et al., 2011).

Conclusions
The 30-years ozone data set from weekly ECC ozone soundings has been homogenized according to the recommendations of 335 comparisons.
-The negative trend of the normalization factor (N T ) calculated using the OHP Dobson and SAOZ total column disappears thanks to the homogenization of the ECC. There is however a remaining -5%-3.7% negative bias which is likely related to an underestimate of the ECC concentrations in the stratosphere above 50 hPa as shown by comparison with the OHP LiO3St lidar and MLS (no bias between ECC and lidar when the ECC is multiplied by N T ). The reason for this bias is 350 still unclear and must be better understood.
-Differences between TOC measured by ECC and by GOME or OMI/OMPS switch from 2%±2% for uncorrected ECC to -2%-1%±2% for homogenized ECC. The negative bias is then smaller than the -3.7% obtained with the OHP TOC measurements, eventhough the time evolution is being consistent with the N T time distribution.
-Direct comparisons of homogenized and uncorrected ECC concentrations in the stratosphere between 18 km and 3026 355 km show limited changes using a subset of 40 days with LiO3St and ECC measurements time difference less than 6 hours.
The mean differences between in 2005 to 2021 ECC and MLS or LIO3St ozone observations using a less restrictive time coincidence < 12 hours are either slightly positive (+2%) for MLS or slightly negative (-2%) for LiO3St meaning that the homogenization is a good compromise for intercomparibility of ECC with other stratospheric O 3 measurements below 20 26 km.but differences between MLS and ECC using all ozonesondes from 2005 to 2021 are more evenly distributed 360 around zero for the homogenized time series than for the uncorrected ECC.
-Both the comparisons with lidar and satellite observations suggest that homogenization increases the negative bias of the ECC up to -10%to values lower than -6% above 26 km While the objective of this paper is to discuss the impact of homogenization on the OHP dataset using lidar and satellite measurements, it is worth checking how such corrections have improved data quality at other sites. The impact of the homog-365 enization is dependent on the site, because different homogenization steps have to be applied at different stations. In general, the additional corrections for the pump temperature will give higher ozone partial pressure amounts in the stratosphere. On the other hand, applying a constant background current subtraction instead of a pressure dependent background current and applying the transfer functions from EnSci-SST 1% will lead to lower ozone partial pressure values above 10 km. Witte et al. (2017) performed an extensive analysis of 7 SHADOZ network stations in the tropics, showing that the mean differences be-370 tween ECC and MLS are reduced from -11.2±13.6% to -3.0±10% at 40 hPa (22 km