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Validation of the Atmospheric Chemistry Experiment by noncoincident MkIV balloon profiles

2011; American Geophysical Union; Volume: 116; Issue: D6 Linguagem: Inglês

10.1029/2010jd014928

2156-2202

Voltaire A. Velazco, Geoffrey C. Toon, Jean-François L. Blavier, A. Kleinböhl, G. L. Manney, W. H. Daffer, P. F. Bernath, K. A. Walker, C. D. Boone,

Atmospheric chemistry and aerosols

Journal of Geophysical Research: AtmospheresVolume 116, Issue D6 Composition and ChemistryFree Access Validation of the Atmospheric Chemistry Experiment by noncoincident MkIV balloon profiles Voltaire A. Velazco, Voltaire A. Velazco [email protected] Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Now at Institute of Environmental Physics, University of Bremen, Bremen, Germany.Search for more papers by this authorGeoffrey C. Toon, Geoffrey C. Toon Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorJean-Francois L. Blavier, Jean-Francois L. Blavier Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorArmin Kleinböhl, Armin Kleinböhl Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorGloria L. Manney, Gloria L. Manney Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorWilliam H. Daffer, William H. Daffer Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorPeter F. Bernath, Peter F. Bernath Department of Chemistry, University of York, York, UKSearch for more papers by this authorKaley A. Walker, Kaley A. Walker Department of Physics, University of Toronto, Toronto, Ontario, CanadaSearch for more papers by this authorChris Boone, Chris Boone Department of Chemistry, University of Waterloo, Waterloo, Ontario, CanadaSearch for more papers by this author Voltaire A. Velazco, Voltaire A. Velazco [email protected] Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Now at Institute of Environmental Physics, University of Bremen, Bremen, Germany.Search for more papers by this authorGeoffrey C. Toon, Geoffrey C. Toon Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorJean-Francois L. Blavier, Jean-Francois L. Blavier Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorArmin Kleinböhl, Armin Kleinböhl Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorGloria L. Manney, Gloria L. Manney Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorWilliam H. Daffer, William H. Daffer Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorPeter F. Bernath, Peter F. Bernath Department of Chemistry, University of York, York, UKSearch for more papers by this authorKaley A. Walker, Kaley A. Walker Department of Physics, University of Toronto, Toronto, Ontario, CanadaSearch for more papers by this authorChris Boone, Chris Boone Department of Chemistry, University of Waterloo, Waterloo, Ontario, CanadaSearch for more papers by this author First published: 25 March 2011 https://doi.org/10.1029/2010JD014928Citations: 26AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract [1] We have compared volume mixing ratio profiles of atmospheric trace gases measured by the Atmospheric Chemistry Experiment (ACE) version 2.2 and the MkIV solar occultation Fourier transform infrared spectrometers. These gases are H2O, O3, N2O, CO, CH4, HNO3, HF, HCl, OCS, ClONO2, HCN, CH3Cl, CF4, CCl2F2, CCl3F, COF2, CHF2Cl, and SF6. Due to the complete lack of close spatiotemporal coincidences between the ACE occultations and the MkIV balloon flights, we used potential temperatures and equivalent latitudes from analyzed meteorological fields to find comparable ACE and MkIV profiles. The results show excellent agreement for CH4, N2O, and other long-lived gases but slightly poorer agreement for shorter-lived species like CO, O3, and HCN. For example, in the upper troposphere (∼400–650 K), maximum differences between MkIV and ACE are 2.4% for CH4, 1.7% for N2O, −12.4% for CO, −15.9% for O3, and −5.6% for HCN. In the lower stratosphere (∼650–900 K), maximum MkIV-ACE differences are 7.6% for CH4, 14.1% for N2O, 7.3% for CO, −9.2% for O3, and 31.5% for HCN. Apart from a small vertical misregistration problem, the overall agreement between MkIV and ACE