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Keywords:

  • FTIR;
  • ILAS-II;
  • Kiruna

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] The Improved Limb Atmospheric Spectrometer-II (ILAS-II), a solar occultation instrument, was developed by the Ministry of the Environment (MOE) of Japan as a successor to ILAS. The ILAS-II was launched on board the Advanced Earth Observing Satellite-II (ADEOS-II) satellite in December 2002 and took measurements between February and October 2003. Ground-based Fourier transform infrared (FTIR) measurements were taken at Kiruna (northern Sweden, 68°N, 20°E) as part of ILAS-II validation. These ground-based observations of vertical profiles of O3, HNO3, N2O, and CH4 were compared to ILAS-II measurements processed by the version 1.4 retrieval algorithm. Nineteen coincident FTIR and ILAS-II observations were determined and analyzed. The ILAS-II profiles had considerably better vertical resolution than the ground-based profiles. The vertical resolution of the ILAS-II profiles was therefore degraded to facilitate comparison between the two sets of profiles. Average relative differences were within 10 to 15% for all four gases. The ILAS-II measurements of O3, N2O, and CH4 had a negative bias relative to the FTIR measurements. However, the bias of ILAS-II measurements relative to the FTIR measurements was positive for HNO3.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] Ozone (O3), nitric acid (HNO3), nitrous oxide (N2O), and methane (CH4) are trace species that play important roles in climate change, ozone depletion processes in the lower stratosphere, and global warming. Of these species, O3 is particularly critical. As a stratospheric gas, with 90% of its column amount in the stratosphere and only 10% in the troposphere, O3 absorbs UV radiation in the stratosphere, keeping this radiation from reaching the Earth. Another key species in stratospheric ozone chemistry is HNO3, which determines chemical and physical properties of polar stratospheric clouds (PSCs) that play a central role in stratospheric ozone depletion. As a long-lived and vertically stratified species, N2O is a useful tracer in dynamical studies. It is also a greenhouse gas with a much larger specific effect on global warming than gases such as CO2. Like N2O, CH4 is a tropospheric greenhouse gas, but it is not as long lived as N2O. Its influence on global warming is larger than that from N2O and its rate of increase is much larger than that of other greenhouse gases.

[3] The Institute of Meteorology and Climate Research (IMK) has operated a ground-based Fourier spectrometer at Kiruna in northern Sweden (68°N, 20°E) since 1996 in cooperation with the Swedish Institute of Space Physics (IRF, Kiruna, Sweden) and Nagoya University in Japan. Measurements from the spectrometer yield profiles and column amounts of O3, HCl, HF, HNO3, ClONO2, N2O, CH4, NO2, CO, NO, ClO, and other gases.

[4] The measurement site at Kiruna can be used to study polar processes because it is frequently inside the polar vortex. The short polar night allows for relatively long periods solar observation. Weather conditions are normally cold and dry, leading to good quality infrared spectral data. Furthermore, Kiruna's well-developed infrastructure and easy access have made it the site for numerous research projects. Finally, the Kiruna station is part of the Network for the Detection of Stratospheric Change (NDSC) and records continuous ground-based measurements that are not limited to specific research projects. Such continuous measurements provide a good statistical basis for the validation of satellite data. As a part of this network the quality of the measurements is ensured by a validation protocol, that has to be followed by all instruments [Kurylo, 1997]. Also, previous independent comparisons with microwave instruments [Kopp et al., 2002], O3 sondes and Brewer spectrometer [Schneider et al., 2005], satellite instruments [Griesfeller, 2004], or side-by-side comparisons [Meier et al., 2005], showed that the FTIR results are tested and well established to be able to validate satellite data.

