An 11-year record of XCO<sub>2</sub> estimates derived from GOSAT measurements using the NASA ACOS version 9 retrieval algorithm

Abstract

Abstract. The Thermal And Near infrared Sensor for carbon Observation – Fourier Transform Spectrometer (TANSO-FTS) on the Japanese Greenhouse gases Observing SATellite (GOSAT) has been returning data since April 2009. The version 9 (v9) Atmospheric Carbon Observations from Space (ACOS) Level 2 Full Physics (L2FP) retrieval algorithm (Kiel et al., 2019) was used to derive estimates of carbon dioxide (CO2) dry air mole fraction (XCO2) from the TANSO-FTS measurements collected over its first 11 years of operation. The bias correction and quality filtering of the L2FP XCO2 product were evaluated using estimates derived from the Total Carbon Column Observing Network (TCCON) as well as values simulated from a suite of global atmospheric inversion systems (models) which do not assimilate satellite-derived CO2. In addition, the v9 ACOS GOSAT XCO2 results were compared with collocated XCO2 estimates derived from NASA's Orbiting Carbon Observatory-2 (OCO-2), using the version 10 (v10) ACOS L2FP algorithm. These tests indicate that the v9 ACOS GOSAT XCO2 product has improved throughput, scatter, and bias, when compared to the earlier v7.3 ACOS GOSAT product, which extended through mid 2016. Of the 37 million soundings collected by GOSAT through June 2020, approximately 20 % were selected for processing by the v9 L2FP algorithm after screening for clouds and other artifacts. After post-processing, 5.4 % of the soundings (2×106 out of 37×106) were assigned a “good” XCO2 quality flag, as compared to 3.9 % in v7.3 (<1 ×106 out of 24×106). After quality filtering and bias correction, the differences in XCO2 between ACOS GOSAT v9 and both TCCON and models have a scatter (1σ) of approximately 1 ppm for ocean-glint observations and 1 to 1.5 ppm for land observations. Global mean biases against TCCON and models are less than approximately 0.2 ppm. Seasonal mean biases relative to the v10 OCO-2 XCO2 product are of the order of 0.1 ppm for observations over land. However, for ocean-glint observations, seasonal mean biases relative to OCO-2 range from 0.2 to 0.6 ppm, with substantial variation in time and latitude. The ACOS GOSAT v9 XCO2 data are available on the NASA Goddard Earth Science Data and Information Services Center (GES-DISC) in both the per-orbit full format (https://doi.org/10.5067/OSGTIL9OV0PN, OCO-2 Science Team et al., 2019b) and in the per-day lite format (https://doi.org/10.5067/VWSABTO7ZII4, OCO-2 Science Team et al., 2019a). In addition, a new set of monthly super-lite files, containing only the most essential variables for each satellite observation, has been generated to provide entry level users with a light-weight satellite product for initial exploration (CaltechDATA, https://doi.org/10.22002/D1.2178, Eldering, 2021). The v9 ACOS Data User's Guide (DUG) describes best-use practices for the GOSAT data (O'Dell et al., 2020). The GOSAT v9 data set should be especially useful for studies of carbon cycle phenomena that span a full decade or more and may serve as a useful complement to the shorter OCO-2 v10 data set, which begins in September 2014.

A 10-year global monthly averaged terrestrial net ecosystem exchange dataset inferred from the ACOS GOSAT v9 XCO2 retrievals (GCAS2021)

“…S7), resulting in the large difference in the posterior NBE between these two studies, while in the second stage, because of the better spatial coverage of OCO-2 XCO 2 , the NBE in CMS-Flux increases significantly, resulting in a shift of NBE from an a priori strong sink to an a posteriori medium source, thus reducing the difference of the posterior NBE in these two studies. We also find that there is also an increase in the NBE in this study, which may be related to the increase of GOSAT XCO 2 data from 2010 to 2019 (Taylor et al, 2022).…”

“…The GOSAT satellite launched in 2009 (Kuze et al, 2009) was developed jointly by the National Institute for Environmental Studies (NIES), the Japanese Space Agency (JAXA), and the Ministry of the Environment (MOE) of Japan, which was designed to retrieve total column abundances of CO 2 and CH 4 . In this study, the GOSAT XCO 2 retrieval is the ACOS Version 9.0 Level 2 Lite product (Taylor et al, 2022) at the pixel level during May 2009-December 2019. The bias correction and quality filtering of this XCO 2 product have been evaluated using estimates derived from the Total Carbon Column Observing Network (TCCON) as well as values simulated from a suite of global atmospheric inverse modeling systems (models); the results show that the differences in XCO 2 between GOSAT v9 and both TCCON and models have an 1σ error of approximately 1 ppm for ocean-glint observations and 1 to 1.5 ppm for land observations, and globally, the mean biases are less than approximately 0.2 ppm (Taylor et al, 2022).…”

“…In this study, the GOSAT XCO 2 retrieval is the ACOS Version 9.0 Level 2 Lite product (Taylor et al, 2022) at the pixel level during May 2009-December 2019. The bias correction and quality filtering of this XCO 2 product have been evaluated using estimates derived from the Total Carbon Column Observing Network (TCCON) as well as values simulated from a suite of global atmospheric inverse modeling systems (models); the results show that the differences in XCO 2 between GOSAT v9 and both TCCON and models have an 1σ error of approximately 1 ppm for ocean-glint observations and 1 to 1.5 ppm for land observations, and globally, the mean biases are less than approximately 0.2 ppm (Taylor et al, 2022). Compared with its previous version (ACOS v7.3), the proportion of data with a "good" XCO 2 quality flag has increased from 3.9 % in v7.3 to 5.4 % in v9.…”

“…GOSAT and OCO-2 XCO 2 have large differences on spatial resolution and coverage, which may lead to discontinuities in the inversion results of certain regions. The ACOS GOSAT v9 XCO 2 data are now available on the NASA Goddard Earth Science Data and Information Services Center (GES-DISC), which spans April 2009 to June 2020 and has been bias-corrected and quality-filtered well (Taylor et al, 2022). In this study, based on the GOSAT v9 XCO 2 retrievals, we generate a 10-year global monthly NEE dataset from 2010 to 2019 (GCAS2021) using the wellconstructed Global Carbon Assimilation System, version 2 (GCASv2) (Jiang et al, 2021;Wang et al, 2021a).…”

Retrieval of greenhouse gases from GOSAT and GOSAT-2 using the FOCAL algorithm

“…The Japanese National Institute for Environmental Studies (NIES) provides operational XCO 2 , XCH 4 (Yoshida et al, 2013) and XH 2 O products (Dupuy et al, 2016). NASA also released an XCO 2 product based on the ACOS v9 retrieval, recently described by Taylor et al (2022). A precursor of the FOCAL XCO 2 product v1.0 from Noël et al (2021) is the BESD v01.04 product, which is also from the Institute of Environmental Physics (IUP), Bremen (Heymann et al, 2015).…”