is very good. Key Points First use of a noncoincident validation method for MkIV and ACE profiles Equivalent latitudes and theta coordinates allow better validation Very good agreement between ACE and MkIV profiles of long-lived gases 1. Introduction ACE-FTS [2] The Atmospheric Chemistry Experiment (ACE) on board SCISAT-1 of the Canadian Space Agency is a solar occultation Fourier transform spectrometer (ACE-FTS). ACE was launched on 12 August 2003 into a 74° inclination, circular, 650 km altitude, Earth orbit, providing coverage that focuses on the polar regions. Coverage of the lower latitudes is relatively sparse (see Figure 1). Also, around the solstices, the spacecraft never enters the shadow of the earth and so there are no occultations. There are ∼29 occultation events per day (∼10,000/year), but not all of these could be telemetered back to Earth, especially in the first year of the ACE mission. Figure 1Open in figure viewerPowerPoint ACE occultation latitudes: sunset (red solid circles) /sunrise (blue triangles). Green ovals represent the fall turn-around periods at Fort Sumner (35°N). The lower density of points in 2004 was due to downlink limitations early in the mission. [3] ACE-FTS operates in the mid infrared range (750–4400 cm−1) with a spectral resolution of 0.02 cm−1 (25 cm maximum optical path difference, OPD). It measures a large number of atmospheric trace gas species with a vertical resolution of 3–4 km from the cloud tops to about 150 km. A detailed description of the ACE mission is provided by Bernath et al. [2005]. A description of ACE data analysis methods is provided by Boone et al. [2005]. JPL MkIV Balloon-Borne Interferometer [4] The JPL MkIV interferometer is a balloon-borne solar occultation FTIR spectrometer [Toon, 1991]. It was designed and built at the Jet Propulsion Laboratory, based on the ATMOS instrument [Farmer, 1987]. The MkIV covers the entire 650–5650 cm−1 region simultaneously at 0.01 cm−1 spectral resolution (56 cm maximum OPD). The JPL MkIV interferometer has performed 21 balloon flights since 1989. Flights are of 6–30 h duration depending on float winds. Each provides one or two occultation events covering altitudes from the cloud tops to the balloon (35–40 km) at 2–4 km vertical resolution. MkIV data analysis methods are summarized by Sen et al. [1996]. The MkIV has an established validation heritage that includes instruments on the UARS in the 1990s [Russell et al., 1996], ILAS-1 [Nakajima et al., 2006] and ILAS-2 [Irie et al., 2006], in situ sensors on board NASA ER-2 aircraft [Toon et al., 1999], POAM3 [Randall et al., 2002] and MLS on AURA [Froidevaux et al., 2006] (for a complete list, please refer to: http://mark4sun.jpl.nasa.gov/paper.html). Review of Prior ACE Validation [5] ACE products have been validated extensively using instruments from different platforms. As an example, De Mazière et al. [2008] provided a thorough validation of ACE CH4 products using measurements from ground-based FTSs, the balloon-borne instrument SPIRALE [Moreau et al., 2005] and satellite instruments MIPAS and HALOE. Carleer et al. [2008] augmented satellite comparisons of H2O with LIDAR and frost point hygrometer data (see http://www.ace.uwaterloo.ca/publications.html for the validation papers). However, most of the validation efforts focused on comparisons with other satellite instruments or comparisons with instruments that use different techniques [see also Walker et al., 2005]. There has so far been no ACE validation performed by another solar occultation FTS. [6] The MkIV instrument is ideally suited instrument for ACE validation since it measures the same gases in the same spectral regions using the same technique (i.e., solar occultation spectrometry). However, to date, the MkIV balloon profiles have not been used much for ACE validation. This is because all the MkIV flights performed since ACE launched (2003) have been from Fort Sumner, New Mexico (35°N), in late September, which unfortunately falls in a gap in the ACE coverage (see Figure 1). The ACE occultations at this time are either at 80°N or 60°S and