Validation of the Improved Limb Atmospheric Spectrometer-II (ILAS-II) Version 1.4 nitrous oxide and methane profiles

Authors


Abstract

[1] This study assesses polar stratospheric nitrous oxide (N2O) and methane (CH4) data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) retrieved by the Version 1.4 retrieval algorithm. The data were measured between January and October 2003. Vertical profiles of ILAS-II volume mixing ratio (VMR) data are compared with data from two balloon-borne instruments, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) and the MkIV instrument, as well as with two satellite sensors, the Odin Sub-Millimetre Radiometer (SMR) for N2O and the Halogen Occultation Experiment (HALOE) for CH4. Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N2O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH4 (from 0 to 2 ppmv) in order to assess systematic differences between the ILAS-II and balloon/satellite data. According to this study, the characteristics of the ILAS-II Version 1.4 N2O and CH4 data differ between hemispheres. For ILAS-II N2O VMR larger than 250 ppbv, the ILAS-II N2O agrees with the balloon/SMR N2O within ±20% in both hemispheres. The ILAS-II N2O in the VMR range from 30–50 to 250 ppbv (corresponding to altitudes of ∼17–30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and ∼13–21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by ∼10–30% than the balloon/SMR N2O. For ILAS-II N2O VMR smaller than 30 ppbv (>∼21 km) in the SH, the differences between the ILAS-II and SMR N2O are within ±10 ppbv. For ILAS-II CH4 VMR larger than 1 ppmv (<∼25 km), the ILAS-II CH4 agrees with the balloon/HALOE CH4 within ±5% in the NH. For ILAS-II CH4 VMR larger than 0.3 ppmv in SH, the ILAS-II CH4 is ∼9% larger than the HALOE CH4; note that this positive systematic difference between the ILAS-II and HALOE CH4 has a seasonal dependence. Also note that the ILAS-II N2O for its VMR smaller than 50 ppbv (>∼30 km) and the ILAS-II CH4 for its VMR smaller than 1 ppmv (>∼25 km) only in the NH, are abnormally small compared to the balloon/satellite data.

1. Introduction

[2] Nitrous oxide (N2O) and methane (CH4) in the stratosphere are long-lived tracer species that originate in the troposphere. These species have natural and anthropogenic sources at ground level [e.g., Intergovernmental Panel on Climate Change (IPCC), 2001] and are well mixed in the troposphere. They enter the stratosphere mainly through the tropical tropopause and are transported upward and poleward. Oxidation and/or photodissociation destroy species in the stratosphere. The photochemical lifetimes of N2O and CH4 are longer than the dynamical timescale in the lower stratosphere, and are shorter in the upper stratosphere than in the lower stratosphere. Such long-lived and vertically stratified species can be useful tracers in dynamical studies. In addition, both species are related to ozone chemistry: N2O is a source of NOx, and CH4 is a source of H2O and consequently HOx [e.g., World Meteorological Organization (WMO), 1999]. Andrews et al. [1987] and Brasseur et al. [1999] have described the dynamical and chemical aspects of N2O and CH4, respectively.

[3] Ground-based spectrometers and instruments on board balloons and aircraft can measure local distributions of stratospheric N2O and CH4 in great detail, but these measurements cover only limited geographical areas. In contrast, satellite measurements can obtain global distributions of stratospheric N2O and CH4.

[4] The Stratospheric and Mesospheric Sounder (SAMS) on board the Nimbus 7 satellite [e.g., Jones and Pyle, 1984; Jones et al., 1986] was the first instrument that measured global distributions of stratospheric N2O and CH4. Many subsequent satellite instruments have yielded further measurements of stratospheric N2O and CH4. The Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) measured N2O and CH4 [e.g., Roche et al., 1996] as well as other species. The Improved Stratospheric and Mesospheric Sounder (ISAMS) on board UARS produced profiles of N2O and CH4 between 7 and 0.08 hPa from September 1991 to July 1992 [e.g., Remedios et al., 1996]. The Halogen Occultation Experiment (HALOE), also on board UARS, measured CH4 as well as other species since October 1991 [e.g., Park et al., 1996]. The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observations Satellite (ADEOS) provided N2O and CH4 data continuously from November 1996 to June 1997 [Kanzawa et al., 2003]. The Sub-Millimetre Radiometer (SMR), on board the Odin satellite launched in February 2001, has continued to provide global N2O measurements [Urban et al., 2005].

[5] The Improved Limb Atmospheric Spectrometer-II (ILAS-II) is a solar occultation instrument that was developed by the Ministry of the Environment (MOE) of Japan to succeed its predecessor ILAS. ILAS-II is one of the instruments on board the Advanced Earth Observing Satellite-II (ADEOS-II) spacecraft that was launched 14 December 2002 by the Japan Aerospace Exploration Agency (JAXA) [Nakajima et al., 2006]. ILAS-II recorded observations until 24 October 2003, measuring N2O and CH4 as well as ozone (O3), nitric acid (HNO3), nitrogen dioxide (NO2), water vapor (H2O), trichlorofluoromethane Freon 11 (CFC-11), dichlorodifluoromethane Freon 12 (CFC-12), chlorine nitrate (ClONO2), dinitrogen pentoxide (N2O5), and aerosol extinction coefficients. ILAS-II operated intermittently from January to March 2003 and continuously from 2 April to 24 October 2003. This paper describes validation of the ILAS-II Version 1.4 N2O and CH4 data by comparisons with independent balloon- and satellite-based measurements.

2. General Characteristics of ILAS-II Nitrous Oxide and Methane Profiles

2.1. ILAS-II Measurements

[6] ILAS-II operated as a solar occultation instrument, consisting of infrared (channel 1, 6.2–11.8 μm), midinfrared (channel 2, 3.0–5.7 μm), narrow-band (channel 3, 12.78–12.85 μm), and visible (753–784 nm) spectrometers and a Sun edge sensor. ILAS-II occultation events were at sunrise and sunset as seen from the ADEOS-II satellite during approximately 14 orbits per day. ILAS-II measurements between January and October 2003 covered latitudes 54° to 71°N and 64° to 88°S with seasonal variations as shown in Figures 1 and 2. Absorptions at around 7.8 and 7.4 μm as measured by the infrared (IR) spectrometer are used to detect N2O and CH4 molecules, respectively. The instantaneous field of view (IFOV) of the IR spectrometer at the tangent point was 1.0 km high and 13 km wide. Vertical profiles of atmospheric constituents, including N2O and CH4, were retrieved using an onion-peeling retrieval method [Yokota et al., 2002]. The retrieval altitude grid interval was 1.0 km, with vertical resolutions of 1.3 to 2.9 km at tangent heights of 15 to 55 km.

Figure 1.

Time-altitude section of the ILAS-II Version 1.4 N2O data from January to October 2003 using all available data in order of measurement time: (top) Northern Hemisphere using sunrise occultation measurements and (bottom) Southern Hemisphere using sunset occultation measurements. White means no data. Temporal changes in the ILAS-II latitude coverage are shown for each panel.

Figure 2.

As in Figure 1, but for ILAS-II Version 1.4 CH4 data.

[7] Figures 1 and 2 show time–altitude cross sections of the N2O and CH4 volume mixing ratios (VMRs) using all available data from January to October 2003. The upper panel shows the Northern Hemisphere (NH) cross section sampled by the ILAS-II sunrise occultations with time variations for the observation latitudes, and the lower panel shows the Southern Hemisphere (SH) cross sections sampled by the ILAS-II sunset occultations. Sunset occultation measurements in the SH extended a few kilometers lower than the sunrise measurements in the NH because Sun tracking by ILAS-II was easier at these lowest altitudes in the sunset mode than in the sunrise mode [Nakajima et al., 2006]. Figures 1 and 2 give the general characteristics of the ILAS-II Version 1.4 N2O and CH4 data. Obvious features include atmospheric descent in the Antarctic stratospheric winter (June–August, around 20–25 km) and mixing of midlatitude air masses with vortex air as the Antarctic polar vortex broke up in spring (October, around 30 km). These atmospheric transport effects are typical of the polar stratosphere.

