Improved Limb Atmospheric Spectrometer Research Articles

MIPAS IMK/IAA CFC-11 (CCl3F) and CFC-12 (CCl2F2) measurements: accuracy, precision and long-term stability

Abstract. Profiles of CFC-11 (CCl3F) and CFC-12 (CCl2F2) of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the European satellite Envisat have been retrieved from versions MIPAS/4.61 to MIPAS/4.62 and MIPAS/5.02 to MIPAS/5.06 level-1b data using the scientific level-2 processor run by Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK) and Consejo Superior de Investigaciones Científicas (CSIC), Instituto de Astrofísica de Andalucía (IAA). These profiles have been compared to measurements taken by the balloon-borne cryosampler, Mark IV (MkIV) and MIPAS-Balloon (MIPAS-B), the airborne MIPAS-STRatospheric aircraft (MIPAS-STR), the satellite-borne Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and the High Resolution Dynamic Limb Sounder (HIRDLS), as well as the ground-based Halocarbon and other Atmospheric Trace Species (HATS) network for the reduced spectral resolution period (RR: January 2005–April 2012) of MIPAS. ACE-FTS, MkIV and HATS also provide measurements during the high spectral resolution period (full resolution, FR: July 2002–March 2004) and were used to validate MIPAS CFC-11 and CFC-12 products during that time, as well as profiles from the Improved Limb Atmospheric Spectrometer, ILAS-II. In general, we find that MIPAS shows slightly higher values for CFC-11 at the lower end of the profiles (below ∼ 15 km) and in a comparison of HATS ground-based data and MIPAS measurements at 3 km below the tropopause. Differences range from approximately 10 to 50 pptv ( ∼ 5–20 %) during the RR period. In general, differences are slightly smaller for the FR period. An indication of a slight high bias at the lower end of the profile exists for CFC-12 as well, but this bias is far less pronounced than for CFC-11 and is not as obvious in the relative differences between MIPAS and any of the comparison instruments. Differences at the lower end of the profile (below ∼ 15 km) and in the comparison of HATS and MIPAS measurements taken at 3 km below the tropopause mainly stay within 10–50 pptv (corresponding to ∼ 2–10 % for CFC-12) for the RR and the FR period. Between ∼ 15 and 30 km, most comparisons agree within 10–20 pptv (10–20 %), apart from ILAS-II, which shows large differences above ∼ 17 km. Overall, relative differences are usually smaller for CFC-12 than for CFC-11. For both species – CFC-11 and CFC-12 – we find that differences at the lower end of the profile tend to be larger at higher latitudes than in tropical and subtropical regions. In addition, MIPAS profiles have a maximum in their mixing ratio around the tropopause, which is most obvious in tropical mean profiles. Comparisons of the standard deviation in a quiescent atmosphere (polar summer) show that only the CFC-12 FR error budget can fully explain the observed variability, while for the other products (CFC-11 FR and RR and CFC-12 RR) only two-thirds to three-quarters can be explained. Investigations regarding the temporal stability show very small negative drifts in MIPAS CFC-11 measurements. These instrument drifts vary between ∼ 1 and 3 % decade−1. For CFC-12, the drifts are also negative and close to zero up to ∼ 30 km. Above that altitude, larger drifts of up to ∼ 50 % decade−1 appear which are negative up to ∼ 35 km and positive, but of a similar magnitude, above.

Ozone loss rates in the Arctic winter stratosphere during 1994-2000 derived from POAM II/III and ILAS observations: Implications for relationships among ozone loss, PSC occurrence, and temperature

