The ACE-FTS instrument (Bernath et al., 2005) is a solar occultation, high-resolution (0.02 cm-1) spectrometer operating between 750 and 4400 cm-1. It was launched in August 2003 into a high-inclination orbit of 74∘ near an altitude of 650 km, and ACE-FTS has been providing volume mixing ratio (VMR) profiles of over 30 atmospheric trace gases and of over 20 isotopologue species since February 2004. During either sunset or sunrise, ACE-FTS makes a measurement approximately every 2 s between ∼ 5 and 150 km with a vertical sampling between ∼ 2 and 6 km, depending on the orbital geometry. The vertical extent of the instrument field of view is ∼ 3–4 km at the tangent point.

The trace species VMR retrieval, as described by Boone et al. (2005), is a nonlinear, least-squares, global-fitting technique that fits the observed spectra in given spectral microwindows (dependent on the retrieved species) to forward modelled spectra. Modelled spectra use line strengths and widths from HITRAN 2004 (Rothman et al., 2005) (with various updates, as detailed by Boone et al., 2013) and use the derived temperature and pressure profiles determined by fitting CO2 lines in the observed spectra. The main updates in v3.5 (compared to v2.2) are improved sets of microwindows for the majority of species, along with an increase in the number of interfering species in their retrievals; improved temperature/pressure retrievals resulting in a reduction of profiles exhibiting unrealistic temperature oscillations; and the inclusion of trace species COCl2, COClF, H2CO, CH3OH, and HCFC-141b and the exclusion of ClO.

The ACE-FTS v3.5 NO retrieval uses 39 microwindows between 1649.3 and 1977.6 cm-1. The main interfering species within the NO microwindows is O3, but spectral features of CO2 and H2O isotopologues and COF2 are also present. The retrieval has a lower altitude limit of 6 km and an upper altitude limit of 107 km. ACE-FTS v2.2 NO was validated by Kerzenmacher et al. (2008), and there were two known issues with the v2.2 results (still present in the v3.5 NO results). At altitudes below ∼ 20 km, NO VMRs suffer from a significant negative bias that causes many unphysical negative results. This is most likely due to strong diurnal variation along the line of sight that is not taken into account in the NO retrievals. Also, in polar winter around 35–50 km, where the NO VMR profile has a large vertical gradient, during times of increased downwelling, NO VMRs can exhibit large negative spikes. Kerzenmacher et al. (2008) found that, on average, ACE-FTS v2.2 NO agreed with coincident HALOE data on the order of 8 % within the altitude range of 22–64 km and exhibited a positive bias of ∼ 10 % from 93 to 110 km, and that the uncertainties were too large for statistically significant comparisons in the 64–93 km region.

The v3.5 NO2 retrieval uses 40 microwindows between 1204.4 and 2950.9 cm-1. The majority of microwindows added since v2.2 were chosen because of their information content with respect to the spectrally interfering isotopologues of CH4 and H2O. Between 7 and 20 km, CO2 and OCS also significantly interfere with the NO2 lines. The retrieval has a lower altitude limit of 7 km and an upper altitude limit of 52 km. ACE-FTS v2.2 NO2 was validated by Kerzenmacher et al. (2008), who concluded that ACE-FTS NO2 typically exhibited a ∼ 15 % low bias with coincident satellite data near the peak (∼ 35 km) and on average was within 20 % in the altitude range of approximately 20–40 km.

The v3.5 HNO3 retrieval uses 41 microwindows between 865.5 and 1977.6 cm-1. Interfering species include CCl2F2, H2O, CO2, OCS, and O3. The retrieval has a lower altitude limit of 5 km and an upper altitude limit of 62 km. ACE-FTS v2.2 HNO3 was validated by Wolff et al. (2008), who found that the ACE-FTS data and all coincident satellite data agreed to within 20 % in the altitude range of 18–35 km.

The v3.5 N2O5 retrieval, with altitude limits of 8 and 45 km, has only one spectral window, 30.0 cm-1 wide and centred at 1244.0 cm-1. Interfering species include O3 and isotopologues of H2O, CO2, CH4, and N2O. ACE-FTS v2.2 update N2O5 profiles (herein v2.2) were compared with MIPAS IMK-IAA N2O5 profiles by Wolff et al. (2008), who used climatological results from a chemical transport model to calculate diurnal scaling factors in order to match the local times of the two instruments. Without the use of diurnal scaling, Wolff et al. (2008) found that ACE-FTS v2.2 N2O5 typically exhibited a low bias on the order of 30–50 %, whereas with diurnal scaling ACE-FTS typically exhibited a ∼ 10–35 % low bias.

The v3.5 ClONO2 retrieval uses five microwindows between 780.2 and 2672.7 cm-1. Interfering species include N2O, CH4, O3, HNO3, and isotopologues of N2O, CO2, H2O, and CH4. The retrieval has a lower altitude limit of 10 km and an upper altitude limit of 41 km at high latitudes and 36 km near the equator. ACE-FTS v2.2 ClONO2 was compared to co-located MIPAS IMK-IAA data by Wolff et al. (2008), who used diurnal scaling factors to match the local times of the two instruments. With the use of diurnal scaling, Wolff et al. (2008) showed that ACE-FTS v2.2 and MIPAS IMK-IAA ClONO2 values typically differed by less than 1 % between 16 and 24 km. Above the peak (∼ 25 km), ACE-FTS exhibited a positive bias with respect to MIPAS of up to 20 % near 33 km.