[5] The Advanced Earth Observing Satellite-II (ADEOS-II) was launched on the H-IIA Launch Vehicle Flight No. 4 from the Tanegashima Space Center at 1031 local time on 14 December 2002 (Japanese Standard Time). One of its five instruments was the Improved Limb Atmospheric Spectrometer-II (ILAS-II) that was developed by the Ministry of the Environment (MOE) of Japan to succeed the ILAS. The ILAS-II was operated on board the ADEOS-II spacecraft of the National Space Development Agency (NASDA) of Japan (recently reorganized as the Japan Aerospace Exploration Agency [JAXA]) [Nakajima et al., 2006]. The ILAS-II retrieved vertical profiles of O3, HNO3, NO2, N2O, CH4, H2O, CFC-11, CFC-12, ClONO2, N2O5, and aerosol extinction coefficients at 780 nm and functioned continuously from April to October, 2003 with some sporadic measurements between January and March 2003. Data obtained using the version 1.4 retrieval algorithm have been recently validated by other independent measurements [Irie et al., 2006; Saitoh et al., 2006; Sugita et al., 2006; M. K. Ejiri et al., Validation of ILAS-II version 1.4 nitrous oxide and methane profiles, submitted to Journal of Geophysical Research, 2005, hereinafter referred to as Ejiri et al., submitted manuscript, 2005].

[6] This study investigated differences between vertical profiles derived from retrievals of Fourier spectrometer data and those retrieved from ILAS-II. Measurement results were compared and differences were investigated in detail.

2. Measurements and Data Analysis

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

2.1. Ground-Based FTIR

[7] FTIR measurements were taken with a Bruker IFS 120HR spectrometer with a spectral resolution of approximately 0.003 cm−1. Two detectors (mercury-cadmium-telluride [MCT] and indium antimonide [InSb]) and an NDSC optical filter set covered the spectral range from 700 to 5000 cm−1 with the Sun as the radiation source. Experimental details have been published elsewhere [Blumenstock et al., 1997; Kopp et al., 2002].

[8] Measured spectra were evaluated using version 9.2 of the retrieval code PROFFIT [Hase, 2000; Hase et al., 2004] and the forward model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) [Stiller et al., 1998]. PROFFIT was developed to analyze solar absorption spectra measured with high-resolution ground-based FTIR spectrometers, and it has been compared to other retrieval codes [Hase et al., 2004]. PROFFIT includes various retrieval options such as scaling of a priori profiles, the Tikhonov-Phillips method [Phillips, 1962; Tikhonov, 1963], or the optimal estimation method [Rodgers, 1976].

[9] Column concentrations of trace gases including O3, H2O, N2O, CH4, HF, HCl, ClONO2, NO, NO2, and HNO3 can be derived from individual absorption lines. Pressure broadening of absorption lines is used for the profile retrieval; thus the vertical volume mixing ratio (VMR) profiles of species with pressure-dependent absorption signatures such as O3, N2O, CH4, HF, HCl, NO, and HNO3 can be derived. The vertical resolution is approximately 8–10 km between the ground and 30 km. The number of independent layers indicated by the degrees of freedom is reflected by the full width at half maximum (FWHM) of the averaging kernels. A set of distinct kernels is expected to equal the degrees of freedom. The subset of kernels was selected according to this guideline (Figure 1). Table 1 shows the spectral signal-to-noise ratio for each microwindow and the degrees of freedom of signal for the four gases measured by FTIR.

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Figure 1. Volume mixing ratio averaging kernels for O3, HNO3, N2O, and CH4 on 10 September 2003.

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Table 1. Spectroscopic Data, Signal-to-Noise Ratio (SNR, Spectral and Column Amounts), and Degrees of Freedom for Signal (DOF) for the FTIR Measurements
GasMicrowindow (MW) RangeSource of FTIR Spectral DataSpectral SNRSNR of Column AmountDOF
O3782.561–782.861 cm−1HITRAN 20042008505
O3788.850–789.369 cm−1HITRAN 20044508505
O31000.000–1005.000 cm−1HITRAN 20045008505
HNO3867.000–869.592 cm−1nearly equivalent to HITRAN 20003601402.3
HNO3872.800–875.200 cm−1nearly equivalent to HITRAN 20004301402.3
N2O1161.339–1161.661 cm−1HITRAN 20005208303.7
N2O1182.400–1182.831 cm−1HITRAN 20004408303.7
N2O1183.250–1183.800 cm−1HITRAN 20004708303.7
N2O1193.900–1194.300 cm−1HITRAN 20005508303.7
CH41202.500–1204.500 cm−1HITRAN 2000 (with corrections)5307503
CH41207.600–1208.100 cm−1HITRAN 2000 (with corrections)4907503
CH41221.561–1223.261 cm−1HITRAN 2000 (with corrections)4807503