therefore far from 35°N. The closest ACE measurements at similar latitudes are 2–3 weeks later than those of MkIV and therefore fail any normal sort of a coincidence criterion. Therefore, directly comparing MkIV and ACE profiles must be done very carefully. [7] The choice of the late September period for MkIV balloon flights is because this is when the stratospheric wind changes from Easterly to Westerly. During this so-called turn-around period, the light float winds make it possible to perform flights of up to 30 h duration with the balloon remaining within telemetry range. The late-September turn-around is preferred to the one in early May because it is more predictable [Wunch et al., 2005] and because the surface winds are usually lighter. 2. Methods [8] In this section, we describe a method of noncoincident validation using, as an example, CH4 from the September 2005 MkIV flight. In this study, we used the ACE version 2.2 profiles with updates to O3. We do not claim that this method is new. The benefit of using a Potential Vorticity/Potential Temperature (PV/Theta) coordinate system for assimilating/comparing data sets has been understood for years [e.g., Lait et al., 2004; Manney et al., 2001]. But to the best of our knowledge, this is the first time that this technique has been applied to the ACE or MkIV data sets. Alternative methods to validate noncoincident measurements exist; for example, Hegglin et al. [2008] used tracer-tracer correlations and vertical tracer profiles relative to tropopause height and showed that the latter method reduced geophysical noise in the UT/LS region (within 6 km of the tropopause). The MkIV profiles, however, extend up to 38 km altitude for many gases and so we did not want to restrict the comparison to the UT/LS altitudes only. Therefore, we present a different approach in this study. [9] Figure 2 (top) shows a comparison of a single MkIV CH4 profile (colored squares) with more than 30 ACE profiles acquired within 6° of latitude and 6 weeks of the MkIV profiles. There is a wide spread of ACE VMRs in the stratosphere. The ACE zonal mean (black dashes) agrees poorly (up to 20% differences) with the MkIV profile. Both the MkIV and ACE data in Figure 2 are color-coded by their Equivalent Latitude (EqL), calculated using the procedures described by Manney et al. [2007]. EqL is calculated from the potential vorticity (PV) field on isentropic surfaces; the EqL of a given PV contour is the latitude that would encompass between it and the pole the same area as is enclosed by the PV contour. Because PV on isentropic surfaces can be regarded as a tracer of atmospheric motions, different EqL values distinguish air with different origins. It can be seen that the MkIV observations match well the ACE observations of the same color/EqL. This suggests that a method of comparing MkIV and ACE observations on the basis of EqL and potential temperature (θ) would do much better than one based on true latitude and altitude. Figure 2Open in figure viewerPowerPoint (top) Comparing the 2005 MkIV CH4 profile (colored squares) with individual ACE profiles (colored dots) and with a zonal mean of the ACE results (black dashed line) acquired within 6° of latitude and 6 weeks of the MkIV. The color of the symbols represents the EqL. (bottom) By resampling the ACE data in θ/EqL space to match the MkIV observation locations, an ACE profile (red) that better matches the MkIV measurements (blue) is obtained. The gray shaded area shows the 1 σ spread in the ACE points. The percentage differences between ACE and MkIV VMR profiles are shown in the inset. [10] Therefore, a method of noncoincident validation was developed. For each MkIV observation at a certain potential temperature (θm) and equivalent latitude (EqLm), we determined the corresponding ACE VMR by fitting a surface to the ensemble of ACE data as a function of θ and EqL and then performing interpolation on this surface to the exact location (θm, EqLm) of the MkIV observation. More specifically, the ACE data are represented by the first-order Taylor expansion in the immediate vicinity