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Kelly Chance - One of the best experts on this subject based on the ideXlab platform.
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cross evaluation of Gems tropospheric ozone retrieval performance using omi data and the use of an ozonesonde dataset over east asia for validation
Atmospheric Measurement Techniques, 2019Co-Authors: Kang Hyeon Baek, Kelly ChanceAbstract:Abstract. The Geostationary Environment Monitoring Spectrometer (Gems) is scheduled to be launched in 2019–2020 on board the GEO-KOMPSAT (GEOstationary KOrea Multi-Purpose SATellite)-2B, contributing as the Asian partner of the global geostationary constellation of air quality monitoring. To support this air quality satellite mission, we perform a cross-evaluation of simulated Gems ozone profile retrievals from OMI (Ozone Monitoring Instrument) data based on the optimal estimation and ozonesonde measurements within the Gems domain, covering from 5 ∘ S (Indonesia) to 45 ∘ N (south of the Russian border) and from 75 to 145 ∘ E. The comparison between ozonesonde and Gems shows a significant dependence on ozonesonde types. Ozonesonde data measured by modified Brewer–Mast (MBM) at Trivandrum and New Delhi show inconsistent seasonal variabilities in tropospheric ozone compared to carbon–iodine (CI) and electrochemical condensation cell (ECC) ozonesondes at other stations in a similar latitude regime. CI ozonesonde measurements are negatively biased relative to ECC measurements by 2–4 DU; better agreement is achieved when simulated Gems ozone retrievals are compared to ECC measurements. ECC ozone data at Hanoi, Kuala Lumpur, and Singapore show abnormally worse agreements with simulated Gems retrievals than other ECC measurements. Therefore, ECC ozonesonde measurements at Hong Kong, Pohang, Naha, Sapporo, and Tsukuba are finally identified as an optimal reference dataset. The accuracy of simulated Gems retrievals is estimated to be ∼5.0 % for both tropospheric and stratospheric column ozone with the precision of 15 % and 5 %, which meets the Gems ozone requirements.
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cross verification of simulated Gems tropospheric ozone retrievals and ozonesonde measurements over northeast asia
Atmospheric Measurement Techniques Discussions, 2019Co-Authors: Kang Hyeon Baek, Kelly ChanceAbstract:Abstract. The Geostationary Environment Monitoring Spectrometer (Gems) is scheduled to be launched in 2019 on board the GEO-KOMPSAT (GEOstationary KOrea Multi-Purpose SATellite)-2B, contributing as the Asian partner of the global geostationary constellation of air quality monitoring. To support this air quality satellite mission, we perform the cross-verification of simulated Gems ozone profile retrievals based on the Optimal Estimation and ozonesonde measurements within the Gems domain, covering from 5° S (Indonesia) to 45° N (south of the Russian border) and from 75° E to 145° E. The comparison between ozonesonde and Gems shows a significant dependence on ozonesonde types. Ozonesonde data measured by Modified Brewer-Master (MB-M) at Trivandrum and New Delhi show inconsistent seasonal-variabilities in the tropospheric ozone, compared to latitudinally adjacent stations with Carbon Iodine (CI) and Electrochemical Condensation Cell (ECC). CI ozonesonde measurements are biased relative to ECC measurements by 2–4 DU; a better agreement with Gems simulations is achieved with ECC measurements. ECC ozone data at Hanoi, Kuala Lump, and Singapore show abnormally worse agreements with simulated Gems retrievals among ECC measurements. Therefore, ECC ozonesonde measurements at Hong Kong, Pohang, Naha, Sapporo, and Tsukuba are finally identified as an optimal reference. The accuracy of simulated Gems retrievals is estimated to be ~ 5.0 % for both tropospheric and stratospheric column ozone with the precision of 15 % and 5 %, which meet the Gems ozone requirements.
