Improved Limb Atmospheric Spectrometer Research Articles

On the distribution of ozone in stratospheric anticyclones

Twelve years (1991–2003) of satellite occultation ozone data are combined with a climatology of stratospheric anticyclones and polar vortices to quantify the climatological ozone differences between air mass types. Ozone data from the Stratospheric Aerosol and Gas Experiment (SAGE) II, the Halogen Occultation Experiment (HALOE), the Polar Ozone and Aerosol Measurement (POAM) II and III missions, and the Improved Limb Atmospheric Spectrometer (ILAS) sensor are used in this study. Daily ozone measurements in the 400–1600 K altitude range are categorized as being either (1) in an anticyclone, (2) in a polar vortex, or (3) in neither (hereinafter referred to as “ambient”). Monthly mean ozone is then calculated in each air mass category from data combined over all years. This study focuses on ozone differences between anticyclones and the ambient environment. A composite annual cycle of ozone in ambient regions is compared to ozone in anticyclones as a function of altitude and latitude. Ozone differences result from both anomalous transport and photochemistry in the vicinity of stratospheric anticyclones. The relative importance of transport versus chemistry is inferred from the long‐term ozone anomalies observed here. In summer, ozone gradients between different air mass types are small. In other seasons, results indicate three distinct vertical regimes at high latitudes. Below ≈600 K, anticyclones coincide with regions where ozone is reduced by 10–50% during the winter. Between 600 and 900 K from fall to midwinter, anticyclones are associated with ≈10% more ozone than ambient air masses. Positive ozone anomalies are also observed in low‐latitude anticyclones within this vertical layer; however, they are half the size of their high‐latitude counterparts and peak several months later. Above 900 K the abundance of ozone in anticyclones is ≈10% less than in the surrounding air. This upper stratospheric regime results from the formation of “low‐ozone pockets” (LOPs) in stratospheric anticyclones. All LOPs that formed between November 1991 and November 2003 and were observed by a satellite used in this study (108 pockets) are documented here in the most comprehensive catalog of LOPs to date.

Time variations of descent in the Antarctic vortex during the early winter of 1997

We analyzed long‐lived chemical constituents observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) to study the stratospheric descent in the Southern Hemisphere polar vortex. The ILAS N2O distribution inside the polar vortex exhibited clear downward motion in February to June 1997. Average descent for the 5 months is estimated to be ∼2.1–1.7 km month−1 in the middle stratosphere. In late April to June when planetary waves are relatively active, the vertical velocity displays time variations with a period of about 10 days. These time variations also synchronize with both time variations of the temperature time change (∂/∂t) and the Eliassen‐Palm flux divergence (DF) in high latitudes. Moreover, a correlation coefficient map in the latitude‐height cross section between the vertical velocity and the temperature time change reveals an interesting four‐box pattern, suggesting warming below 10 hPa and cooling above 10 hPa in the polar region (70°–90°S) and an opposite distribution in midlatitudes (40°–70°S), when large descent is observed inside the polar vortex. It is just like the meridional circulation in response to DF induced by planetary waves, which was first illustrated by Matsuno's stratospheric sudden warming theory.

Monthly averaged ozone and nitrous oxide from the Improved Limb Atmospheric Spectrometer (ILAS) in the Northern and Southern Hemisphere polar regions