[1] Quantitative chemical ozone loss rates at the 475 K isentropic surface inside the Arctic polar vortex are evaluated for six winters (January through March) using a satellite-based Match technique. Satellite observational data are taken from the Polar Ozone and Aerosol Measurement (POAM) II for 1994–1996, the Improved Limb Atmospheric Spectrometer (ILAS) for 1997, and the POAM III for 1999–2000. The largest ozone loss rates were observed in the end of January 1995 (∼50 ± 20 ppbv d−1), February 1996 (∼40–50 ± 15 ppbv d−1), February 1997 (∼40 ± 8 ppbv d−1), January 2000 (∼60 ± 30 ppbv d−1), and early March 2000 (∼40 ± 10 ppbv d−1). The probability of polar stratospheric cloud (PSC) existence is estimated using aerosol extinction coefficient data from POAM II/III and ILAS. Ozone loss and the PSC probability are strongly correlated and an absolute increase of 10% in the PSC probability is found to amplify the chemical ozone loss rate during Arctic winter by approximately 25 ± 6 ppbv per day or 3.2 ± 0.7 ppbv per sunlit hour. Relationships between average Arctic winter ozone loss rates and various PSC- and temperature-related indices are investigated, including the area of polar vortex that is colder than the threshold temperature for PSC existence (APSC), the PSC formation potential (PFP), and the potential for activation of chlorine (PACl). Of these three, PACl provides the best proxy representation of interannual variability in Arctic ozone loss at the 475 K level. Large ozone loss occurred primarily for air masses that experienced low temperatures between 187 K and 195 K within the previous 10 days and the ozone loss rates clearly increase with decreasing the minimum temperature. The particularly large ozone losses of ∼9 ± 3 ppbv per sunlit hour in February 1996 and January 2000 were associated with low minimum temperatures of 187–189 K, simultaneously with high PSC probabilities.

Evaluation of CLaMS, KASIMA and ECHAM5/MESSy1 simulations in the lower stratosphere using observations of Odin/SMR and ILAS/ILAS-II

Abstract. 1-year data sets of monthly averaged nitrous oxide (N2O) and ozone (O3) derived from satellite measurements were used as a tool for the evaluation of atmospheric photochemical models. Two 1-year data sets, one solar occultation data set derived from the Improved Limb Atmospheric Spectrometer (ILAS and ILAS-II) and one limb sounding data set derived from the Odin Sub-Millimetre Radiometer (Odin/SMR) were employed. Here, these data sets are used for the evaluation of two Chemical Transport Models (CTMs), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) and the Chemical Lagrangian Model of the Stratosphere (CLaMS) as well as for one Chemistry-Climate Model (CCM), the atmospheric chemistry general circulation model ECHAM5/MESSy1 (E5M1) in the lower stratosphere with focus on the Northern Hemisphere. Since the Odin/SMR measurements cover the entire hemisphere, the evaluation is performed for the entire hemisphere as well as for the low latitudes, midlatitudes and high latitudes using the Odin/SMR 1-year data set as reference. To assess the impact of using different data sets for such an evaluation study we repeat the evaluation for the polar lower stratosphere using the ILAS/ILAS-II data set. Only small differences were found using ILAS/ILAS-II instead of Odin/SMR as a reference, thus, showing that the results are not influenced by the particular satellite data set used for the evaluation. The evaluation of CLaMS, KASIMA and E5M1 shows that all models are in agreement with Odin/SMR and ILAS/ILAS-II. Differences are generally in the range of ±20%. Larger differences (up to −40%) are found in all models at 500±25 K for N2O mixing ratios greater than 200 ppbv, thus in air masses of tropical character. Generally, the largest differences were found for the tropics and the lowest for the polar regions. However, an underestimation of polar winter ozone loss was found both in KASIMA and E5M1 both in the Northern and Southern Hemisphere.

Longitudinally Dependent Ozone Increase in the Antarctic Polar Vortex Revealed by Balloon and Satellite Observations

Abstract The horizontal structure of processes causing increases in ozone in the Antarctic polar vortex was examined using data measured in 2003 from an ozonesonde observation campaign at Syowa Station (39.6°E, 69.0°S) and from the Improved Limb Atmospheric Spectrometer II (ILAS-II) onboard the Advanced Earth Observing Satellite II. The ILAS-II data are daily and distributed uniformly at 14 points in the zonal direction, mostly at polar latitudes. The Antarctic ozone hole that developed in 2003 was one of the largest recorded. The period of focus in this study is 26 September through 24 October, when a strong polar vortex was situated in the stratosphere. An ozone mixing ratio contour (1.0 ppmv) moved downward near a height of 20 km during the period of focus. This increase in ozone is likely to result from downward transport of ozone-rich air originating from lower latitudes by Brewer–Dobson circulation. First, the descent rate of the mixing ratio contour was estimated by taking the geometric height as the vertical coordinate for the deep vortex interior around 20 km. A significant longitudinal dependence was observed. An analysis using ECMWF operational data shows that this dependence can be approximately explained by longitudinally dependent vertical movements of the isentropes caused by a zonal wavenumber-1 quasi-stationary planetary wave with amplitude and phases varying on a seasonal time scale. Next, the descent rate was calculated around 500 K (around 20 km) by taking the potential temperature (isentrope) as the vertical coordinate. The longitudinal dependence was still present using this coordinate, meaning that the ozone mixing ratio and its increase are not constant on the isentropic layer even in the interior of the polar vortex. A backward trajectory analysis showed that air parcels with large ozone mixing ratios were mostly transported from the polar vortex boundary region. This result suggests that lateral transport/mixing is important even before the breakup of the polar vortex. Results from a tracer–tracer correlation analysis of O3 and long-lived constituent N2O were also consistent with this inference. The contribution of lateral mixing to the increase in ozone was estimated at about 17% ± 4% that of the Brewer–Dobson circulation around 20 km, using the calculated descent rates. The results of this study also imply that Lagrangian downward motions in the vortex interior are not correctly estimated without accounting for lateral mixing, even if the polar vortex is dynamically stable.