2.2. Errors

2.2.1. Errors in the ILAS-II Version 1.4 N2O and CH4 Products

[8] In the ILAS Version 5.2 data, two error components were evaluated. The total error was calculated using the root sum square (RSS) of the two components [Yokota et al., 2002]. One component was the “internal error,” which was determined by residuals in the spectral fitting procedure. The other was the “external error,” which was determined by sensitivity tests assuming uncertainties in the data used for computing the transmittance. Basically, internal errors were estimated based on the assumption that spectral residuals were not systematic but were due to random errors. Recently, however, an in-depth study of spectral residuals has revealed that systematic spectral residuals of nonnegligible magnitude exist in common in the ILAS and ILAS-II measurement events (T. Yokota et al., unpublished manuscript, 2006). The internal errors estimated for the ILAS and ILAS-II data have therefore been overestimated as random errors when processing these data products. Accordingly, instead of the conventional internal error calculations based on spectral residuals, a “repeatability error” (see below) is employed as a measure of the random error (or measurement precision) for processing the ILAS-II Version 1.4 data products.

[9] Measurement repeatability is defined as the “closeness of the agreement between the results of successive measurements of the same measurand” [International Organization for Standardization, 1993]. The “repeatability error” in the ILAS-II Version 1.4 data product is based on measurement repeatability, calculated empirically as follows. The mean (equation image) and one sigma standard deviation (σ) of the ILAS-II N2O and CH4 VMRs were calculated for 100 consecutive occultation events (OEs) (∼7–8 days) at every 50 OEs repeatedly from April to October at each altitude level for each gas in each hemisphere. The smallest relative standard deviation (RSD), defined as equation image = σ/equation image was selected for each altitude level and defined as the measurement repeatability of the altitude level. In this way, the ɛ value calculated for the period when the variability in the N2O and CH4 VMRs was smallest during ILAS-II operations was selected as the measurement repeatability. The measurement repeatability represents an upper limit to the measurement precision because it may also include real geophysical variability. In practice, the retrieval value (ppmv) multiplied by the fractional value of the lowest RSD is set to the repeatability error (ppmv) for all of the events separately for both hemispheres in the ILAS-II Version 1.4 data files. The derivation of the repeatability error is described in detail by T. Yokota et al. (unpublished manuscript, 2006).

[10] The “external error”, which is also taken into consideration for the ILAS data retrieval, is calculated based on the errors from uncertainties in the United Kingdom Meteorological Office (MetO) temperature data that are used; the assumed uncertainties in temperature are ±2 K at 10 km and ±5 K at 70 km [Yokota et al., 2002]. Finally, the “total error” is calculated from the RSS of the repeatability error and the external error as the uncertainty for the ILAS-II Version 1.4. Table 1 shows the median values of retrieved VMRs, repeatability errors, and total errors in both mixing ratio units and percentages, for the ILAS-II Version 1.4 N2O and CH4 data. Repeatability errors are comparable to external errors below 20 km and exceed external errors above 30 km for both N2O and CH4 data in both hemispheres. The median values of the relative total errors for both N2O and CH4, except for N2O in the SH, are less than 10% below ∼30 km. The relative total errors for N2O below 30 km in the SH are larger than others; however, the absolute values of the total errors are smaller for N2O in the SH than for N2O in the NH. It is because the median values of the N2O VMRs below 30 km in the SH are much smaller than those in the NH that the relative total errors for N2O in the SH have such large values. There are two reasons for the small N2O VMRs in the SH. One reason is the effect of stratospheric descent in the Antarctic winter. ILAS-II made continuous observations for less than a full year, from April to October; therefore the median values of N2O VMRs in the SH are estimated from the ILAS-II data that were obtained mainly in winter, whereas the median values of N2O VMRs in the NH are estimated from data obtained mainly in summer. The other reason is that the negative systematic differences between ILAS-II and SMR N2O data in the SH are larger than those in the NH (see section 4.1.2). However, there is little difference in the median values of the ILAS-II CH4 VMRs below 30 km between hemispheres, even though CH4 VMRs in the SH are affected by the stratospheric descent. One reason for this situation may be the difference in the positive biases of the ILAS-II CH4 VMRs between the hemispheres; positive biases are seen in the SH but not in the NH (see section 4.2.2). The median values of VMRs, repeatability errors, and total errors in ppmv units of CH4 data above 30 km in the NH are much smaller than those in the SH. This is probably because the retrieved CH4 VMRs were always small (almost 0 ppmv) due to an ILAS-II instrument problem (see section 4.2).

Table 1. Summary of Error Analysis for the ILAS-II Version 1.4 N2O and CH4 Datad
Altitude, kmN2OCH4
NHSHNHSH
VMR,a ppbvRep. Err.or,b ppbvTotal ErrorcVMR,a ppbvRep. Err.or,b ppbvTotal ErrorVMR,a ppmvRep. Error,b ppmvTotal Error.cVMR,a ppmvRep. Error,b ppmvTotal Errorc
Parts per Million by VolumePercentParts per Million by VolumePercentParts per Million by VolumePercentParts per Million by VolumePercent
  • a

    Median of the volume mixing ratio (VMR) for all retrievals.

  • b

    Median of the repeatability error for all retrievals.

  • c

    Median of the total error for all retrievals.

  • d

    Median of the percentage of total error for all retrievals.

600.0010.040.041390.150.060.0634
50133100144950.020.010.01360.210.040.0417
4011146211390.040.010.01130.230.030.0314
3058346411190.430.030.0380.430.030.0410
20177814899612121.220.040.0541.230.020.065
152691025920571681.500.040.0751.350.040.086

2.2.2. Tangent Height Registration

[11] The tangent heights of the measurements were determined using the combined method (Comb-M), which combines a Sun edge sensor method (SES-M) above 30 km and a transmittance spectrum method (TS-M) below 30 km (for more details, see Nakajima et al. [2002] and T. Tanaka et al. (New tangent height registration method with the Version 1.4 data retrieval algorithm for the solar occultation sensor ILAS-II, manuscript in preparation, 2006)). This tangent height registration can introduce errors into the retrieved VMR profiles. The uncertainty in the tangent height registration below 30 km for the ILAS-II Version 1.4 retrieval includes systematic errors ranging from −180 to 180 m and random errors of ±30 m. A sensitivity study using the ILAS-II data shows that a height assignment of 100 (300) m higher than in the current retrieval causes increases of 5 (12)% and 5 (13)% in N2O and increases of 2 (7)% and 4 (14)% in CH4 at altitudes of 20 and 30 km, respectively. In contrast, height assignments 100 m lower than in the current retrieval cause 5 and 5% decreases in N2O and 2 and 4% decreases in CH4 at altitudes of 20 and 30 km, respectively.

2.2.3. Aerosol Extinction Evaluation of the IR Spectral Elements

[12] The nongaseous contribution correction method [Yokota et al., 2002] is required for derivations of vertical profiles of gaseous concentrations in altitude ranges where extinction cannot be neglected due to aerosol particles such as sulfate aerosols or polar stratospheric clouds (PSCs). Transmittances by gaseous components of four IR spectral elements for which absorption due to gaseous species is relatively low (“window spectral elements”) are evaluated to determine the nongaseous contribution in the simulated transmittance. To evaluate the gaseous contribution for each window spectral element, we use climatological values of gas VMR profiles (reference atmosphere model), as shown by T. Yokota et al. (unpublished manuscript, 2006). Subtraction of the gaseous contribution from the total extinction coefficient measured at each window spectral element yields the nongaseous contribution. The nongaseous contributions at the four IR window spectral elements are then linearly interpolated and extrapolated in wave number to yield the nongaseous transmittance for all 40 IR spectral elements.

