Abstract. The Thermal and Near Infrared Sensor for Carbon Observation (TANSO)-Fourier Transform Spectrometer (FTS) on board the Greenhouse Gases Observing Satellite (GOSAT) has been observing carbon dioxide (CO 2 ) concentrations in several atmospheric layers in the thermal infrared (TIR) band since its launch. This study compared TANSO-FTS TIR version 1 (V1) CO 2 data and CO 2 data obtained in the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) project in the upper troposphere and lower stratosphere (UTLS), where the TIR band of TANSO-FTS is most sensitive to CO 2 concentrations, to validate the quality of the TIR V1 UTLS CO 2 data from 287 to 162 hPa. We first evaluated the impact of considering TIR CO 2 averaging kernel functions on CO 2 concentrations using CO 2 profile data obtained by the CONTRAIL Continuous CO 2 Measuring Equipment (CME), and found that the impact at around the CME level flight altitudes (∼ 11 km) was on average less than 0.5 ppm at low latitudes and less than 1 ppm at middle and high latitudes. From a comparison made during flights between Tokyo and Sydney, the averages of the TIR upper-atmospheric CO 2 data were within 0.1 % of the averages of the CONTRAIL CME CO 2 data with and without TIR CO 2 averaging kernels for all seasons in the Southern Hemisphere. The results of comparisons for all of the eight airline routes showed that the agreements of TIR and CME CO 2 data were worse in spring and summer than in fall and winter in the Northern Hemisphere in the upper troposphere. While the differences between TIR and CME CO 2 data were on average within 1 ppm in fall and winter, TIR CO 2 data had a negative bias up to 2.4 ppm against CME CO 2 data with TIR CO 2 averaging kernels at the northern low and middle latitudes in spring and summer. The negative bias at the northern middle latitudes resulted in the maximum of TIR CO 2 concentrations being lower than that of CME CO 2 concentrations, which led to an underestimate of the amplitude of CO 2 seasonal variation.

Abstract. CO 2 observations in the free troposphere can be useful for constraining CO 2 source and sink estimates at the surface since they represent CO 2 concentrations away from point source emissions. The thermal infrared (TIR) band of the Thermal and Near Infrared Sensor for Carbon Observation (TANSO) Fourier transform spectrometer (FTS) on board the Greenhouse Gases Observing Satellite (GOSAT) has been observing global CO 2 concentrations in the free troposphere for about 8 years and thus could provide a dataset with which to evaluate the vertical transport of CO 2 from the surface to the upper atmosphere. This study evaluated biases in the TIR version 1 (V1) CO 2 product in the lower troposphere (LT) and the middle troposphere (MT) (736-287 hPa), on the basis of comparisons with CO 2 profiles obtained over airports using Continuous CO 2 Measuring Equipment (CME) in the Comprehensive Observation Network for Trace gases by AIrLiner (CONTRAIL) project. Biascorrection values are presented for TIR CO 2 data for each pressure layer in the LT and MT regions during each season and in each latitude band: 40-20 • S, 20 • S-20 • N, 20-40 • N, and 40-60 • N. TIR V1 CO 2 data had consistent negative biases of 1-1.5 % compared with CME CO 2 data in the LT and MT regions, with the largest negative biases at 541-398 hPa, partly due to the use of 10 µm CO 2 absorption band in conjunction with 15 and 9 µm absorption bands in the V1 retrieval algorithm. Global comparisons between TIR CO 2 data to which the bias-correction values were applied and CO 2 data simulated by a transport model based on the Nonhydrostatic ICosahedral Atmospheric Model (NICAM-TM) confirmed the validity of the bias-correction values evaluated over airports in limited areas. In low latitudes in the upper MT region (398-287 hPa), however, TIR CO 2 data in northern summer were overcorrected by these bias-correction values; this is because the bias-correction values were determined using comparisons mainly over airports in Southeast Asia, where CO 2 concentrations in the upper atmosphere display relatively large variations due to strong updrafts.

Abstract. The thermal infrared (TIR) band of the Thermal and Near Infrared Sensor for Carbon Observation (TANSO)–Fourier Transform Spectrometer (FTS) on board the Greenhouse Gases Observing Satellite (GOSAT) has been observing carbon dioxide (CO2) concentrations in several atmospheric layers since its launch. This study compared TANSO-FTS TIR V1.0 CO2 data and CO2 data obtained in the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) project in the upper troposphere and lower stratosphere (UTLS), where the TIR band of TANSO-FTS is most sensitive to CO2 concentrations, to validate the quality of the TIR V1.0 UTLS CO2 data from 287 to 162 hPa. From a comparison made during flights between Tokyo and Sydney, the averages of the TIR upper atmospheric CO2 data agreed well with the averages of the data obtained by the CONTRAIL Continuous CO2 Measuring Experiment (CME) within 0.1 % for all of the seasons in the Southern Hemisphere. The results of a comparison for all of the eight airline routes showed that the agreement between the TIR and CONTRAIL CO2 data was within 0.5 % on average in the Northern Hemisphere, which was better than the agreement between a priori and CONTRAIL CO2 data. The quality of TIR lower stratospheric CO2 data depends largely on the information content, and therefore has a seasonal dependence. In high latitudes, TIR V1.0 lower stratospheric CO2 data are only valid in the summer. The magnitude of bias in the TIR upper atmospheric CO2 data did not have a clear longitudinal dependence. The comparison results for flights in northern low and middle latitudes showed that the agreement between TIR and CONTRAIL CO2 data in the upper troposphere was worse in the spring and summer than in the fall and winter. This could be attributed to a larger negative bias in the upper atmospheric a priori CO2 data in the spring and summer and a seasonal dependence of spectral bias in TANSO-FTS TIR Level 1B (L1B) radiance data. The negative bias in northern middle latitudes made the maximum of TIR CO2 concentrations lower than that of CONTRAIL CO2 concentrations, which leads to underestimate the amplitude of CO2 seasonal variation.