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evaluation of ozone profile and tropospheric ozone retrievals from Gems and omi spectra
Atmospheric Measurement Techniques, 2013Co-Authors: J Bak, Kelly Chance, Xiong Liu, J H Kim, Jhoon KimAbstract:Abstract. South Korea is planning to launch the Gems (Geostationary Environment Monitoring Spectrometer) instrument into the GeoKOMPSAT (Geostationary Korea Multi-Purpose SATellite) platform in 2018 to monitor tropospheric air pollutants on an hourly basis over East Asia. Gems will measure backscattered UV radiances covering the 300–500 nm wavelength range with a spectral resolution of 0.6 nm. The main objective of this study is to evaluate ozone profiles and stratospheric column ozone amounts retrieved from simulated Gems measurements. Ozone Monitoring Instrument (OMI) Level 1B radiances, which have the spectral range 270–500 nm at spectral resolution of 0.42–0.63 nm, are used to simulate the Gems radiances. An optimal estimation-based ozone profile algorithm is used to retrieve ozone profiles from simulated Gems radiances. Firstly, we compare the retrieval characteristics (including averaging kernels, degrees of freedom for signal, and retrieval error) derived from the 270–330 nm (OMI) and 300–330 nm (Gems) wavelength ranges. This comparison shows that the effect of not using measurements below 300 nm on retrieval characteristics in the troposphere is insignificant. However, the stratospheric ozone information in terms of DFS decreases greatly from OMI to Gems, by a factor of ∼2. The number of the independent pieces of information available from Gems measurements is estimated to 3 on average in the stratosphere, with associated retrieval errors of ~1% in stratospheric column ozone. The difference between OMI and Gems retrieval characteristics is apparent for retrieving ozone layers above ~20 km, with a reduction in the sensitivity and an increase in the retrieval errors for Gems. We further investigate whether Gems can resolve the stratospheric ozone variation observed from high vertical resolution Earth Observing System (EOS) Microwave Limb Sounder (MLS). The differences in stratospheric ozone profiles between Gems and MLS are comparable to those between OMI and MLS below ~3 hPa (~40 km), except with slightly larger biases and larger standard deviations by up to 5%. At pressure altitudes above ~3 hPa, Gems retrievals show strong influence of a priori and large differences with MLS, which, however, can be sufficiently improved by using better a priori information. The Gems-MLS differences show negative biases of less than 4% for stratospheric column ozone, with standard deviations of 1–3%, while OMI retrievals show similar agreements with MLS except for 1% smaller biases at middle and high latitudes. Based on the comparisons, we conclude that Gems will measure tropospheric ozone and stratospheric ozone columns with accuracy comparable to that of OMI and ozone profiles with slightly worse performance than that of OMI below ~3 hPa.
Jhoon Kim - One of the best experts on this subject based on the ideXlab platform.
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Spectral Calibration Algorithm for the Geostationary Environment Monitoring Spectrometer (Gems)
Remote Sensing, 2020Co-Authors: Mina Kang, Myoung-hwan Ahn, Xiong Liu, Ukkyo Jeong, Jhoon KimAbstract:The Geostationary Environment Monitoring Spectrometer (Gems) onboard the Geostationary Korean Multi-Purpose Satellite 2B was successfully launched in February 2020. Gems is a hyperspectral spectrometer measuring solar irradiance and Earth radiance in the wavelength range of 300 to 500 nm. This paper introduces the spectral calibration algorithm for Gems, which uses a nonlinear least-squares approach. Sensitivity tests for a series of unknown algorithm parameters such as spectral range for fitting, spectral response function (SRF), and reference spectrum were conducted using the synthetic Gems spectrum prepared with the ground-measured Gems SRF. The test results show that the required accuracy of 0.002 nm is achievable provided the SRF and the high-resolution reference spectrum are properly prepared. Such a satisfactory performance is possible mainly due to the inclusion of additional fitting parameters of spectral scales (shift, squeeze, and high order shifts) and SRF (width, shape and asymmetry). For the application to the actual Gems data, in-orbit SRF is to be monitored using an analytic SRF function and the measured Gems solar irradiance, while a reference spectrum is going to be selected during the instrument in-orbit test. The calibrated Gems data is expected to be released by the end of 2020.