It should be noted that ACE-FTS also derives VMR profiles of HNO4; however, because HNO4 does not contribute substantially to the overall NOy budget and due to a lack of multiple correlative satellite data sets with which to validate, it is not included in this study. All ACE-FTS data used in this study were screened for physically unrealistic outliers using the recommended quality flags version 1.1, as described by Sheese et al. (2015). Any profile known to be affected by instrument or processing errors (flag values of 7) or any profile containing a data point determined to be an extreme outlier (flag value in the range of 4–6) was excluded from the analysis.

The MAESTRO instrument (McElroy et al., 2007) consists of two spectrographs designed to cover the spectral range 210–1025 nm – with a 1.5–2.5 nm spectral resolution, which observe direct solar radiation occulted by the Earth's atmosphere. The MAESTRO solar occultation measurements are used to retrieve profiles of O3, NO2, H2O, aerosol extinction, and other various atmospheric properties. The instrument has been providing measurements since February 2004. The NO2 retrieval algorithm, described by McElroy et al. (2007), uses a two-step process. The spectral fitting of apparent optical depth spectra is used to derive slant column densities, assuming temperature-independent NO2 and O3 absorption cross sections from Burrows et al. (1998, 1999). Then an iterative Chahine inversion technique (Chahine, 1968) is used to retrieve NO2 VMR profiles from the slant column values. The spectral fitting algorithm is performed over a spectral range of 420–750 nm, and NO2 profiles are retrieved in an altitude range of ∼ 5–52 km, with a vertical resolution on the order of 1–2 km.

Version 1.2 of the NO2 data (used in this study) was validated by Kerzenmacher et al. (2008), who found that between 25 and 40 km, when comparing to correlative satellite measurements, diurnally scaled MAESTRO NO2 tends to exhibit a bias within -20 and +10 %. In the same altitude region, scaled MAESTRO NO2 also tends to exhibit a high bias of 0–50 % when compared to correlative ground- and balloon-based measurements. The poorer comparison with ground-based instruments was attributed to not accounting for diurnal variations along the MAESTRO line of sight in the NO2 retrieval algorithm. It should be noted that this issue would similarly affect ACE-FTS NO2 retrievals.

The ACE-FTS outlier detection method described by Sheese et al. (2015) was used to detect physically unrealistic outliers in the MAESTRO NO2 data set. Any profile that was found to contain such an outlier was rejected prior to any comparisons. This method was ineffective at removing many of the outliers below 19 km. Therefore at altitude levels below 19 km, NO2 VMR values greater than 3 ppb were screened out. At all altitude levels, any values with a corresponding fractional error of 1 or greater were also removed. Only data between February 2004 and September 2010 were used in the analysis.

The GOMOS instrument (Kyrölä et al., 2004) on the Envisat satellite employed a grating spectrometer that observed the attenuation of stellar emission, from the ultraviolet (UV) to the near-infrared, through the limb of the Earth's atmosphere. The stellar occultation technique was employed to retrieve vertical profiles of nighttime O3, NO2, NO3, H2O, OClO, BrO, O2, and aerosol extinction nominally between altitudes of 5 and 150 km, using three different bands within the spectral range of 248–954 nm. GOMOS was capable of obtaining hundreds of occultations each day with a vertical sampling typically between 0.4 and 1.7 km. GOMOS measurements span from March 2002 to April 2012.

The NO2 retrieval algorithm is described by Kyrölä et al. (2010) and makes use of a Tikhonov-type regularization (Tikhonov, 1963), which leads to a retrieval vertical resolution of 4 km. Version 6 of the GOMOS NO2 data set is used in this study. Version 5 of the NO2 retrievals was validated by Verronen et al. (2009), who compared the GOMOS profiles to nighttime MIPAS ESA NO2 data (described below). It was found that in the low to midlatitudes, between approximately 25 and 60 km, GOMOS NO2 tended to exhibit a positive bias with respect to MIPAS on the order of 0–25 %. In the high latitudes, the two data sets agreed within 35 % at altitudes above ∼ 45 km where nighttime NO2 VMR was at a maximum. However, at lower altitudes (in the high latitude regions) the bias reached up to 65 %, which was greater than the combined systematic errors. Since the ACE-FTS NO2 profiles only extend up to 52 km, GOMOS comparisons have been limited to between 60∘ S and 60∘ N.

Only GOMOS profiles where the local solar zenith angle is greater than 97∘ at altitudes below 50 km and greater than 110∘ at altitudes below 100 km were used in the analysis. In order to eliminate the presence of extreme outliers, any GOMOS NO2 profile that contained an absolute VMR value greater than 0.5 ppm in the altitude range of 0–52 km was also rejected; in the limited latitude region this rejected less than 1 % of the GOMOS profiles.