[10] Spectroscopic data from Wagner et al. [2002] and Wagner and Birk [2003] were used in the PROFFIT O3 retrieval, and there is thus a difference in the spectroscopic data between ILAS-II and FTIR. Data by Wagner et al. [2002] and Wagner and Birk [2003] have also been added to the High Resolution Transmission (HITRAN) 2004 database, so that the FTIR spectroscopic data were equal to HITRAN 2004 data for O3. The HITRAN 2000 database was used for the N2O retrieval [Rothman et al., 2003]. The HNO3 spectral compilation used for the FTIR analysis was nearly equivalent to the line list used by ILAS-II (HITRAN 2000); because these spectroscopic data were not exactly the same for FTIR and ILAS-II, the line strengths were compared and found to agree within 1.5% or better. HITRAN 2000 was used for the CH4 retrieval, with some corrections applied from Brown et al. [2003]. The FTIR data are also shown in Table 1.

[11] The pressure and temperature profiles used to evaluate the FTIR data were acquired from the automailer system of the Goddard Space Flight Center. The climatological profiles were based on data from the National Centers for Environmental Prediction (NCEP) and were compiled by J. Remedios and I. Parkes (private communication, 2003).

2.2. ILAS-II

[12] The Improved Limb Atmospheric Spectrometer-II (ILAS-II) solar occultation instrument was developed by the Ministry of the Environment (MOE) of Japan. It was operated on board the Advanced Earth Observing Satellite-II (ADEOS-II) during its Sun-synchronous polar orbit at an inclination angle of 98.7° and a height of 802.9 km. Measurements were made approximately 14 times daily in each hemisphere from January to October 2003. Latitudinal coverage was from 54–71°N and 64–88°S, varying seasonally. The instantaneous field of view (IFOV) for the infrared (IR) spectrometer at the tangent point was 1.0 km in the vertical and 13.0 km in the horizontal. The ILAS-II included four spectrometers that measured in the IR (850–1610 cm−1), the midinfrared (MIR, 1754–3333 cm−1), high-resolution IR (778–782 cm−1), and visible (VIS, 12,755–13,280 cm−1). The ILAS-II used a solar occultation technique to measure stratospheric vertical profiles of O3, HNO3, NO2, N2O, CH4, H2O, ClONO2, N2O5, CFC-11, CFC-12, and aerosol extinction coefficients. Vertical VMR profiles of atmospheric constituents were derived with an onion-peeling retrieval method [Yokota et al., 2002; T. Yokota et al., manuscript in preparation, 2006]. The ILAS-II data from the version 1.4 retrieval algorithm were used. The retrieval vertical grid interval was 1 km. Vertical resolutions were 1.3–2.9 km at tangent heights of 15 to 55 km [Nakajima et al., 2006]. Spectroscopic data (Table 2) were adopted from the year 2000 edition of the HITRAN database that included updates through the end of 2001 [Rothman et al., 2003]. The spectral range used by the ILAS-II was 850–1610 cm−1 (channel 1) for all four gases. The spectral signal-to-noise ratio was between 300 and 3400, depending on the wavelength. Further details about the ILAS-II have been reported by Nakajima et al. [2006] and T. Yokota et al. (manuscript in preparation, 2006).

Table 2. Spectroscopic Data and Spectral Signal-to-Noise Ratio for the ILAS-II Measurements
GasILAS-II Spectral RangeSource of ILAS-II Spectral DataSpectral SNR
O3850–1610 cm−1 (channel 1)HITRAN 2000300–3400 (depending on the wavelength)
HNO3850–1610 cm−1 (channel 1)HITRAN 2000300–3400 (depending on the wavelength)
N2O850–1610 cm−1 (channel 1)HITRAN 2000300–3400 (depending on the wavelength)
CH4850–1610 cm−1 (channel 1)HITRAN 2000300–3400 (depending on the wavelength)

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[13] At Kiruna FTIR observations were made on 89 days between January and October 2003. During this period, ILAS-II observations occurred within 500 km of Kiruna on 50 days. Using coincidence criteria of 500 km in space and 12 hours in time, 19 coincidences were found. These coincidence criteria were chosen to increase the number of coincidences and obtain more reliable statistics. All four gases used for this comparison are long-lived species with little spatial and temporal variability. The coincidences are listed in Table 3. Table 3 also shows the isentropic potential vorticity (IPV) differences at the θ = 475 K level (with θ representing the potential temperature) and the lowest tangent altitudes of the ILAS-II.

