Bias assessment of lower and middle tropospheric CO₂ concentrations of GOSAT/TANSO-FTS TIR version 1 product

Abstract. CO₂ observations in the free troposphere can be useful for constraining CO₂ source and sink estimates at the surface since they represent CO₂ 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 CO₂ concentrations in the free troposphere for about 8 years and thus could provide a dataset with which to evaluate the vertical transport of CO₂ from the surface to the upper atmosphere. This study evaluated biases in the TIR version 1 (V1) CO₂ product in the lower troposphere (LT) and the middle troposphere (MT) (736–287 hPa), on the basis of comparisons with CO₂ profiles obtained over airports using Continuous CO₂ Measuring Equipment (CME) in the Comprehensive Observation Network for Trace gases by AIrLiner (CONTRAIL) project. Bias-correction values are presented for TIR CO₂ 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 CO₂ data had consistent negative biases of 1–1.5 % compared with CME CO₂ data in the LT and MT regions, with the largest negative biases at 541–398 hPa, partly due to the use of 10 µm CO₂ absorption band in conjunction with 15 and 9 µm absorption bands in the V1 retrieval algorithm. Global comparisons between TIR CO₂ data to which the bias-correction values were applied and CO₂ 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 CO₂ 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 CO₂ concentrations in the upper atmosphere display relatively large variations due to strong updrafts.