Technical Note: Intercomparison of ILAS-II version 2 and 1.4 trace species with MIPAS-B measurements

Abstract. The Improved Limb Atmospheric Spectrometer (ILAS)-II sensor aboard the Japanese ADEOS-II satellite was launched into its sun-synchronous orbit on 14 December 2002 and performed solar occultation measurements of trace species, aerosols, temperature, and pressure in the polar stratosphere until 25 October 2003. Vertical trace gas profiles obtained with the balloon version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) provide one of the sparse data sets for validating ILAS-II version 2 and 1.4 data. The MIPAS-B limb emission spectra were collected on 20 March 2003 over Kiruna (Sweden, 68° N) at virtually the same location that has been sounded by ILAS-II about 5.5 h prior to the sampling of MIPAS-B. The intercomparison of the new ILAS-II version 2 (Northern Hemispheric sunrise) data to MIPAS-B vertical trace gas profiles shows a good to excellent agreement within the combined error limits for the species O3, N2O, CH4, H2O (above 21 km), HNO3, ClONO2, and CFC-11 (CCl3F) in the compared altitude range between 16 and 31 km such that these data appear to be very useful for scientific analysis. With regard to the previous version 1.4 ILAS-II data, significant improvements in the consistency with MIPAS-B are obvious especially for the species CH4 and H2O, but also for O3, HNO3, ClONO2, NO2, and N2O5. However, comparing gases like NO2, N2O5, and CFC-12 (CCl2F2) exhibits only poor agreement with MIPAS-B such that these species cannot be assumed to be validated at the present time.

Temporal evolution of ClONO2 observed with Improved Limb Atmospheric Spectrometer (ILAS) during Arctic late winter and early spring in 1997

The temporal evolution of the volume mixing ratio (VMR) of chlorine nitrate (ClONO2) observed with the Improved Limb Atmospheric Spectrometer (ILAS) is described for the Arctic late winter and early spring of 1997. The temporal development of ClONO2 on the 475‐K isentropic surface during winter and spring is characterized by high variability in the VMR with seasonal enhancement to about 2 ppbv. In February, depleted values of ClONO2 were also observed; some of these low values are attributable to denitrification or to occurrence of polar stratospheric clouds. After mid‐March, when ClONO2 reached peak values, ClONO2 decreased and showed much less variability. Comparison of ClONO2 with HCl observed by the Halogen Occultation Experiment/Upper Atmospheric Research Satellite (HALOE/UARS) suggests a conversion of ClONO2 into HCl earlier at high altitudes than at lower altitudes. During the period a marked enhancement in NO2 was observed with a reduction in ClONO2 in the vortex, providing the first evidence from space of the NO2 time evolution in conjunction with ClONO2. Continuous measurements of ClONO2 through winter and spring over the Arctic are limited to date. The ILAS measurements reported in this paper will be useful for reanalyzing the seasonal variation of chlorine activation/deactivation processes in the Arctic lower stratosphere that control the degree of ozone destruction.

Temporary Denitrification in the Antarctic Stratosphere as Observed by ILAS-II in June 2003

To examine the characteristics of polar stratospheric clouds (PSCs) in the Antarctic, we have analyzed short-time (≤ 5 days) changes in nitric acid (HNO3) and aerosol extinction coefficient (AEC) at 780 nm, focusing near 20 km altitude in June 2003 as observed by the Improved Limb Atmospheric Spectrometer (ILAS)-II. The Match technique based on the air parcel trajectory was applied to the ILAS-II data. The several Match pairs have revealed decreased HNO3 values with increased AEC values within short times, indicating “temporary” denitrification. It is also suggested that the observed PSCs could be nitric acid trihydrate (NAT) particles, considering that the temperatures were above existence temperatures for supercooled ternary solution, but below those for NAT. Given appropriate size distributions for NAT particles, it is suggested that the median radius of particles was less than 3 µm.

