Naoko Saitoh

Arctic polar stratospheric clouds observed with the Improved Limb Atmospheric Spectrometer during winter 1996/1997

The newest retrieval (version 4.20) of the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) captured more than 60 polar stratospheric cloud (PSC) profiles during the winter and early spring of 1997 in the Northern Hemisphere. That winter is well known for its long-lasting polar vortex and significant ozone loss over the Arctic. The ILAS PSC measurements were the only spaceborne measurements made on a regular basis (about 14 times daily) during that period. PSC events were selected by comparing an individual profile with a threshold value at each altitude that was defined as an average of the extinction coefficient of background aerosols plus five standard deviations. Many of the selected PSC events correspond to temperatures lower than the nitric acid trihydrate (NAT) temperature, which was calculated using nitric acid and water vapor data observed with ILAS. The correlation between the aerosol extinction coefficient and temperature shows that the extinction data increase as the temperature decreases to a point several degrees lower than the NAT temperature, suggesting the formation of particles of a supercooled ternary solution. Some of the nitric acid profiles corresponding to intense PSC events showed a decreased mixing ratio, suggesting the uptake of nitric acid in the gas-phase into particles. The highest probability of sighting PSCs was obtained in mid-January at an altitude of approximately 23 km, and subsequent occurrences of PSCs were found intermittently at lower altitudes until mid-March. The 1997 Arctic winter was characterized by the prolonged appearance of PSCs until mid-March, associated with a long-lasting polar vortex. The PSC data presented in this paper compensate for the gap in the long-term PSC record from space and help to reveal the chemical mechanisms that caused the Arctic ozone loss observed that season.

ILAS observations of chemical ozone loss in the Arctic vortex during early spring 1997

Chemical ozone loss rates were estimated for the Arctic stratospheric vortex by using ozone profile data (Version 3.10) obtained with the Improved Limb Atmospheric Spectrometer (ILAS) for the spring of 1997. The analysis method is similar to the Match technique, in which an air parcel that the ILAS sounded twice at different locations and at different times was searched from the ILAS data set, and an ozone change rate was calculated from the two profiles. A statistical analysis indicates that the maximum ozone loss rate was found on the 450 K potential temperature surface in February, amounting to 84 ppbv/day. The integrated ozone loss for two months from February to March 1997 showed its maximum of 1.5±0.1 ppmv at the surface that followed the diabatic descent of the air parcels and reached the 425 K level on March 31. This is about 50% of the initial (February 1) ozone concentration. The present study demonstrated that data from a solar occultation sensor with a moderate altitude resolution can be used for the Match analysis.

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.

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

This study assesses polar stratospheric nitrous oxide (N(2)O) and methane (CH(4)) 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 N(2)O and the Halogen Occultation Experiment (HALOE) for CH(4). Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N(2)O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH(4) (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 N(2)O and CH(4) data differ between hemispheres. For ILAS-II N(2)O VMR larger than 250 ppbv, the ILAS-II N(2)O agrees with the balloon/SMR N(2)O within +/- 20% in both hemispheres. The ILAS-II N(2)O in the VMR range from 30-50 to 250 ppbv (corresponding to altitudes of similar to 17-30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and similar to 13-21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by similar to 10-30% than the balloon/SMR N(2)O. For ILAS-II N(2)O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N(2)O are within +/- 10 ppbv. For ILAS-II CH(4) VMR larger than 1 ppmv ( similar to 30 km) and the ILAS-II CH(4) for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.

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 O 3 , HNO 3 , CH 4 , H 2 O, NO 2 , N 2 O, N 2 O S , ClONO 2 , 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 CH 4 , H 2 O, NO 2 , N 2 O, and CFC-12 from PSC observations.

Retrieval of minor constituents from thermal infrared spectra observed by GOSAT TANSO-FTS sensor

The thermal infrared band of the main sensor of the greenhouse gas observing satellite (GOSAT), the TANSO-FTS, must be calibrated with accuracy higher than 0.3 K in the brightness temperature Tbb for retrieving CO2 concentration with accuracy of 1% in the upper atmosphere. However, that accuracy has not been achieved because of some error sources. One is the systematic bias in the radiance spectrum resulting from effects of radiation emitted from internal optics and multiple scattering of target signals. Another is the polarization effect of the pointing mirror. Both effects can be merged into two parameters, gain and offset, in the two point calibration procedure. They can be tuned by comparing the spectrum with well-calibrated spectra such as those from the AIRS sensor. Based on the corrected radiance spectra, global CO2 concentrations were processed. However, they show peculiar latitudinal distribution implying the existence of temporally variant parameters that can affect the calibration. This bias can be reduced by referring to housekeeping data of the satellite in the calibration procedure. The stratospheric ozone distribution is also analyzed. The sensor demonstrated the difference in the ozone hole feature between spring 2009 and 2010 over the South Pole.

