Algorithm update of the GOSAT/TANSO-FTS thermal infrared CO₂ product (version 1) and validation of the UTLS CO₂ data using CONTRAIL measurements

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 (CO₂) concentrations in several atmospheric layers in the thermal infrared (TIR) band since its launch. This study compared TANSO-FTS TIR version 1 (V1) 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 UTLS CO₂ data from 287 to 162 hPa. We first evaluated the impact of considering TIR CO₂ averaging kernel functions on CO₂ concentrations using CO₂ profile data obtained by the CONTRAIL Continuous CO₂ 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 CO₂ data were within 0.1 % of the averages of the CONTRAIL CME CO₂ data with and without TIR CO₂ 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 CO₂ 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 CO₂ data were on average within 1 ppm in fall and winter, TIR CO₂ data had a negative bias up to 2.4 ppm against CME CO₂ data with TIR CO₂ 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 CO₂ concentrations being lower than that of CME CO₂ concentrations, which led to an underestimate of the amplitude of CO₂ seasonal variation.