The GOMOS instrument on board the ESA's Envisat satellite was launched in 2002 and it monitored ozone, other trace gases (such as NO2, NO3, H2O and O2) and aerosols until 2012 using stars as light sources . The spectral range of the spectrometers is in the ultraviolet-visible wavelengths (248–690 nm) and in the near-infrared (755–774 and 926–954 nm).

GOMOS utilized a stellar occultation technique to measure the vertical distribution of ozone at the altitudes between 10 and 100 km . The retrieval is based on the maximum likelihood method and it uses only minimally a priori information of the vertical distribution of ozone, namely smoothness that results in vertical resolution of 2–3 km .

The accuracy of the ozone profile derived from the GOMOS retrieval depends on the star magnitude and temperature of the star . The most accurate ozone profiles are retrieved using the hot and bright stars. The GOMOS nighttime measurements have high precision with a retrieval error in the stratosphere around 0.5–4 % and in the mesosphere around 2–10 % . The stratospheric ozone precision estimates from the GOMOS nighttime measurements have been validated by using the differential method presented by . The summary of the geophysical validation of GOMOS measurements is presented in .

In this comparison study, we have used nighttime (with solar zenith angle > 107∘) ozone profiles from the GOMOS processor version 6 data (Table ). We have selected only stars that are medium to bright (magnitude Mv≤2) and stars that are hot (temperature ≥7000 K). Additionally, GOMOS data were filtered for outliers and unreliable data using the recommendations of the readme document (https://earth.esa.int/web/sppa/mission-performance/esa-missions/envisat/gomos/products-and-algorithms/products-information).

The OSIRIS instrument on board the Swedish Odin satellite, launched in 2001, measures limb scattered solar light at the 280–810 nm wavelength region with around 1 nm spectral resolution . OSIRIS also includes an infrared imager with three channels but in this study we only use the spectrograph data. The OSIRIS spectrograph data can be used to retrieve various trace gases such as ozone, NO2, OClO, BrO, and aerosols. OSIRIS measures the atmosphere near 06:00 p.m. local solar time on the ascending node and near 06:00 a.m. local solar time on the descending node.

The OSIRIS ozone profile data used in this paper are retrieved using an onion peeling type inversion method developed at the Finnish Meteorological Institute (FMI). The FMI-OSIRIS Level 2 version is 3.2 and it uses the Level 1 version 55092 data. In the FMI-OSIRIS ozone retrieval, around 300 wavelengths between 280 and 680 nm are used to produce ozone profiles between 10 and 70 km. Generally, the FMI-OSIRIS ozone profiles agree within ∼5 % with the GOMOS night time and MLS data . The difference between the FMI-OSIRIS ozone and the other OSIRIS ozone product retrieved using the SaskMART algorithm is typically a few percentage points in the stratosphere .

In this comparison we only use OSIRIS morning measurements, which is convenient because GOME-2 measures in the morning too. There were some OSIRIS ozone profiles containing unrealistic ozone concentration values which were omitted from the analysis.

The MLS instrument on board the Earth Observing System (EOS) Aura satellite, launched in 2004, measures microwave thermal emission from the limb . The Aura satellite's equator crossing time is near 01:45 p.m. The MLS instrument observes emissions in spectral regions centred near 118, 190, 240, 640 GHz and 2.5 THz. The retrieved vertical profiles of trace gases include ozone, BrO, ClO, CO, H2O, HCl, HCN, HNO3, N2O and SO2. The MLS limb-viewing measurements are processed using an optimal-estimation-based retrieval method .

Ozone is retrieved using the 240 GHz retrieval. The vertical ozone profiles are reported as mixing ratios at pressure levels. The comparison in this paper includes MLS version 3.3 Level 2 ozone profiles . The useful pressure range for the MLS version 3.3 ozone profiles is between 261 and 0.02 hPa (∼ 10–80 km). The MLS ozone comparison in the stratosphere with other profiles from satellite, balloon, aircraft and ground-based data resulted in an overall agreement of 5–10 % .

We used MLS ozone profile data filtered using data quality metrics provided along with the profile data and as recommended in the document by . In addition, we only used MLS data with the SZA<80∘. High SZA observations often have additional uncertainties, but with MLS we can avoid these measurements because the spatiotemporal coverage is very good, anyway.

