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 A flight of the balloon-borne version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) was performed from Kiruna (Sweden, 68°N, 21°E) on 20/21 March 2003 as part of the validation program of the chemistry instruments MIPAS, GOMOS, and SCIAMACHY aboard the European environmental satellite ENVISAT. The 15 hour long duration of this flight provided a good match with the Japanese Improved Limb Atmospheric Spectrometer (ILAS)-II sensor aboard the ADEOS-II satellite, launched in December 2002, in addition to the primary goal of ENVISAT validation. The MIPAS-B flight data coincided nicely with one of the early operational periods of the ILAS-II instrument, offering one of the sparse opportunities to validate the whole set of trace species measured by ILAS-II during its unfortunately short lifetime. Radiance spectra were observed by MIPAS-B at nearly the same location that was observed by ILAS-II about 5.5 hours prior to the sampling of MIPAS-B. The intercomparison of ILAS-II atmospheric target parameters (version 1.4) to profiles measured by MIPAS-B has shown that for the species O3, N2O, CH4 (below about 22 km), HNO3, ClONO2, and CFC-11 (CCl3F), a predominantly good consistency with MIPAS-B has been achieved within the combined errors. However, atmospheric parameters like temperature, H2O, NO2, and N2O5 are widely characterized by low biases compared to MIPAS-B results while CFC-12 (CCl2F2) exhibits a high bias in comparison to the balloon-borne observations. Therefore ILAS-II profile retrievals of temperature, H2O, NO2, N2O5, and CFC-12 cannot be assumed to be validated at the present time.
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 Satellite measurements play an essential role in monitoring the Earth's atmosphere and surface within the context of naturally and anthropogenically induced climate changes. From these remote sensing measurements, a global set of simultaneously derived atmospheric parameters can be obtained.
 The necessity of validating satellite instrument products is obvious from experience with prior space instruments [see, e.g., Gille et al., 1996; Sasano et al., 1999]. Apart from satellite observations, balloon-borne measurements are the only tool to obtain distributions of a large number of molecules with sufficiently high vertical resolution over most of the stratospheric altitude region. They provide high precision and accuracy because of the quasi-Lagrangian measurement situation which, e.g., allows longer integration times. Furthermore, systematic errors can be kept small since a complete calibration and characterization of the instruments can be performed and checked close to the time of the measurement. Therefore balloon observations are to be regarded as a key component in any satellite validation task in spite of the limited number of profiles that can be obtained. Since the number of balloon launches is restricted because of logistical and financial constraints and thus any statistics is very limited, the quality of coincidence in time and space between the balloon measurement and the satellite measurement is crucial. Otherwise, modeling techniques such as backward trajectory mapping of observed air masses need to be applied which may reduce the significance of the validation. In this paper measurements of the ILAS-II (Improved Limb Atmospheric Spectrometer-II) satellite sensor are compared to observations with the MIPAS-B (Michelson Interferometer for Passive Atmospheric sounding, Balloon-borne version) instrument.
2. ILAS-II Instrument and Observations
 The second version of the Japanese Advanced Earth Observing Satellite (ADEOS-II) was launched on 14 December 2002 and operated sporadically for initial checkout between January and March 2003, followed by a continuous and routinely operation between 2 April and 25 October 2003. Unfortunately, the operation had to be terminated on 25 October 2003 because of the malfunction of the solar power subsystem. The Improved Limb Atmospheric Spectrometer-II (ILAS-II) has been designed similarly to its predecessor [Sasano et al., 1999; Nakajima et al., 2006] and was one of five sensors for Earth observation aboard ADEOS-II. The objectives of the ILAS-II project has been to monitor and study chemical and dynamical processes affecting the stratospheric ozone layer. ILAS-II was a spectrometer that observed the atmospheric limb absorption spectrum from the upper troposphere to the stratosphere using sunlight as a light source (solar occultation technique). The spectrometer covered the infrared region (3–13 μm) and the near visible region (753 to 784 nm). The spectral resolution is dependent on the spectral region and ranges from 0.129 μm up to 0.15 nm. Observations were confined to high-latitude (57°N to 73°N, and 64°S to 90°S) regions because of the geometrical relation of the solar occultation events with the sun synchronous orbit. Vertical profiles of species related to ozone depletion phenomena, including ozone (O3), nitric acid (HNO3), nitrogen dioxide (NO2), nitrous oxide (N2O), methane (CH4), water vapor (H2O), CFC-11 (CCl3F), CFC-12 (CCl2F2), chlorine nitrate (ClONO2), dinitrogen pentoxide (N2O5), as well as aerosol extinction coefficients at 780 nm, temperature, and pressure can be derived from these measurements.