Preface to special section on ILAS‐II: The Improved Limb Atmospheric Spectrometer–II

The Improved Limb Atmospheric Spectrometer–II (ILAS‐II) was a solar‐occultation satellite sensor designed to measure minor constituents associated with polar ozone depletion. ILAS‐II was placed on board the Advanced Earth Observing Satellite–II (ADEOS‐II, “Midori‐II”), which was successfully launched on 14 December 2002 from the Tanegashima Space Center of the Japan Aerospace Exploration Agency (JAXA). After an initial check of the instruments, ILAS‐II made routine measurements for about 7 months, from 2 April 2003 to 24 October 2003, a period that included the formation and collapse of an Antarctic ozone hole in 2003, one of the largest in history. This paper introduces a special section containing papers on ILAS‐II instrumental and on‐orbit characteristics, several validation results of ILAS‐II data processed with the version 1.4 data processing algorithm, and scientific analyses of polar stratospheric chemistry and dynamics using ILAS‐II data.

Intercomparison of ILAS‐II version 1.4 aerosol extinction coefficient at 780 nm with SAGE II, SAGE III, and POAM III

The Improved Limb Atmospheric Spectrometer (ILAS)‐II on board the Advanced Earth Observing Satellite (ADEOS)‐II observed stratospheric aerosol in visible/near‐infrared/infrared spectra over high latitudes in the Northern and Southern hemispheres, intermittently from January to March and continuously from April through October 2003. This study assesses the data quality of ILAS‐II version 1.4 (V1.4) aerosol extinction coefficient at 780 nm. In the Northern Hemisphere (NH), aerosol extinction coefficient (AEC) from ILAS‐II agreed with extinctions from SAGE II and SAGE III within ±10% and with extinction from POAM III within ±15% at heights below 20 km. From 20 to 26 km, ILAS‐II AEC was smaller than extinctions from the other three sensors; differences between ILAS‐II and SAGE II ranged from 10% at 20 km to 34% at 26 km in the NH. Over the Southern Hemisphere (SH), ILAS‐II AEC from 20 to 25 km in February was 12–66% below SAGE II extinction. The difference increased with increasing altitude. Comparisons between ILAS‐II and POAM III from January to May in the SH (“non‐PSC season”) yielded qualitatively similar results. From June to October (“PSC season”), ILAS‐II extinction was also smaller than POAM III extinction above 17 km; however, ILAS‐II extinction agreed with POAM III extinction to within ±15% from 12 to 17 km during the PSC season. The comparisons indicate that in both hemispheres the ILAS‐II V1.4 AEC is comparable to extinctions from other measurements below approximately 20 km and systematically low above approximately 20 km although the mean difference is as small as ∼2 × 10−5 km−1 during the non‐PSC season.

Measurements of ClONO2 by the Improved Limb Atmospheric Spectrometer (ILAS) in high‐latitude stratosphere: New products using version 6.1 data processing algorithm

We report the first continuous measurements of chlorine nitrate (ClONO2) in high‐latitude regions taken by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) and processed using the latest data retrieval algorithm (version 6.1). Performance of the measurements, validation with three balloon‐borne sensors, and seasonal variation of ClONO2 in the Arctic and Antarctic stratosphere are presented, as well as a brief description of the version 6.1 algorithm and data characteristics for both the Arctic and Antarctic. Although the ILAS‐measured ClONO2 data show, on average, ∼30% lower values than the validation data, they agree with validation data within the combined total error (∼20–40%) of the ClONO2 measurements at ∼15‐ to 32‐km altitudes. In the Arctic, enhancement of ClONO2 amounts was observed in spring 1997 after the appearance of polar stratospheric clouds (PSCs) inside the polar vortex. This is the result of preference for ClONO2 formation rather than HCl after the activation of ClOx in this Arctic spring of 1997. In the Antarctic, ClONO2 amounts showed strong local time/latitudinal dependence around the austral fall equinox in 1997.