of a MkIV observation. The index i represents different ACE observations. The three unknowns, Y0, α, and β were then obtained by minimizing the cost function where Yi = CH4 VMR for the ith ACE observation, ɛi = uncertainty in Yi (the ACE-supplied uncertainty), Y0, α, β = coefficients to be determined, and wi = weights given for each ACE observation. Note that the Y0 = Y(θm, EqLm) is simply the value of the surface fitted to the ACE data at the coordinates of the MkIV observation, the quantity that we are seeking. The weights (wi) depend on the closeness (in θ-EqL space) of the ACE and MkIV observation locations, and are given by [11] So the weights are ∼1 for ACE measurements within Δθ and ΔEqL of the MkIV observation but fall off rapidly outside this range. The Δθ and ΔEqL criteria used here are 10 K and 4°, respectively. A Δθ of 10 K is chosen so that the weighting function's full width at half maximum (FWHM ∼40 K) in the stratosphere would still overlap with the ACE vertical resolution. A ΔEqL of 4° means that the weighting function has a FWHM of ∼8°. This roughly corresponds to the ensemble of equivalent latitudes that the MkIV samples during its flight (Figure 3). Without this weighting, the ACE points that are most distant from the MkIV observation would have the most leverage in determining the slope of the fitted surface. This weighting has been applied to all ACE data within the 6 week criteria. This time interval still falls within the stratospheric lifetime of CO, which is a suitable tracer. Minschwaner et al. [2010] showed that on average, the production lifetime of CO is 40–60 days throughout most of the sunlit stratosphere and mesosphere. The loss lifetime is about 10–60 days at midlatitudes in September [Minschwaner et al., 2010, Figure 9]. These numbers are consistent with the findings of Rinsland et al. [2000], where the CO lifetime at 800 K from 20°N to 35°N are reported to be 40 days. Moreover, the time scales for "nonconservative" changes in EqL and theta are primarily related to the diabatic descent rates. Their time scales are shortened at times and places where there is strong diabatic descent such as the fall/winter high-latitude regions, and more in the upper to middle stratosphere than lower down where descent rates are always lower. The MkIV and ACE measurements compared in this work are at a time (Sept) and place (35°N) where diabatic descent rates are small; hence the approximation of conservation of EqL and theta in an air mass over a period of even six weeks should be a good assumption. Note that all the Y values in all the equations above represent ACE VMRs. No use has been made yet of the MkIV VMRs–only the locations of the MkIV observations (θm, EqLm) have been used so far. Figure 3Open in figure viewerPowerPoint The θ, EqL coordinates of all the (left) 2004 and (right) 2005 MkIV and ACE points used in this study (only the 2005 points were used in Figure 2). ACE sunrise measurements are represented by circles, and sunsets are represented by open squares. The color coding represents the days before (blue) and after (red) the MkIV flight, which is represented by the solid green squares. Note that the MkIV observations fall within the range of the ACE observations at all altitudes. In 2004 the available ACE observations were solely sunrise data from before the MkIV flight. In 2005 a mix of sunrise/sunset data from before and after the MkIV flight was available. [12] Y0, α and β are obtained using the method of weighted least squares. This is done independently for each MkIV point. We could have fitted a single surface to the entire ensemble of ACE measurements, but this would have been a complicated and nonlinear function. Instead we have assumed that the ACE data are locally linear functions of θ and EqL in the vicinity (Δθ, ΔEqL) of each MkIV data point, so the three-parameter least squares solution is straightforward, requiring no iteration. We have no interest in the retrieved values of α and β. We fit these parameters only to minimize potential biases in Y0 arising from any asymmetry in the distribution of