We selected collocated profiles with the spatiotemporal separation presented in Table . They are the following: ±12 h, ≤400km for GOMOS, ±6 h, ≤200km for OSIRIS and ±6 h, ≤100km for MLS. The geographic distance between GOMOS, as well as OSIRIS, and the GOME-2 ground pixel centre is calculated approximately at 30 km tangent point location. For MLS, the distance is calculated between the GOME-2 pixel centre and MLS geolocation. We would like to note that the geophysical distance in the atmosphere can differ from the ground distance by 20–30 km .

For each reference instrument, the collocation criterion is a compromise of having the smallest spatiotemporal separation whilst having sufficient amount of collocations. The separation in time is dictated by the local time of the measurements. For the spatial separation with GOMOS (separation less than 400 km), we used the effective horizontal resolution of the considered limb/occultation measurements (e.g. ). Decreasing of the allowed geographic distance or time difference reduces the number of collocated ozone profile pairs with GOMOS significantly, and also influences the spatial coverage by the collocated profiles and statistical significance of the results. More dense measurements by OSIRIS and MLS allow tighten the collocation criterion. Formally, the same collocation criterion (as for GOMOS) can be applied for all reference instruments, but this will lead to multiple collocations. In order to get statistically independent pairs, only the closest one in time or in space should be used in the analysis. Thus, even if the collocation criteria would be formally the same, the real spatiotemporal difference will be smaller in collocations with denser samplers. Figure  shows latitude-time distribution of the collocated measurements. Similarity of biases (see Sect. ) with respect to all reference instruments indicates that the selected collocation criteria are adequate for the evaluation of the GOME-2 profiles.

The agreement between GOME-2 and the reference instruments for the whole time period is shown in Fig. . The ozone profiles were retrieved from GOME-2 measurements, which are not corrected for instrument degradation (see Sect.  about degradation correction).

The vertical pattern of the mean relative differences across the tropical and midlatitude regions is similar with all three instruments. The comparison shows an agreement of GOME-2 ozone profiles with those of GOMOS, OSIRIS and MLS within ±15 % in the altitude range from 15 km up to ∼ 35–40 km depending on latitude. On average, the GOME-2 ozone profiles have a positive bias from the surface up to ∼25 km and a negative bias above ∼30 km apart from the higher latitudes. The agreement between the GOME-2 and reference ozone profiles is the best at midlatitudes. Around the equator, between 20∘ S and 20∘ N, there is a higher positive bias reaching 20 % at altitudes below ∼20 km. The black line (upper panels) shows the mean climatological tropopause altitude. We can notice that the high positive relative bias occurs ∼3 km above the tropopause and below.

At southern high latitudes the averaged relative differences between GOME-2 and the reference profiles are significantly different from the midlatitudes and tropics, and they are also different from each other. In comparisons with both GOMOS and MLS, a positive bias of ∼ 10 % is observed at altitudes 30–40 km, and a negative bias ∼ 20 % is observed at altitudes below 30 km (Fig. ). These features at high southern latitudes are not seen in comparison with OSIRIS (Fig. , center). In addition, a positive bias in the zone ∼ 55–75∘ S at altitudes 18–28 km is observed solely in the comparisons with MLS. These differences can be explained by sampling issues related to the seasonal dependence at high latitudes as verified and discussed below in Sect. . Note that southern high latitudes have non-uniform seasonal coverage by collocated data, especially for GOMOS and OSIRIS.

The seasonal variation of the mean relative differences between GOME-2 ozone profiles and the averaging kernel smoothed GOMOS, OSIRIS and MLS profiles are shown in Fig. . As in Sect. , the GOME-2 ozone profiles are based on measurements that are not corrected for instrument degradation. In these plots, the collocated data are divided into five latitude zones. The seasons are defined as DJF (December, January, February), MAM (March, April, May), JJA (June, July, August) and SON (September, October, November). Here we consider only 2 years of data: 2010 and 2011. A period of 2 years enables a good representation of the latitudinal and seasonal dependent relative differences without excessive averaging. Note that due to large samples the detected biases are statistically significant, except a few cases of small number of collocated profiles (e.g. Fig.  upper left corner, N=10 for OSIRIS).

The seasonal variation of the bias with respect to all reference profiles is very similar, especially at the tropical and midlatitudes. At the high latitudes the divergence in comparison results may occur due to the different temporal sampling by the collocated profiles. The different air masses, being inside or outside polar vortex, can also contribute to the varying mean relative differences in the polar spring season.

In general, at tropical latitudes (30∘ S–30∘ N) the seasonal variation of the mean relative difference is low. However, the negative peak in the bias just above 40 km is on average about 10 % deeper in DJF than in the other seasons. The reason for this seasonal increase in the relative difference is not yet understood. At midlatitudes, there is a moderate seasonal variation of the GOME-2 biases in the stratosphere, yet more evident at southern midlatitudes.