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Spectral Calibration Algorithm for the Geostationary Environment Monitoring Spectrometer (Gems)
2020Co-Authors: Mina Kang, Myoung-hwan Ahn, Xiong Liu, Ukkyo Jeong, Jhoon KimAbstract:The Geostationary Environment Monitoring Spectrometer (Gems) onboard the Geostationary Korean Multi-Purpose Satellite 2B was successfully launched in February 2020. Gems is a hyperspectral spectrometer measuring solar irradiance and Earth radiance in the range of 300 to 500 nm. This paper introduces the spectral calibration algorithm for Gems, which uses a nonlinear least-squares approach. To assess the performance of the algorithm, sensitivity tests for a series of spectral parameters such as shift, spectral range for fitting, signal-to-noise ratio, spectral response function (SRF), and reference spectrum have been conducted. To improve the assessment, a synthetic Gems spectrum using the prelaunch Gems SRF is adopted here. The test results show that the required accuracy (0.002 nm) is achievable for the expected uncertainties of the parameters except for the SRF and the choice of high-resolution reference spectrum, which degrade the algorithm performance by an order magnitude. To mitigate the sensitivity to SRF, retrieval of in-orbit SRF using an analytic function is suggested. Finally, a few candidates for the high-resolution solar reference spectrum are prepared for testing by the instrument during in-orbit tests.
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description of a formaldehyde retrieval algorithm for the geostationary environment monitoring spectrometer Gems
Atmospheric Measurement Techniques, 2019Co-Authors: Hyeong Ahn Kwon, Jhoon Kim, Rokjin J Park, Gonzalo Gonzalez Abad, K Chance, T P Kurosu, Isabelle De Smedt, Michel Van Roozendael, Enno PetersAbstract:Abstract. We describe a formaldehyde (HCHO) retrieval algorithm for the Geostationary Environment Monitoring Spectrometer (Gems) that will be launched by the Korean Ministry of Environment in 2019. The algorithm comprises three steps: preprocesses, radiance fitting, and postprocesses. The preprocesses include a wavelength calibration, as well as interpolation and convolution of absorption cross sections; radiance fitting is conducted using a nonlinear fitting method referred to as basic optical absorption spectroscopy (BOAS); and postprocesses include air mass factor calculations and bias corrections. In this study, several sensitivity tests are conducted to examine the retrieval uncertainties using the Gems HCHO algorithm. We evaluate the algorithm with the Ozone Monitoring Instrument (OMI) Level 1B irradiance/radiance data by comparing our retrieved HCHO column densities with OMI HCHO products of the Smithsonian Astrophysical Observatory (OMHCHO) and of the Quality Assurance for Essential Climate Variables project (OMI QA4ECV). Results show that OMI HCHO slant columns retrieved using the Gems algorithm are in good agreement with OMHCHO, with correlation coefficients of 0.77–0.91 and regression slopes of 0.94–1.04 for March, June, September, and December 2005. Spatial distributions of HCHO slant columns from the Gems algorithm are consistent with the OMI QA4ECV products, but relatively poorer correlation coefficients of 0.52–0.76 are found compared to those against the OMHCHO products. Also, we compare the satellite results with ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations. OMI Gems HCHO vertical columns are 9 %–25 % lower than those of MAX-DOAS at Haute-Provence Observatory (OHP) in France, Bremen in Germany, and Xianghe in China. We find that the OMI Gems retrievals have less bias than the OMHCHO and OMI QA4ECV products at OHP and Bremen in comparison with MAX-DOAS.