The MIPAS instrument (Fischer and Oelhaf, 1996; Fischer et al., 2008) on the Envisat satellite was a limb-viewing Fourier transform spectrometer that observed atmospheric emissions. The spectrometer had five spectral bands in the range of 685–2410 cm-1 and scanned the Earth's limb between altitudes from approximately 6 to 70 km in nominal mode and up to 170 km in special modes. The MIPAS vertical field of view was 3 km and the instrument had a vertical sampling that ranged from 1.5 to 5 km, depending on the altitude. Prior to 2005, MIPAS operated at its full spectral resolution of 0.025 cm-1, with a sampling time of 4.5 s. In 2004, an anomaly occurred in the interferometer mirror slide mechanism and it was determined that the spectral resolution needed to be downgraded to 0.0625 cm-1 with a consequent reduction of the sampling time to 1.8 s, exploited to allow for a finer vertical sampling. In order to avoid any discontinuities that may arise from switching the observation mode, only MIPAS measurements from the period of January 2005 to April 2012 were used in this study.

Two different MIPAS level 2 products, based on two different retrieval algorithms, were used in this study – the first is from the ESA and the second is the result of a collaboration between the Institute of Meteorology and Climate Research at the Karlsruhe Institute of Technology and the Instituto de Astrofísica de Andalucía (IMK-IAA). The ESA algorithm that produces version 6 of the level 2 retrievals (used in this study) is described by Raspollini et al. (2013). It is a least-squares, global-fitting technique, using the regularized Levenberg–Marquardt method (Hanke, 1997), which fits spectra in species-dependent microwindows to a forward model. A parameter setting has been chosen that leaves results largely independent from the initial guess profiles. The forward model assumes horizontal homogeneity and local thermodynamic equilibrium at all altitudes. An a posteriori regularization, using a self-adapting regularization constraint, is then applied to the retrieved profile (Ceccherini, 2005; Cecccherini et al., 2007).

The IMK-IAA algorithm is described by von Clarmann et al. (2009) and Funke et al. (2014), and the most recent version of the level 2 data (used in this study) is version 5. The IMK-IAA algorithm uses an iterative variant of Tikhonov regularization (Tikhonov, 1963) on species-dependent sets of microwindows. This inversion technique is implemented to constrain the shape of the resulting profile without pushing the values towards an a priori profile. The retrieval is performed on a 1 km grid, and the altitude-dependent strength of the smoothing constraint was chosen in order to optimize vertical resolution in the upper troposphere to lower mesosphere while still minimizing artificial oscillations in the retrieved profile. The NO and NO2 retrievals are performed in log(VMR) space, and the forward model allows for horizontal variation in temperature. In the forward model, NO and NO2 line-of-sight variations are considered and a line-of-sight NOx gradient is retrieved concurrently. Further, the forward model can allow for deviations from local thermodynamic equilibrium (LTE), which mainly affects mesospheric retrievals, and LTE is assumed for all NOy species except NO and NO2. The NOy microwindows are chosen, in part, in order to reduce non-LTE effects.

MIPAS IMK-IAA NOx retrievals (only in the original resolution mode) were compared to HALOE measurements by Funke et al. (2005b). It was found that the two NOx data sets typically agreed within 20 % between 25 and 50 km. Wetzel et al. (2007) found that, in the mid-stratospheric MIPAS ESA version 4.6 NO2, diurnally scaled using data from a 1-D photochemical model agreed best with balloon-borne measurements, with biases typically better than 10 %. In similar comparisons with correlative satellite-based solar occultation measurements, the MIPAS ESA profiles typically agreed within 10–30 %.

Wang et al. (2007a, b) assessed the quality of the MIPAS IMK-IAA version 3 and MIPAS ESA version 4.6 HNO3 data sets, respectively. Comparing MIPAS ESA HNO3 with correlative data sets from ground-based and balloon-borne instruments, both Wang et al. (2007a, b) studies determined that relative differences were typically better than 10 %. In their comparisons with ACE-FTS v2.2 HNO3, relative differences in the lower to mid-stratosphere were on the order of 5–15 %.

MIPAS IMK-IAA ClONO2 profiles were validated by Höpfner et al. (2007), who showed that the MIPAS data set agreed well with correlative balloon and airborne data sets, typically to better than 10 %. Höpfner et al. (2007) also compared the MIPAS IMK-IAA profiles to ACE-FTS v2.2 ClONO2 using diurnal correction factors obtained from a chemical transport model. The diurnally corrected MIPAS data and ACE-FTS typically agreed within 10 % at altitudes between 15 and 27 km. However, above 27 km, the ACE-FTS exhibited a ∼ 20 % low bias with the diurnally corrected MIPAS data and a ∼ 20 % high bias with the uncorrected data.

Neither the MIPAS ESA nor IMK-IAA N2O5 data set has been the focus of a MIPAS validation study, but MIPAS ESA N2O5 and ClONO2 data were compared with the balloon-based MIPAS-B instrument by Wetzel et al. (2013). It was found that N2O5 concentrations typically agree within ±40 % and ClONO2 concentrations typically within ±30 %. Also, the IMK-IAA N2O5 data set was used in the ACE-FTS v2.2 N2O5 validation study of Wolff et al. (2008), the results of which are summarized in Sect. 2.1.1.

All MIPAS vertical resolutions, listed in Table 1, were calculated as the full-width, half-maximum of the retrieval averaging kernels. MIPAS IMK-IAA data were used only where the corresponding averaging kernel diagonal values were greater than 0.03.