 Retrieval calculations of the target parameters (version 1.4) were performed with an onion-peeling algorithm in combination with a multiparameter nonlinear least squares fitting procedure [Yokota et al., 2002; T. Yokota et al., manuscript in preparation, 2006]. The HITRAN spectroscopic database [Rothman et al., 2003] was used for the radiative transfer forward model calculations. For heavy species like ClONO2, N2O5, CFC-11, and CFC-12, pseudoline parameters provided by G. C. Toon (private communication, 2003) have been used. The sources of these spectroscopic data are the same as those used for the MIPAS-B data analysis (see section 3 and T. Yokota et al. (manuscript in preparation, 2006)). The retrieval grid was set to 1 km, the typical vertical resolution ranges between 1.3 and 2.9 km.
 Tangent altitudes of the measurements were determined using a sun edge sensor method for altitudes above 30 km and a transmittance method for altitudes below this level [see Nakajima et al., 2002; T. Tanaka et al., manuscript in preparation, 2006]. A “repeatability error” was estimated and provided in the ILAS-II version 1.4 product on the basis of measurement repeatability which is defined as the closeness of the agreement between the results of successive measurements of the same measurand [International Organization for Standardization, 1993]. This measurement repeatability was empirically calculated in the ILAS-II version 1.4 retrieval procedure (T. Yokota et al., manuscript in preparation, 2006). In addition, an “external error” was estimated by considering temperature uncertainties and inaccuracies in the nongaseous correction due to aerosols. Root-sum-square of the repeatability and external error is defined as the “total error.” A detailed description of the error estimation is given by T. Yokota et al. (manuscript in preparation, 2006).
3. MIPAS-B Instrument and Data Analysis
 The balloon-borne version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) is a limb-emission sounder for atmospheric research. The heart of the instrument is a Fourier spectrometer that covers the midinfrared spectral range (4 to 14 μm) and operates at cryogenic temperatures. Essential for this application is the sophisticated line of sight stabilization system, which is based on an inertial navigation system and supplemented with an additional star reference system. A comprehensive overview and description of the MIPAS-B instrument is given by Friedl-Vallon et al.  and references therein. Beside the high performance of the pointing system which virtually avoids any mapping of pointing errors into retrieval errors, the suitability of MIPAS-B to validate ILAS-II is based mainly on its high spectral resolution (about 0.07 cm−1 after apodization) allowing the distinction of spectral lines due to molecular transitions from continuum-like emissions. Averaging several spectra during one single elevation angle yields to a reduction of the noise equivalent spectral radiance (NESR) and therefore to an improvement of the signal-to-noise ratio. MIPAS-B is capable of simultaneously measuring vertical profiles of all target parameters ILAS is covering with an accuracy of about 10%.
 The MIPAS-B data processing from the raw interferograms and the instrument housekeeping data to the calibrated spectra is described by Friedl-Vallon et al. [2004, and references therein]. It includes instrument characterization in terms of the instrumental line shape, field of view, noise equivalent spectral radiance, line of sight of the instrument, detector nonlinearity and a complete error budget of the calibrated spectra. Retrieval calculations of the target parameters are performed with a least squares fitting algorithm using analytical derivative spectra calculated by KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) [Stiller et al., 2002; Höpfner et al., 2002]. Spectroscopic parameters were taken from the HITRAN database [Rothman et al., 2003]. Data for chlorofluorocarbons (CFC-11 and CFC-12) originate from Varanasi and Nemtchinov . ClONO2 cross sections have been measured by Wagner and Birk  while cross sections of N2O5 originate from Cantrell et al. . The measurements were performed with a vertical grid of about 1.5 km while the retrieval grid was set to 1 km. Regularization was based on the Tikhonov-Phillipps approach [Phillips, 1962; Tikhonov, 1963] constraining with respect to the form of an a priori profile. The number of degrees of freedom of the retrieval (trace of the averaging kernel matrix [see, e.g., Steck, 2002]) was between 6 and 14, dependent on the parameter and the number of tangent altitudes used during the retrieval. The resulting vertical resolution lies typically between 1.5 and 3 km and is therefore comparable to the vertical resolution of ILAS-II. Spectra were fitted in MIPAS-B proven microwindows [see, e.g., Stowasser et al., 1999; Wetzel et al., 2002]. Temperature, volume mixing ratios of trace species, frequency shift, and radiance offset were subject to fitting. The error estimation includes random noise as well as the mutual influence of the fitted parameters, temperature errors, and line of sight inaccuracies. The resulting total error refers to the 1-σ confidence limit.