Northern and southern hemispheric averaged ozone (O3) and nitrous oxide (N2O) measured by the Improved Limb Atmospheric Spectrometer (ILAS) were used to examine photochemical and dynamical changes in high‐latitude O3 distributions. Using correlations of O3 versus N2O, the ILAS data are organized monthly in both hemispheres by partitioning these data into equal bins of altitude or potential temperature. The resulting families of curves help to differentiate O3 changes due to photochemistry from those due to transport. Our study extends the work of Proffitt et al. [2003] for the Northern Hemisphere to the Southern Hemisphere. Further, our study confirms and extends their results for the Northern Hemisphere by applying their analysis to a significantly greater altitude range. As in the Northern Hemisphere, the families of curves for the altitude, and potential temperature bins in the Southern Hemisphere are separated and generally do not cross. In both hemispheres a better separation is found for the potential temperature binning. In the Southern Hemisphere November and December data, preserved photochemical O3 loss is evident in the lower stratosphere. Further, summer ozone loss is evident in the Southern Hemisphere from January to March. In the Arctic, ongoing photochemical O3 loss is evident in the Northern Hemisphere spring data. While at higher altitudes the correlation between N2O and O3 is generally positive (increasing N2O with increasing O3), at lower levels the correlation is negative. This change of correlation from positive to negative can be interpreted in terms of photochemical and dynamical processes. Strong descent causes a steepening of the positively correlated curves, while the curves change their slope from positive to negative if photochemical destruction of O3 is present and descent is weak. The level of slope change is also photochemically influenced and therefore changes with season. Data sets such as the one derived here may be useful for testing atmospheric models and for identifying future changes in stratospheric ozone.

Validation of CFC‐12 measurements from the Improved Limb Atmospheric Spectrometer (ILAS) with the version 6.0 retrieval algorithm

Measurements of CFC‐12 were made by the Improved Limb Atmospheric Spectrometer (ILAS) between 57°N and 72°N in the Northern Hemisphere and between 64°S and 89°S in the Southern Hemisphere. ILAS was launched on 17 August 1996 on board the Advanced Earth Observing Satellite (ADEOS). The ILAS validation balloon campaigns were carried out from Kiruna, Sweden (68°N, 21°E), in February and March 1997 and from Fairbanks, Alaska (65°N, 148°W), in April and May 1997. During these validation balloon campaigns, CFC‐12 was measured with the in situ instruments ASTRID, BONBON, and SAKURA and the remote sensing spectrometers MIPAS‐B, FIRS‐2, and MkIV. ILAS version 6.0 CFC‐12 profiles obtained at the nearest location to the validation balloon measurement are compared with these validation balloon measurements. The quality of ILAS CFC‐12 data processed with the version 6.0 algorithm improved significantly compared to previous versions. Low relative differences between ILAS CFC‐12 and the correlative measurements of about 10% were found between 13 and 20 km. The comparison of vertical profiles shows that ILAS CFC‐12 data are useful below about 20–22 km inside the vortex and below about 25 km outside the vortex. However, at greater altitudes the relative percentage difference increases very strongly with increasing altitude. Further, correlations of CFC‐12 with N2O show a good agreement with the correlative measurements for N2O values of N2O > 150 ppbv. In summary, ILAS CFC‐12 data are now suitable for scientific studies in the lower stratosphere.

Ozone perturbations in the Arctic summer lower stratosphere as a reflection of NOX chemistry and planetary scale wave activity

Ozone concentration perturbations in the high‐latitude lower stratosphere in the Northern Hemisphere were observed by Improved Limb Atmospheric Spectrometer (ILAS) after the polar vortex breakdown at the beginning of May 1997 and until the end of June of that same year. Simulations and a passive tracer experiment using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) show that the low‐ozone perturbations observed in May were caused by the Arctic polar vortex debris, while those after the end of May resulted from a dynamical elongation due to zonal wave number 2 planetary waves of the low‐ozone region in the summer polar stratosphere, which had been developed by the catalytic ozone destruction cycle of NOX. These low‐O3 air masses of different origin were advected or elongated from the polar region to the ILAS measurement points. An episodic event of a dynamical O3 perturbation in June 1997 on a chemically induced meridional O3 gradient is described. These results show that a timing of the polar vortex breakdown and activity of planetary waves after the breakdown may affect the O3 background gradient in the summer lower stratosphere at middle and high latitudes.