The seasonal dependence of the mean relative differences can be clearly seen at the high latitudes. There is a negative bias with respect to all reference data just above 20 km in JJA (i.e. local winter) at southern high latitudes. The same vertical structure in the relative difference is seen at the northern high latitudes in the local winter (DJF).

In SON (30–90∘ S) and in MAM (60–90∘ S), the shape of the bias profile is a bit different between the comparisons with the separate reference instruments. In SON (30–90∘ S) the collocated data with GOMOS includes only September, whereas the collocated data with both OSIRIS and MLS include all three “SON” months, and the comparison results from November are very different (not shown here). In MAM (60–90∘ S), collocated data for MLS includes March and April, whereas for GOMOS and OSIRIS only March is included (note that biases with respect to OSIRIS and GOMOS are very close to each other).

At the northern high latitudes (60–90∘ N) the mean relative differences vs. MLS data at the upper altitude levels deviate from others in MAM and JJA. In addition, in SON (60–90∘ N) the bias above 40 km is higher against OSIRIS and MLS than against GOMOS. The reason for this might be that collocations for GOMOS includes only September.

At lower altitudes, the accuracy of the ozone profiles from limb-viewing instruments worsens due to lower signal-to-noise ratio thus causing a wide standard deviation around the mean relative differences (Fig. ). When the comparison is done with the actual reference profiles, i.e. without averaging kernel smoothing, the bias is very similar than can be seen in Fig.  except the highest altitude layers and the tropical lowest altitude layers, and this can be expected.

Figures – show a temporal evolution of the monthly median of relative differences (%) starting from March 2008 for each latitude zone with respect to the smoothed GOMOS, OSIRIS and MLS profiles, respectively. The GOME-2 ozone profiles in these figures are based on measurements that are not corrected for instrument degradation.

These figures confirm that the relative difference has a definite seasonal cycle at polar and midlatitudes. This can be seen especially in Figs.  and since both OSIRIS and MLS have temporally denser data set. In the tropics, the seasonal variations in the GOME-2 biases are milder, as expected.

We can notice a change to the better agreement in the upper stratosphere and the lower mesosphere after October 2009, and even more clearly in the whole stratosphere after the end of April 2010. These changes correspond to OPERA algorithm versions 1.01 and 1.10 updates, and software version 1.20 and 1.23 updates .

There can be several possible reasons for the mainly systematic differences encountered with respect to the reference data. The GOME-2 ozone profiles and the reference profiles used from the GOMOS, OSIRIS and MLS instruments are collocated at different local solar times (see Table ). It has been shown that the natural variability of ozone is affected by the photo-chemical reactions using solar light above 40 km e.g.. The large relative differences in the upper stratosphere with respect to GOMOS data, and to a lesser extent against OSIRIS and MLS data, could be partly caused by this diurnal effect (see Fig. ).

The comparisons done here with occultation and limb measurements show a turning point of bias with the maximum relative differences at around 40–50 km. In the paper by it is discussed that above 2 hPa (∼45 km) the implemented additive offset (to partially correct the influence of instrument degradation) has the largest influence on the number of pixels passing the quality control. In the validation report it has been noticed that in the course of time the GOME-2/MetopA data become noisier due to the instrument degradation, mainly at higher altitudes above 40 km. The effect of degradation correction and its influence on observed biases is discussed below in Sect. .

We have studied the dependence of GOME-2 bias on the solar zenith angle (SZA). The SZA is provided with GOME-2 ozone profile retrieval. Typically the bias increases with increasing SZA. Figures  and  show the SZA dependence of the GOME-2 bias compared to smoothed GOMOS and OSIRIS data for the altitude layers located around 23, 30, 37 and 45 km at the southern high latitudes (60–90∘ S). We show here bias only for one latitude zone, 60–90∘ S, since only there do exceptionally high bias values that depend on SZA occur. The results were similar for all years; thus we only show data from 2010.

At the southern high latitudes the negative bias increases with increasing SZA around 23 km altitude. This is also observed at northern high latitudes, 60–90∘ N (not shown here). A similar dependence on SZA at tropical and midlatitude regions was registered around 45 km (not shown here). On the contrary, at the southern midlatitudes around 37 km there is a tendency for the (negative) bias to get smaller; i.e. there is better agreement when the SZA increases (not shown here).