ACE measurements in θ-EqL space about the MkIV observations. [13] Results of this method for the CH4 case are shown in Figure 2 (bottom). The zonal mean (black dashed line) deviates from the MkIV measurements at altitudes above the 800 K theta level. However, the resampled (i.e., interpolated in θ-EqL space) ACE VMRs ("ACE fit" red) agrees well with the MkIV profile (blue) and is within the MkIV error bars (blue). The percentage differences at each MkIV level are shown in the inset. The 1 σ errors in the % difference (σ%diff) between MkIV and the 2-D interpolated ACE profile (green lines) were calculated by taking into account the 1 σ error bars of the two VMR profiles, such that σ%diff = (√(σMkIV2 + σACE2)/VMRave)*100%, where VMRave is the average VMR from ACE and MkIV. [14] Figure 3 shows the EqL and θ of the MkIV observations (green) superimposed on those of the selected subset of ACE observations, i.e., taken within ±6 weeks and 6° of latitude of the MkIV measurements (no constraint was applied to the longitudes). Figure 3 shows that, in terms of EqL and θ, the MkIV measurements are well represented by the selected subset of ACE measurements; that is, there are no MkIV points outside the region of EqL and θ space occupied by ACE data. Although the ACE occultations during sunset (open squares) happened after the MkIV flight, they are very much evenly scattered in terms of equivalent latitude. [15] A more complicated expression for the surface fitted to the ACE data was tried in which a fourth unknown γ was added representing the cross term between θ and EqL This allows some curvature of the fitted surface. This equation is analogous to that used for bilinear interpolation. But we found no significant improvement in the quality of the fits using equation (4) compared with equation (2), indicating that the curvature of the surface fitting the ACE data within [Δθ, ΔEqL] of the MkIV observations is small. We therefore used the simpler equation (2) for the analysis described below. 3. Results and Discussion [16] We have applied the noncoincident validation method discussed above to 18 atmospheric gases and will provide a short discussion in this section. The plots of selected gases shown in Figures 4–21 are separated into two panels, with those on the left-hand side corresponding to the MkIV balloon flight in September 2004 and those on the right-hand side to September 2005. The plots and legends are the same as in Figure 2 (bottom). Table 1 summarizes the percentage biases for three atmospheric layers: 400–650 K, 650–900 K and 900 K up to the highest useable MkIV and ACE value (defined as positive VMR values with uncertainties less than 50%). The uncertainties of the biases in percent are given in the "Error" column. Table 1. Percentage Bias (MkIV Minus ACE) of Various Gases for Three Atmospheric Layers, 400–650 K, 650–900 K, and 900 K up to the Highest Useful ACE Valuea Gas Upper Troposphere (400–650 K) Lower Stratosphere (650–900 K) Midstratosphere (900 K to Useful ACE Value) 2004 2005 2004 2005 2004 2005 Bias (%) Error (%) Bias (%) Error (%) Bias (%) Error (%) Bias (%) Error (%) Bias (%) Error (%) Bias (%) Error (%) H2O −3.05 6.07 1.33 9.03 −1.44 2.7 0.48 0.67 −3.82 1.94 2.6 2.75 O3 −15.89 5.57 −11.85 7.65 −9.16 1.15 −1.31 1.03 −9.68 1.06 −2.19 1.12 N2O 1.72 2.03 1.25 1.24 14.1 5.52 1.82 3.54 13.44 5.17 0.39 6.93 CO −6.32 10.72 −12.39 4.26 7.26 5.31 5.97 2.46 15.35 11.34 16.76 8.45 CH4 2.45 1.88 1.14 1.68 7.59 2.95 0.14 1.62 0.18 4.82 −1.57 4.57 HNO3 −4.34 11.24 −13.62 8.35 5.97 7.01 −2.21 3.37 6.26 5.79 −1.65 3.32 HF −13.06 11.29 −7.59 13.12 −20.44 6.14 −10.82 5.29 −16.28 4.74 −7.91 1.54 HCl −5.94 7.31 −3.65 6.01 −13.41 4.85 −5.77 2.14 −7.24 3.43 −6.62 1.75 OCS 15.78 15.78 15.16 5.04 NaN NaN NaN NaN NaN NaN NaN NaN ClONO2 −22.01 15.03 −18.55 19.44 −14.44 7.75 −1.94 9.15 −49.05 4.51 −42.84 10.32 HCN −5.57 1.99 −4.45 3.81 31.52 26.95 −1.11 1.54 NaN NaN NaN NaN CH3Cl −2.28 14.49 −10.19 9.84 NaN NaN NaN NaN NaN NaN NaN NaN CF4 9 5.12 10.44 3.87 −0.66 3.48 1.49 3 −4.74 2.93 −3.45 1.91 CCl2F2 11.64 2.6 10.31 3.75 12.84 1.17 6.97 7.7 NaN NaN NaN NaN