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evaluation of ozone profile and tropospheric ozone retrievals from Gems and omi spectra
Atmospheric Measurement Techniques, 2013Co-Authors: J Bak, Kelly Chance, Xiong Liu, J H Kim, Jhoon KimAbstract:Abstract. South Korea is planning to launch the Gems (Geostationary Environment Monitoring Spectrometer) instrument into the GeoKOMPSAT (Geostationary Korea Multi-Purpose SATellite) platform in 2018 to monitor tropospheric air pollutants on an hourly basis over East Asia. Gems will measure backscattered UV radiances covering the 300–500 nm wavelength range with a spectral resolution of 0.6 nm. The main objective of this study is to evaluate ozone profiles and stratospheric column ozone amounts retrieved from simulated Gems measurements. Ozone Monitoring Instrument (OMI) Level 1B radiances, which have the spectral range 270–500 nm at spectral resolution of 0.42–0.63 nm, are used to simulate the Gems radiances. An optimal estimation-based ozone profile algorithm is used to retrieve ozone profiles from simulated Gems radiances. Firstly, we compare the retrieval characteristics (including averaging kernels, degrees of freedom for signal, and retrieval error) derived from the 270–330 nm (OMI) and 300–330 nm (Gems) wavelength ranges. This comparison shows that the effect of not using measurements below 300 nm on retrieval characteristics in the troposphere is insignificant. However, the stratospheric ozone information in terms of DFS decreases greatly from OMI to Gems, by a factor of ∼2. The number of the independent pieces of information available from Gems measurements is estimated to 3 on average in the stratosphere, with associated retrieval errors of ~1% in stratospheric column ozone. The difference between OMI and Gems retrieval characteristics is apparent for retrieving ozone layers above ~20 km, with a reduction in the sensitivity and an increase in the retrieval errors for Gems. We further investigate whether Gems can resolve the stratospheric ozone variation observed from high vertical resolution Earth Observing System (EOS) Microwave Limb Sounder (MLS). The differences in stratospheric ozone profiles between Gems and MLS are comparable to those between OMI and MLS below ~3 hPa (~40 km), except with slightly larger biases and larger standard deviations by up to 5%. At pressure altitudes above ~3 hPa, Gems retrievals show strong influence of a priori and large differences with MLS, which, however, can be sufficiently improved by using better a priori information. The Gems-MLS differences show negative biases of less than 4% for stratospheric column ozone, with standard deviations of 1–3%, while OMI retrievals show similar agreements with MLS except for 1% smaller biases at middle and high latitudes. Based on the comparisons, we conclude that Gems will measure tropospheric ozone and stratospheric ozone columns with accuracy comparable to that of OMI and ozone profiles with slightly worse performance than that of OMI below ~3 hPa.
J. P. Bradley - One of the best experts on this subject based on the ideXlab platform.
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comment on the shape and composition of interstellar silicate grains
arXiv: Astrophysics, 2008Co-Authors: J. P. Bradley, H. A. IshiiAbstract:This Comment is in response to an Erratum to the paper entitled "The shape and composition of interstellar silicate grains" (Min et al., A&A, 462, 667-676 (2007). The Erratum corrects erroneously cited data but upholds the paper's original conclusions that Gems are not remnant interstellar grains and that most Gems formed in the early solar system. In our Comment, we show that the correct data in fact strengthen the relationship between Gems and interstellar silicates, contrary to the conclusions of Min et al. in their Erratum. We also explore the validity of the isotopic and bulk chemical bases for comparison and present additional indicators of past ISM radiation processing of silicates.