[13] The linear interpolation used in the nongaseous contribution correction method can introduce systematic errors into derived gas profiles; this interpolation can result in systematic residuals in the spectral fitting if linearly interpolated extinction spectra differ greatly from true extinction spectra. Systematic errors due to linear interpolation are evaluated by simulating transmittances using several types of IR absorption spectra for sulfuric acid aerosols and PSCs as the nongaseous component and using the reference atmosphere model profiles (a priori profiles) as the gaseous component. These simulated transmittances are then used to retrieve vertical profiles of the gaseous concentration following linear interpolation to determine the nongaseous contribution for the Version 1.4 retrieval algorithm. Differences between a priori and retrieved N2O and CH4 profiles describe systematic errors produced by the linear interpolation. Table 2 summarizes the sulfate aerosol and PSC composition assumed in the simulations, a proportionality factor (α) that relates the systematic error of the gas number density to the aerosol extinction coefficient (AEC) at 780 nm (T. Yokota et al., unpublished manuscript, 2006), and the estimated systematic errors of N2O and CH4 concentrations at 20 km for the AEC at 780 nm of 5 × 10−4 km−1. Estimated systematic errors that arise from nitric acid trihydrate (NAT) are the largest among the errors of the several types of PSCs; the systematic errors are −43 ppbv and 0.48 ppmv for N2O and CH4, respectively. These estimated systematic errors cannot be neglected because they are as large as typical N2O and CH4 VMRs at 20 km in winter. The systematic errors for N2O and CH4 VMRs for any altitude can be estimated from air number densities and the AEC at 780 nm at the same altitude, using a proportional factor α.

Table 2. Systematic Errors in N2O and CH4 Caused by Optically Thick PSCs at 20 kma
 ICENATSTS(5, 37)bSTS(33, 15)bSTS(47, 3)bSTS(60, 0.5)bS(75)cS(50)c
  • a

    AEC = 5 × 10−4 km−1.

  • b

    STS(5, 37), STS(33, 15), STS(47, 3), and STS(60, 0.5) denote 5, 33, 47, and 60 wt % H2SO4/37, 15, 3, and 0.5 wt % HNO3/H2O supercooled ternary solutions of polar stratospheric clouds (PSCs), respectively.

  • c

    S(75) and S(50) are sulfuric acid aerosols whose components are 75 and 50 wt % H2SO4/H2O, respectively.

  • d

    The α is proportional factor that relates the systematic error of the number density of a gas to the 780 nm aerosol extinction coefficient (T. Yokota et al., unpublished manuscript, 2006).

  • e

    Mixing ratios were calculated assuming the typical air number density at 20 km (T. Yokota et al., unpublished manuscript, 2006).

HNO3, wt %371530.5  
H2SO4, wt %53347607550
N2O        
α,d10123.1−72−158.315112711
Systematic errors, 109 cm−33.1−72−158.31511135.5
Systematic errors,e ppbv1.8−43−9.25.09.16.98.03.3
CH4        
α,d 1013−5.58026−12−20−12−29−20
Systematic errors, 1010 cm−3−5.58026−12−20−12−15−9.8
Systematic errors,e ppmv−0.030.480.16−0.07−0.12−0.07−0.09−0.06

3. Data for Comparison

3.1. Balloon Data

[14] The ILAS-II Version 1.4 N2O and CH4 vertical profiles are first compared to profiles measured by two balloon-borne instruments. The Michelson Interferometer for Passive Atmospheric Sounding-Balloon-borne (MIPAS-B) instrument [Friedl-Vallon et al., 2004; Wetzel et al., 2006] is a cryogenic Fourier transform infrared (FTIR) spectrometer that measures thermal emissions from the limb of the atmosphere. The MkIV [Toon, 1991; Sen et al., 1998] instrument is a solar occultation FTIR spectrometer. Table 3 describes the characteristics of MIPAS-B and MkIV and also compares these balloon-borne sensors to ILAS-II.

Table 3. List of Balloon-Based Measurements Near Kiruna, Sweden (67.9°N, 21.1°E)a
SensorMIPAS-BMkIV
  • a

    From Friedl-Vallon et al. [2004], Wetzel et al. [2006], Toon [1991], and Sen et al. [1998].

  • b

    One-sigma values at 20 km.

  • c

    These were original measurement locations at 20 km. The location of MkIV measurements at 20 km after calculating forward isentropic trajectories was (61.9°N, 84.4°E). The locations of the coincident ILAS-II measurements at 20 km were (65.8°N, 26.1°E) and (64.5°N, 92.8°E) for the MIPAS-B and MkIV measurements, respectively.

  • d

    These values were at 20 km.

  • e

    Average of PV differences between 12 and 19 km.

MethodLimb Emission Fourier Transform Infrared SpectrometerSolar Occultation Fourier Transform Infrared Spectrometer
Vertical resolution, km (altitude range, km)2–3 (9–31)2 (7–33)
Accuracy,b %8 (N2O), 7 (CH4)5 (N2O), 5 (CH4)
Precision,b %6 (N2O), 5 (CH4)3 (N2O), 4 (CH4)
Observation date20 March 20031 April 2003
Measurement locationc(65.4°N, 28.2°E)(67.9°N, 32.9°E)
Distance,d1042500
Time differenced hours5.833.6
PV difference, %5d9e

[15] The MIPAS-B profiles have a vertical resolution of 2 to 3 km; the profiles were retrieved on a 1-km vertical grid. The accuracy and precision of the MIPAS-B measurements at 20 km are about 8 and 6% for N2O and 7 and 5% for CH4, respectively. The MIPAS-B data that are compared with the ILAS-II data were obtained near Kiruna, Sweden, (67.9°N, 21.1°E) on 20 March 2003. The ILAS-II and MIPAS-B observations have a time difference of 5.8 hours. The two observation points are 104 km apart and have a potential vorticity (PV) difference at 20 km altitude of 5%.

[16] The MkIV profiles have a vertical resolution of 2 km; the profiles were retrieved on a 1-km vertical grid. The accuracy and precision of the MkIV measurements at 20 km are about 5 and 3% for N2O and 5 and 4% for CH4, respectively. No ILAS-II measurements occurred on 1 April 2003; MkIV did measure data at Kiruna on that day. Forward isentropic trajectories from the time and location of the MkIV measurements on 1 April calculated using MetO wind fields are within 600 km and 1 hour of the ILAS-II measurements on 2 April between 12 and 19 km altitudes [Irie et al., 2006] (Figure 1). Average PV differences are 9% between the ILAS-II measurement point and the original MkIV measurement times and locations between 12 and 19 km altitudes.

3.2. Satellite Data

[17] Satellite N2O data derived from the Sub-Millimetre Radiometer (SMR) on board the Odin satellite and CH4 data derived by the Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) were compared to the ILAS-II Version 1.4 N2O and CH4 data. Tables 4 and 5 summarize the characteristics of the satellite-borne sensors (SMR, HALOE, and ILAS-II) and the coincident measurements, respectively.