The SCIAMACHY instrument (Burrows et al., 1995; Bovensmann et al., 1999) was an eight-channel grating spectrometer that observed the Earth's atmosphere in the wide spectral range of 240–2400 nm, using three different viewing geometries – limb viewing of scattered sunlight, solar occultation, and nadir viewing. The NO2 data used in this study are the profiles retrieved from limb-viewing observations in the channel that observed in the spectral window of 394–620 nm (spectral channel 3). The instrument scanned the Earth's limb from the surface up to 100 km with a 2.5 km vertical field of view and a ∼ 3 km vertical sampling. The NO2 retrieval algorithm, detailed by Rozanov et al. (2005) and summarized by Bauer et al. (2012), uses limb-scattered radiances measured from 420 to 470 nm and solves the inverse problem using the DOAS technique and Tikhonov regularization (Tikhonov, 1963). In each profile, the spectra are normalized by the limb radiances nearest 43 km. The regularization matrix smooths the retrievals using an empirically determined height-dependent smoothing parameter, chosen in order to minimize physically unrealistic oscillations in profiles while maximizing vertical resolution. The retrieval makes use of a forward model that takes into account absorption by O3 (simultaneously retrieved) and O2–O2 and uses pressure and temperature profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF). The NO2 and O3 absorption cross sections were obtained from Bogumil et al. (1999). The algorithm retrieves NO2 profiles between 10 and 40 km with a typical vertical resolution of 3–5 km, degrading to ∼ 10 km at the upper and lower retrieval altitude limits.

This study used v3.1 of the SCIAMACHY level 2 NO2 profiles, which was validated by Bauer et al. (2012). The NO2 profiles were compared to correlative satellite measurements that were diurnally scaled to the SCIAMACHY local times. It was found that in the altitude range of 25–35 km SCIAMACHY NO2 tends to exhibit a 2 % low bias with respect to HALOE v19 profiles and tends to exhibit a 5 % high bias with respect to ACE-FTS v2.2 profiles.

Only SCIAMACHY data below 40 km with a retrieval response greater than 0.8 were used in the analysis.

The HALOE instrument (Russell et al., 1993), on the UARS, was a solar occultation instrument that provided observations of the Earth's limb between October 1991 and November 2005. The UARS precessing orbit allowed for HALOE measurements to observe all latitudes between 80∘ S and 80∘ N approximately every 36 days. Profiles of O3, HCl, HF, CH4, H2O, NO, NO2 concentrations, temperature, and aerosols were derived from observations within four radiometric channels and four radiometric/gas-filter correlation channels.

The HALOE NO measurements use a gas-filter correlation method with a spectral filter band pass near 1900 cm-1 and are virtually insensitive to interfering absorbers. The NO2 measurements are made using a broadband radiometric channel centred near 1600 cm-1 and the effects of interfering species O2, H2O, and CH4 are accounted for in the retrieval. The interfering species N2O is not accounted for, although the effect on NO2 is very small. Retrievals of NO and NO2 profiles use a modified onion peel approach and account for aerosol extinction and interfering attenuation. The NO retrievals have a vertical resolution of 4 km at altitudes below ∼ 60 km (degrading to 7 km at higher altitudes), and the NO2 retrievals have a vertical resolution of 2 km. The HALOE version 17 NO and NO2 data were validated by Gordley et al. (1996). This study uses HALOE version 19 NO and NO2, which have very small differences relative to v17 (James M. Russell III, Hampton University, personal communication, December 2015). Gordley et al. (1996) found that above 25 km HALOE v17 NO tended to agree with correlative satellite and balloon-based measurements within 15 %, but with a maximum low bias reaching 35 %. Also, above 25 km HALOE v17 NO2 agreed with correlative satellite, balloon, and ground-based measurements to within 15 %.

The POAM III instrument (Lucke et al., 1999) was a nine-channel photometer that viewed the Earth's limb in solar occultation. POAM III, on board the Satellite Pour l'Observation de la Terre (SPOT) 4 satellite, was launched in March 1998 into a sun-synchronous orbit with a descending node of 10:30 LT, at an altitude of ∼ 830 km and a 98.7∘ inclination. Designed to measure atmospheric profiles of O3, H2O, NO2, and aerosol extinction, POAM III observed the limb at tangent heights between cloud-top and 60 km in nine different narrow passbands in the near-UV to near-infrared spectral region, with a total spectral range from 354 to 1018 nm. POAM III started taking measurements in April 1998, and measurements stopped in December 2005 due to instrument failure.

NO2 profiles were retrieved between 20 and 45 km from differential measurements in the 439.6 nm (NO2 “on”) and 442.2 nm (NO2 “off”) channels, both with a full-width, half-maximum passband of 2.1 nm. The vertical resolution of retrieved NO2 was ∼ 1.5 km from 25 to 35 km, increasing to nearly 3 km at 20 km and > 7 km at 45 km. The retrieval algorithm is described in detail by Lumpe et al. (2002). The algorithm inverts slant column densities to vertical profiles using the Newtonian optimal estimation technique (Rodgers, 2008) for all target species. The forward model assumes horizontal homogeneity. Randall et al. (2002) validated POAM III version 3.0 NO2 measurements through comparisons with data from multiple instruments. They found no evidence for any systematic bias below 35 km; e.g. differences with respect to HALOE were within approximately ±0.2 ppbv (∼ 6 %). Relative to HALOE, POAM III NO2 mixing ratios were shown to be higher by up to 0.7 ppbv (∼ 17 %) from 35 to 42 km; about 5 % of that bias was attributed to an error in HALOE retrievals, but no explanation for the remaining 12 % was identified. Although the version 4 NO2 data (used in this study) have not been the focus of a validation study, it was used by Kerzenmacher et al. (2008) in comparison with ACE NO2. It was shown that above 25 km, POAM III typically agreed within ±6 % with respect to ACE-FTS v2.2 and within ±8 % with respect to MAESTRO v1.2.