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Comment on "The shape and composition of interstellar silicate grains"
2007Co-Authors: J. P. Bradley, H. A. IshiiAbstract:In the paper entitled 'The shape and composition of interstellar silicate grains' (A & A, 462, 667-676 (2007)), Min et al. explore non-spherical grain shape and composition in modeling the interstellar 10 and 20 {micro}m extinction features. This progression towards more realistic models is vitally important to enabling valid comparisons between dust observations and laboratory measurements. Min et al. proceed to compare their model results with Gems (glass with embedded metals and sulfides) from IDPs (interplanetary dust particles) and to discuss the nature and origin of Gems. Specifically, they evaluate the hypothesis of Bradley (1994) that Gems are interstellar (IS) amorphous silicates. From a comparison of the mineralogy, chemical compositions, and infrared (IR) spectral properties of Gems with their modeling results, Min et al. conclude: 'Gems are, in general, not unprocessed leftovers from the diffuse ISM'. This conclusion is based, however, on erroneous and incomplete Gems data. It is important to clarify first that Bradley (1994) never proposed that Gems are unprocessed leftovers from the diffuse ISM, nor did he suggest that individual subnanogram mass Gems are a representative sampling of the enormous mass of silicates in the diffuse ISM. Bradley (1994) simply showed that Gems properties are consistent with those of IS amorphous silicates. It is widely accepted that circumstellar outflows are important sources of IS silicates, and whether Gems are processed or not, the circumstellar heritage of some has been rigorously confirmed through measurements of non-solar oxygen (O) isotope abundances (Messenger et al., 2003; Floss et al., 2006). Keller et al. (2000) assert that even Gems without detectable O isotope anomalies are probably also extrasolar IS silicates because they are embedded in carbonaceous material with non-solar D/H isotopic composition. (Much of the silicate dust in the ISM may be isotopically homogenized (Zhukovska et al., 2007)). Recent measurements show that the elemental compositions of Gems with non-solar isotopic compositions are 'remarkably similar' to those with solar isotopic compositions (Keller & Messenger, 2007). About 80% of all isotopically anomalous IS silicates identified to date are Gems with detectable and variable O isotopic memories of a circumstellar ancestry (Messenger, 2007). Bradley (1999) proposed that Gems are IS silicates from 'a presolar interstellar molecular cloud, presumably the local molecular cloud from which the solar system formed'. Although based on incorrect data (detailed below), Min et al. propose that most Gems actually formed in the presolar molecular cloud, and they further propose that none of them are IS silicates. IS silicate sources include molecular clouds, circumstellar outflows, supernovae, and even recently discovered black hole winds (Molster & Waters; 2003; Jones, 2005; Zhukovska et al. 2007; Markwick-Kemper et al. 2007). The average IS 10 {micro}m extinction feature observed along lines of sight towards the galactic center (modeled by Min et al.) presumably provides a good average for IS silicates, but it cannot distinguish amorphous silicates originating in the presolar molecular cloud from amorphous silicates originating in other interstellar molecular clouds or indeed other sources of amorphous IS silicates. Even if most Gems accreted in the presolar molecular cloud, then they must also be representatives of some portion of the IS amorphous silicate population. Laboratory heating experiments indicate it is highly unlikely that Gems were modified in a protoplanetary accretion disk environment (Brownlee et al. 2005). In summary, Min et al. conclude from their modeling of the shape and composition of IS silicates that the properties of Gems are generally inconsistent with those of IS silicates. First, it has been rigorously confirmed via ion microprobe measurements that some Gems are indeed presolar IS silicates. Second, regardless of whether Gems, or components of Gems, originated in presolar circumstellar outflows or a presolar molecular cloud they are all IS silicates. Third, key Gems data reported in Min et al. are inaccurate. Had complete isotopic, chemical, mineralogical and infrared (IR) spectral properties of Gems been considered, Min et al. may have concluded that the properties of Gems, although not an exact match, are generally consistent with those of amorphous silicates in the ISM.
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Origin and properties of Gems
2006Co-Authors: Z R Dai, J. P. BradleyAbstract:Gems are to the outer solar system what chondrules are to the inner solar system. Ten years after it was first proposed that Gems are the long-sought interstellar amorphous silicates, ion microprobe measurements have confirmed that some of them are indeed interstellar amorphous silicates. The new challenges are to obtain even higher precision isotope measurements from these submicrometer-sized objects and to clarify how and where they originally formed. Individual Gems exhibit a strikingly narrow (0.1-0.5 {micro}m diameter) size distribution and they are systematically depleted from solar abundances in S/Si, Mg/Si, Ca/Si and Fe/Si, implying that they formed by a common mechanism. Mineralogical and petrographic evidence suggest that irradiation processing may be that mechanism. Recent nanometer-scale compositional mapping using new-generation transmission electron microscopes reveals that truly pristine Gems may be relatively rare and new metrics need to be developed to distinguish the primordial properties of Gems from more recent secondary alteration effects.