Table 4. List of Satellite Measurements
Satellite/SensorOdin SMRUARS/HALOEADEOS-II/ILAS-II
MethodLimb Emission SounderSolar Occultation SensorSolar Occultation Sensor
Latitudinal coverage82.5°N–82.5°S80°N–80°S54–71°N, 64–88°S
Measurement frequency90 s/measurement, 10 days/month15 measurements/day/occultation mode14 measurements/day/hemisphere
Vertical resolution, km (altitude range, km)≤4 (7–70)4.5 (15–75)∼2 (≤70)
Wavelength for N2O, μm596.87.8
Wavelength for CH4,μm3.37.4
Table 5. Summary of Coincident Measurements From January to 21 October 2003
SensoraPeriodNumber of MatchesAverage Distance, kmTime Difference, hoursHemisphereOccultation Mode
HALOEILAS-II
  • a

    Criteria for SMR are r ≤ 300 km, dt ≤ 12 hours and for HALOE are r ≤ 600 km, dt ≤ 12 hours, dPV ≤ 20%difference at 20 km, where r is distance at 20 km and dt is time, and dPV is PV difference at 20 km. SR, sunrise; SS, sunset.

SMRJan–Oct, except Feb2742016.3NHSR
SMRJan–Oct, except Mar7842016.5SHSS
HALOEApr144389.5NHSRSR
HALOEMay153830.4NHSSSR
HALOESep504679.6NHSRSR
HALOESep445140.3NHSSSR
HALOEJan13160.6SHSSSS
HALOEOct25905.4SHSRSS

[18] Odin/SMR was the first space-borne sensor to use passive submillimeter wave heterodyne spectroscopy to observe the global distribution of stratospheric N2O. The Odin satellite that carries SMR is a Swedish-led project that is supported by France, Canada, and Finland. Regular measurements started in November 2001 and consist of ten observation days per month. Version 1.2 (Chalmers v1.2) data, a product of the Chalmers University of Technology, Göteborg (Sweden), are based on a processing scheme that focuses on fast operational analysis of the Odin/SMR measurements. Urban et al. [2005] compared Version 1.2 N2O VMRs with balloon-, aircraft-, and satellite-based N2O measurements. Compared with balloon-based and MIPAS/Envisat measurements, Odin/SMR Version 1.2 N2O VMRs have positive biases (from a few percentage points to 20%) for N2O VMRs exceeding 75 ppbv; the root-mean-square (RMS) deviation is ∼10%. Coincident ILAS-II and SMR data pairs meet time and space difference criteria of ±12 hours and 300 km at 20 km altitude. Coincident matches of 274 and 784 occur in the NH and SH, respectively, throughout the ILAS-II observation period.

[19] The HALOE Version 19 CH4 data were also compared to the ILAS-II Version 1.4 CH4 data. HALOE is on an inclined-orbit satellite and can measure global solar occultation events. HALOE data are collected 15 times daily for both sunrises and sunsets and provide coverage of one hemisphere in a month long period. The total error, including systematic and random components, is less than 15%, and the precision is better than 7% between 0.3 and 50 hPa (approximately 20–75 km altitudes) [Park et al., 1996]. Coincident ILAS-II and HALOE data pairs meet the time and space difference criteria of ±12 hours and 600 km at 20 km altitude. In April, May, and September, 123 coincident pairs occur in the NH, and 3 coincident pairs occur in January and October in the SH. The ILAS-II and HALOE data were compared at altitudes where PV differences between the two satellite observation points are less than 20%.

4. Comparison With Balloon and Satellite Data

[20] The difference (Da) and relative percentage difference (Dp), defined as

equation image
equation image

are used to evaluate the ILAS-II Version 1.4 vertical profiles of N2O and CH4. The Da and Dp values are given as absolute values and percentages (%), respectively, for each altitude. [VMR]ILAS-II and [VMR]Validation denote VMRs of N2O or CH4 from the ILAS-II and from the balloon/satellite measurements, respectively. We use quartiles (the first and third quartiles) to show variability of the VMR data and Dp values. The first (third) quartile of the sample data is the median of the former (latter) half of the data that are smaller (larger) than the overall median. (Note that the second quartile coincides with the overall median.). Consequently, 50% of the data is between the first and third quartiles.

[21] PSCs in the line of sight of ILAS-II may have introduced systematic errors into the retrieved VMRs. These systematic errors depend on the types and particle size distributions of the PSCs because of the nongaseous contribution correction in the ILAS-II Version 1.4 algorithm (see section 2.2.3). Data that may include the systematic errors are removed as follows. The systematic errors are estimated assuming that all observed nongaseous components (the ILAS-II Version 1.4 AEC at 780 nm) are NAT that yield the largest systematic errors (see Table 2). Data are not used in this validation when the systematic errors exceed a value equivalent to 15% of the retrieved VMR below 25 km.

[22] The Royal Greenwich Observatory (RGO) sunspot data provided by the U.S. National Oceanic and Atmospheric Administration (NOAA) and U.S. Air Force (USAF) have extremely large sunspots on the solar disk during the final 3 days (22–24 October 2003) of the ILAS-II observation period. During these 3 days, the ILAS-II IFOV included sunspots that affected the observed solar scan data. Most ILAS-II N2O and CH4 profiles on these 3 days also show abnormal oscillations. The large sunspots likely affected the ILAS-II products, including N2O and CH4 data; thus the data from 22 to 24 October are discarded from the validation analysis.

4.1. N2O

4.1.1. Comparison With Balloon-Borne Measurements

[23] Figure 3 compares ILAS-II N2O profiles with MIPAS-B (Figure 3a) and MkIV (Figure 3b) profiles (see Table 3 for coincidence details). The left-hand panels show vertical profiles of N2O and error bars. Error bars for the ILAS-II and balloon measurements correspond to the total error and the one-sigma measurement precision, respectively. The middle and right-hand panels show Da and Dp values against the balloon measurements, respectively. The dotted lines represent absolute values of the RSSs for both errors in the middle panels and relative RSSs in the right-hand panels. According to the PV map on 20 March 2003, based on the MetO data, the measurement locations of MIPAS-B and ILAS-II were inside the polar vortex at 475 K (corresponding to an altitude of ∼20 km). The ILAS-II and MIPAS-B N2O VMRs above ∼20 km are almost zero, because air masses at higher altitudes characterized by a low N2O VMR (e.g., above 30–40 km), had descended to around 20 km. The ILAS-II and MIPAS-B VMR profiles have similar shapes, especially below 18 km, where the ILAS-II N2O VMRs exceed 100 ppbv (Figure 3a). The ILAS-II N2O VMRs are slightly smaller than those of MIPAS-B between 14 and 31 km. The Dp values for ILAS-II and MIPAS-B are within −20% below 18 km. The ILAS-II N2O VMRs and the magnitude of Da values (∣Da∣) for ILAS-II and MIPAS-B are smaller than ∼20 and ∼10 ppbv above 20 km, respectively, while the Dp values oscillate between ∼−50 and ∼30%.

Figure 3.

Vertical profiles of the N2O volume mixing ratio (VMR) from ILAS-II and two balloon measurements for (a) MIPAS-B and (b) MkIV near Kiruna, Sweden (67.9°N, 21.1°E), and the absolute and relative percentage differences, respectively (Da and Dp, see section 4 for the definition). The ILAS-II error bar shows the total error as defined in the text. The error bars in the balloon measurements have one-sigma measurement precision. Dotted lines in the middle and right show the absolute and relative values of the root sum squares (RSS) of both errors, respectively.

[24] Figure 3b compares the N2O profile from ILAS-II with that from MkIV. According to the PV map on 2 April 2003, the measurement location of the ILAS-II and the forward isentropic trajectories from the measurement location of MkIV were outside the polar vortex at 475 K (corresponding to an altitude of ∼20 km). The ILAS-II N2O VMRs are significantly smaller than the MkIV N2O VMRs from 12 to 19 km. The N2O VMR profile from ILAS-II agrees with that from MkIV between 18 and 19 km, and the Dp values for ILAS-II and MkIV are approximately −11%, with the absolute value being almost equal to the combined error bars. The N2O VMR profiles from the ILAS-II and MkIV differ noticeably below 18 km; the Dp values for the ILAS-II and MkIV are ∼−13% between 12 and 13 km and exceed ∼−25% from 14 to 17 km.