A Lagrangian method to study stratospheric nitric acid variations in the polar regions as measured by the Improved Limb Atmospheric Spectrometer

Denitrification is well known to affect the severity of springtime ozone depletion in Polar Regions. In winter 1996/1997 in the Northern Hemisphere and winter 1997 in the Southern Hemisphere, the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) detected denitrification in both hemispheres. Here the Match technique and a Lagrangian model are used to analyze the nitric acid variation between a pair of measurements belonging to the same air parcel. Eleven cases are studied in Antarctica and seventeen in the Arctic, permitting testing of the laboratory‐measured homogeneous freezing rate of liquid ternary aerosol into nitric acid hydrates, thought to be the determining step of the denitrification process. Over the Antarctica, eight cases out of eleven lead to results in agreement with the measurements, taking into account uncertainties of the measurement and possible acceptable bias in the temperature field used for the modeling study. Over the Arctic, more cases remain unexplained even after taking into account the possible scavenging of nitric acid by large nitric acid trihydrate (NAT) particles falling from the layers above. These disagreements are mainly due to relatively high temperatures along the trajectories that do not lead to significant NAT or nitric acid dihydrate (NAD) freezing. Thus it appears that some additional mechanisms are required to explain the denitrification in the Arctic winter. Moreover, the occurrence of denitrification due to the few NAT particles with very large radius (few μm), the so‐called “NAT rocks,” over the Antarctic winter in 1997 is suggested.

Analysis of ozone loss in the Arctic stratosphere during the late winter and spring of 1997 using the Chemical Species Mapping on Trajectories (CSMT) technique

A scheme to create synoptic maps of stratospheric minor species from asynoptic satellite measurements using a photochemical box model and trajectory analysis was developed and named Chemical Species Mapping on Trajectories (CSMT). CSMT combined with Improved Limb Atmospheric Spectrometer (ILAS) data were used to study the Arctic ozone loss mechanism in the late winter and early spring of 1997. Long‐ and short‐lived species in the stratosphere were successfully mapped by the CSMT initialized with ILAS‐observed ozone, nitric acid, and nitrous oxide. Comparisons of CSMT‐derived data and ozonesonde data, and/or other satellite data, validated the scheme. The comparisons showed the reliability of the scheme in mapping long‐ and short‐lived species. A chemical ozone loss amount was estimated using the chemical model; the maximum ozone loss rate was about 34 ppbv/day in late February. The integrated ozone loss from 13 January to 31 March was 41%, averaged over the entire polar vortex. The effects of differences in polar stratospheric cloud composition and a possible warm bias in the temperature data set were examined, and only minor differences were found in the ozone loss amount. The derived ozone loss rates and integrated ozone loss are consistent with results from other studies. The Arctic ozone loss in the late winter and early spring of 1997 depended significantly on latitude and showed complex features: ozone loss occurred mainly at lower latitudes until late February; the region of significant ozone loss shifted to higher latitudes in March. The CSMT scheme shows good potential for various applications including detailed analyses of chemical mechanisms in the atmosphere.

Validation and data characteristics of nitrous oxide and methane profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the Version 5.20 algorithm

Vertical profiles of nitrous oxide and methane at high latitudes (57–72°N; 64–89°S) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard Advanced Earth Observing Satellite. These measurements were made continuously from November 1996 through June 1997 with some additional periods in September–October 1996. A validation study of the nitrous oxide and methane data processed with the Version 5.20 ILAS retrieval algorithm is presented in this paper. Comparisons are made with (1) nitrous oxide and methane obtained by the ILAS validation balloon campaigns at Kiruna, Sweden, and at Fairbanks, Alaska, in the Arctic; (2) nitrous oxide and methane by the Photochemistry of Ozone Loss in the Arctic Region in Summer aircraft campaign in the Arctic; (3) nitrous oxide by the ground‐based spectroscopic measurements and by the aircraft‐based remote sensing measurements in the Arctic; and (4) methane by satellite measurements of the Version 19 Halogen Occultation Experiment in the Arctic and Antarctic. Comparisons of ILAS nitrous oxide and methane with Upper Atmosphere Research Satellite Reference Atmosphere data are also made. The results of the comparisons and additional ILAS internal consistency analyses are as follows: (1) the uncertainty of ILAS nitrous oxide is better than 10% over 10–30 km in altitude, and is larger than 50% over 30–40 km, which is comparable to the expected total errors of the ILAS measurements; (2) the uncertainty of ILAS methane is better than 10% over 10–50 km, except for 15–30 km in winter with positive biases exceeding 20%, which is smaller than or comparable to the expected total errors of the ILAS measurements (the quality of ILAS methane in the polar lower stratosphere is better in summer than in winter). In summary, the characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution, along with the good quality of ILAS Version 5.20 nitrous oxide for 10–40 km and the good quality of ILAS Version 5.20 methane for 10–50 km except for 15–30 km in winter, make the ILAS nitrous oxide and methane data set valuable for scientific study of various polar stratospheric phenomena.

