Abstract
Abstract. The Total Carbon Column Observing Network (TCCON) is a network of ground-based Fourier transform spectrometers (FTSs) that record near-infrared (NIR) spectra of the sun. From these spectra, accurate and precise observations of CO2 column-averaged dry-air mole fractions (denoted XCO2) are retrieved. TCCON FTS observations have previously been used to validate satellite estimations of XCO2; however, our knowledge of the short-term spatial and temporal variations in XCO2 surrounding the TCCON sites is limited. In this work, we use the National Institute for Environmental Studies (NIES) Eulerian three-dimensional transport model and the FLEXPART (FLEXible PARTicle dispersion model) Lagrangian particle dispersion model (LPDM) to determine the footprints of short-term variations in XCO2 observed by operational, past, future and possible TCCON sites. We propose a footprint-based method for the collocation of satellite and TCCON XCO2 observations and estimate the performance of the method using the NIES model and five GOSAT (Greenhouse Gases Observing Satellite) XCO2 product data sets. Comparison of the proposed approach with a standard geographic method shows a higher number of collocation points and an average bias reduction up to 0.15 ppm for a subset of 16 stations for the period from January 2010 to January 2014. Case studies of the Darwin and Reunion Island sites reveal that when the footprint area is rather curved, non-uniform and significantly different from a geographical rectangular area, the differences between these approaches are more noticeable. This emphasises that the collocation is sensitive to local meteorological conditions and flux distributions.
Highlights
Satellite observations of the column-averaged dry-air mole fraction of CO2 (XCO2) have the potential to significantly advance our knowledge of carbon dioxide (CO2) distributions globally and provide new information on regional CO2 sources and sinks
National Institute for Environmental Studies (NIES) transport model (TM) CO2 concentrations sampled at the location of Total Carbon Column Observing Network (TCCON) sites at the level of 1 km above ground at 13:00 local time were used to initialise backward tracer simulations with the FLEXPART model
There are no TCCON sites in Alaska or on the east coast of North America, which is a region of intense anthropogenic activity (Fig. 1)
Summary
Satellite observations of the column-averaged dry-air mole fraction of CO2 (XCO2) have the potential to significantly advance our knowledge of carbon dioxide (CO2) distributions globally and provide new information on regional CO2 sources and sinks. Observations of XCO2 are available from space-based instruments such as the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY; data available for the period 2002– 2012; Bovensmann et al, 1999), the Greenhouse Gases Observing Satellite (GOSAT; data available since 2009; Kuze et al, 2009, 2016; Yokota et al, 2009) and the Orbiting Carbon Observatory-2 (OCO-2; available since the middle of 2014; Crisp et al, 2004) These satellites provide unprecedented spatial coverage of the variability in XCO2 around the world, with the exception of polar regions and areas with dense clouds. FTS observations are used to validate satellite retrievals in order to assess bias, variability and other key parameters (e.g. Wunch et al, 2011; Lindqvist et al, 2015)
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