The SAGE III instrument (SAGE III ATBD Team, 2002a) was a solar and lunar occultation atmospheric sounder on board the Russian Meteor 3M satellite, which was launched in December 2001 and was operational until March 2006. Meteor 3M was launched into a 1020 km altitude, sun-synchronous orbit with a descending node of ∼ 09:00 LT. In solar occultation mode, SAGE III was designed to retrieve vertical profiles of O3, NO2, H2O, and aerosol extinction (plus NO3 and OClO in lunar mode) throughout the stratosphere from observations in the near-UV to near-infrared spectral region. The instrument consisted of a grating spectrometer that observed in the spectral range of 280–1040 nm and an InGaAs infrared detector that observed in a band pass between 1530 and 1560 nm.

The SAGE III NO2 retrieval algorithm is detailed by SAGE III ATBD Team (2002b). The algorithm first uses a multiple linear regression technique to derive slant column densities for both O3 and NO2 simultaneously from calculated slant column optical depths. The O3 and NO2 region wavelength-dependent optical depths are derived from observations in two spectral channels spanning 433–450 and 563–622 nm. The NO2 column densities are inverted into vertical density profiles (on a 0.5 km grid between 0 and 100 km with a vertical resolution of 1–2 km) using a modified Chahine technique (Chahine, 1968), assuming horizontal homogeneity.

There has not yet been a rigorous SAGE III NO2 validation study. Kar et al. (2007) found that SAGE III NO2 version 3 data (used in this study) typically exhibited a high bias (within ∼ 10–15 %) above 25 km with respect to v1.2 MAESTRO data. Similarly, Kerzenmacher et al. (2008) found that the SAGE III v3 data also tended to exhibit a high bias (typically within ∼ 10 %) with respect to v2.2 ACE-FTS data. These results are consistent with Polyakov et al. (2005), who reported that their SAGE III NO2 product, derived using the Newtonian iterative optimal estimation technique, was systematically lower than the SAGE III operational product.

The optical spectrograph of the OSIRIS instrument operates in the spectral range of 280–810 nm, with ∼ 1 nm spectral resolution, and observes Rayleigh and Mie scattered sunlight in the Earth's limb between altitudes of ∼ 7 and 110 km with a vertical field of view of approximately 1 km. The NO2 retrievals, described by Haley and Brohede (2007), use the DOAS technique to calculate NO2 slant columns. These are calculated in the spectral window of 435–451 nm and between altitudes of 10 and 46 km, with the OSIRIS 46–60 km averaged radiances as the reference spectrum. The slant columns are then inverted into density profiles using the optimal estimation technique (Rodgers, 2008), using LIMBTRAN (Griffioen and Oikarinen, 2000) for the forward model. The NO2 retrievals have a vertical resolution of approximately 2 km at all altitudes.

Version 3 of the data set (used in this study) was validated by Brohede et al. (2007a), who found that OSIRIS NO2 typically agrees with correlative satellite and balloon-borne data sets within 20 % between 25 and 35 km for all seasons and latitudes. Between 35 and 45 km, the agreement was within 30 %, with smaller absolute systematic differences for comparisons in the high latitudes than for those nearer the equator.

SMR observes thermal emission in the Earth's limb using four tunable receivers in the spectral range of 486–581 GHz and a millimetre-wave receiver near 119 GHz. The HNO3 profile retrieval algorithm (Urban et al., 2005) uses observations in a 1 GHz band centred at 544.6 GHz and is based on the Newtonian Levenberg–Marquardt optimal estimation technique (Rodgers, 2008). The forward model used is that of the MOLIERE-5 forward/inversion model (Urban et al., 2004). HNO3 is retrieved at altitudes above 18 km, with vertical resolutions on the order of 2–3 km. As discussed by Urban et al. (2009), the SMR HNO3 data exhibit a ∼ 1–1.5 km vertical bias. Therefore, in this study the version 2.1 HNO3 data were offset upwards by 1.5 km prior to any analysis.

Urban et al. (2009) showed that the SMR HNO3 climatology exhibits reasonably good agreement with UARS/MLS climatology from measurements taken between 1991 and 1998. Wolff et al. (2008) showed that SMR HNO3 profiles exhibit a ∼ 20 % high bias with respect to ACE-FTS v2.2 HNO3 at altitudes below 30 km, and exhibit systematic differences within ±20 % between 30 and 35 km.

Only profiles that had retrieval response values greater than 0.75 were used in the analysis. Due to a level 2 processing error that affected SMR data for May 2009 and onwards, only SMR data before May 2009 were used in this study.