Comparison of microphysical modeling of polar stratospheric clouds against balloon‐borne and Improved Limb Atmospheric Spectrometer (ILAS) satellite observations

A size‐segregated stratospheric aerosol model with detailed polar stratospheric cloud (PSC) microphysics has been developed to study the formation and evolution of PSCs. For comparison with satellite and balloon‐borne observations, one‐dimensional simulations have been performed in Esrange/Kiruna (67.93°N, 21.07°E), Sweden during the period of winter 1996/1997. The modeled proportions of PSCs are consistent with the observations of volume, number, size distributions, and extinction coefficients derived from the in situ balloon‐borne optical particle counter and the Improved Limb Atmospheric Spectrometer (ILAS) satellite sensor, demonstrating the capability of the model to capture PSC events. Model simulations successfully reproduce three major PSC events in the period from January to February 1997. The model is able to produce particle size distribution and number densities typical of the field observations in the Arctic stratosphere. Large HNO3‐containing particles with a median radius of 6 μm and number densities in the range of 10−5 to 10−3 cm−3 are predicted to occur over a broad range of altitudes. Model simulations suggest that the homogeneous nucleation mechanism of type Ia PSCs out of type Ib PSCs plays a critical role in the formation of large HNO3‐containing particles. The present model shows good potential for interpreting and validating the balloon‐borne and satellite observations.

Chemical ozone loss and related processes in the Antarctic winter 2003 based on Improved Limb Atmospheric Spectrometer (ILAS)–II observations

In this study, ILAS‐II (Improved Limb Atmospheric Spectrometer) measurements were used to analyze chemical ozone loss during the entire Antarctic winter 2003, using the tracer‐tracer correlation technique. The temporal evolution of both the accumulated local chemical ozone loss and the loss in column ozone in the lower stratosphere is in step with increasing solar illumination. Half of the entire loss in column ozone of 157 DU occurred during September 2003. By the end of September 2003, almost the total amount of ozone was destroyed between 380 and 470 K. Further, ozone loss rates increased strongly during September for the entire lower stratosphere. The values of accumulated ozone loss and ozone loss rates are strongly dependent on altitude. Once ozone loss is saturated during September, especially at latitudes between 380 and 420 K, ozone loss rates decrease, and accumulated ozone loss can no longer increase. Moreover, at altitudes above 470 K, accumulated ozone loss depends on the amount of PSCs occurring during winter and spring. During September, ozone mixing ratios show a large day to day variation. Box model simulations by the Chemical Lagrangian Model of the Stratosphere (CLaMS) show that this is a result of the different histories of the observed air masses. Further, the box model supports the general evolution of ozone loss values during September as a result of the strong increase of halogen catalyzed ozone destruction.

Intercomparison and validation of ILAS‐II version 1.4 target parameters with MIPAS‐B measurements

A flight of the balloon‐borne version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS‐B) was performed from Kiruna (Sweden, 68°N, 21°E) on 20/21 March 2003 as part of the validation program of the chemistry instruments MIPAS, GOMOS, and SCIAMACHY aboard the European environmental satellite ENVISAT. The 15 hour long duration of this flight provided a good match with the Japanese Improved Limb Atmospheric Spectrometer (ILAS)‐II sensor aboard the ADEOS‐II satellite, launched in December 2002, in addition to the primary goal of ENVISAT validation. The MIPAS‐B flight data coincided nicely with one of the early operational periods of the ILAS‐II instrument, offering one of the sparse opportunities to validate the whole set of trace species measured by ILAS‐II during its unfortunately short lifetime. Radiance spectra were observed by MIPAS‐B at nearly the same location that was observed by ILAS‐II about 5.5 hours prior to the sampling of MIPAS‐B. The intercomparison of ILAS‐II atmospheric target parameters (version 1.4) to profiles measured by MIPAS‐B has shown that for the species O3, N2O, CH4 (below about 22 km), HNO3, ClONO2, and CFC‐11 (CCl3F), a predominantly good consistency with MIPAS‐B has been achieved within the combined errors. However, atmospheric parameters like temperature, H2O, NO2, and N2O5 are widely characterized by low biases compared to MIPAS‐B results while CFC‐12 (CCl2F2) exhibits a high bias in comparison to the balloon‐borne observations. Therefore ILAS‐II profile retrievals of temperature, H2O, NO2, N2O5, and CFC‐12 cannot be assumed to be validated at the present time.