[25] Figure 4 shows Dp values as a function of ILAS-II N2O VMRs to reveal systematic differences between the ILAS-II and balloon data in the NH. The Dp values vary between −60 and 30% when ILAS-II N2O VMRs are smaller than 50 ppbv. When ILAS-II N2O VMRs exceed 100 ppbv, however, the Dp values vary little, and the average Dp values between ILAS-II and MIPAS-B or MkIV are −10 or −18%, respectively.

Figure 4.

Dp values between ILAS-II and balloon measurements as a function of the ILAS-II N2O VMR. Dots (triangles) show the Dp values between the ILAS-II and MIPAS-B (MkIV). The dotted line shows the average of the RSSs of both errors divided by the balloon N2O VMR.

4.1.2. Comparison With Odin/SMR

[26] Figure 5 compares ILAS-II N2O profiles with SMR profiles statistically in the NH (Figure 5a) and SH (Figure 5b) (see Table 5 for coincidence details). The median and quartile values of the N2O VMRs and the average errors are calculated at each altitude level for each satellite profile. These values are shown in the left-hand panels in Figure 5. The median values of Da and Dp (Damed, Dpmed) between the ILAS-II and SMR N2O VMRs and the average of absolute and relative RSSs of both errors are calculated at each altitude level and are shown in the middle and right-hand panels, respectively. Figure 5a (left) shows that the ILAS-II N2O is smaller than the SMR N2O at all altitude levels in the NH. The Dpmed values show that the ILAS-II N2O is ∼5–25% smaller than the SMR N2O below ∼30 km. The magnitude of the Dpmed values (∣Dpmed∣) increases to ∼95% above ∼30 km altitude in the NH, while the ILAS-II N2O VMRs and the magnitude of the Damed values (∣Damed∣) are smaller than ∼10 ppbv and ∼10 ppbv above ∼40 km, respectively. Figure 5b (left) indicates that the ILAS-II N2O is smaller than the SMR N2O at all altitude levels in the SH, as in the NH. The Dpmed values in the SH show that the ILAS-II N2O is ∼25% smaller than the SMR N2O in the altitude range from 15 to 20 km. The Dpmed values below 15 km in the SH are ∼−10–20%; these Dpmed values are within the RSSs of both errors. The Dpmed values above ∼20 km are ∼−10–70%, while the ILAS-II N2O VMRs and ∣Damed∣ values are smaller than ∼30 and ∼10 ppbv, respectively.

Figure 5.

Comparison of median profiles between ILAS-II and Odin/SMR (Version 1.2) N2O VMRs in the (a) Northern and (b) Southern hemispheres and the median Da and Dp values. The error bars show the average total error for ILAS-II N2O and the average one-sigma measurement precision for SMR N2O at each altitude level; the dotted lines show quartiles of the ILAS-II and SMR N2O VMRs. (middle and right) The dashed lines show the average of the absolute and relative values of the RSSs of both errors at each altitude level, respectively.

[27] Figure 6 compares ILAS-II and SMR N2O VMRs in the NH (left panel) and SH (right panel). As in Figure 4, the Dp values in Figure 6 are shown as a function of ILAS-II N2O VMRs on the vertical axis. ILAS-II N2O VMRs from 0 to 400 ppbv are divided into 40 bins of 10 ppbv widths. Dpmed and quartiles of Dp values for the ILAS-II and SMR N2O VMRs and the average of the relative RSSs of both errors are calculated in each bin and are shown in Figure 6. Systematic differences between the ILAS-II and SMR N2O data are estimated using the Dpmed values. The ILAS-II N2O VMRs are smaller than the SMR N2O VMRs in both hemispheres when the ILAS-II N2O VMRs are smaller than 300 ppbv.

Figure 6.

Shown are the Dp values between the ILAS-II and Odin/SMR N2O VMRs in the (left) Northern and (right) Southern hemispheres. The vertical and horizontal axes are the ILAS-II N2O VMR and the Dp values, respectively. The ILAS-II N2O VMR between 0 and 400 ppbv was divided into 40 bins of 10 ppbv. Median (dots) and quartile (solid lines) of Dp values for the ILAS-II and SMR N2O and the average of the relative RSS of both errors (dotted lines) are calculated for every bin.

[28] The average Dpmed is calculated for four ranges of the ILAS-II N2O VMRs to give rough estimates of the systematic differences between the ILAS-II and SMR N2O products. The four ranges are <50, 50–250, 250–300, and >300 ppbv. The ILAS-II N2O agrees with the SMR N2O within ±20% in both hemispheres when ILAS-II N2O VMRs exceed 250 ppbv (Figure 6). The Dpmed values are negative when the VMRs are between 250 and 300 ppbv and positive when the VMRs exceed 300 ppbv. For the smaller VMR range (50–250 ppbv), the average Dpmed is ∼−17 and ∼−26% in the NH and SH, respectively. For the smallest VMR range (<50 ppbv), the ∣Dpmed∣ values increase as ILAS-II N2O VMRs decrease in the NH. In the SH, the average Dpmed is ∼−33% in the smallest VMR range; however, Dp values vary widely for N2O VMRs smaller than 30 ppbv (the difference between the first and third quartiles is ∼60%), while Dpmed values and quartiles of Dp values are within the RSS errors.

4.2. CH4

4.2.1. Comparison With Balloon-Borne Measurements

[29] Figure 7 compares ILAS-II CH4 profiles with MIPAS-B (Figure 7a), and MkIV (Figure 7b) profiles (see Table 3 for coincidence details). (Figures 7a left and 7b left) show vertical profiles of CH4 and error bars. Error bars for ILAS-II and balloon measurements correspond to the total error and the one-sigma measurement precision, respectively. (Figures 7a middle and right and 7b middle and right) show Da and Dp values against the balloon measurements, respectively. The dotted lines represent absolute values of RSSs for both errors in the middle panels and relative RSSs in (Figures 7a right and 7b right). The vertical profiles of CH4 from ILAS-II and MIPAS-B have similar shapes from 14 to 21 km (Figure 7a). The ILAS-II CH4 VMRs agree particularly well with the MIPAS-B CH4 VMRs below 18 km, where the ILAS-II CH4 VMRs exceed 1 ppmv; in that region, the Dp values are as low as −5 to 3%. Above 19 km, the ILAS-II CH4 VMRs decrease rapidly with altitude compared to the MIPAS-B CH4 VMRs. For example, the ILAS-II CH4 VMR at 30 km is 0.06 ppmv, while the MIPAS-B CH4 VMR is 0.4 ppmv.

Figure 7.

As in Figure 3, but for the ILAS-II Version 1.4 CH4 data.

[30] Figure 7b compares the CH4 profile from ILAS-II with that from MkIV. The ILAS-II CH4 is ∼8% smaller than the MkIV CH4 below 18 km, but the profiles show very good agreement at 18–19 km. The measurement location of ILAS-II on 2 April 2003 was outside the polar vortex. Above 25 km, where ILAS-II CH4 VMRs are smaller than ∼1 ppmv, the ILAS-II CH4 VMRs decrease rapidly with altitude (−0.12 ppmv/km) compared to the expected CH4 VMR profile outside the polar vortex.

[31] Figure 8 shows Dp values as a function of ILAS-II CH4 VMR to reveal systematic differences between the ILAS-II and balloon data. When ILAS-II CH4 VMRs exceed 1 ppmv, the ILAS-II and balloon measurements agree well, and the Dp values are ±∼10%. When ILAS-II CH4 VMRs are smaller than 1 ppmv, the Dp values against the MIPAS-B measurements exceed −15%. For ILAS-II CH4 VMRs smaller than 0.3 ppmv, the ∣Dp∣ values increase as VMRs decrease.