Note that only HALOE and MIPAS IMK-IAA are being compared with ACE-FTS (MIPAS ESA does not have an NO data product). Figure 3 shows the mean NO VMR profiles for coincident ACE-FTS and HALOE profiles and coincident ACE-FTS and MIPAS IMK-IAA profiles. Since ACE-FTS and HALOE are both solar occultation instruments and only overlapped between 2004 and 2005, there are not many coincident measurements. As such, the spatial coincidence criterion was kept somewhat lax, within 500 km, in order to ensure a statistically significant number of coincidences. It was found that a temporal coincidence criterion of within 3 h also led to a statistically significant number of coincidences (47 profiles). Comparisons with a less stringent criterion (greater than 3 h) led to a larger number of coincidences but significantly reduced the correlation and increased the standard deviation of the relative differences between the data sets. Using a tighter spatial criterion, e.g. within 350 km, also yields a significant number of coincidences but does not significantly improve the comparison results.

At all altitudes, with any temporal coincidence criterion, it was found that using the diurnal scaling factors did not greatly improve the HALOE comparison results. This is likely due to the fact that both ACE-FTS and HALOE are solar occultation instruments, and hence measurements at a common geographic location do not differ greatly in local time. Figure 4a shows the ACE-FTS and HALOE NO comparison results, with and without diurnal scaling. The two data sets are only strongly correlated in the altitude region of approximately 25–55 km. In this region, the relative difference shows that ACE-FTS NO tends to exhibit a low bias of less than 10 %, with standard deviations on the order of 10 % with respect to HALOE. Of the 47 coincident profiles 41 are local sunset occultations, and the remaining 6 are local sunrise occultations. Due to the lack of sunrise measurements, it was not possible to determine whether or not there is a significant bias between the sunrise and sunset (or similarly local morning and local evening) NO profiles.

It was found that the temporal and spatial coincidence criteria that optimized the comparison results for diurnally scaled ACE-FTS and MIPAS IMK-IAA NO profiles were within 3 h and within 100 km. Similar to the HALOE comparisons, using the diurnal scaling factors did not greatly improve the comparison results at most altitudes. However, for temporal differences larger than 3 h, using the diurnal scaling factors worsened comparison results at all altitudes.

Figure 4b shows the ACE-FTS and MIPAS IMK-IAA NO comparison results with and without diurnal scaling, using coincidence criteria of within 3 h and 100 km. Throughout the middle stratosphere, the diurnal scaling generally increased the correlation coefficients by ∼ 0.05 and lowered the standard deviations by ∼ 3 %. Relatively strong correlation is seen above 25 km, where ACE-FTS exhibits a negative bias within -10 and -22 % between 25 and 35 km and an approximate -5 % bias between 40 and 50 km. The lowest standard deviations are observed in the 30–50 km region, on the order of 35–50 %. The higher standard deviations (relative to comparisons with HALOE, Fig. 4a) reflect the higher variance within the MIPAS IMK-IAA NO data set. Below 25 km, the relative differences get more negative with decreasing altitude – more negative than -100 % below 21 km. An ACE-FTS NO low bias with respect to non-solar occultation instruments, on the order of ∼ 10–40 %, is expected in this region due to not accounting for diurnal variations of NO along the line of sight (Brohede et al., 2007a).

Figure 5a shows NO comparison results for data separated by local time using all available MIPAS IMK-IAA data. It can be seen that there is an apparent significant local time bias in the ACE-FTS-MIPAS IMK-IAA comparison results. Between 19 and 52 km, the correlation coefficients are better for local evening (PM) comparisons than for local morning (AM) comparisons by up to 0.4, and at all altitudes the evening comparisons exhibit lower standard deviations by ∼ 15–50 %. This leads to improved relative differences when only using the evening data in the 25–34 and 52–60 km ranges. However, due to the orbital geometries and the MIPAS IMK-IAA retrieval sensitivity to NO, the only coincident PM data are during November–January in the Southern Hemisphere (SH) and May–July in the Northern Hemisphere (NH), hereafter referred to as “summer” months. Figure 5b shows NO comparison results between ACE-FTS and MIPAS IMK-IAA for data separated by local time and using only the summer months (both NH and SH). It can be seen that correcting for this seasonal bias greatly improves the AM comparison results, as there is less NO variation in the polar summer regions than in the winter. At most altitudes the summer PM comparisons still tend to exhibit better correlation than the AM, but the summer AM and PM standard deviation profiles are rather similar – values of ∼ 100 % near 18 km, then decreasing with altitude to ∼ 15–20 % near 45 km, and from there increasing with altitude. Between 22 and 52 km, the summer AM and PM relative difference profiles are also quite similar. ACE-FTS exhibits a negative bias with respect to MIPAS IMK-IAA of approximately -100 to -10 % between 22 and 27 km. Above 27 km, up to ∼ 50 km, ACE-FTS NO is typically systematically lower than MIPAS IMK-IAA by 0–10 %. Above 52 km, the summer PM results (correlation coefficients and standard deviations) are typically better than the AM; the PM relative differences are between 0 and +7 %, and the AM relative differences decrease with altitude from 0 to -32 % between 53 and 60 km.