Validation of stratospheric nitric acid profiles observed by Improved Limb Atmospheric Spectrometer (ILAS)–II

The Improved Limb Atmospheric Spectrometer‐II (ILAS‐II) was launched aboard the Advanced Earth Observing Satellite‐II (ADEOS‐II) in December 2002. Stratospheric vertical profiles of nitric acid (HNO3) concentration observed by ILAS‐II (version 1.4) are validated using coincident HNO3 measurements by balloon‐borne instruments (MIPAS‐B2 and MkIV) in March and April 2003. Further validation is performed by making climatological comparisons of lower stratospheric HNO3‐ozone (O3) correlations obtained by ILAS‐II and ILAS (the predecessor of ILAS‐II) for specific potential vorticity‐based equivalent latitudes and seasons where and when ILAS data showed very compact correlations in 1997. The reduced scatter of ILAS‐II HNO3 values around the reference HNO3, which is derived from ILAS‐II O3 using the ILAS HNO3‐O3 correlation, shows that the precision of the ILAS‐II HNO3 data is better than 13–14%, 5%, and 1% at 15, 20, and 25 km, respectively. Combining all of the comparisons made in the present study, the accuracy of the ILAS‐II HNO3 profiles at 15–25 km is estimated to be better than −13%/+26%.

Ozone profiles in the high‐latitude stratosphere and lower mesosphere measured by the Improved Limb Atmospheric Spectrometer (ILAS)‐II: Comparison with other satellite sensors and ozonesondes

A solar occultation sensor, the Improved Limb Atmospheric Spectrometer (ILAS)‐II, measured 5890 vertical profiles of ozone concentrations in the stratosphere and lower mesosphere and of other species from January to October 2003. The measurement latitude coverage was 54–71°N and 64–88°S, which is similar to the coverage of ILAS (November 1996 to June 1997). One purpose of the ILAS‐II measurements was to continue such high‐latitude measurements of ozone and its related chemical species in order to help accurately determine their trends. The present paper assesses the quality of ozone data in the version 1.4 retrieval algorithm, through comparisons with results obtained from comprehensive ozonesonde measurements and four satellite‐borne solar occultation sensors. In the Northern Hemisphere (NH), the ILAS‐II ozone data agree with the other data within ±10% (in terms of the absolute difference divided by its mean value) at altitudes between 11 and 40 km, with the median coincident ILAS‐II profiles being systematically up to 10% higher below 20 km and up to 10% lower between 21 and 40 km after screening possible suspicious retrievals. Above 41 km, the negative bias between the NH ILAS‐II ozone data and the other data increases with increasing altitude and reaches 30% at 61–65 km. In the Southern Hemisphere, the ILAS‐II ozone data agree with the other data within ±10% in the altitude range of 11–60 km, with the median coincident profiles being on average up to 10% higher below 20 km and up to 10% lower above 20 km. Considering the accuracy of the other data used for this comparative study, the version 1.4 ozone data are suitably used for quantitative analyses in the high‐latitude stratosphere in both the Northern and Southern Hemisphere and in the lower mesosphere in the Southern Hemisphere.

ILAS data processing for stratospheric gas and aerosol retrievals with aerosol physical modeling: Methodology and validation of gas retrievals

This paper presents initial results of simultaneous gas and aerosol retrievals from Improved Limb Atmospheric Spectrometer (ILAS) observations taken between November 1996 and June 1997. The solar occultation measurements were processed by an inversion method that included aerosol physical modeling and permitted simultaneous retrieval of O3, HNO3, CH4, H2O, NO2, N2O, N2O5, ClONO2, CFC‐11, and CFC‐12 trace species and particle volume size distributions for key aerosol/polar stratospheric cloud (PSC) components such as liquid ternary solution, nitric acid trihydrate, nitric acid dihydrate, and water ice. The retrieval method was designed for the version 7.0 ILAS data processing algorithm. Gas retrieval results for the entire ILAS data set were compared to results from the earlier version 6.0 retrieval algorithm that was based on aerosol/PSC contribution estimates in the gas window channel. Gas data from nearby balloon‐borne validation measurements and new data on the aerosol retrievals helped explain discrepancies between the two algorithms. The new version 7.0 methodology proved effective for simultaneous retrievals of all trace gases and showed a significant advantage when retrieving CH4, H2O, NO2, N2O, and CFC‐12 from PSC observations.