Figure 8.

As in Figure 4, but for the ILAS-II Version 1.4 CH4 data.

4.2.2. Comparison With UARS/HALOE

[32] Figure 9 compares ILAS-II CH4 profiles with HALOE profiles statistically in the NH (Figure 9a) and SH (Figure 9b) (see Table 5 for coincidence details). As in Figure 5, the median and quartile values of CH4 VMRs and the average errors are calculated at each altitude level for each satellite profile and are shown in the left-hand panels. The Damed and Dpmed values between the ILAS-II and HALOE CH4 VMRs and the average of absolute and relative RSSs of both errors are calculated at each altitude level and are shown in the middle and right-hand panels, respectively. Figure 9a (left) shows that the ILAS-II CH4 agrees with the HALOE CH4 below ∼25 km very well in the NH. Above 25 km in the NH, however, the ILAS-II CH4 decreases and the ∣Damed∣ values increase drastically with increasing altitude. The Dpmed values in the NH show that the ILAS-II and HALOE CH4 VMRs agree within ±5% below ∼25 km. However, the ∣Damed∣ and ∣Dpmed∣ values above ∼25 km increase rapidly as altitude increases. According to monthly profiles of the Dpmed values in the NH, the altitude at which the ∣Dpmed∣ values start to increase depends on the month (season), i.e., 23 km for April, 24 km for May, and 26 km for September (data not shown). Figure 9b shows the results of the statistical comparison between the ILAS-II and HALOE CH4 VMR profiles obtained in January (one match) and October (two matches) in the SH. The median profiles of CH4 VMRs in the left-hand panel show very small VMRs at around 30 km because these profiles reflect mainly characteristics of the CH4 VMR profiles obtained in October, when the upper part (above ∼30 km altitude) of the polar vortex broke up but the lower part (below ∼30 km altitude) of the polar vortex still existed. Figure 9b shows that the ILAS-II CH4 is approximately 0.1 ppmv larger than the HALOE CH4 at all altitude levels in the SH, while most of the Damed and Dpmed values are within the RSSs of both errors.

Figure 9.

As in Figure 5, but for the ILAS-II and HALOE (Version 19) CH4 data.

[33] Figure 10 compares ILAS-II and HALOE CH4 VMRs in the NH (Figure 10, left) and SH (Figure 10, right). The Dp values are shown as a function of the ILAS-II CH4 VMR on the vertical axis as in Figure 8. The ILAS-II CH4 VMRs between 0 and 2.0 ppmv are divided into 40 bins, each 0.05 ppmv (50 ppbv) wide; Dpmed and quartiles of Dp values for the ILAS-II and HALOE CH4 VMRs and the average of the relative RSSs of both errors are calculated for every bin. Figure 10 shows the Dpmed, the quartiles of Dp values, and the average RSS errors as dots, solid lines, and dotted lines, respectively. The Dpmed values differ noticeably between hemispheres. When the ILAS-II CH4 VMRs are smaller than 1 ppmv in the NH, the ∣Dpmed∣ values increase drastically as the ILAS-II CH4 VMRs decrease. When the ILAS-II CH4 VMRs are larger than 1 ppmv in the NH, the ILAS-II CH4 agrees with the HALOE CH4 within ±∼5%. The ILAS-II CH4 in the SH are 6 and 13% larger than the HALOE CH4 when the ILAS-II CH4 VMRs are smaller and larger than 1 ppmv, respectively; all Dpmed values are within the combined error bars. Note that the HALOE CH4 data are available for coincident comparison with ILAS-II CH4 VMRs exceeding 0.3 ppmv only in January and October in the SH (see Table 5).

Figure 10.

As in Figure 6, but for the ILAS-II and HALOE CH4 data. The ILAS-II CH4 VMR data between 0 and 2 ppmv were divided into 40 bins of 0.05 ppmv.

5. Discussion

5.1. Northern Hemisphere (NH)

[34] This subsection for the NH focuses on negative systematic differences between the ILAS-II and balloon/SMR N2O data for ILAS-II N2O VMRs between 50 and 250 ppbv, unrealistically small VMRs in the ILAS-II N2O and CH4 compared to the balloon/satellite data in the smallest VMR ranges (ILAS-II N2O VMR < 50 ppbv [corresponding to altitudes above ∼30 km outside the polar vortex]; ILAS-II CH4 VMR < 1 ppmv [corresponding to altitudes above ∼25 km outside the polar vortex]), and characteristics of the ILAS-II N2O and CH4 data that are revealed by the N2O-CH4 correlation for January through October.

[35] The ILAS-II N2O has negative systematic differences in comparison to three validated measurements. For ILAS-II N2O VMRs between 50 and 250 ppbv, the ILAS-II data differ from the MIPAS-B, MkIV, and SMR data by −12% (average Dp), −23% (average Dp), and −17% (average Dpmed), respectively. When N2O VMRs exceed 75 ppbv, the SMR Version 1.2 N2O data have a positive bias (from a few percentage points to 20%) in comparison with balloon-based and MIPAS/Envisat measurements [Urban et al., 2005]. The systematic difference between the ILAS-II and SMR N2O (−17%) is consistent with that in the ILAS-II N2O versus the MIPAS-B N2O (−12%), if the positive biases in the SMR N2O is considered. The systematic difference between the ILAS-II and MkIV N2O profiles (−23%) is more negative than in the other cases (−12%, −17%) because ILAS-II and MkIV sampled different air masses, as detailed below. As shown in Figure 3b, the ILAS-II and MkIV N2O values obtained at 18–19 km agree within the combined error bars; however, the VMR profiles from the ILAS-II and MkIV measurements have different shapes below 18 km. The ILAS-II CH4 VMR profile also agrees with the MkIV CH4 VMR profile at 18–19 km and has a different shape below 18 km, as shown in Figure 7b. In addition, a similar tendency occurs in O3 VMR profiles based on the ILAS-II and MkIV measurements (data not shown). The similarities between N2O, CH4, and O3 are reasonable because O3 is also a long-lived tracer; little or no ozone loss likely occurred in the 34 hours between the ILAS-II and MkIV measurements (Table 3). Below 18 km, the ILAS-II VMR profiles of N2O, CH4, and O3 all differ in shape from the MkIV measurement profiles. Such differences suggest that the air mass properties observed by ILAS-II differed from those observed by MkIV. As other supporting evidence, PV differences between the ILAS-II measurement point and the original MkIV measurement times and locations are small between 17 and 19 km, i.e., 6% at 18–19 km and 7% at 17 km, but oscillate from −11 to 17% below 16 km (data not shown). This validation of the ILAS-II data by comparison with the MkIV data therefore focuses on measurements taken between 18 and 19 km only. Figure 3b shows that the average Dpmed is −11% at 18–19 km (within the N2O VMR ranges from 50 to 250 ppbv); these values are consistent with other coincident pairs. In conclusion, the systematic difference between the ILAS-II and balloon/SMR N2O in the NH is approximately −10% when the ILAS-II N2O VMRs are between 50 and 250 ppbv.