From Fig. 6 it is apparent that in the comparisons with all other NO2 data sets, the diurnally scaled ACE-FTS profiles have a low bias near the NO2 peak, ∼ 33 km. It can be seen in Fig. 7a that using coincidence criteria of within 350 km and within 4 h without any diurnal scaling leads to relatively poor agreement between ACE-FTS and many instruments in the middle to upper stratosphere. Near the NO2 peak, without diurnal scaling mean relative differences between ACE-FTS and INST data range from -38 to +2 %, with standard deviations that reach up to ∼ 50 %. With diurnal scaling (Fig. 7b), near the NO2 peak the ACE-FTS low bias is on the order of -20 to -5 %, with standard deviations on the order of 7–35 %. Between 30 and 40 km, the weighted-average mean relative differences are on the order of -10 %, with weighted-average standard deviations within 18–43 %. Within these altitudes, most comparisons typically yielded correlation coefficients that were greater than 0.8, the exception being GOMOS which measures at nighttime. The weighted-average correlation coefficients are better than 0.8 between 15 and 40 km and better than 0.9 between 17 and 35 km.

Below 25 km, an ACE-FTS NO2 positive bias is expected with respect to instruments that do not use the solar occultation viewing geometry due to not accounting for diurnal variations in NO2 along the line of sight in the forward model. In solar occultation viewing geometry, not accounting for this diurnal effect is expected to lead to a ∼ 10–40 % positive bias (Brohede et al., 2007a). It can be seen from Fig. 7b that, below 25 km, ACE-FTS does have a positive bias on the order of 5–40 % with respect to MIPAS IMK-IAA, OSIRIS, and SCIAMACHY. As well, below 22 km, ACE-FTS exhibits a positive bias with respect to HALOE, which is a solar occultation instrument but accounts for the diurnal effect in the NO2 retrieval algorithm.

Diurnal scaling has less of an effect on comparisons with the solar occultation instruments (HALOE, POAM III, SAGE III) than on those with other viewing geometries, as there is less of a difference in measurement local times, and diurnal scaling has no effect on the ACE-FTS comparisons with MAESTRO as measurements are co-located (although they do have differing vertical and horizontal resolutions). In order to determine biases in the comparisons due to local time or hemispheric coverage, comparisons were made in the 20–40 km region where the NO2 peak is well sampled and the majority of instruments have sufficient coverage. For local time differences, GOMOS data have been excluded, as it only contains local evening data, and the solar occultation instruments have been excluded as they tend to only have a significant number of coincidences in either local morning or local evening. For hemispheric differences only HALOE data were excluded, as the vast majority of HALOE data are from the NH.

As can be seen in Fig. 8, at all altitudes within the 20–40 km range, the weighted-average results are generally better for the evening comparisons than for the morning comparisons. The weighted-average standard deviations are better by up to 18 % and the correlation coefficients are better by ∼ 0.05 in the evening comparisons. The evening average relative differences are ∼ 0 % near 20 km, reach -11 % near 35 km, and 6 % near 40 km. Whereas for morning results, compared to evening results, average relative differences are more negative above 35 km (reaches -13 %) and more positive below 30 km (up to +40 %). The better evening results are likely due to differences in the diurnal variation along the line of sight between sunrise and sunset observations. For sunrise (local morning) observations, ACE-FTS samples a region of the atmosphere that has yet to be sunlit long enough for NO2 to be in equilibrium. For sunset (local evening), however, the entire sampled area should be relatively stable. As can be seen in Fig. 9, there were no major differences in the weighted-average results between the NH comparisons and the SH comparisons. The only significant difference in the weighted-average relative differences is below 25 km, with the SH exhibiting larger values by up to 7 %.

Due to the relatively weak diurnal variation of HNO3 in the stratosphere, using the photochemical box model did not improve the HNO3 comparison results at any altitude level. In addition, a lax temporal coincidence criterion of within 6 h was used, as tightening the criterion did not significantly improve comparison results. As such, it was possible to use a spatial coincidence criterion of within 100 km, which optimized the comparison results.

Figure 10 shows the mean coincident ACE-FTS and INST HNO3 profiles along with the 1σ measurement variation. There is typically good agreement between ACE-FTS and the other instruments, and HNO3 comparison results are shown in Fig. 11. Near the HNO3 peak, ∼ 20–25 km, there is excellent agreement, with weighted-average relative differences within -1 %, correlation coefficients of ∼ 0.97, and standard deviations of ∼ 8 %.

The weighted-average correlation coefficients are greater than 0.5 for altitudes of 7–40 km and greater than 0.9 for altitudes of 12–31 km. Between 9 and 38 km the weighted-average standard deviations are below 50 %, reaching a minimum of 7 % near 24 km. The weighted-average relative differences are within ±6 % between 9 and 29 km. Above 30 km, the average relative differences increase with altitude to 37 % at 40 km; however, at that altitude only the MIPAS IMK-IAA comparisons exhibit standard deviations below 50 %, and the ACE-FTS-MIPAS IMK-IAA relative difference at 40 km is on the order of 20 %. Below 30 km, the ACE-FTS-MIPAS ESA relative differences are within ±10 %, and the ACE-FTS-MLS differences are typically on the order of -5 to 10 %. Between 20 and 38 km ACE-FTS typically exhibits a high bias with respect to SMILES, which is on the order of 1 % near 20 km and increases to 55 % near 34 km. With respect to SMR, ACE-FTS exhibits a negative bias on the order of -9 to -13 % between 25 and 30 km. This is an improvement from the ∼ 20 % low bias exhibited in the ACE-FTS v2.2 and SMR v2.0 comparisons reported by Wolff et al. (2008). Mean relative differences between ACE-FTS and SMR are also within ±10 % between 30 and 35 km.