 Within the framework of the validation activities of the instruments MIPAS, GOMOS, and SCIAMACHY aboard ENVISAT, a validation flight of MIPAS-B was performed from Kiruna (Sweden, 68°N, 21°E) on 20/21 March 2003. After a flight duration of more than 15 hours from 1822 UT (20 March) to 0938 UT (21 March) touch down of the gondola was only about 50 km away from the launch site. The long duration of this flight allowed not only the measurement of several limb sequences matching the evening and morning overpasses of ENVISAT but also to look for a good match with the ILAS-II sensor aboard ADEOS-II. Radiance spectra were observed at the same location ILAS-II had measured about 5.5 hours prior to the sampling by MIPAS-B.
 The long duration measurement of MIPAS-B was possible since the gondola was situated right in the center of the arctic vortex where winds are generally weak. Both instruments, MIPAS-B and ILAS-II measured nearly the same air masses inside the polar vortex over a wide altitude range in the absence of polar stratospheric clouds. At altitudes below about 18 km, both observations were performed near the vortex edge region. Geolocations of the two sensors are depicted in Figure 1 together with the difference in Ertel's potential vorticity as calculated from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. Even though the geolocations are close together, the time difference of more than 5 hours in combination with a small displacement of the vortex toward northeast within this time period, leads to slightly higher values in potential vorticity for the ILAS-II sensor compared to MIPAS-B.
4. Profile Comparison of Target Parameters
 In this section, profiles of atmospheric parameters measured by MIPAS-B are compared to ILAS-II version 1.4 data. Temporal and spatial differences of the observations of both sensors are reflected in small differences of Ertel's potential vorticity (compare Figure 1). To account for these deviations, ILAS-II measured quantities were linearly interpolated to the potential temperature levels corresponding to the MIPAS-B altitude grid. Differences of measured parameters are displayed together with the combined error σcomb of both instruments (95% confidence limit) which is defined as:
where σM is the total error of MIPAS-B and σI is the total error for ILAS-II measured quantities.
 The comparison for the temperature is shown in Figure 2. Besides the profiles observed by MIPAS-B and ILAS-II, meteorological data of the United Kingdom Meteorological Office (UKMO) and ECMWF, interpolated to the ILAS-II and MIPAS-B geolocations, are shown. At altitudes below about 27 km the meteorological analyses coincide well with each others. However, temperature differences are obvious above this altitude level caused not only by different geolocations of the sensors but also as a result of differences and potential inaccuracies in the analyses themselves [Knudsen, 2003]. The MIPAS-B profile exhibits some systematic deviations to the meteorological analyses which may at least partly be explained by the fact that the retrieved temperature profile (especially at the lower-altitude levels) is representative for a horizontal region of a few hundred kilometers and not for a single point. This is important to mention since strong horizontal temperature gradients occurred during the time of observation. Nevertheless, the ILAS-II profile derived from the O2 A-band absorption in the visible channel is up to 6 K off the MIPAS-B profile with a large cold bias especially below about 23 km. Because of these temperature retrieval problems the ILAS-II gas retrievals were performed with assimilated UKMO temperatures.
 The results for the comparison of the trace species ozone are shown in Figure 3. Besides the retrieved profiles of MIPAS-B and ILAS-II, the in situ profile of an ozone sonde, launched shortly after the MIPAS-B launch is shown, too. To allow a more realistic comparison in terms of altitude resolution, the ozone sonde profile was smoothed with the averaging kernel matrix and the a priori profile of MIPAS-B. The smoothed in situ profile xs is calculated as described by Rodgers ,
where xa is the a priori profile of MIPAS-B, xa* the a priori profile interpolated to the altitude grid of the in situ profile x, and A is the averaging kernel matrix of MIPAS-B.
 The smoothed in situ profile agrees well with MIPAS-B over most altitude regions. The same holds for the profile intercomparison of MIPAS-B and ILAS-II. The absolute difference of both sensors amounts about 12% in average and is still within the combined errors at most altitudes with a small low bias in the ILAS-II data over a larger altitude region. Above 28 km ILAS-II and MIPAS-B profiles exhibit higher O3 values compared to the sonde. However, studies have shown that ozone sonde measurements are less reliable above about 28 km [see World Meteorological Organization, 1998, and references therein].