Table 3. Coincidences of FTIR and ILAS-II Measurements
Day of MeasurementΔ Space, kmΔ Time, hoursΔ IPV, 10−6 Km2/kgs, at θ = 475 KLowest Tangent Altitude, km
030215165.225:30514
030320339.093:4710 (strong gradient)14
030403424.415:1112
030406433.588:2811
030902488.628:29n/a17
030904414.606:37n/a14
030908463.5511:03n/a15
030910137.837:11n/a13
030916337.078:49n/a14
030917499.897:05n/a14
030924197.216:11n/a13
030925275.807:21n/a14
030926444.727:04n/a11
030930337.365:55n/a12
031001481.817:4313
031010391.534:3113
031013424.555:4115
031015386.914:1015
031020364.615:0913

[14] On one of these 19 days, 20 March 2003, Kiruna was at the edge of the polar vortex, and the ILAS-II measurement point was inside the vortex. The difference in isentropic potential vorticity (IPV) was approximately 10 × 10−6 Km2/kgs and the gradient was very large (Table 3). The IPV data, which were based on data from the European Centre for Medium-Range Weather Forecasts (ECMWF), were obtained from the Norwegian Institute for Air Research database (NILU). This day was omitted from the comparison of ILAS-II and FTIR data because the large difference in IPV suggested a large air mass difference between the two points. On the other 18 days, differences arising from relative positions within the polar vortex were deemed unimportant (Table 3). There was no polar vortex after the end of March 2003 [Griesfeller, 2004]. Between May and September no data for the polar vortex are available in the NILU database because during this period no polar vortex is expected. In the Arctic winter of 2003/04 the polar vortex formed quite late in November [Raffalski et al., 2004], so that in October no Δ IPV occurred.

[15] The retrievals from the two measurements were compared directly, and it was obvious that the ILAS-II profiles had considerably better vertical resolution than the ground-based profiles. The vertical resolution of the ILAS-II profiles was therefore degraded to facilitate a comparison between the two sets of profiles. Synthetic spectra were calculated for this reduction in vertical resolution by inputting ILAS-II profiles into the forward calculation code KOPRA [Stiller et al., 1998]. FTIR VMR profiles were added below the lowest tangent altitude of ILAS-II, shown in Table 3. The lowest tangent altitude was between 13 and 15 km on most days. A profile was retrieved from these synthetic spectra with PROFFIT, as was done for FTIR observations. The procedure yielded an ILAS-II profile at FTIR resolution; the same procedure has been used for comparisons with ENVISAT data [Griesfeller, 2004] and O3 sonde data [Schneider et al., 2005]. This smoothing procedure was different from the procedure used by Rodgers and Connor [2003], where averaging kernels were used for the smoothing. The method by Rodgers and Connor [2003] assumed linearity of the forward and inverse models. The method used in this study was not influenced by possible nonlinearities.

[16] Figure 1 shows the averaging kernels of the VMR profiles measured on 10 September 2003. These data provided information on the spectral sensitivity and vertical resolution of the retrieval. Spectral information for O3 was found at heights between ground level and 40 km. Most spectral information was obtained from five independent layers between 15 and 40 km. Similarly, HNO3 was sensitive between 15 and 30 km in three independent layers. N2O is a tropospheric gas; therefore spectral information from the ground-based FTIR measurements existed only up to approximately 25 km. Some spectral sensitivity was even present at the surface, and there were four independent layers above 5 km. Spectral sensitivity was also revealed close to the surface for CH4, and three independent layers were found above the surface.