Properties of polar stratospheric clouds observed by ILAS in early 1997

The relative extinction coefficients for Arctic polar stratospheric clouds (PSCs) are determined using a transmittance‐ratio technique from spectral data observed in early 1997 by the Improved Limb Atmospheric Spectrometer (ILAS). The observed relative extinction coefficients are compared with theoretical coefficients calculated by the Mie theory for single mode lognormal size distributions of PSCs. The observed PSCs are then classified into three types: nitric acid droplets (HNO3/H2O), and nitric acid trihydrate (NAT) solid particles in the form of α‐NAT and β‐NAT. However, the definitive detection of α‐NAT is difficult due to uncertainties of atmospheric temperature measurements and laboratory determination of refractive indices.

Correction to “Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm” by H. Kanzawa et al.

Correction to “Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm” by H. Kanzawa et al.

Evidence for the nucleation of polar stratospheric clouds inside liquid particles

The Improved Limb Atmospheric Spectrometer on board the Advanced Earth Observing Satellite first detected the onset of denitrification at 19 and 20 km in the 1996/1997 Arctic winter stratosphere. Box model calculations along back trajectories were used to estimate the degree of denitrification caused by the formation of nitric acid trihydrate (NAT) particles inside liquid aerosols. The loss of reactive nitrogen calculated assuming conversion of nitric acid dihydrate to NAT and direct NAT formation explains well the observed loss, demonstrating that such NAT particle formation mechanisms play a critical role in Arctic denitrification.

Calculation of chemical ozone loss in the Arctic winter 1996–1997 using ozone‐tracer correlations: Comparison of Improved Limb Atmospheric Spectrometer (ILAS) and Halogen Occultation Experiment (HALOE) results

The ozone‐tracer correlation method is used to deduce the stratospheric ozone loss in the Arctic winter 1996–1997. Improvements of the technique are applied, such as a new calculation of the vortex edge [Nash et al., 1996] and an improved early vortex reference function. Winter 1996–1997 is characterized by a late formation and an unusually long lifetime of the polar vortex. Remnants of vortex air were found until May. Chemical ozone losses deduced from two satellite data sets, namely Improved Limb Atmospheric Spectrometer (ILAS) and Halogen Occultation Experiment (HALOE), are discussed. The ILAS observations allow a detailed analysis of the temporal evolution of the ozone‐tracer correlation inside the polar vortex and, in particular, of the development of the early vortex. For November and December 1996, it is shown that horizontal mixing still influences the ozone‐tracer relation. Significant PSC related chemical ozone loss occurred beginning at mid‐February, and the averaged column ozone loss is increasing toward the middle of May. From April onwards, ozone profiles in the vortex became more uniform. The decrease of ozone in the vortex remnants in April and May occurred due to chemistry. HALOE observations are available for March to May 1997. In the period 4–16 March 1997, the calculated ozone loss deduced from HALOE and ILAS is in good agreement. The average of the result from the two instruments is 15 ± 7 Dobson units (DU) inside the vortex core, in the altitude range of 450–550 K. At the end of March, a discrepancy between HALOE and ILAS ozone loss arises due to a significant difference (0.6 ppmv) between the two data sets in the relatively low ozone minimum measured at 475 K. Nonetheless, both data sets consistently show an inhomogeneity in ozone loss inside the vortex core at the end of March. The vortex is separated in two parts, one with a large ozone loss (HALOE 40–45 DU, ILAS 30–35 DU) and one with a moderate ozone loss (HALOE 15–30 DU, ILAS 5–25 DU) for 450–550 K. The ozone loss from HALOE in 380–550 K at that time was calculated to be 90–110 DU for the large ozone loss and 20–80 DU for the moderate ozone loss. The vortex average of column ozone loss from HALOE inside the vortex core at the end of March is 61 ± 20 DU, which is an increase of about 20% compared to the earlier study by Müller et al. [1997b] brought about by the improvement of the technique.

Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm

Vertical profiles of water vapor concentration at high latitudes (57–72°N; 64–89°S) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard the Advanced Earth Observing Satellite (ADEOS). These measurements were made continuously from November 1996 through June 1997 with some additional periods in September to October 1996. A validation study of the water vapor data processed with the version 5.20 ILAS retrieval algorithm is presented in this paper. Uncertainty and general characteristics of the ILAS water vapor measurements are briefly reviewed. Comparisons are made with data obtained by (1) the ILAS validation balloon campaigns at Kiruna, Sweden and at Fairbanks, Alaska; (2) the aircraft measurements under the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign; and (3) available satellite measurements of the version 19 Halogen Occultation Experiment (HALOE) and the version 6 Stratospheric Aerosol and Gas Experiment II (SAGE II). The agreement between ILAS water vapor and all independent reliable correlative measurements in the altitude region of 15–60 km is better than 10% for the majority of cases and better than 20% for all comparisons, with the exception of some isolated cases detailed in this paper. Climatological comparisons of ILAS data with Upper Atmosphere Research Satellite (UARS) climatology and HALOE data show the overall consistency of ILAS water vapor data considering the known features of atmospheric circulation. The characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution along with the good data quality, make the ILAS water vapor data set valuable for various polar stratospheric research applications.

Improved Limb Atmospheric Spectrometer (ILAS) data retrieval algorithm for Version 5.20 gas profile products

The Improved Limb Atmospheric Spectrometer (ILAS), a sensor for stratospheric ozone layer observation using a solar occultation technique, was mounted on the Advanced Earth Observing Satellite (ADEOS), which was put into a Sun‐synchronous polar orbit in August 1996. Operational measurements were recorded over high‐latitude regions from November 1996 to June 1997. This paper describes the data processing algorithm of Version 5.20 used to retrieve vertical profiles of gases such as ozone, nitric acid, nitrogen dioxide, nitrous oxide, methane, and water vapor from the infrared spectral measurements of ILAS. To simultaneously derive mixing ratios of individual gas species as a function of altitude, the nonlinear least squares method was utilized for spectral fitting, and the onion peeling method was applied to perform vertical profiling. This paper also discusses in detail estimation of errors (internal and external errors) associated with the derived gas profiles and compares the errors with repeatability. The internal error estimated from residuals in spectral fitting was generally larger than the repeatability, which suggests either that some unknown factors have not been incorporated into the forward model for simulating observed transmittance data or that some parameters in the model are inaccurate. The external error was almost comparable in magnitude to the repeatability. Numerical simulations were carried out to investigate performance of the nongaseous correction technique. The results showed that the background level of sulfuric acid aerosols has little effect on the retrieved profiles, while polar stratospheric clouds (PSCs) with extinction coefficients of the order of 10−3 km−1 at a wavelength of 780 nm have nonnegligible effects on the profiles of some gas species. Despite the problems that require further investigations, it is shown that the ILAS Version 5.20 algorithm generates scientifically useful products.