[36] A possible cause of this negative systematic difference between the ILAS-II and balloon/SMR N2O is the uncertainty in the tangent height registration (see section 2.2.2). The uncertainty has a maximum of ±210 m (systematic error ranging from −180 to 180 m and random error of ±30 m). If the height assignment were 200 m higher than in the current retrieval, the magnitude of the negative difference between the ILAS-II and balloon/SMR N2O would be smaller than the current difference (∼−10%) and a positive difference (several percentages) would be appear in the ILAS-II CH4 versus the balloon/HALOE CH4. In this case, the ILAS-II N2O (>50 ppbv) and CH4 (>1 ppmv) may possibly agree with the balloon/satellite data within the RSS errors. However, the ILAS-II N2O and CH4 in the smallest VMR ranges (N2O VMR < 50 ppbv; CH4 VMR < 1 ppmv) would still be abnormally small compared to the balloon/satellite data in the NH, even if the 200 m higher altitude were employed for the height assignments. The ILAS-II N2O and CH4 VMRs at the smallest VMR ranges decrease rapidly compared to the balloon/satellite data as altitudes increase; therefore the ∣Dp∣ values between the ILAS-II and balloon/satellite measurements increase drastically with increasing altitude. Such a drastic increase in the ∣Dp∣ values suggests that unresolved problems remain in the ILAS-II Version 1.4 retrieval algorithm. One is that the detected spectrometer signal changes abnormally with time because the entrance slit could be distorted by solar energy under direct sunlight [Nakajima et al., 2006]. This abnormal change in the signal is especially prominent during sunrise occultations, i.e., in the NH measurements. Sugita et al. [2006] reported that a similar increase in ∣Dp∣ values also occurs in O3 data at high altitudes. The magnitude of the effect induced by this abnormal change in the signal depends on the species, the VMR values of each species, and the tangent height (T. Yokota et al., unpublished manuscript, 2006). Monthly comparisons of the ILAS-II and balloon/satellite data versus altitudes (the same format as in Figures 5 and 9) and VMR levels (the same format as in Figures 6 and 10) show that the drastic increase in ∣Dp∣ values correlates to ILAS-II N2O and CH4 VMRs rather than to altitude (data not shown). Note that the ILAS-II Version 1.4 N2O and CH4 in the smallest VMR ranges (N2O VMR < 50 ppbv; CH4 VMR < 1 ppmv) in the NH are abnormally small.

[37] Figure 11 shows monthly correlations between N2O and CH4 in March (winter) and June (summer) in the NH. Figure 11 includes reference curves from the Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS), which measured N2O and CH4 VMRs over a wide range of altitudes and latitudes in both hemispheres from 1992 to 1994 [Michelsen et al., 1998]. The MIPAS-B and MkIV measurements taken near Kiruna, Sweden, in 2003, are also plotted in Figure 11 for reference. Kanzawa et al. [2003] noted that when N2O VMRs exceed 20–30 ppbv (corresponding to altitudes below ∼25–35 km), the ILAS CH4 are much larger in winter than expected from the N2O-CH4 correlations of ATMOS and balloon measurements. However, such large differences in winter are not present in the ILAS-II CH4 for winter in the NH. On the contrary, ILAS-II CH4 VMRs smaller than 1 ppmv are smaller than expected from N2O-CH4 correlations of the ATMOS and balloon data in both summer and winter. One explanation for the difference between the ILAS and ILAS-II N2O-CH4 correlations is that CH4 VMRs in the NH have a positive bias in the ILAS data but no positive bias in the ILAS-II data. In addition, the ILAS-II N2O and CH4 are abnormally small compared to the balloon/satellite data in the smallest VMR ranges (ILAS-II N2O VMR < 50 ppbv; ILAS-II CH4 VMR < 1 ppmv). For CH4 VMRs exceeding 1 ppmv, ILAS-II N2O-CH4 correlations show good agreement with the N2O-CH4 correlation from the ATMOS and balloon data in both summer and winter.

Figure 11.

Correlations (shown as dots) between N2O and CH4 data for March and June in the NH. Data are inside (red), on the boundary of (green), or outside (blue) the polar vortex, or are not classified (black) because the polar vortex cannot be identified. Reference curves obtained from the ATMOS space shuttle measurements are shown. The MIPAS-B and MkIV balloon measurements over Kiruna in 2003 are also plotted. It should be noted that several abnormally small data as compared to the reference curves found in CH4 around 300 ppbv N2O (corresponding to altitudes below 15 km) for June are due to inadequateness of the nongaseous contribution correction method.

5.2. Southern Hemisphere (SH)

[38] This subsection for the SH discusses the negative systematic differences between the ILAS-II and balloon/SMR N2O between 30 and 250 ppbv, the positive systematic difference between the ILAS-II and HALOE CH4, and the seasonal dependence of the positive systematic difference in the ILAS-II CH4 revealed by the N2O-CH4 correlation.

[39] The average Dpmed for the ILAS-II and SMR N2O in the SH is −26% when ILAS-II N2O VMRs are between 50 and 250 ppbv. The quality of the SMR N2O data does not differ between hemispheres; therefore the negative systematic difference between the ILAS-II Version 1.4 N2O and the SMR N2O is approximately −20% in this N2O VMR range if the positive bias in the SMR version 1.2 N2O data (see section 5.2.) is considered. In Figure 6, when ILAS-II N2O VMRs are smaller than 50 ppbv, all Dpmed are within the RSS errors. For ILAS-II N2O VMRs smaller than 30 ppbv (corresponding to altitudes above ∼20 km for inside the polar vortex), the Dp values vary (the difference between the first and third quartiles is ∼60%), while the ∣Damed∣ values are smaller than ∼10 ppbv. However, when ILAS-II N2O VMRs are between 30 and 50 ppbv, the quartiles of the Dp values are relatively small (±∼18%), and all Dpmed values are around 36%. Therefore the negative systematic difference between the ILAS-II Version 1.4 N2O between 30 and 50 ppbv and the SMR N2O is ∼30% after considering the positive bias of the SMR N2O.

[40] The ILAS-II CH4 in the SH are 6 and 13% larger than the HALOE CH4 when the ILAS-II CH4 VMRs are smaller and larger than 1 ppmv, respectively. As shown in the right panel of Figure 10, the ILAS-II and HALOE CH4 agree within the combined error bars at all CH4 VMR levels. However, most Dpmed values are around ∼9%, which is the average Dpmed for all VMR levels. We therefore conclude that the positive systematic difference between the ILAS-II and HALOE CH4 is ∼9% in the SH. Note that the HALOE CH4 data are available for coincident comparison with ILAS-II CH4 VMRs exceeding 0.3 ppmv only in January and October in the SH.

[41] Figure 12 shows monthly correlations between N2O and CH4 in March (summer) and June (winter) in the SH. Figure 12 also includes reference curves obtained by ATMOS, as in Figure 11, and reference data obtained by SAKURA, a cryogenic sampler that measured N2O and CH4 VMRs over the Syowa Station (69°S, 49°S), Antarctica, on 3 January 1998 [Nakazawa et al., 2002]. Where ILAS-II N2O VMRs range from 30 to 100 ppbv (corresponding to altitudes of ∼20–25 km), the ILAS-II CH4 VMRs in winter are larger than in summer; i.e., the differences between the reference curves and the ILAS-II N2O-CH4 correlation in winter exceed the differences in summer. Monthly comparison of the ILAS-II N2O with the SMR N2O from January to October in the SH reveals no remarkable seasonal dependence on the negative systematic differences between the ILAS-II N2O between 30 and 250 ppbv and the SMR N2O (data not shown). Therefore the positive systematic difference between the ILAS-II and HALOE CH4 must change with the season. The ILAS data also included a similar seasonal dependence on the degree of high bias in CH4 data [Kanzawa et al., 2003]. The cause of this seasonal dependence remains unknown.

Figure 12.

As in Figure 11, but for the SH. SAKURA balloon measurements over Syowa, Antarctica, in 1998 are included in the plot.