Characterization of stratospheric liquid ternary solution aerosol from broadband infrared extinction measurements

Characterization of stratospheric liquid ternary solution (LTS) aerosol is studied as an inverse problem in retrieving particle volume size distribution and chemical composition of ternary droplets using the extinction coefficient over wavelengths of 3 to 12 μm. The retrieval method comprises a constrained, weighted least squares minimization procedure between measured and modeled spectra. The constraints restrict the first derivative of the particle size distribution with respect to particle size and restrict the relationship between weight percents of discrete partials (H2SO4/H2O/HNO3) in the ternary droplets using the Carslaw equilibrium model. In the inverse modeling, emphasis is given to minimize the number of reference spectra up to consideration of two mostly binary components composed of H2SO4/H2O and HNO3/H2O solutions. This permits the retrieval of particle volume size distribution independently of particle chemical composition and therefore simply adopts the inversion method for the processing of spaceborne occultation measurements when other aerosol/cloud components along with trace gases are subject to identification. We initially carry out numerical tests by applying the inversion method to synthetic extinction measurements in order to study the limitations of the retrievals over wide ranges of microphysical parameters. A case study in processing actual space‐borne data obtained by the Improved Limb Atmospheric Spectrometer (ILAS) demonstrates the efficiency of the retrievals.

Intercomparison between GOME Ozone Profiles Retrieved by Neural Network Inversion Schemes and ILAS Products

Abstract The Global Ozone Monitoring Experiment (GOME) is a spectrometer boarded on the European Space Agency (ESA) Remote Sensing Satellite (ERS)-2, which measures, in a nadir-viewing geometry, the solar radiation that is scattered by the earth’s atmosphere. Measurements are performed in the wavelength range of 240–790 nm, with a spectral resolution of 0.2–0.4 nm. Vertical ozone profiles can be retrieved from observations of the backscattered light using neural network inversion schemes, which are able to provide real-time estimations with an accuracy that is comparable with the accuracy obtained by means of traditional optimal estimation (OE) methods, which are high-computation demanding and generally offline. In this study, neural network–estimated profiles have been compared to the ozone profiles that are retrieved by the Improved Limb Atmospheric Spectrometer (ILAS) boarded on the Japanese Advanced Earth Observing Satellite (ADEOS). From November 1996 to June 1997 more than 3000 coincident profiles have been found in the northern and southern high-latitude regions of the globe. The relative difference between GOME and ILAS profiles over the entire dataset and over subsets corresponding to different measurement and seasonal conditions have been calculated, and the results have been critically discussed.

Monte Carlo approach to identification of the composition of stratospheric aerosols from infrared solar occultation measurements

We describe an inversion method for determining the composition, density, and size of stratospheric clouds and aerosols by satellite remote sensing. The method, which combines linear least-squares minimization and Monte Carlo techniques, is tested with pure synthetic IR spectra. The synthetic spectral data are constructed to mimic mid-IR spectra recorded by the Improved Limb Atmospheric Spectrometer (ILAS-I and ILAS-II) instruments, which operate in the solar occultation mode and record numerous polar stratospheric cloud events. The advantages and limitations of the proposed technique are discussed. In brief we find that stratospheric aerosol in the size range from 0.5 to 4.0 02114 microm can be retrieved to an accuracy of 30%. We also show that the chemical composition of common stratospheric aerosols can be determined, whereas identification of their phases from mid-IR satellite remote-sensing data alone appears to be questionable.

Simultaneous stratospheric gas and aerosol retrievals from broadband infrared occultation measurements

The inversion method for simultaneous gas (O3, NO2, HNO3, N2O, CH4, H2O, CFC-11, CFC-12, N2O5, and ClONO2) and aerosol retrievals from broadband continuous IR spectra of occultation measurements is described. Both gas and aerosol physical modeling with consideration of the multicomponent character of aerosol and polar stratospheric clouds (PSCs) are used to minimize the difference between measured and modeled transmittance spectra under smoothness constraints imposed on particle-size distributions for each PSC component and positive constraints on all gas and aerosol parameters. The method is tested by numerical simulations in which synthetic occultation measurements inherent to the improved limb atmospheric spectrometer are used. The study reveals that the method has significant advantages over other approaches based on offset or gas-window-channel aerosol correction for accurate gas retrievals and provides additional information on the particle-size composition, volume density, and chemical component character of PSCs.