Characteristics and performance of the Improved Limb Atmospheric Spectrometer (ILAS) in orbit

The Improved Limb Atmospheric Spectrometer (ILAS) was a satellite‐based solar occultation sensor that was developed by the Environment Agency of Japan (EA) to monitor and study the stratospheric ozone layer. This paper describes the characteristics of the ILAS instrument and its performance in orbit. ILAS measured the vertical distribution of ozone, nitric acid, nitrogen dioxide, nitrous oxide, methane, water vapor, temperature, pressure, and aerosol extinction coefficients at 1.6‐km vertical resolution. ILAS was equipped with two spectrometers: an infrared (IR) spectrometer with an uncooled pyroelectric linear array detector to sense between 6.21 and 11.76 μm and a visible spectrometer to monitor 753–784 nm. In addition, a Sun‐edge sensor (SES) assigned the tangent height of the instantaneous field‐of‐view (IFOV). A two‐axis gimbals control system on ILAS used two Sun position sensors to track the center of brightness of the Sun during occultation measurements. Before launch onboard the Advanced Earth Observing Satellite (ADEOS), the performance of ILAS was checked on the ground using several methods, including gas‐cell measurements, time response measurements, Sun‐tracking tests, and hollow‐cathode lamp measurements. After the launch of ADEOS on 17 August 1996, ILAS functioned successfully for 8 months of routine operation, from 30 October 1996 to 30 June 1997, collecting more than 6700 solar occultation measurements, after which time the satellite failed due to a failure in a solar paddle. The time delay response of the IR channel was characterized using stepwise IR input. Instrument functions of the ILAS IR and visible spectrometers were determined by combining theoretical optical calculations, experimental measurements using a gas‐cell before launch, and in‐orbit data. The signal‐to‐noise ratio (SNR) of each element in the IR channel was estimated to be 400–1200. In the visible channel, it was 1600–1800 for a 100% direct Sun signal. At sunset occultation, ILAS was able to track the Sun below a tangent height of 10 km in some cases. The method of determining the solar edges from the SES data worked correctly, giving adequate tangent height information for observations. Output signal levels of the SES, visible channel, and IR channel showed slight degradation during the period that ILAS was operational, which is attributed to space‐borne contaminants. However, changes in absolute signal levels do not affect data retrieval, because the solar occultation technique was self‐calibrating. Overall, ILAS worked as designed during its operation in orbit and gathered valuable data for ozone layer studies.

Variability of polar stratospheric water vapor observed by ILAS

We present an analysis of polar stratospheric water vapor from measurements of the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS). The variability of water vapor is examined using contemporaneous potential vorticity fields to separate the vortex from the extravortex air. The overall water vapor distribution from the ILAS measurements agrees qualitatively with the water vapor climatology in the UARS reference atmosphere (covering 1992–1999), and the space‐time variability is consistent with the known stratospheric circulation. Quantitatively, comparisons between ILAS and Halogen Occultation Experiment (HALOE) equivalent latitude zonal means show agreement to within 5% in the lower stratosphere (400–800 K potential temperature or 15–30 km). In the upper stratosphere (800–1800 K potential temperature or 30–45 km), however, ILAS data have a 10% to 20% high bias compared to the 8‐year (1992–1999) UARS climatology and to HALOE data for the same time period. The ILAS data, covering the time period November 1996 to June 1997, were measured during a winter‐spring period when the Arctic polar stratosphere was anomalously cold. The ILAS water vapor data suggest an Arctic vortex that is more isolated and persistent in the spring than that seen in the UARS climatology. The spatial and temporal extent of the Arctic vortex this year was very similar to that of the Antarctic vortex. This feature is supported by HALOE data for the same measurement period. Using a trajectory model, we show that the NH vortex in spring 1997 was significantly more isolated compared to other years in the decade, and this explains the unusual confinement of high water vapor within the Arctic vortex observed by ILAS.