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Mechanism of Gems formation
The Astrophysical Journal, 2004Co-Authors: J. P. Bradley, Z R DaiAbstract:Gems (glass with embedded metal and sulfides) in interplanetary dust particles (IDPs) were examined using 200 keV analytical transmission electron microscopy. The morphologies and crystallography of embedded relict grains reveal that Gems are pseudomorphs formed by irradiation processing of crystals free-floating in space. Some Gems retain a compositional and morphological ''memory'' of the crystal from which they formed. Pseudomorphism rules out condensation, annealing, flash heating, or shock melting as alternative mechanisms of Gems formation. A significant and often dominant fraction of the atoms in Gems were sputtered deposited from other grains. Therefore, a normal (solar) isotopic composition is not a reliable indicator of whether Gems formed in the solar system or in presolar interstellar or circumstellar environments.
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analysis of a deuterium rich interplanetary dust particle idp and implications for presolar material in idps
Journal of Geophysical Research, 2000Co-Authors: L P Keller, S Messenger, J. P. BradleyAbstract:Deuterium (D)-rich interplanetary dust particles (IDPs) are the most primitive extraterrestrial materials available for laboratory studies in terms of their mineralogy, chemistry, and isotopic compositions. Transmission electron microscopy analysis of one such D-rich IDP shows it to be a highly porous object that consists of crystalline grains (Mg-rich pyroxene and olivine, and FeNi sulfides) and glass with embedded metal and sulfides (Gems) enclosed within a matrix of amorphous carbonaceous material. The nonsolar H isotopic anomaly measured in this IDP provides direct evidence for the incorporation of partially preserved cold molecular cloud material that predates the solar system. These results indicate that organic molecules associated with the carbonaceous matrix of the IDP are the most likely D carrier phase in the particle. Gems are a major constituent of this D-rich IDP. The physical and chemical characteristics of Gems show many similarities to the properties of interstellar amorphous silicates inferred from astronomical observations including: size (diameters of 0.1–0.5 um), solar abundances for heavy elements, presence of superparamagnetic FeNi metal, and an amorphous silicate matrix. Infrared transmission spectra from Gems-rich IDPs show marked similarities to astronomical data for interstellar silicates. The close association of Gems and mineral grains to D-rich matter of probable interstellar origin suggests that these inorganic materials are themselves interstellar grains.
Kang Hyeon Baek - One of the best experts on this subject based on the ideXlab platform.
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cross evaluation of Gems tropospheric ozone retrieval performance using omi data and the use of an ozonesonde dataset over east asia for validation
Atmospheric Measurement Techniques, 2019Co-Authors: Kang Hyeon Baek, Kelly ChanceAbstract:Abstract. The Geostationary Environment Monitoring Spectrometer (Gems) is scheduled to be launched in 2019–2020 on board the GEO-KOMPSAT (GEOstationary KOrea Multi-Purpose SATellite)-2B, contributing as the Asian partner of the global geostationary constellation of air quality monitoring. To support this air quality satellite mission, we perform a cross-evaluation of simulated Gems ozone profile retrievals from OMI (Ozone Monitoring Instrument) data based on the optimal estimation and ozonesonde measurements within the Gems domain, covering from 5 ∘ S (Indonesia) to 45 ∘ N (south of the Russian border) and from 75 to 145 ∘ E. The comparison between ozonesonde and Gems shows a significant dependence on ozonesonde types. Ozonesonde data measured by modified Brewer–Mast (MBM) at Trivandrum and New Delhi show inconsistent seasonal variabilities in tropospheric ozone compared to carbon–iodine (CI) and electrochemical condensation cell (ECC) ozonesondes at other stations in a similar latitude regime. CI ozonesonde measurements are negatively biased relative to ECC measurements by 2–4 DU; better agreement is achieved when simulated Gems ozone retrievals are compared to ECC measurements. ECC ozone data at Hanoi, Kuala Lumpur, and Singapore show abnormally worse agreements with simulated Gems retrievals than other ECC measurements. Therefore, ECC ozonesonde measurements at Hong Kong, Pohang, Naha, Sapporo, and Tsukuba are finally identified as an optimal reference dataset. The accuracy of simulated Gems retrievals is estimated to be ∼5.0 % for both tropospheric and stratospheric column ozone with the precision of 15 % and 5 %, which meets the Gems ozone requirements.