[17] Figure 2 shows the a priori profiles by the FTIR of O3, HNO3, N2O, and CH4 (top left, top right, bottom left, and bottom right, respectively). Figure 2 includes three a priori profiles and three profiles retrieved using these three a priori profiles with the same single measured FTIR spectrum (recorded on 10 September 2003). The climatological profiles normally used for the FTIR retrieval (dotted line in Figure 2) were based on NCEP data and compiled by J. Remedios and I. Parkes (private communication, 2003). The solid and dashed lines in Figure 2 were based on Upper Atmosphere Research Satellite (UARS) data observed over Kiruna at 67.5–72.5°N [Nakajima et al., 1999]. The retrieved profiles showed some dependency on a priori profiles. Retrievals for the three different a priori profiles were different for all four gases, as expected, owing to the limited information content of the ground-based measurements. Note, however, that the chosen constraints applied to the ILAS-II results in the smoothing procedure described above were identical to the settings used in the ground-based retrieval, so that the smoothing error, which depends on these chosen constraints (averaging kernels), canceled out. In this smoothing procedure, the same a priori profiles and the same vertical profiles of temperature and pressure were used for both the smoothed ILAS-II VMR profile and the FTIR VMR profile, so that both VMR profiles yielded the same averaging kernels.

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Figure 2. O3, HNO3, N2O, and CH4 VMR profiles measured by FTIR at Kiruna on 10 September 2003. The solid and dashed lines with squares show the a priori profile from the UARS climatological data for September (December) [Nakajima et al., 1999] and the dotted line with squares shows the profile normally used for the FTIR retrieval (J. Remedios and I. Parkes, private communication, 2003). The solid, dashed and dotted lines without squares show the retrieved profiles using the same spectrum but different a priori profiles.

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[18] The grid spacing of the a priori and retrieved profiles was chosen on the basis of forward calculation by KOPRA. For the forward calculation, a much smaller height grid was used than necessary for the retrieval. This procedure has been discussed in more detail by Hase et al. [2004].

3.1. Ozone

[19] Figure 3 compares O3 profiles from ILAS-II (retrieved by version 1.4) with profiles from ground-based FTIR measurements taken at Kiruna on 10 September 2003. This example was chosen because the polar vortex had little influence on the trace species in September. The horizontal difference was only 138 km, and the lowest tangent altitude of ILAS-II was 13 km, one of the lowest tangent altitudes in the data set (Table 3). The right-hand side of Figure 3 shows the differences between the smoothed ILAS-II profile and the FTIR profile. The ILAS-II measurements were slightly smaller than FTIR measurements for nearly the entire profile. The largest absolute and relative discrepancies were 0.7 ppmv at 35 km and 12.5% between 26 and 29 km, respectively.

image

Figure 3. Comparison of O3 profiles from the FTIR and ILAS-II at Kiruna on 10 September 2003. The solid line with error bars represents the FTIR measurement, and the squares represent the a priori profile used for the FTIR retrieval. The circles represent the ILAS-II measurement, and the dashed line represents the ILAS-II measurement in FTIR height resolution as described in the text. Differences between the FTIR and the smoothed ILAS-II are shown on the right-hand side, with units in volume mixing ratio (solid lines) and % (dotted lines).

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[20] Figure 4 shows the statistics for all 18 coincidences for O3. The black line indicates the mean of the differences between the FTIR and smoothed ILAS-II profiles. Error bars represent the standard deviation. As in the example of 10 September 2003 (Figure 3), smoothed ILAS-II measurements were smaller than FTIR measurements for all 18 coincidences. Differences below 22 km were quite small (<0.1 ppmv and <5%). Above 22 km, differences increased to 0.9 ppmv and 20%.

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Figure 4. Statistics for 18 matched O3 profiles from FTIR and smoothed ILAS-II measurements at Kiruna. Means of the differences and standard deviations are shown.