Tangent height registration for the solar occultation satellite sensor ILAS: A new technique for Version 5.20 products

The Improved Limb Atmospheric Spectrometer (ILAS) was a solar occultation satellite sensor that was developed by the Environment Agency of Japan to monitor the stratospheric ozone layer. This paper describes methods of registering tangent heights for ILAS vertical profiles of gas mixing ratio and aerosol extinction coefficient. Accurate tangent height registration is crucial for the retrieval of accurate gas mixing ratios from atmospheric absorption spectra. Three methods for tangent height registration have been applied to retrieved ILAS data. The first method is the transmittance spectrum method (TS‐M), which uses absorption spectra of oxygen molecules at around 760 nm (O2A band) measured by the ILAS visible channel and compares the average transmittance with that calculated theoretically from temperature and pressure using meteorological data. A tangent height is derived from these data. Version 3.10 ILAS data products use the TS‐M. A second method is the Sun‐edge sensor method (SES‐M). This method for registering tangent heights was in mind when ILAS was originally designed. The SES‐M geometrically determines the direction of the instantaneous field‐of‐view (IFOV) of the spectrometer from the angular difference between the top edge of the Sun determined with the SES and the spectrometer's IFOV. Information on the satellite's orbital position and solar position relative to the center of the Earth is used to register the tangent height. Version 4.20 ILAS data products use SES‐M. The third method is a hybrid method (Hybrid‐M) that was developed to correct for seasonal differences in tangent heights computed by the SES‐M. The Hybrid‐M assumes that the TS‐M can correctly determine the tangent height at 30 km. Version 5.20 ILAS data products (the latest version) use Hybrid‐M. Random and systematic errors in the Hybrid‐M tangent height registration were estimated. The root‐sum‐square (RSS) total random error is 30 m, while the total systematic error is +300 ± 360 m in tangent height. Actual errors in tangent height registration in Version 5.20 algorithm are considered to be small judging from the results of comparisons with independent validation data sets. The Hybrid‐M gives good estimates of tangent heights in Version 5.20 of the ILAS data processing algorithms.

Validation of ozone measurements from the Improved Limb Atmospheric Spectrometer

Vertical profiles of ozone concentration in the high latitudes were observed by the Improved Limb Atmospheric Spectrometer (ILAS) aboard the Advanced Earth Observing Satellite (ADEOS) from November 1996 to June 1997. The ozone data obtained by the version 5.20 ILAS retrieval algorithm are compared with those obtained by the version 19 Halogen Occultation Experiment (HALOE), the version 6 Stratospheric Aerosol and Gas Experiment (SAGE) II, and the version 6 Polar Ozone and Aerosol Measurement (POAM) II retrieval algorithms. The ILAS data are also compared with ozone data measured by ozonesondes, instruments on board balloons or an aircraft, and ground‐based instruments. The ILAS ozone data generally agree with its correlative data between 11 and 64 km with some exceptions. Quantitatively, the median value of the relative difference (absolute difference divided by its mean value) for these comparisons was within ±10%. Relative differences (18%) exceeding the combined measurement errors were found around 45–55 km altitude from comparisons with the HALOE and SAGE II data in January 1997 in the Southern Hemisphere (SH). Larger relative differences (around 50%) were also found below 15 km from comparisons with the HALOE and POAM II data in November 1996 in the SH, but these absolute differences were 0.10–0.16 ppmv as the median value. The ozone data processed by the version 5.20 were improved compared to the former version 3.10, which is available to the general public. The version 5.20 ozone data can be used for scientific analysis purposes based on the accuracy of the data in comparison with these other instruments.

Trajectory hunting as an effective technique to validate multiplatform measurements: Analysis of the MLS, HALOE, SAGE‐II, ILAS, and POAM‐II data in October–November 1996