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cross verification of simulated Gems tropospheric ozone retrievals and ozonesonde measurements over northeast asia
Atmospheric Measurement Techniques Discussions, 2019Co-Authors: Kang Hyeon Baek, Kelly ChanceAbstract:Abstract. The Geostationary Environment Monitoring Spectrometer (Gems) is scheduled to be launched in 2019 on board the GEO-KOMPSAT (GEOstationary KOrea Multi-Purpose SATellite)-2B, contributing as the Asian partner of the global geostationary constellation of air quality monitoring. To support this air quality satellite mission, we perform the cross-verification of simulated Gems ozone profile retrievals based on the Optimal Estimation and ozonesonde measurements within the Gems domain, covering from 5° S (Indonesia) to 45° N (south of the Russian border) and from 75° E to 145° E. The comparison between ozonesonde and Gems shows a significant dependence on ozonesonde types. Ozonesonde data measured by Modified Brewer-Master (MB-M) at Trivandrum and New Delhi show inconsistent seasonal-variabilities in the tropospheric ozone, compared to latitudinally adjacent stations with Carbon Iodine (CI) and Electrochemical Condensation Cell (ECC). CI ozonesonde measurements are biased relative to ECC measurements by 2–4 DU; a better agreement with Gems simulations is achieved with ECC measurements. ECC ozone data at Hanoi, Kuala Lump, and Singapore show abnormally worse agreements with simulated Gems retrievals among ECC measurements. Therefore, ECC ozonesonde measurements at Hong Kong, Pohang, Naha, Sapporo, and Tsukuba are finally identified as an optimal reference. The accuracy of simulated Gems retrievals is estimated to be ~ 5.0 % for both tropospheric and stratospheric column ozone with the precision of 15 % and 5 %, which meet the Gems ozone requirements.
Xiong Liu - One of the best experts on this subject based on the ideXlab platform.
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Spectral Calibration Algorithm for the Geostationary Environment Monitoring Spectrometer (Gems)
Remote Sensing, 2020Co-Authors: Mina Kang, Myoung-hwan Ahn, Xiong Liu, Ukkyo Jeong, Jhoon KimAbstract:The Geostationary Environment Monitoring Spectrometer (Gems) onboard the Geostationary Korean Multi-Purpose Satellite 2B was successfully launched in February 2020. Gems is a hyperspectral spectrometer measuring solar irradiance and Earth radiance in the wavelength range of 300 to 500 nm. This paper introduces the spectral calibration algorithm for Gems, which uses a nonlinear least-squares approach. Sensitivity tests for a series of unknown algorithm parameters such as spectral range for fitting, spectral response function (SRF), and reference spectrum were conducted using the synthetic Gems spectrum prepared with the ground-measured Gems SRF. The test results show that the required accuracy of 0.002 nm is achievable provided the SRF and the high-resolution reference spectrum are properly prepared. Such a satisfactory performance is possible mainly due to the inclusion of additional fitting parameters of spectral scales (shift, squeeze, and high order shifts) and SRF (width, shape and asymmetry). For the application to the actual Gems data, in-orbit SRF is to be monitored using an analytic SRF function and the measured Gems solar irradiance, while a reference spectrum is going to be selected during the instrument in-orbit test. The calibrated Gems data is expected to be released by the end of 2020.