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[21] The FTIR O3 data have previously been compared with other measurements made by different instruments, such as the Brewer spectrometer [Schneider et al., 2005], with very small differences, or compared with satellite measurements from MIPAS-ENVISAT [e.g., Griesfeller, 2004], where small relative differences of less than 5% for the column amounts and approximately 10% for the vertical VMR profiles were observed. Other instruments (e.g., LIDAR, O3 sondes) that also validated this satellite data found the same differences, which confirmed the FTIR-MIPAS comparison. A side-by-side comparison [Meier et al., 2005] also showed very good accuracy for all four gases discussed here. The ILAS-II O3 data were also compared to other instruments [e.g., Sugita et al., 2006]. Sugita et al. [2006] compared ILAS-II measurements with O3 sonde measurements and four satellite-borne occultation sensors (SAGE II, SAGE III, HALOE, and POAM III). In the Northern Hemisphere the data agreed within 10% in altitudes between 11 and 40 km, which is similar to the results presented here. The differences found by these validations were comparable to the differences found in this study.

[22] Some of the differences between the FTIR and ILAS-II could have arisen from the different measurement times. Table 3 shows the temporal and spatial differences between the FTIR and the ILAS-II measurements, although there is no large variability in space and time expected for O3, especially in September and October, when most of the coincidences were found. Another reason might be that the spectroscopic windows used by the FTIR and ILAS-II were different. The ILAS-II measured absorption over a large spectral range. The four gases examined in this paper were measured with channel 1 data at wavelengths from 850 to 1610 cm−1 and at low spectral resolution. In contrast, the FTIR measurements had high spectral resolution and narrow wavelength bands (microwindows). Narrow regions of the spectra were used in the VMR profile retrievals.

3.2. Nitric Acid

[23] Figure 5 compares HNO3 profiles from smoothed ILAS-II with ground-based FTIR profiles measured at Kiruna on 10 September 2003. The smoothed ILAS-II HNO3 VMR showed larger values than the FTIR profile above 18 km. The largest differences between FTIR and the smoothed ILAS-II profiles (dashed line) occurred around 24 to 30 km where the bias was approximately 1.3 ppbv (20 to 22%).

image

Figure 5. As in Figure 3 but for HNO3.

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[24] Figure 6 shows the statistical relationship of all 18 coincidences for HNO3. The left-hand side shows the absolute differences (ppbv), and the right-hand side shows relative differences (%). Differences were less than 20% (1.5 ppbv), except above 26 km where differences increased to 23%. Smoothed ILAS-II measurements had a positive bias above 17 km. A 20% difference in the VMR profiles was also detected in ILAS-II comparisons using other instruments; Irie et al. [2006] compared HNO3 VMR profiles of ILAS-II to balloon-borne measurements by MIPAS-B2 and MkIV over the Arctic in March and April 2003 with differences between 13/26% at 15 to 25 km in altitude.

image

Figure 6. As in Figure 4 but for HNO3.

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3.3. Nitrous Oxide

[25] Figure 7 shows measurements of the VMR profiles of the tropospheric gas N2O. As was the case for O3, the VMR for N2O obtained from the ILAS-II were smaller than those by the FTIR for all heights. The largest absolute and relative differences exceeded 0.02 ppmv between 15 and 20 km and were nearly 12% from 20 to 22 km, respectively. The example from 10 September 2003 was typical. Smoothed VMR profiles from ILAS-II were smaller than those from the FTIR measurements for the entire ensemble at all heights (Figure 8). The largest differences were between 19 and 22 km and were 0.03 ppmv (16 to 18%). Differences were smaller above 22 km. Percentage differences for N2O resembled those for O3 and HNO3.

image

Figure 7. As in Figure 3 but for N2O.

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Figure 8. As in Figure 4 but for N2O.

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[26] As with HNO3, the N2O measurements taken by ILAS-II had been previously compared to other measurements. Ejiri et al. (submitted manuscript, 2005) compared ILAS-II measurements with measurements taken by the balloon-borne MIPAS-B and MkIV sensors and also by the Sub-Millimeter Radiometer (SMR) sensor on board the Odin satellite. The differences between these three instruments and ILAS-II were comparable to those in this study. In the Northern Hemisphere, all three instruments had larger VMR values than ILAS-II, with differences up to 20%.