The goal of this study is to show that trajectory hunting is an effective technique for comparison of multiplatform measurements. In order to achieve this goal, we (1) describe in detail the trajectory hunting technique (THT), (2) perform several consistency tests for THT (self‐hunting and reversibility), (3) estimate uncertainties of this technique, and (4) validate THT results against those obtained by the traditional correlative analysis (TCA). THT launches backward and forward trajectories from the locations of measurements and finds air parcels sampled at least twice within a prescribed match criterion during the course of several days. TCA finds matched profiles for a chosen match criterion, averages them for each instrument separately, and compares the averaged profiles. As an example, we consider the 22 October to 30 November 1996 period in the Southern Hemisphere and compare the latest versions of relevant measurements made by the following five instruments: Microwave Limb Sounder (MLS, version 5 (v.5)), Halogen Occultation Experiment (HALOE, v.19), Polar Ozone and Aerosol Measurement II (POAM‐II, v.6), Stratospheric Aerosol and Gas Experiment II (SAGE‐II, v.6.1), and Improved Limb Atmospheric Spectrometer (ILAS, v.5.20). We present results for O3, H2O, CH4, HNO3, and NO2, which show that (1) ozone measurements from all five instruments agree to better than 0.4 (0.2) ppmv below (above) 30 km; (2) water vapor measurements agree within ±5–10% above 22 km; (3) methane measurements by HALOE and ILAS agree to better than 10% above 30 km with a possible positive offset of up to 10–15% by ILAS in the lower stratosphere; (4) MLS HNO3 data corrected to account for some excited vibrational lines omitted in the v.5 HNO3 retrieval agree with ILAS HNO3 measurements to within ∼0.5 ppbv (∼10–20%) over the range ∼450–750 K; (5) ILAS sunset NO2 measurements are larger than both POAM‐II and SAGE‐II values by up to 10–15% below 30 km. The self‐hunting tests show that the THT RMS noise is of the order of 1–2% for O3, CH4, and H2O and 4% for NO2 and HNO3 measurements in the stratosphere. Total THT‐related uncertainties may be 3–5% for O3 measurements when photochemical effects and sensitivities of the results to duration of trajectories and match criteria are taken into account. Good agreement is found between the THT and TCA results for each of these products and for each possible pair of instruments, with considerably better statistics (typically by at least an order of magnitude) in the THT case. This agreement validates the THT results.

Stratospheric ozone loss in the 1996/1997 Arctic winter: Evaluation based on multiple trajectory analysis for double‐sounded air parcels by ILAS

Quantitative chemical ozone loss rates and amounts in the Arctic polar vortex for the spring of 1997 are analyzed based on ozone profile data obtained by the Improved Limb Atmospheric Spectrometer (ILAS) using an extension of the Match technique. In this study, we calculated additional multiple trajectories and set very strict criteria to overcome the weakness of the satellite sensor data (lower vertical resolution and larger sampling air mass volume) and to identify more accurately a double‐sounded air mass. On the average inside the inner edge of the vortex boundary (north of about 70°N equivalent latitude), the local ozone loss rate was 50–80 ppbv/day at the maximum during late February between the levels of 450 and 500 K potential temperatures. The integrated ozone loss during February to March reached 2.0 ± 0.1 ppmv at 475–529 K levels, and the column ozone loss between 400 and 600 K during the 2 months was 96 ± 0.3 DU. Using a relative potential vorticity (rPV) scale, the vortex was divided into some rPV belts, and it was shown that the magnitude of the ozone loss increased gradually toward the vortex center from the edge. The maximum ozone loss rate of 6.0 ± 0.6 ppbv/sunlit hour near the vortex center was higher than near the vortex edge by a factor of 2–3. When we expanded the area of interest to include all the data obtained inside the vortex edge (north of about 65°N equivalent latitude), the local ozone loss rate was about 50 ppbv/day at the maximum. This value is slightly larger than that estimated by the Match analysis using ozonesondes for the same winter by ∼10 ppbv/day. Temperature histories of double‐sounded air parcels indicated that the extreme ozone loss in the innermost part of the vortex was observed when the air parcel experienced temperatures below TNAT during the two soundings and had experienced temperatures near Tice in the 10 days prior to the first sounding. These facts suggest that the high ozone loss rate deep inside the vortex in the 1997 Arctic early spring correlates with the presence of type Ia polar stratospheric clouds (PSCs).