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Spectral Calibration Algorithm for the Geostationary Environment Monitoring Spectrometer (Gems)
2020Co-Authors: Mina Kang, Myoung-hwan Ahn, Xiong Liu, Ukkyo Jeong, Jhoon KimAbstract:The Geostationary Environment Monitoring Spectrometer (Gems) onboard the Geostationary Korean Multi-Purpose Satellite 2B was successfully launched in February 2020. Gems is a hyperspectral spectrometer measuring solar irradiance and Earth radiance in the range of 300 to 500 nm. This paper introduces the spectral calibration algorithm for Gems, which uses a nonlinear least-squares approach. To assess the performance of the algorithm, sensitivity tests for a series of spectral parameters such as shift, spectral range for fitting, signal-to-noise ratio, spectral response function (SRF), and reference spectrum have been conducted. To improve the assessment, a synthetic Gems spectrum using the prelaunch Gems SRF is adopted here. The test results show that the required accuracy (0.002 nm) is achievable for the expected uncertainties of the parameters except for the SRF and the choice of high-resolution reference spectrum, which degrade the algorithm performance by an order magnitude. To mitigate the sensitivity to SRF, retrieval of in-orbit SRF using an analytic function is suggested. Finally, a few candidates for the high-resolution solar reference spectrum are prepared for testing by the instrument during in-orbit tests.
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evaluation of ozone profile and tropospheric ozone retrievals from Gems and omi spectra
Atmospheric Measurement Techniques, 2013Co-Authors: J Bak, Kelly Chance, Xiong Liu, J H Kim, Jhoon KimAbstract:Abstract. South Korea is planning to launch the Gems (Geostationary Environment Monitoring Spectrometer) instrument into the GeoKOMPSAT (Geostationary Korea Multi-Purpose SATellite) platform in 2018 to monitor tropospheric air pollutants on an hourly basis over East Asia. Gems will measure backscattered UV radiances covering the 300–500 nm wavelength range with a spectral resolution of 0.6 nm. The main objective of this study is to evaluate ozone profiles and stratospheric column ozone amounts retrieved from simulated Gems measurements. Ozone Monitoring Instrument (OMI) Level 1B radiances, which have the spectral range 270–500 nm at spectral resolution of 0.42–0.63 nm, are used to simulate the Gems radiances. An optimal estimation-based ozone profile algorithm is used to retrieve ozone profiles from simulated Gems radiances. Firstly, we compare the retrieval characteristics (including averaging kernels, degrees of freedom for signal, and retrieval error) derived from the 270–330 nm (OMI) and 300–330 nm (Gems) wavelength ranges. This comparison shows that the effect of not using measurements below 300 nm on retrieval characteristics in the troposphere is insignificant. However, the stratospheric ozone information in terms of DFS decreases greatly from OMI to Gems, by a factor of ∼2. The number of the independent pieces of information available from Gems measurements is estimated to 3 on average in the stratosphere, with associated retrieval errors of ~1% in stratospheric column ozone. The difference between OMI and Gems retrieval characteristics is apparent for retrieving ozone layers above ~20 km, with a reduction in the sensitivity and an increase in the retrieval errors for Gems. We further investigate whether Gems can resolve the stratospheric ozone variation observed from high vertical resolution Earth Observing System (EOS) Microwave Limb Sounder (MLS). The differences in stratospheric ozone profiles between Gems and MLS are comparable to those between OMI and MLS below ~3 hPa (~40 km), except with slightly larger biases and larger standard deviations by up to 5%. At pressure altitudes above ~3 hPa, Gems retrievals show strong influence of a priori and large differences with MLS, which, however, can be sufficiently improved by using better a priori information. The Gems-MLS differences show negative biases of less than 4% for stratospheric column ozone, with standard deviations of 1–3%, while OMI retrievals show similar agreements with MLS except for 1% smaller biases at middle and high latitudes. Based on the comparisons, we conclude that Gems will measure tropospheric ozone and stratospheric ozone columns with accuracy comparable to that of OMI and ozone profiles with slightly worse performance than that of OMI below ~3 hPa.