In Figs.  and  exceptionally large positive relative differences with respect to both GOMOS and OSIRIS data appear above ∼30 km when SZA>85∘ and at the same time large negative differences occur around 23 km. We have marked these pixels by grey and they represent the same pixels at all the altitude layers. We can notice that at ∼30 km (top right panel in Fig. ) these pixels do not have divergent relative difference values. In addition, the very high positive relative differences with respect to OSIRIS profiles are encountered when SZA is between 68 and 80∘ (marked with grey dots in Fig. ). This happens only in the southernmost latitude levels around 87∘ S during the southern polar spring and summer months. The reason for these exceptionally high bias values is not known at the moment; this feature will be investigated in the future studies. The collocated pixels with GOMOS data have the southernmost latitudes around 69∘ S thus these high difference values are not seen against GOMOS data (Fig. ).

In order to demonstrate the usefulness of the GOME-2 data we considered the ozone profiles retrieved from the GOME-2 measurements during the Arctic ozone depletion in spring 2011. In the Antarctic region the ozone hole event recurs annually, but in the Arctic region severe ozone loss happens less frequently. The exceptionally reduced transport of ozone from midlatitudes into the Arctic region together with enhanced chemical ozone loss inside polar vortex caused the anomalously low ozone concentration over the North Pole in March 2011 . The Arctic ozone depletion is not represented by the typical a priori profile used for this season and latitude and hereby offers a chance to verify the retrieved profiles under the challenging circumstances.

As the air masses inside the polar vortex can be very different compared to outside polar vortex atmosphere, the comparisons have been done separately for the pixels being inside and outside the polar vortex. We have done the comparison over Arctic region (60–90∘ N) with collocated OSIRIS data and used 475 K potential vorticity associated with OSIRIS data to determine the collocated GOME-2 pixel to be outside or inside polar vortex area. In this special case, we require pixels to a SZA≤65∘ because the disagreement with reference data increases as SZA increases at high latitudes when considering the altitude range around 20 km (see previous Sect. ). The number of collocated profiles inside vortex is 17 and outside vortex 96, respectively. In reality, as the division is not the same at all altitudes, some part of the profile can be inside, and the rest of the profile can be outside of the polar vortex.

Figure  shows the monthly averages for March 2011 of collocated ozone profiles from GOME-2 and OSIRIS measurements separately for profiles inside (left hand panel) and outside polar vortex (middle panel). The retrieved GOME-2 mean profile inside polar vortex is far away from a priori profile and is rather close to the OSIRIS profile. Outside polar vortex, all profiles are close to each other. Interestingly, in this case the OSIRIS mean profiles agree better with a priori values, especially in outside polar vortex situation. Probably, this happened by chance.

The difference between inside and outside polar vortex monthly mean values scaled by outside polar vortex monthly mean is evident as seen for GOME-2 (blue) and OSIRIS (red and black) (Fig.  right panel). In this figure are shown also the mean GOME-2 relative retrieval errors outside and inside polar vortex separately.

This case study shows that the GOME-2 can observe the ozone depletion well and thus provides very useful information. We would like to note that the horizontal coverage by GOME-2 is much more dense than by limb-viewing instruments.

The GOME-2 vertical ozone profile algorithm depends strongly on good absolute-calibrated solar and earthshine spectra. Unfortunately, GOME-2 is affected by instrument degradation that is strongest in UV . This instrumental degradation increases over time, thus setting additional challenges to the algorithm to adapt to these changes. An experimental correction has been developed, which aims to rectify an initial bias and the time dependent degradation for a number of relevant wavelengths and scan angles. This degradation correction is applied to the measurement, leaving all other Opera ozone profile algorithmic features intact. The data produced with the degradation correction is only used for comparisons in this section, not in the operational data shown earlier in this paper. This limited reprocessed GOME-2 data set was generated with OPERA version 1.32.

To demonstrate its potential we have done some comparisons of GOME-2 vertical ozone profiles using a newly developed experimental degradation correction with the GOMOS, OSIRIS and MLS data. The preliminary results for March 2008, shown in Fig. , indicate good functionality of the implemented correction. The large underestimation of ozone above 40 km has been turned into a smaller overestimation, rectifying the deviation considerably. The operational ozone profiles (dashed curves) used here are from the offline ozone product (OOP) with software version 1.14 in coarse resolution, whereas the degradation corrected profiles (solid curves) have been retrieved in higher resolution. Thus the number of collocated profiles in comparison is higher when using higher resolution GOME-2 data. Since the beginning of the O3M SAF, the operationally available vertical ozone profile product has been under continuous development and improvement . This can be seen also in timeline comparison results of the upper stratosphere, e.g. after 2010 (Figs. –).