CCl3F 10.71 4.08 12.31 4.84 NaN NaN NaN NaN NaN NaN NaN NaN COF2 −3.59 3.56 −0.3 8.16 3.55 10.5 6.99 5.9 NaN NaN NaN NaN CHF2Cl −5.46 13.34 −1.4 7.13 NaN NaN NaN NaN NaN NaN NaN NaN SF6 −2.30b 17.6 −1.94b 21.40 NaN NaN NaN NaN NaN NaN NaN NaN a Values with higher ACE bias are boldface. NaN means that there are either no MkIV measurements or there are no useable ACE measurements for that region. b SF6 "Upper Troposphere" values are calculated at 350–550 K. [17] For most gases, the differences between MkIV and ACE for 2004 are slightly larger than for 2005. We attribute this to: (1) more ACE data used for the 2005 comparison, (2) the ACE data for 2004 were solely sunrise measurements and (3) the ACE measurements for 2004 were all taken 6 weeks before the MkIV flight (see Figure 3). CH4 [18] De Mazière et al. [2008] reported a CH4 validation accuracy within 10% in the upper troposphere to lower stratosphere and within 25% in the mid and higher stratosphere to the lower mesosphere. With our noncoincident method, we show an agreement better than 5% between ACE and MkIV from the upper troposphere to the midstratosphere and within 10% in the mid to upper stratosphere (Figure 2, inset, and Figure 4) for CH4 in 2005. Figure 4Open in figure viewerPowerPoint MkIV and ACE CH4 comparisons with ACE data resampled in θ/EqL space (ACE fit, red lines) to match the locations of the MkIV measurements (blue squares). The gray shaded area shows the 1 σ spread in the ACE points. The percentage differences between ACE and MkIV VMR profiles are shown in the insets. H2O [19] H2O is difficult to compare in the upper troposphere. First it can condense and is therefore short-lived and not a conserved tracer, especially near the tropical tropopause layer or TTL [Notholt et al., 2010; Steinwagner et al., 2010]. Second, there are large vertical gradients in H2O which cause any intercomparison to be susceptible to vertical misregistration (error in altitude) of the data sets and to differences in vertical resolution (especially at the hygropause). So it should be no surprise to see MkIV/ACE differences of up to 20% below the hygropause (Figure 5). In the stratosphere, where H2O is much longer lived and has much smaller vertical gradients, the agreement is much better with differences generally <5%. Carleer et al. [2008] compared ACE H2O data with SAGE II, HALOE, POAM III, MIPAS and Odin SMR. They found that the difference between ACE H2O profiles and that of the other instruments were about ±20% in the upper troposphere and better in the stratosphere. Figure 5Open in figure viewerPowerPoint MkIV and ACE H2O comparisons. Details as for Figure 4. O3 [20] Dupuy et al. [2009] presented an extensive bias determination analysis of ozone observations from the ACE satellite instruments (FTS, MAESTRO). They compared the Ozone version 2.2 updated products with coincident observations from nearly 20 satellite, airborne, balloon-borne and ground-based instruments. MkIV data were not included in the comparison, probably due to the strict coincidence criteria the MkIV data would have not fulfilled during the comparison period (2004–2006). Dupuy et al. [2009] stated that the ACE-FTS version 2.2 O3 updated products report more O3 than most correlative measurements from the upper troposphere to the lower mesosphere. At altitude levels from 16 km to 44 km, the average values of the mean relative differences are nearly all within +1% to +8%. These values are consistent with the O3 comparisons presented here (Figure 6). Above 600 K (∼24 km), MkIV and ACE O3 have differences ranging from about −1% to −10%, with better agreement for 2005, probably due to more ACE data. The ACE O3 is consistently larger than the MkIV O3 values, consistent with the observations presented by Dupuy et al. [2009]. Figure 6Open in figure viewerPowerPoint MkIV and ACE O3 comparisons. Details as for Figure 4. N2O [21] ACE-FTS N2O measurements have been validated by Strong et al. [2008] using profiles from satellite measurements (SMR, MLS, MIPAS), aircraft (ASUR), balloon (SPIRALE, FIRS-2) and partial columns from ground-based