5.3. Hemispheric Differences in the Systematic Difference in N2O and CH4 Data

[42] The negative systematic difference between the ILAS-II N2O between 50 and 250 ppbv and the balloon/SMR N2O is ∼10% larger in the SH than in the NH. The positive systematic difference between the ILAS-II and HALOE CH4 is ∼9% in the SH. In contrast, there is no systematic difference between the ILAS-II and balloon/HALOE CH4 in the NH when ILAS-II CH4 VMRs exceed 1 ppmv. Such differences in the systematic difference between hemispheres are not found in the ILAS N2O and CH4 data [Kanzawa et al., 2003]. Two factors may have affected the systematic differences in the ILAS-II N2O and CH4 data between the hemispheres. One possible cause is that PSCs may have still affected the results of the comparison, although systematic errors that arise from the presence of PSCs are partly eliminated in this validation by the following methods. Data were not considered when the systematic errors exceeded a value equivalent to 15% of the retrieved VMRs below 25 km. These systematic errors are estimated using the observed AEC at 780 nm assuming that NAT yields the largest systematic errors (see Table 2). The other cause is that the abnormal change in the detected spectrometer signal in the NH measurements may have affected not only the ILAS-II N2O and CH4 VMRs in the smallest VMR ranges (ILAS-II N2O VMR < 50 ppbv; ILAS-II CH4 VMR < 1 ppmv) but also those in the larger VMR ranges (ILAS-II N2O VMR range of 50 to 250 ppbv; ILAS-II CH4 VMR exceeding 1 ppmv). However, the extent of these effects and the precise cause of the differences in the data quality of the ILAS-II N2O (where the N2O VMR was 50–250 ppbv) and CH4 (where the CH4 VMR exceeded 1 ppmv) between hemispheres are still under investigation.

6. Summary

[43] To evaluate the quality of the ILAS-II Version 1.4 N2O and CH4 data, comparisons to N2O and CH4 measurements from the balloon-borne instruments MIPAS-B and MkIV and to data obtained from two satellite-based sensors, Odin/SMR (for N2O) and UARS/HALOE (for CH4) were performed. Systematic errors that arise from the nongaseous contribution corrections in the ILAS-II Version 1.4 algorithm are partly suppressed by eliminating data from the validation when the systematic errors, estimated using the observed AEC at 780 nm assuming NAT, exceed 15% of the retrieved VMRs below 25 km. In addition, data affected by large sunspots (22–24 October 2003) are not used in this validation. The systematic differences between the ILAS-II and balloon/satellite data are quantified after calculating the median (Dpmed) of the relative percentage differences (Dp) of these data in 10-ppbv-wide bins for N2O (between 0 and 400 ppbv) and 0.05-ppmv-wide bins for CH4 (between 0 and 2.0 ppmv). Table 6 shows the average Dpmed for four ranges of the ILAS-II N2O VMRs (<50, 50–250, 250–300, and ≥300 ppbv) and for two ranges of the ILAS-II CH4 VMRs (<1 and ≥1 ppmv).

Table 6. Summary of Comparisons
N2O, ppbvCorresponding AltitudeBalloonsOdin SMR
Outside PV, kmInside PV, kmMIPAS-B, %MkIV,a%NH, %SH, %
  • a

    ILAS-II data were compared with MkIV data at 18–19 km altitudes.

  • b

    No coincidence.

  • c

    ILAS-II data have abnormal tendencies.

<50∼30<∼20<−31bc−33
50–250∼17–∼30∼13–∼20−12−11−17−26
250–300<∼17<∼13−5b−7−4
300<bb313
CH4, ppmvCorresponding AltitudeBalloonsUARS/HALOE
Outside PV, kmInside PV, kmMIPAS-BMkIVaNHSH
<1.0∼25<∼17<−49c6
1.0<<∼25<∼17−30113

[44] In both hemispheres, when ILAS-II N2O VMRs exceed 250 ppbv (corresponding to altitudes of <∼17 km and <∼13 km for outside and inside the polar vortex, respectively), the ILAS-II and SMR N2O data agree within ±∼20% and most Dpmed values are smaller than half of the RSS errors. When ILAS-II N2O VMRs are between 50 and 250 ppbv (∼17–30 km for outside the polar vortex and ∼13–21 km for inside the polar vortex), the ILAS-II N2O is ∼10% smaller in the NH and ∼20% smaller in the SH than the balloon and/or SMR N2O, and the quartiles of the Dp values are within ±∼10% in both hemispheres. In the SH, the ILAS-II N2O is ∼30% smaller than the SMR N2O and the quartiles of Dp values are within ±∼18% at the N2O VMR range from 30 to 50 ppbv (∼20–21 km). For ILAS-II N2O smaller than 30 ppbv (above ∼20 km inside the polar vortex) in the SH, the Dp values vary widely (the difference between the first and third quartiles is ∼60%), while the ∣Damed∣ values are smaller than ∼10 ppbv and the Dpmed and quartiles of Dp are within the RSS errors.

[45] When ILAS-II CH4 VMRs exceed 1 ppmv in the NH (<∼25 km), the ILAS-II CH4 agrees with the balloon/HALOE CH4 within ±∼5%. In the SH, the ILAS-II CH4 is ∼9% larger than the HALOE CH4 values at all CH4 VMR levels. Note that the HALOE data in the SH are available for coincident comparisons with data exceeding 0.3 ppmv in January and October, only. Also note that the positive systematic difference between the ILAS-II and HALOE CH4 has a seasonal dependence in the VMR range corresponding to altitudes of ∼20–25 km in the SH and the positive systematic difference is slightly larger in winter than in summer, as also seen in the ILAS CH4 data [Kanzawa et al., 2003].

[46] The ILAS-II Version 1.4 N2O and CH4 VMRs in the smallest VMR ranges (ILAS-II N2O VMR < 50 ppbv [above ∼30 km outside the polar vortex]; ILAS-II CH4 VMR < 1 ppmv (above ∼25 km outside the polar vortex)) in the NH are unrealistically small compared to the balloon/satellite data, and the ∣Dp∣ values of the ILAS-II data from the balloon/satellite data increase drastically with decreasing ILAS-II VMRs. These abnormalities arise from an unresolved problem that is not considered in the ILAS-II Version 1.4 algorithm, i.e., abnormal changes in the detected spectrometer signal that were caused by thermal distortion of the entrance slit.

[47] In summary, the quality of the ILAS-II Version 1.4 data for N2O when VMRs are between 50 and 340 ppbv in the NH and between 0 and 350 ppbv in the SH and for CH4 when VMRs are between 1 and 1.6 ppmv in the NH and between 0.3 and 1.4 ppmv in the SH (except for CH4 VMRs in winter around ∼20–25 km) is sufficient for most scientific purposes such as studies of polar stratospheric phenomena. The retrievals at high altitudes (>∼30 km for N2O and >∼25 km for CH4) and low VMRs (<∼50 ppbv for N2O and <1 ppmv for CH4) in the NH (sunrise occultation mode) still require some more effort before being fully consistent.

Acknowledgments

[48] We sincerely thank the ILAS-II science team and validation experiment team members and their associates. We are grateful to P. Ricaud and J. de La Noe of the Observatoire de Bordeaux and to Cathy Boonne of the Service d'Aeronomie, Paris, for their kind assistance in accessing Odin SMR data through the French atmospheric database ETHER (http://ether.ipsl.jussieu.fr/). Odin is a Swedish-led satellite project that is funded jointly by Sweden (SNSB), Canada (CSA), Finland (TEKES), and France (CNES). We also thank Y. Kasai for help in arranging the collaborative research with the SMR science team. The MetO data are regularly supplied to the ILAS-II project by R. Swinbank of MetO. The ILAS-II data retrieval processing was done at the ILAS Data Handling Facility (DHF) at the National Institute for Environmental Studies (NIES), Japan. The ILAS-II project has been funded by the Ministry of the Environment of Japan (MOE). Part of this research was supported by the Global Environment Research Fund provided by the MOE.

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