There were no major local time biases found in the HNO3 comparisons. In the altitude range 12–28 km, weighted-average mean relative differences were within ±4 % for the local morning comparisons, whereas local evening comparisons yielded weighted-average mean relative differences within ±7 % (not shown). Between 16 and 38 km, there was no significant hemispheric bias found in the HNO3 comparisons, with SMILES data excluded (due to asymmetric hemispheric coverage).

Before showing the N2O5 validation results, it should be noted that a significant difference was found between local morning and local evening MIPAS (both ESA and IMK-IAA) N2O5 comparisons with ACE-FTS: the evening comparisons exhibited much worse agreement than the morning comparisons. Figure 12a shows results for comparisons between local evening diurnally scaled ACE-FTS and MIPAS profiles using coincidence criteria of within 3 h and within 100 km. Near 20–25 km, the relative differences are on the order of ±10 % with standard deviations of ∼ 50–80 % and correlation coefficients of ∼ 0.65–0.75. However, outside of this region, comparison results yield poorer results, with weak correlation, standard deviations greater than 100 %, and relative differences beyond ±100 %. In order to highlight that this poor agreement is not an issue with differences due to diurnal variation, Fig. 12b shows comparisons using non-scaled ACE-FTS profiles and with a much tighter temporal coincidence of within 20 min (and within 200 km). In comparing to both MIPAS data products in this case, there are large systematic differences from ACE-FTS. The MIPAS ESA differences range from approximately -60 to 200 % and the MIPAS IMK-IAA differences range from approximately -130 to 200 %. The poor agreement in the evening is mostly due to the low signal-to-noise ratio in the ACE-FTS measurements due to the lower N2O5 concentrations at sunset than at sunrise.

Figures 13 and 14 show the results of the morning comparisons. At coincidence criteria of within 3 h and 100 km, with diurnal scaling, ACE-FTS and MIPAS tend to agree best in the altitude range of 22–34 km. In this region, weighted-average correlation coefficients are better than 0.8, weighted-average standard deviations are between 16 and 40 %, and weighted-average mean relative differences are typically better than -7 %. Above 34 km, ACE-FTS exhibits a positive bias that is within 10 % up to 38 km and increases with altitude, up to 33 % at 43 km. This positive bias in the upper altitudes is not reduced when tighter temporal coincidence criteria are chosen (down to within 20 min) and exists both with and without diurnal scaling. Also shown in Fig. 14 are the weighted-average comparison results for non-scaled ACE-FTS profiles. It can be seen that using the photochemical box model does improve the comparison results, especially in the 23–38 km region, where it leads to an improvement to the average standard deviations on the order of 5 %. Diurnal scaling also reduces the positive bias above 33 km by up to 16 %.

Although there was very poor agreement between local evening ACE-FTS and local evening MIPAS N2O5 profiles, comparisons between diurnally scaled morning ACE-FTS and evening MIPAS N2O5 profiles yield much better agreement. This indicates that the poor agreement seen in the evening data is most likely due to the high level of noise in the evening ACE-FTS N2O5 data and is unlikely an issue with the MIPAS data. Figure 15 shows the weighted-average results (scaled) from Fig. 14, along with comparison results between diurnally scaled morning ACE-FTS and evening MIPAS (both ESA and IMK-IAA) profiles using coincidence criteria of within 12 h and within 100 km. Between 22 and 37 km, the morning/evening weighted-average correlation coefficients are greater than 0.8 and the standard deviations are less than 50 %. In this altitude range, the weighted-average relative differences are better than 10 %.

Figures 16 and 17 show the results of the ACE-FTS and MIPAS, both ESA and IMK-IAA, comparisons. Comparisons were found to be optimized at coincidence criteria of within 4 h and 100 km. With diurnal scaling, ACE-FTS and MIPAS tend to agree best in the altitude range of 17–34 km. In this region, weighted-average correlation coefficients are better than 0.7, weighted-average standard deviations are between 13 and 32 %, and weighted-average mean relative differences tend to exhibit a negative bias within -1 and -10 %, except at the lower altitudes where the low bias reaches -20 % near 17 km. Also shown in Fig. 17 are the weighted-average comparison results for non-scaled ACE-FTS profiles. It can be seen that using the photochemical box model does improve the comparison results, especially above 26 km, where it leads to an improvement to the average correlation coefficients by up to 0.15 and to the average standard deviations by up to 4 %.

Similar to the case for the MIPAS IMK-IAA NO comparisons in Sect. 4.2.1, separating the coincident MIPAS ClONO2 data into morning and evening subsets seasonally biases the data. Due to the orbital geometries and the MIPAS retrievals' sensitivity to ClONO2, there is typically only coincident evening data between February and April in the NH and August and October in the SH (henceforth referred to as “spring” months). In examining the differences between spring morning and evening comparison results, shown in Fig. 18, between 17 and 36 km there are no major differences in the weighted-average relative difference profiles. In the 13–23 km region, where the comparison results are more consistent for the evening results, both the morning and evening results tend to exhibit a -10 % bias. Above 25 km, the comparison results are typically better for the morning results. Between 25 and 33 km the morning relative differences are typically between -5 and 0 % and evening relative differences are typically between -1 and 4 %. Near 36 km, both the morning and evening values are ∼ 10 % and then increase with altitude, up to 74 % at 40 km for morning data and 130 % for evening data.