Bias assessment of lower and middle tropospheric CO 2 concentrations of GOSAT/TANSO-FTS TIR version 1 product
CO 2 observations in the free troposphere can be useful for constraining CO 2 source and sink estimates at the surface due to their representativeness being away from local point sources of CO 2 . 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. Bias-correction 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°S–20°S, 20° S–20° N, 20° N–40° N, and 40° N–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-μm 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 Nonhydrostatic ICosahedral Atmospheric Model (NICAM)-based transport model (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 East Asia where CO 2 concentrations in the upper atmosphere display relatively large variations due to strong updrafts.