Bias assessment of lower and middle tropospheric CO<sub>2</sub> concentrations of GOSAT/TANSO-FTS TIR version 1 product

Abstract

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.

Important evidence of constant low CO2 windows and impacts on the non-closure of the greenhouse effect

“…3 to improve the display of the quality of the figure to avoid a cluttered image and the omitted pressure levels don’t add value (not showing different distribution than the other used pressure levels). The results are different from those in other studies, almost all of which focus on regional characteristics 19 or individual heights 13 . A recent published paper developed a new monthly zonal mean carbon dioxide (CO 2 ) distribution from the upper troposphere to the stratosphere (5–42 km) over 2000–2010 based on a Lagrangian backward trajectory model driven by ERA-Interim reanalysis meteorology and tropospheric CO 2 measurements 29 .…”

“…In previous published work, the largest negative biases in the Thermal and Near Infrared Sensor for Carbon Observation-Fourier Transform Spectrometer (TANSO-FTS) TIR V1 CO 2 profile of GOSAT were found in the mid-troposphere region centred at 500–400 hPa 13 , and the monitoring accuracy of the GOSAT CO 2 concentrations has been determined experimentally to be 4 ppmv 37,38 , which is less sensitive to CO 2 in the mid-troposphere (500 hPa) 26 . The accuracy of AIRS is approximately 1–2 ppmv 36 .…”

“…However, the long-range vertical transport process of atmospheric CO 2 has not been fully investigated using observations from satellite sensors, especially based on Thermal Infrared (TIR) 9,13 , although the CO 2 column abundance based on Shortwave Infrared (SWIR) and its retrieval algorithms have enhanced our knowledge of the global carbon cycle 14–17 . How this process operates in the mid-troposphere and near the surface from a global and regional perspective remains unclear due to several reasons.…”

Carbon dioxide variations in the upper troposphere and lower stratosphere from GOSAT TANSO-FTS TIR profile data

“…In summary, Saitoh et al (2016) found that the CO 2 profile data had consistent negative biases of 1.0%-1.5% as compared with the CME CO 2 data in the LT and MT regions (736-287 hPa). Saitoh et al (2017) found that the CO 2 profile data at the CME altitude range (approximately 11 km) was less than 0.5 ppmv at lower latitudes and 1.0 ppmv at middle and high latitudes. In addition, after applying the bias correction coefficient based on a comparison with CONTRAIL, both the trends and seasonal fluctuations showed good agreement with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM)-Transport Model (TM) (Niwa et al, 2011(Niwa et al, , 2012 after inverse analysis.…”

The Significance of Fast Radiative Transfer for Hyperspectral SWIR XCO2 Retrievals

“…While there are several other space-based instruments which obtain CO 2 absorption spectra in the thermal infrared region (TIR), such as Infrared Atmospheric Sounding Interferometer (IASI), we focus only on measurements from the SWIR and relevant instruments. SWIR measurements have sensitivity to the total CO 2 column down to the surface, whereas TIR measurement sensitivity is peaked at the middle and upper troposphere [3,4]. Both GOSAT and GOSAT-2 feature Fourier transform spectrometers (TANSO-FTS and TANSO-FTS-2) that point to specific locations for each single measurement.…”