Tangent height registration method for the Version 1.4 data retrieval algorithm of the solar occultation sensor ILAS-II

The Improved Limb Atmospheric Spectrometer-II (ILAS-II) is a satellite-borne solar occultation sensor onboard the Advanced Earth Observing Satellite-II (ADEOS-II). The ILAS-II succeeded the ILAS. The ILAS-II used four grating spectrometers to observe vertical profiles of gas volume mixing ratios of trace constituents and was also equipped with a Sun-edge sensor to determine tangent heights geometrically with high precision. The accuracy of gas volume mixing ratios depends on the accuracy of the tangent height determination. The combination method is a tangent height registration method that was developed to give appropriate tangent heights for the ILAS-II Version 1.4 data retrieval algorithm. This study describes the method used in the ILAS-II Version 1.4 retrieval algorithm to register tangent heights. The root-sum-square total random error is estimated to be 30 m, and the total systematic error is 180 m at an altitude of 30 km. The influence of the tangent height errors on the vertical profiles of gas volume mixing ratios in ILAS-II Version 1.4 is estimated by using the relative difference. The relative difference for each species is within 7% (20%) for an altitude shift of +/-100 m(+/-300 m).

A comparative study of stratospheric temperatures between ILAS-II and other data

The successor of the Improved Limb Atmospheric Spectrometer (ILAS), ILAS-II, aboard the Advanced Earth Observing Satellite-II (ADEOS-II) measured atmospheric absorption spectra at a wavelength region from 753 nm to 784 nm, including the molecular oxygen (O2) A-band centered at 762 nm, with a FWHM spectral resolution of 0.06 nm. Temperature and pressure profiles between ~10 km and 80 km were retrieved from the solar occultation measurements of the O2 A-band spectra during the operational period of ADEOS-II in 2003. Based on the actual measured data during the smallest atmospheric variability, the repeatability of the measurement, which is a measure of the measurement precision, for temperature and pressure was estimated to be 1-2 K and 0.5-2%, respectively. Comparisons between ILAS-II and the U.K. Met. Office (UKMO) stratospheric analyses or the NASAs UARS/HALOE and TIMED/SABER temperature data are performed. Regardless of the good precision, it is found that the ILAS-II temperatures are systematically lower in the stratosphere and significantly higher in the lower mesosphere.

Radiometric calibration accuracy of GOSAT-TANSO-FTS (TIR) relating to CO2 retrieval error

Radiometric calibration accuracy of 0.3 K in Tbb is necessary to retrieve CO2 concentration profile with accuracy of 1 % in the upper atmosphere. In case of the thermal infrared (TIR) band (band 4) of GOSAT-TANSO-FTS, interferometric phase correction procedure is very important because the total transmittance of the optics at the band is about 70 % because of opacity of dichroic mirrors of band 1-3 placed obstructing the field of view of band 4, and the mirrors reflect the radiation emitted from inside of the optics. Based on the results from the thermal vacuum tests (TVTs) of the sensor, it is found that interferometric signal is almost zero when the sensor view a target of which temperature is about 280- 300K because the radiation emitted from inside of the spectrometer controlled at about 296 K has completely opposite phase to that of the target. It is also found that the interferometric final phase of the calibrated signal varies when the total signal is almost zero because of weak signals that have phases differ from both of those of targets and calibrators. A candidate phase correction procedure is proposed based on that adopted for a previous space FTS sensor, IMG/ADEOS. Non-linearity correction for the detector and polarization efficiency correction are also desccussed.

Stratospheric background aerosols and polar stratospheric clouds observed with satellite sensors: inference of particle composition and sulfate amount

The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) successfully observed atmospheric profiles over the Arctic and Antarctic from November 1996 through June 1997. It revealed the frequent occurrence of Polar Stratospheric Clouds (PSCs) over the Arctic between January and mid-March 1997. The ILAS provides a unique data set, including aerosol extinction at 780 nm, nitric acid, water vapor, and nitrous oxide, simultaneously. This paper demonstrates the validity of the ILAS aerosol data and presents an approach to estimate the chemical composition of PSCs. Comparisons are made with data from the Stratospheric Aerosol and Gas Experiment (SAGE) II.