4.3. Nitrous Oxide and Methane
 The profiles of N2O and CH4 are considered together because the behavior of these long-lived trace gases in the stratosphere in terms of transport and lifetime is similar. Figures 4 and 5show the intercomparison of N2O and CH4, respectively. The measured N2O profiles (see Figure 4) exhibit very low values around 22 km pointing to strongly subsided air masses originating from the mesosphere [Wetzel et al., 2004]. Not only the minimum observed by MIPAS-B around 22 km but also the profile above and below is pretty well reproduced by ILAS-II. All measured differences are clearly within the combined error bars with a mean absolute deviation of roughly 17% (the percentage deviation is larger at low N2O values and vice versa). This example shows that ILAS-II and MIPAS-B actually sounded air masses with similar characteristics. The same holds also for CH4 (Figure 5) for altitudes below about 22 km. However, if we compare altitudes above this level, we find a significant low bias in the ILAS-II CH4 data.
 Measured N2O-CH4 correlations are presented in Figure 6. This correlation serves as self-consistency check and is not influenced by any degree of subsidence of air masses which have been sounded by different sensors. Both correlations, ILAS-II and MIPAS-B, are quite close to the standard relationships [Engel et al., 1996; Michelsen et al., 1998] which have been adjusted to the year 2003 taking into account mean increase rates of N2O and CH4. However, at high altitudes (CH4 less than 0.3 ppmv) ILAS-II CH4 values are too low such that the mesh points are off the expected standard correlation. At least parts of the problem can be caused by the temporal change of the ILAS-II spectrometer signal because the entrance slit is distorted by solar heat energy [Nakajima et al., 2006]. The magnitude of this effect caused by this signal change (not yet considered in the 1.4 algorithm) depends on tangent height, volume mixing ratio and species (T. Yokota et al., manuscript in preparation, 2006). A detailed discussion of ILAS-II data problems for CH4 less than 0.5 ppmv (and N2O less than 50 ppbv) is given by M. K. Ejiri et al. (Validation of the Improved Limb Atmospheric Spectrometer-II (ILAS-II) version 1.4 nitrous oxide and methane profiles, submitted to Journal of Geophysical Research, 2005).
4.4. Water Vapor and Hydrogen Budget
Figure 7 displays the H2O profiles observed by ILAS-II and MIPAS-B. The subsidence of the air masses can be seen in the balloon profile by the strong H2O volume mixing ratio gradient above the hygropause at 13 km. The shape of the profile measured by ILAS-II is similar to that observed by MIPAS-B but mixing ratios are significantly lower by more than 1 ppmv in average. Unreasonably low H2O values in the higher stratosphere are quite often seen in version 1.4 profiles.
 The oxidation of CH4 in the stratosphere yields about two molecules of H2O. The sum H = [H2O] + 2[CH4] is therefore a good measure for the hydrogen budget in the stratosphere. Figure 8 shows the hydrogen budget as measured by ILAS-II and MIPAS-B in comparison to formerly carried out observations [Engel et al., 1996; Herman et al., 2002]. Between about 13 and 27 km, the MIPAS-B hydrogen budget profile is quite close to these observations. The decreasing values above 27 km are mainly connected with the decreasing mixing ratios of H2O. High values of CH4 around 26 km and above (compare Figure 5) give a hint that this dry air originates from low latitudes where H2O is frozen out when penetrating the very cold tropopause. A minimum H2O stratospheric entry value can be estimated to a realistic value of 3.1 ppmv with the help of a H2O-CH4 relationship as measured by Stowasser et al.  inside the late winter arctic vortex assuming that methane oxidation produces nearly two H2O molecules from one CH4 molecule. This minimum entry value corresponds to the lowest hydrogen budget value at 29 km altitude. The ILAS-II profile exhibits a low bias to MIPAS-B of roughly 1.6ppmv in average which has propagated into the hydrogen budget from the distinct low bias in H2O in combination with a low bias in CH4 which occurs above about 22 km. In view of the unrealistic low values of the ILAS-II hydrogen budget, error bars of H2O (and the hydrogen budget) appear to be underestimated by the ILAS-II error calculation.
4.5. Nitrogen Species
 The intercomparison results for the reservoir species HNO3 are shown in Figure 9. The shape of the measured profiles of both sensors is very similar including the dip near 24 km altitude but ILAS-II measures slightly higher (about 0.6 ppbv) HNO3 volume mixing ratios compared to MIPAS-B. However, except the altitude region above 26 km the difference is close to the combined error limits.