FTIR spectrometers. Satellite comparisons at 6 km to 30 km yielded a mean absolute difference of ±15%. Strong et al. [2008] also showed that ACE-FTS measurements are consistently smaller than the SPIRALE balloon-borne measurements between 17 km to 24 km, with relative differences of up to 19%. ACE-FTS also had a low bias relative to the balloon-borne measurements of FIRS-2 between 11 km to 13 km. Below 20 km, Strong et al. [2008] showed that the relative differences between ACE and SPIRALE measurements range from −6% to +17%. [22] Comparisons with the MkIV measurements in this study (Figure 7) show some consistency with the results of Strong et al. [2008]. Overall, the agreement here is very good up to about 27 km, above that, MkIV measurements were higher than ACE in 2004 and lower than ACE in 2005. In 2004, the differences from this study are within 10% below ∼27 km and within 20% below ∼38 km for 2004. For 2005, the relative differences are within 10% or better than 4% at ∼17 km to about 27 km. The comparison for the 2005 flight was better because ACE measurements for 2004 were taken 6 weeks after the MkIV flight. Although the MkIV equivalent latitude (EqL) values fall within the ACE EqLs, MkIV VMRs at altitudes of 700 K to 1100 K are at the edge of the ACE VMR distribution. This probably led to the relatively larger differences. Figure 7Open in figure viewerPowerPoint MkIV and ACE N2O comparisons. Details as for Figure 4. HCl, HF, ClONO2, COF2, and HNO3 [23] Photolysis of man-made chlorofluorocarbons (CFC) in the stratosphere leads to the release of chlorine and fluorine atoms. This results in the formation of reservoir gases like HF, HCl, COF2 and ClONO2. These molecules have VMR profiles that increase with altitude in the lower and middle stratosphere. Their long lifetimes should also mean that ACE and MkIV profiles would agree better compared to profiles of gases that have shorter lifetimes. However, we see a strong negative MkIV-ACE bias for these gases in contrast to positive MkIV-ACE biases for gases that decrease in altitude, indicating vertical shifts in the retrieved profile. [24] Mahieu et al. [2008] compared ACE-FTS measurements of HCl and HF with MkIV profiles taken over Fort Sumner during the 2003, 2004 and 2005 campaigns. Due to the absence of direct coincidences between ACE and MkIV, Mahieu et al. [2008] used zonal means from 90 ACE profiles over a latitude bin of ±5° width centered near Fort Sumner, between August and October in 2004, 2005 and 2006. They showed that above 20 km, ACE and MkIV HCl profiles are in good agreement (to better than ±7%) but differences become extremely large below 17 km. ACE also measured systematically higher HCl than FIRS-2 (20% to 60%) at 12 to 31 km for the Arctic comparison done by Mahieu et al. [2008]. However, they stated that the large discrepancies may have been influenced by the presence of polar stratospheric clouds (PSCs). In this comparison, ACE also shows consistently larger HCl values than the MkIV (Figure 8). For 2005, the relative differences range from approximately 0% to −10%. For 2004, most of the relative differences are within 0% to 10% as well, except for the region around 700 K (between 24 and 30 km) where ACE measures up to 20% more HCl than MkIV. In this altitude region, a "fold" in the profile can also be observed. It is more prominent in the ACE profile's zonal mean compared to the MkIV and the "ACE Fit" profile. Figure 8Open in figure viewerPowerPoint MkIV and ACE HCl comparisons. Details as for Figure 4. [25] Comparisons with HF zonal means from ACE with MkIV done by Mahieu et al. [2008] reported relative differences of about 10% above 19 km, with ACE profiles biased high. The same bias was shown by Mahieu et al. [2008] for comparisons with FIRS-2 and HALOE HF profiles. These findings are consistent with the results shown here, with ACE HF always larger in values (up to 30%) compared to MkIV above 500 K (Figure 9). Figure 9Open in figure viewerPowerPoint MkIV and ACE HF comparisons. Details as for Figure 4. [26] Wolff et al. [2008] compared ACE and MIPAS ClONO2 profile