3.4. Methane

[27] Figure 9 shows CH4 measurements at Kiruna on 10 September 2003. Again, the smoothed ILAS-II VMR were smaller than those from the FTIR profile. The largest differences occurred between 20 and 23 km with absolute and relative differences of 0.12 ppmv and 12%, respectively. The smoothed ILAS-II profile had smaller VMR values than the original ILAS-II profile between approximately 14 and 26 km. We determined that these differences were not due to the VMR values of the lowest tangent altitudes but, rather, may have been caused by the small VMR value of the ILAS-II profile above 25 km.

image

Figure 9. As in Figure 3 but for CH4.

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[28] The observed differences for this one example closely resembled differences for the other 18 coincidences. In general, the largest differences occurred between 20 and 26 km, with absolute and relative values of 0.16 ppmv and 21%, respectively (Figure 10).

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Figure 10. As in Figure 4 but for CH4.

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[29] Ejiri et al. (submitted manuscript, 2005) also compared CH4 data from the ILAS-II with balloon-based MIPAS-B and MkIV data and UARS/HALOE satellite data. As with N2O, those differences were comparable to the differences found in this study with approximately 10% larger VMR values by the three instruments compared to ILAS-II.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[30] Vertical VMR profiles of O3, HNO3, N2O, and CH4 were measured by a ground-based Fourier spectrometer at Kiruna and compared to profiles derived from ILAS-II measurements on 19 different days. The resolution of the ILAS-II measurement profiles was degraded to the FTIR resolution to compare the measurements.

[31] Table 4 shows the mean differences between the smoothed ILAS-II VMR profiles and the FTIR VMR profiles and the altitude ranges over which these means were calculated. Differences were between 10 and 15% for all four gases. The VMR profiles of O3, N2O, and CH4 were smaller for ILAS-II compared with the FTIR data; only for HNO3 was the bias positive.

Table 4. Comparison of Results
GasMean Difference ILAS-II − FTIR, % (Altitude Range)
O3−10 (lowest tangent altitude − 35 km)
HNO315 (lowest tangent altitude − 30 km)
N2O−10 (lowest tangent altitude − 30 km)
CH4−15 (lowest tangent altitude − 30 km)

[32] Measurement differences for the four gases have also been shown by other studies and could be compared with the results of this study. Sugita et al. [2006] compared ILAS-II O3 VMR profiles with satellite-borne instruments and O3 sondes and found differences of 10%, which are comparable to the differences found in the comparison of O3 VMR profiles from the FTIR. Irie et al. [2006] found HNO3 differences of approximately 20% in the VMR profile, while Ejiri et al. (submitted manuscript, 2005) compared N2O and CH4 VMR profiles with ILAS-II profiles. The differences of 20% and 10%, respectively, are comparable to the differences found in this study. Other previous independent comparisons [e.g., Griesfeller, 2004; Kopp et al., 2002; Meier et al., 2005; Schneider et al., 2005] showed that the FTIR results are well established for validating satellite data.

[33] Differences in measurement time and location were constrained to less than 12 hours and 500 km. These criteria yielded 19 coincident measurements. A larger number of coincidences could have decreased differences and made the statistical results more robust.

[34] Some of the differences between the FTIR and ILAS-II could have arisen from local and spatial differences in the measurements, although no large variability in space and time is expected for the four gases, especially in September and October, when most of the coincidences were found. Another reason may be that the spectroscopic windows used by the FTIR and ILAS-II were different.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[35] We gratefully acknowledge the Goddard Space Flight Center for providing temperature and pressure profiles via the automailer system. The profiles were used for the inversion analysis. The ILAS-II project was funded by the Ministry of the Environment of Japan (MOE). A part of this research was supported by a Global Environment Research Fund provided by the MOE.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Measurements and Data Analysis
  5. 3. Results
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
jgrd12499-sup-0001-t01.txtplain text document1KTab-delimited Table 1.
jgrd12499-sup-0002-t02.txtplain text document0KTab-delimited Table 2.
jgrd12499-sup-0003-t03.txtplain text document1KTab-delimited Table 3.
jgrd12499-sup-0004-t04.txtplain text document0KTab-delimited Table 4.

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