 The comparison for the temporary reservoir species ClONO2 is illustrated in Figure 10. Again, the profile shape measured by MIPAS-B is reproduced fairly well by ILAS-II. The volume mixing ratio maxima are located at the same altitude with high values around 2.3 ppbv which are expected in the late arctic winter after passivation of active chlorine [Wetzel et al., 2002]. Some deviations appear especially at higher altitudes. Below 26 km differences are generally small and well within the combined errors. The mean absolute difference is less than 20% in average.
 The intercomparison of the short-lived nitrogen species NO2 is depicted in Figure 11. NO2 is in photochemical equilibrium with NO and is known to have a strong diurnal variation. Since ILAS-II measured during local sunset and MIPAS-B in the early night, the NO2 volume mixing ratios measured by the latter sensor are expected to be higher compared to the ones observed by the previous sensor. To balance these temporal variations the MIPAS-B results have been transferred to the time and location of the ILAS-II measurement with the help of the three-dimensional Chemistry Transport Model (CTM) KASIMA (Karlsruhe Simulation model of the Middle Atmosphere) [Kouker et al., 1999] which was run in a 2.0° × 2.0° horizontal resolution. The MIPAS-B NO2 values were corrected by scaling them with the altitude-dependent NO2 ratio determined from the modeled NO2 profiles for the MIPAS-B and ILAS-II measurement times and locations. However, this could not be done below 23 km because modeled values are close to zero in this altitude region because of a very strong denoxification in the CTM. In spite of this correction, there is still a significant low bias of roughly 1 ppbv visible in the ILAS-II profile compared to the transferred MIPAS-B data. Investigations of the influence of the photolysis rates used in the CTM on the NO2 distribution show only small differences on the modeled NO2 result and can therefore hardly help to explain the differences between ILAS-II and MIPAS-B measured NO2 quantities. Only about 4% mean absolute differences in NO2 above 22 km are obtained by using photolysis rates calculated online in KASIMA by Fast-J2 [Bian and Prather, 2002] instead of the KASIMA standard photolysis rates being stored in a lookup table [see Ruhnke et al., 1999].
 Another nitrogen species exhibiting a diurnal variation is N2O5. Since ILAS-II mixing ratios are negative over nearly the complete altitude region under comparison (not shown), a sound comparison to MIPAS-B results is not yet possible.
 The retrieved profiles of two important chlorine source gases, CFC-11 (CCl3F) and CFC-12 (CCl2F2) are depicted in Figures 12 and 13, respectively. The intercomparison of CFC-11 profiles shows that below 20 km the agreement between ILAS-II and MIPAS-B is pretty good and differences are clearly within the combined errors. The increase in CFC-11 as measured by ILAS-II above this altitude region is not seen in the shape of the inferred CFC-12 profile (see Figure 13) and appears to be unrealistic. This feature appears quite often in CFC-11 profiles measured by ILAS-II. Studies have shown that this feature will disappear when an appropriate signal correction of the distortion of the entrance slit by solar heat energy is applied. The CFC-12 profile exhibits the same shape as the one measured by MIPAS-B. However, ILAS-II values show a significant high bias of nearly 100 pptv compared to MIPAS-B.
 The intercomparison of ILAS-II target parameters to MIPAS-B measurements has shown that for the species O3, N2O, CH4 (below about 22 km), HNO3, ClONO2, and CFC-11, a predominantly good agreement between MIPAS-B and ILAS-II (data version 1.4) could be achieved. A summary of the assessment of the individual target parameters is given in Table 1. However, temperature, H2O, NO2, and N2O5 are widely characterized by low biases compared to MIPAS-B results while CFC-12 exhibits a high bias in comparison to the balloon-borne observations. These target parameters can therefore not be assumed to be validated at the present time.
Table 1. Quality of Agreement Between ILAS-II (Version 1.4) and MIPAS-B Target Parameters
 Future ILAS-II data releases are expected to have an improved accuracy in the line of sight determination since a new altitude registration method is being developed. Furthermore, the changing spectrometer signal due to the distortion of the entrance slit by solar heat energy can be taken into account during the retrieval process. These new data versions will again be compared to MIPAS-B data and may ameliorate the consistency of the measured data sets, especially for gases like CH4 and H2O.
 We are grateful to the CNES launching team for the excellent balloon operations, the Esrange team of SSC for logistical support and the FU Berlin for meteorological support.